Laser has the advantages of high brightness, directivity, energy, and beam quality. In recent years, laser technology has been widely used in industrial sensing, communication, and medical treatment, particularly in treating vascular diseases. Thrombosis is a serious vascular disease with a complicated pathogenesis. Thrombosis can cause blood clots in blood vessels, resulting in insufficient blood supply to vital organs. Ischemic stroke is an acute cerebrovascular disease, mainly occasioned by atherosclerosis of the arteries supplying blood to the brain, which in turn results in blockage of blood vessels and insufficient blood supply to the brain; long-term obstruction can lead to brain tissue necrosis. If a blood clot flows into the heart, it can cause a myocardial infarction. Moreover, if lower extremity thrombosis is serious, it can cause a blood circulation disorder at the end of the extremity and even gangrene. Venous vascular injury, endothelial dysfunction, and slow blood flow are all critical factors in developing deep vein thrombosis. Severe consequences of deep vein thrombosis can lead to pulmonary embolism and amputation. Therefore, thrombus is a vascular disease that seriously endangers human life, health, and safety, and its treatment is the fundamental method for recovery.

The rapid development of laser technology has promoted research progress in laser medical treatment. In particular, pulsed laser has broad application prospects in the fields of industry, medicine, and communication owing to its high repetition rate, energy peak power, and beam quality. The effect of laser on thrombus is mainly realized by the photothermal, photochemical, and photomechanical effects between the thrombus and biological tissue to achieve laser thrombolysis. Laser thrombolysis has the advantage of accurate localization, which can eliminate the thrombus at the site of the blood vessels, restore blood flow, avoid severe injury caused by surgery, and reduce postoperative complications. Because the ablation time is short, postoperative patients recover quickly and reduce hospital stay and medical costs. Therefore, the study of laser thrombolysis is of great significance.

With the development of laser technology and the continuous improvement of the interaction mechanism between laser and biological tissue, the application of laser in thrombus ablation has progressed in some aspects. With the same sample and conditions, the thermal cautery of continuous-wave laser to tissue is significantly higher than that of a pulsed laser. Hence, a pulsed laser is mostly used in laser thrombolysis to avoid unnecessary damage to surrounding tissue. Optimization of pulsed-laser parameters—such as wavelength, pulse width, power, and energy—is the future development direction of pulsed-laser thrombolysis. Furthermore, in laser thrombolysis, the direct use of bare optical fibers to generate laser has certain adverse effects on biological tissues, whereas the use of radial fibers requires lower energy, which significantly reduces the adverse effects. Consequently, various launch fibers have been developed. Moreover, laser thrombolysis combined with various other thrombolysis methods, such as mechanical thrombectomy, can increase the success rate of surgery and reduce the recurrence rate of restenosis, which is of great significance for thrombolysis. In-vitro and clinical tests have shown that the light emitted by 308 nm and 355 nm excimer lasers is a cold light source that generates less heat and penetrates less deeply into tissues, especially for calcified thrombus. Several studies support this conclusion. The use of multiple laser wavelengths to ablate thrombi has also been verified by various experiments. Different wavelengths of lasers absorb different tissue components differently and have specific ablation effects on different thrombus types and sites. Therefore, many research institutions have studied and developed multi-band laser thrombolytics.

This paper reviews the application status of laser in the treatment of thrombosis, mainly from in-vitro and clinical settings. Further, it summarizes the application progress of laser thrombolysis and the possible future development direction. Currently, research on laser medical treatment is developing in the direction of multi-band and multi-application fields. Meanwhile, laser technologies such as multi-fiber and multi-diameter types are being developed to reduce the incidence of complications. For excimer lasers, realizing high power, full fiber structure, and low cost is the main research prospect. Moreover, deepening the research on the mechanism of interaction between laser and tissue while understanding every process and stage of organizational change can optimize the laser parameters to realize real-time adjustment and dynamically control the process of laser thrombolysis. This forms an essential part of theoretical research to guide the actual application process.

Over the past few decades, endoscopes have been used to view the interior of cavities in the human body or the surfaces of internal human organs noninvasively for diagnosis or treatment. However, white light endoscopy and magnifying endoscopy widely used in clinical practice have poor resolution and contrast and require pathological biopsy examination to confirm the diagnosis. In recent years, narrow-spectrum technology has used blue light via optical or digital filtering to irradiate tissues and enhance the microstructural and microvascular morphology of the mucosal surface, improving the imaging contrast. However, it still exhibits poor resolution. White light and narrow-spectrum endoscopy cannot achieve cellular-level resolution; therefore, a purely optical biopsy cannot be performed, significantly reducing diagnosis accuracy. Confocal endoscopy has emerged owing to its submicron resolution and optical sectioning capability. Cell morphology observed using confocal endoscopy is highly consistent with the biopsy pathology. Since its introduction in 2004, confocal laser endomicroscopy (CLE) has become a vital technique in gastrointestinal endoscopic imaging. Confocal laser endoscopy enables endoscopists to perform cellular imaging and tissue structure assessments at the focal plane during endoscopic testing. Thus, real-time in vivo histological information can be obtained, enabling "optical biopsy."

Confocal microscopy was first developed in 1957 by Minsky, who used pinholes on the illumination and detection sides in the same conjugate image plane to achieve "confocal." In 1967, Egger and Petrǎn successfully used confocal microscopy for label-free imaging of neural tissues. The key to confocal microscopy imaging technology is that the "double focus" of the two pinholes can shield all signals from the nonfocal plane, and the photomultiplier tube behind the detection pinhole can detect only the signal from the focal plane to achieve optical sectioning. Depending on the source of the image contrast, laser scanning confocal microscopy can be performed in the fluorescence or reflectance mode. Fluorescence confocal microscopy requires fluorescent contrast agents to generate contrast, yielding spatial and functional information about endogenous autofluorescence and exogenously labeled molecules and structures. Reflection confocal microscopy relies on differences in the refractive indices of cellular structures to generate natural contrast.

Based on the scanning method, confocal endoscopy in confocal endoscopic imaging technology is divided into endoscopy-integrated and probe-based confocal endoscopy. As shown in Figure 2, the endoscopy-integrated confocal endoscope adopts the distal scanning mode [Fig. 2(b)], whereas the probe-based confocal endoscope adopts the proximal scanning mode [Fig. 2(a)]. The eCLE uses a point-scanning method to drive a single optical fiber to scan through a scanning device, achieving high-resolution confocal endoscopic imaging. Because the eCLE adopts a distal scanning method and the mechanical scanning device is included in the imaging probe, it is necessary to miniaturize the mechanical scanning device. However, the miniaturization of this device required for confocal endoscopy is technically challenging and expensive. Therefore, eCLE is limited to clinical applications because of the limited size of the mechanical scanning device. The pCLE probe does not contain a scanning device, and the scanning device does not have size limitation. However, its resolution is limited by the distance between the cores, and the imaging quality is affected by the honeycomb structure of the fiber bundle.

As both eCLE and pCLE are based on traditional confocal microscopy imaging techniques, they use a single excitation wavelength. However, the traditional confocal endoscope requires mechanical scanning to complete three-dimensional imaging, and the imaging speed is low. Therefore, traditional confocal endoscopic microscopy-imaging solutions cannot achieve rapid three-dimensional deep tissue imaging or real-time optical diagnosis in clinical practice. Spectral-encoded confocal microscopy (SECM) is a reflection confocal microscopy technique. It can be used to determine the spatial position of a sample by measuring the spectrum of light reflected from the sample. It can significantly increase the confocal imaging speed, enabling large-area imaging within a short time. The high imaging rate of SECM can potentially increase the confocal field of view, but the imaging depth of the focus is still limited to 200 μm. At more significant imaging depths, the effective resolution of SECM is significantly reduced owing to light scattering and optical aberrations.

In recent years, chromatic confocal technology has been used to achieve high-resolution, fast, and multi-depth imaging. Chromatic confocal endoscopy solves the problem of insufficient imaging depth in traditional confocal endoscopy, and shows significant potential in gastric cancer diagnosis. However, it cannot guarantee large chromatic and small spherical aberration simultaneously in miniaturization, owing to the limitations of lens-manufacturing technology, resulting in limited axial resolution.

Confocal endoscopes enable in vivo and real-time imaging of different tissues, cells, molecules, and even bacteria owing to the higher magnification and resolution of confocal endoscopes than those of conventional endoscopes. In particular, confocal endoscopic imaging technology has promising applications in diagnosing diseases in the human body, such as the gastrointestinal tract, skin, cervix, and eye.

In this paper, confocal endoscopic imaging technology is briefly described. A comparative introduction is presented for fluorescence confocal imaging, reflectance confocal imaging, and probe-based and endoscope-integrated confocal endoscopic imaging. Furthermore, the application of confocal endomicroscopy in biomedical science is discussed.

The incidence and mortality rates of digestive tract cancers are rising quickly globally, greatly endangering human life and health. Most digestive tract tumors come from precancerous lesions, and the development of early cancer detection and diagnosis technology is crucial to improving people’s health. To date, histopathological examination is still the "gold standard" for the clinical diagnosis of cancer, but this method has limitations, such as time-consuming and in vitro detection. Additionally, while biopsy sampling can examine the pathological characteristics of the suspected lesion area at the cellular scale, it cannot achieve full coverage of the suspected lesion area, so there is a certain risk of missed detection and false detection. Therefore, there is an urgent need to develop real-time, in vivo, in situ histological diagnostic techniques at the cellular scale to achieve early diagnosis of GI (gastrointestinal) cancers.

Two-photon endomicroscopy is a new type of endomicroscopic imaging technology based on the principle of two-photon excitation, with the technical advantages of optical-sectioning capability, deep penetration, low phototoxicity, and label-free imaging. This technique can realize structural imaging and functional imaging, which has great potential for applications in life science and clinical medicine.

Piezoelectric ceramic scanning two-photon endomicroscopy is the current preferred solution for two-photon endomicroscopy imaging technology. In recent years, this technique has achieved technological breakthroughs and new applications. This paper summarizes piezoelectric ceramic scanning two-photon endomicroscopic imaging technology and the research progress and introduces its application in the field of biomedical imaging.

Section 2 introduces three typical two-photon endomicroscopy systems: fiber bundle proximal scanning scheme, MEMS distal scanning scheme, and piezoelectric ceramic-driven fiber distal scanning scheme (Fig. 1). Subsequently, the system structure and breakthroughs in core device technology of piezoelectric ceramic scanning two-photon endomicroscopy in recent years are summarized (Fig. 2). It mainly includes low-dispersion low-loss transmission double-cladding fiber, high-imaging resolution miniature objective, and high resonant frequency piezoelectric ceramic fiber scanner.

On this basis, we introduce in Section 3 the recent research progress of the representative piezoelectric ceramic scanning two-photon endomicroscopy in this field. In the abroad research progress, the works from the following research groups are summarized, including Chris Xu’s group from Cornell University (Fig. 3), Frédéric Louradour’s group from Université de Limoges (Fig. 4), Xingde Li’s group from Johns Hopkins University (Fig. 5), Ki-Hun Jeong’s group from the KAIST (Fig. 6), and a joint team of Bernhard Messerschmidt’s and Juergen Popp’s groups from the GRINTECH and the Leibniz Institute of Photonic Technology, respectively (Fig. 7). In the domestic research progress, the work from the following research groups is summarized, including Ling Fu’s group from the Huazhong University of Science and Technology (Fig. 8), and a joint team of Lishuang Feng’s and Aimin Wang’s groups from the Beihang University and the Peking University, respectively (Fig. 9). It can be concluded that the capability of this technology for in situ, real-time, noninvasive, and high-resolution structural and functional imaging of biological tissues and organs has been fully verified. A part of the research units continues to focus on the research of a two-photon endomicroscopy integrated probe. The capability of the piezoelectric ceramic scanning two-photon endomicroscopy technology can be improved further by optimizing the core device and introducing new principles and methods; parts of the research units have conducted the development of a miniaturized endomicroscopy system to meet the clinical biosafety and compatibility requirements and develop its application in the biomedical imaging field.

In Section 4, we summarize two-photon endomicroscopy applications in structural and functional imaging of tissues and brain imaging of freely-moving animals. The following research groups’ work, including Xingde Li’s group from the Johns Hopkins University [Fig. 10 (a)-(i) and Fig. 12], a joint team of Liwei Liu and Junle Qu’s group from Shenzhen University [Fig. 10 (j)-(r)], and Heping Cheng’s group from the Peking University (Fig. 11), is summarized.

As a subcellular-scale optical biopsy technology, two-photon endomicroscopy can achieve real-time structural and functional imaging of biological tissues in situ, which has important scientific research value and broad clinical application prospects. The following recommendations are considered for the future development of two-photon endomicroscopy: 1) further breakthroughs in core device performance to improve the imaging capability and throughput of piezoelectric ceramic scanning two-photon endomicroscopy; 2) research on two-photon endomicroscopy technology based on MEMS scanning mirrors; 3) research on disposable endomicroscopy technology; 4) exploration of two-photon imaging technology-based multimodal imaging technology. It is foreseeable that piezoelectric ceramic scanning two-photon endoscopic imaging technology, as one of the important research directions of two-photon imaging technology, is expected to open a new paradigm of optical biopsy imaging applications for life science research and clinical medicine applications.

Gyroscopes, which are the critical components of inertial navigation devices, determine the navigation positioning and attitude control of spacecraft, cranes and vehicle load, and unmanned systems. Due to its high precision, the demand for a miniaturized integrated gyroscope has become urgent. As the market demand for inertial sensor systems grows in the future, micro-nano integrated optical gyroscope technology should be the first choice of the next generation optical gyroscope. Gyroscopes are not only suitable for spacecraft attitude control but can also be used for automatic vehicle navigation systems and safety protection. Integrated optical gyroscopes may fill in the gaps because the global navigation satellite system can be blocked and the weight of laser or optical fiber gyros is extremely large. Resonator micro-nano optical gyroscopes can be used to achieve all-optoelectronic integration, where different separation devices, such as light source, modulator, resonator, and detector, are integrated on the same chip to reduce the volume, weight, and cost of the device. Micro-nano integrated optic gyroscopes should satisfy both the demand for integration and precision.

The main problem is that the small area of the closed optical path at the micro/nanoscale decreases gyroscope sensitivity. To achieve a high-precision-resonant integrated optical gyroscope, new principles and technologies must be explored. The research progress of the integrated optical gyroscopes with optical gain, those that are dispersion-enhanced, and non-Hermitian optical gyroscopes are reviewed.

Because of the limitations of fabrication technology, the propagation loss of passive-optical waveguides cannot meet the requirement of high-precision micro-nano optical gyroscopes. To solve this problem, methods for exciting two laser beams in opposite directions or using the optical gain to compensate for the losses in the resonator are proposed. Several methods for achieving active resonators are introduced, such as semiconductor Ⅲ-Ⅴ materials, stimulated Brillouin scattering, stimulated Raman scattering, and rare earth ion doping. The development and problems of the gyroscopes with optical gain resonators are discussed. The main problems with the development of a resonant gyroscope with optical gain are as follows: (1) the mode competition in the active cavity causes nonreciprocity of the two beams, thereby making it difficult to establish the bidirectional operation of lasers; (2) for a gyroscope lock-in effect, the rough side of the wall of the processed optical waveguide causes backscattering and cross-coupling of clockwise and counterclockwise beams, resulting in no frequency difference in the output signal of the gyroscope at low rotation. (3) The optical pump technique is more complex because of the need to introduce a pump laser. However, an electric pump is more compact and suitable for developing the entire chip integration.

The dispersion relationship directly affects the propagation velocity of light, which can be divided into phase and group velocities. The Sagnac effect can be enhanced in an optical resonator by manipulating the group velocity of light. Slow and fast lights correspond to normal and abnormal dispersions, respectively. Through numerous theoretical and experimental studies, several methods have regulated the group velocity of light, which mainly fall into two categories: material and structural dispersions. Several academic papers have discussed whether the precision of a slow-light gyroscope can be enhanced under normal dispersion conditions. The Sagnac frequency shift of a resonant optical gyroscope could be enhanced under the anomalous dispersion condition. The main problem of the dispersion-enhanced gyroscope is achieving optical dispersion relation, generation mechanism of fast-light, and its influence on the Sagnac effect in active waveguide resonators. However, resonance linewidth is broadened using material or structural anomalous dispersion, which may counteract the dispersion enhancement effect of a resonant optical gyroscope.

Optical microcavities, with high-Q factor and small-mode volume, can significantly enhance the light-matter interaction. They play an important role in the research of non-Hermitian optics and have become an important platform for their research. Presently, achieving ultra-high sensitivity optical sensing based on exceptional points is a research hotspot of non-Hermitian optical systems. We are committed to achieving highly sensitive non-Hermitian optical sensing by combining the theory of non-Hermitian optics and whispering gallery mode (WGM) microcavities. Under the same perturbation, the non-Hermitic optical system at the exceptional point has a larger response to external perturbations than the traditional optical system, thus providing a new method for achieving high-performance optical sensors. The exceptional surface provides additional degrees of freedom that can shift the working point along the exceptional point (EP) surface when the system experiences undesired perturbations, thus improving the robustness of the non-Hermetic optical system. An exceptional surface, composed of chiral exceptional points, can be achieved by breaking the time-reversal symmetry of microcavity.

By combing the research progress of micro-nano integrated technology and optical gyroscope in this study, we summarize the research status, bottlenecks, and future development of the integrated resonant optical gyroscopes. Based on the possible solutions, which include new materials, structures, and physical effects, we emphasize the influence and research progress of optical gain compensation, dispersion control, and exceptional points of the non-Hermitian optical system on the sensitivity of optical gyroscopes. Finally, the related research prospect for integrated optical gyroscopes are summarized.

The advantages of photonic bandgap fiber (PBF) in terms of temperature, radiation, magnetic field, and other aspects of environmental adaptability make it an important development direction of fiber optic gyroscope technology. PBF has attracted extensive attention from research institutions worldwide. However, the fiber loss of seven-core PBF which is suitable for fiber-optic gyroscopes is large, and the PBF cannot meet the low-loss application requirements of fiber-optic gyroscopes for fibers.

The PBF loss is mainly caused by the coupling between the fundamental mode and surface modes of the core wall, and the scattering loss caused by the roughness of the inner wall of the fiber core. In this study, an isolated anti-resonant core photonic bandgap fiber (IAC-PBF) is developed, in which the fiber structure can isolate the core from the cladding, thereby the coupling between the fundamental mode and surface mode is suppressed through the anti-resonant effect of the core wall. An IAC-PBF structural model is established and the loss of the fiber is calculated using the F parameter method. The optimized fiber structure is obtained by scanning the structural parameters. The mode characteristics of the fiber and the loss reduction principle are determined by mode coupling analysis, and it is verified that the proposed fiber structure has low theoretical loss. Finally, the fiber is fabricated using the stacking-drawing method. Although the core size is small and the bandgap is offset, the feasibility of low-loss fiber is proved.

The IAC-PBF structure is proposed in this article (Fig. 1). Its cladding structure is the same as that of 19-cell PBF, and the photonic crystal structure is still formed through the periodic arrangement of hexagonal air holes to generate the photonic bandgap effect to prevent light leakage from the cladding. A hexagonal anti-resonance layer is used to isolate the core and cladding layers spatially. This reduces the mode coupling and scattering loss. The mode field diameter is approximately 8 μm, and the fiber loss is less than 3.5 dB/km, which is achievable with the current fiber fabrication process (Fig. 2). The reason for the increase in loss caused by structural changes is studied based on the relationship between the mode coupling and the structure of the PBF (Fig. 4). The simulation results show that the fundamental mode couples with the surface mode of the core wall when the core wall is thin. When the thickness of the core wall is greater than 0.4 μm, the surface mode of the core wall is coupled with the higher-order mode, and the mode refractive index is far from that of the fundamental mode, ensuring the decoupling of the fundamental mode from the surface mode and higher-order mode. For the bending loss, when the bending radius is greater than 4 mm, the bending loss of the IAC-PBF varies within 0.3 dB/km, maintaining the excellent bending characteristics of the PBF (Fig. 5). The IAC-PBF is fabricated using the stacking-drawing method (Fig. 7). The feasibility and loss reduction effect of the proposed fiber structure are verified by fiber testing and theoretical analysis, laying a foundation for the subsequent development of long-distance PBFs with a small mode field and low loss (Fig. 8). Theoretical simulation and experimental results show that IAC-PBF exhibits small fiber loss, a small mode field diameter, and bending resistance.

Compared with anti-resonance fiber, the PBF has relatively large loss but a small mode field and bending resistance, making it an ideal fiber for a high-stability fiber-optic gyroscope. In this study, an isolated anti-resonance core photonic bandgap fiber is designed. In this configuration, the core and cladding are spatially isolated, and the coupling between the fundamental and cladding surface modes is significantly reduced. An anti-resonance core is used to enhance the confinement of the fundamental mode, compress the fundamental mode field, and reduce the scattering loss. At the same time, the core structure of the IAC-PBF is formed by a stack of independent capillaries, unlike the traditional bandgap fiber which is formed by the capillary surrounding the fiber core. In the process of drawing, it is easy to control the fiber core thickness and reduce the roughness of the core wall, which can significantly reduce the fiber loss. The theoretical analysis results show that the loss of the PBF with a mode field diameter of approximately 8 μm can be reduced to less than 3.5 dB/km. The structure of the fiber is basically reproduced, but the difference in the quartz wall thickness causes a shift in the bandgap. The minimum loss of the fiber is approximately 25 dB/km @ 1200 nm.

Multi-wavelength optical carriers play a critical role in applications such as wavelength-division multiplexing (WDM) optical commutation, photonic generation of radio-frequency signals, and photonic wireless communications. Conventional approaches are limited by either the insufficient coherence between optical carriers (e.g., using an independent-running laser array) or the poor flexibility in choosing the number of optical tones and their spacing in the spectral domain, such as in an optical frequency comb. In this study, we present a technique for generating highly coherent multi-wavelength optical carriers and verify the superiority of the synthesized signals through demonstrative applications in low-noise THz-wave generation, photonic THz wireless communication, and jointly processed WDM optical communication.

For a single-wavelength output, a continuous-wave pump laser is frequency-locked to an optical cavity consisting of a fiberized coupler and a spool of standard single-mode fiber. As the frequency of the pump laser is in resonance with one of the longitudinal modes, the optical field inside the optical cavity accumulates exponentially, reaching the threshold of the stimulated Brillouin scattering (SBS), which would otherwise require a much higher pump power if the pump laser is not locked to the cavity mode. A Brillouin spectral gain would appear ~10 GHz redshifted from the pump wavelength, and if the free spectral range (FSR) of the cavity matches the width of the spectral gain, only one cavity mode will start to oscillate and produce a narrow-linewidth SBS output in the direction opposite to the pump. For a multi-wavelength output, pump lasers with the desired wavelengths and spectral separations are pumped into the same optical cavity. Through independent frequency locking, the one-to-one correspondence described above allows the generation of the same number of SBS signals while maintaining the frequency separation of the pumps. Coherence between the output optical tones is ensured because the oscillations originate from the same optical cavity. The linewidth of the output is significantly reduced compared with that of the pump owing to the equivalent spectral filtering from the SBS gain and high Q of the optical cavity.

The proposed multi-wavelength SBS optical carrier generation technique is validated in three different applications. In synthesizing frequency-stabilized low-noise THz waves, two optical carriers separated by ~300 GHz are generated by the proposed SBS system and subsequently transformed to a THz wave through a large-bandwidth uni-travelling-carrier photodiode, as shown in Fig. 4. To stabilize the frequency of the THz wave, the FSR of the SBS fiber cavity is locked to an external frequency reference through both mechanical and thermal feedback, resulting in an root-mean-square (RMS) frequency drift of only 0.47 mHz in 60 min and Allen deviation of ~10^-15 at an average time of 1 s, as shown in Fig. 5. Moreover, by comparing the electrical waves generated at 300 and 11 GHz, the phase noise is confirmed to not be governed by the quadratic dependence on the frequency and stays at the same level of ~-90 dBc/Hz at 10 kHz offset (Fig. 6). In real-time photonic THz wireless communication, two-wavelength optical carriers generated by the proposed SBS system are used to replace the conventional light source generated by the electro-optic frequency comb. As the bit-error-rate traces and constellation diagrams shown in Fig. 8 indicate, the novel multi-wavelength SBS optical carriers reduce the required THz power at the transmitter by 6 dB at the same baud and bit error rates as the conventional source and remove the ellipse of the constellation diagram owing to the improved phase noise. Multi-wavelength optical carriers are also applied in the WDM optical communication demonstration, where the inter-channel joint signal processing technique is adopted. Owing to the high coherence between the SBS tones, the independent phase estimations of each communication channel standard in the WDM post signal processing can be replaced by only one estimation of a master channel shared with other channels, effectively reducing the computation requirements by 34%, as shown in Fig. 9. Such alleviation of the digital signal processing (DSP) complexity can considerably boost the communication capacity in scenarios where the computation power is restricted.

This study proposes a method for generating multi-wavelength optical carriers. Owing to the in-resonance pumping of the high-Q optical fiber cavity, arbitrary selection of pumping wavelength, and spectral filtering of the SBS effect, this technique is advantageous compared with the conventional approaches in terms of coherence, phase noise, and adjustability. In the experiments, the proposed method allows the synthesis of frequency-stabilized low-noise THz waves, whose phase noise does not scale up quadratically with the output frequency, and the efficient photonic THz wireless data transmission, where the required transmitted power decreases by 6 dB. The optical carriers generated by the proposed method also enable the application of joint DSP in WDM optical communication, which has only been demonstrated with light sources from an optical frequency comb. The high performance and low system complexity of the proposed method are expected to assist in the development of research fields such as broadband signal synthesis and high-efficiency communication, where highly coherent, narrow-linewidth optical carriers are desired.

Stimulated Brillouin scattering (SBS) is a typical third-order optical nonlinear effect describing the coupling between coherent light and phonon, which is the strongest nonlinear effect in the materials. Since the first realization in quartz and sapphire crystals in 1964, more work has been performed to study the SBS in various media, such as optical fibers, optical microcavities, gases and fluids. For the properties of narrow-linewidth and opposite-direction of the light generated by SBS, SBS lasers have many potential applications in the fields of laser, optical sensing, and optoelectronic devices related to fast and slow lights.

Optical gyroscope is an all solid-state angular velocity sensor based on the Sagnac effect with the advantages of high precision and high reliability. It has three main categories: ring laser gyroscope (RLG), fiber optical gyroscope (FOG) and micro optical gyroscope (MOG). RLG is sensitive to angular velocity by resonant laser beat frequency detection, which has the advantage of high stability of scale factor, but it has harsh requirements on the fabrication process. Interferometric fiber optical gyroscope (IFOG) owns the advantages of relatively simple structure and easy process, but the scale factor is not as stable as that of RLG. To improve the precision, hundreds or even thousands of kilometers of fiber are required to be accurately wound onto the ring-pillar. After decades of study, RLG and IFOG have been widely used in many optical sensor fields and navigation systems. In recent years, with the rapid development of autonomous driving, unmanned aerial vehicle and satellite platform, the demand for miniaturization and high precision of gyroscope is put forward, while the traditional gyroscope is difficult to meet these new requirements.

The sensing element of a resonant optical gyroscope is fiber ring or waveguide resonator. It only needs tens of meters of fiber or an on-chip microcavity with the diameter of millimeters to realize high-precision gyroscope, which is helpful for the miniaturization of the whole gyroscope devices theoretically. Based on the criterion that whether or not there is a laser source generated inside the fiber cavity, resonant optical gyroscope can be classified into active and passive ones, respectively. Subject to the disadvantages in linewidth, stability, various optical noise and complexity of the system, the passive fiber gyroscope is still under research in the laboratory, not completely used in real devices. On the other hand, with the continuous progress of nanophotonics and microfabrication technology, the active optical gyroscope based on SBS laser generation in the microcavity, combining both the advantages of laser gyroscope and traditional fiber optical gyroscope, provides a new way for the study of the miniaturized and integrated optical gyroscope. Theoretically, it could realize the excellent gyroscope with the advantages of easy integration, low threshold, high gain, large dynamic range and high sensitivity. However, the study on the micro-resonator Brillouin gyroscope has not reached such a high level yet. Therefore, it is meaningful to advance the study on the micro-resonator (such as microcavity, waveguide) Brillouin gyroscope for the development of integrated optical gyroscope and extend their applications in the real civil devices, weapons, and navigation systems.

The basic parameters to characterize the stimulated Brillouin scattering effect mainly include the full width at half maximum (FWHM) of Brillouin gain spectrum, peak Brillouin gain and threshold. For solids and fluids, they are expressed by different equations, as shown in Eqs. (1)-(5). The Brillouin gain coefficient can be increased by doping and designing device structure. The corresponding Brillouin parameters in different types of fiber, microcavity, gas and liquid are summarized in Table 1 and, at the same time, their typical applications are introduced (Figs. 2-5). The research progress of miniaturized and integrated optical gyroscope based on SBS in the fiber and microcavity is thoroughly introduced (Fig. 6), including the study of exceptional point to achieve optical gyroscope with ultra-high sensitivity (Fig. 7).

Stimulated Brillouin scattering laser plays an important role in the field of miniaturization and integration of active resonating optical gyroscope. In this paper, the SBS effect and its corresponding applications in different media are detailedly summarized. Especially, the SBS effect is an important tool for the integration and miniaturization of optical gyroscope both in the fiber and micro-resonator systems. Hence, the studies on the SBS fiber optical gyroscope and micro-resonator gyroscope are reviewed to provide the recent progress of the integrated optical gyroscope. More interestingly, exceptional point excited in the optical microcavity has been found to own the possibility of significantly enhancing the sensitivity of the gyroscope, paving a new avenue for the study of a highly sensitive integrated optical gyroscope.

The wavefront of any object can be recorded and reproduced using computer-generated holography, regardless of whether the object is real or not. Due to a computer-generated holographic display provides all of the object’s depth cues, it naturally resolves the vergence-accommodation conflict. The development of color computer-generated holographic three-dimensional(3D) display technology has received a lot of attention as one of the key technologies of the computer-generated holographic 3D display.

The types and principles of chromatic aberration in the color computer-generated holographic 3D display are investigated in this paper. The technologies of color computer-generated holographic 3D display are then classified and outlined.

The time-division multiplexing technology makes it possible to use only one spatial light modulator (SLM) for color holographic display. Some researchers, for example, use the time-division multiplexing method to construct a full-color holographic 3D display system based on liquid crystal displays (LCD). In this system, a rotating wheel plate is used to cause red light, green light, and blue laser light to alternately irradiate the LCD. At the same time, the red, green, and blue holograms are correspondingly loaded on the LCD. The resolution of each color hologram is the same as that of the LCD. The recording distances of the red, green, and blue holograms are compensated to eliminate the axial chromatic aberration introduced by the 4f system. To achieve color holographic 3D reconstruction, the time-division multiplexing technology fully utilizes the refresh rate of the SLM and the persistence of vision effect of the human eye. The demanding requirements on the refresh rate of the SLM and the performance of the synchronization control module limit the realization of the dynamic display effect to some extent.

The space-division multiplexing technology based on multiple SLMs has lower requirements on the refresh rate of the SLMs, so there is no image flickering. Red, green, and blue holograms are loaded on multiple SLMs in this method, and the resolution of the hologram is equal to that of the SLM. The red, green, and blue images reproduced are spatially coincident by precisely adjusting the optical path. The image can then be reproduced in color. However, the system structure of this technology is typically complex, and the cost is high. The spatial alignment of the reconstructed red, green, and blue images is extremely difficult.

The space-division multiplexing technology based on a single SLM has the merits of the relatively simple optical path structure, the flicker-free reconstructed image, and being free from rainbow effect. The red, green, and blue lasers illuminate one-third of the area of a single SLM in this method. Each color hologram has a resolution that is one-third of the resolution of the SLM. However, the image resolution is reduced to one-third of the original, the spatial bandwidth product of the color reproduced image is reduced, and the perspective is also affected.

Some researchers have proposed the target image separation method based on a single SLM to avoid reducing the resolution of the color reproduction image. When the SLM is irradiated by red, green, and blue light at the same time, the target color reproduction image and the interference image are separated, and the target color reproduction image can be obtained by filtering. In addition to designing and improving the optical system structure, using the excellent properties of new materials is another way to realize color holographic display. Other technologies, such as the use of metasurfaces, fully exploit the advantages of the new materials and have promising future prospects.

Many methods have been proposed to realize color computer-generated holographic 3D display. This paper summarizes the color computer-generated holographic 3D display realization technology. Among these methods, the space-division multiplexing with multiple spatial light modulators and time-division multiplexing with a single spatial light modulator are frequently used. The quality of the color image is affected since the resolution is reduced when using the space-division multiplexing technology based on a single SLM. The technique that using a single SLM to realize the color holographic reconstruction based on the angle compensation method has certain advantages in resolution because it saves space and time resources. The various method proposals provide novel ideas for the perfect reconstruction of color holograms. However, based on the current state of holographic 3D display development, the high-quality holographic 3D display has higher spatial bandwidth product requirements. While improving the spatial bandwidth product, more research and exploration are needed to determine how to make the color computer-generated holographic 3D display system simple and compact without a complex synchronous control system.

Single pixel imaging is a new imaging technique which is able to obtain imaging information through a single pixel detector. Compared with the traditional array detection imaging techniques, single pixel imaging has the advantages of high sensitivity and anti-interference ability, and has broad application prospects in many fields. Various modulation schemes and reconstruction algorithms for single pixel imaging have been proposed for all kinds of scenario. However, the defect of large time consumption in the modulation and reconstruction process detracts single pixel imaging from practical applications. Recently, many strategies have been proposed to address this problem from the aspects of modulation and reconstruction. The comparison of different modulation schemes and algorithms in various aspects can establish guidelines for practical single pixel imaging.

The single pixel imaging uses a single pixel detector to record the light intensities of the scene illuminated by a sequence of resolved patterns. The spatial information of the scene can be recovered from correlation of the sequential measurements and patterns. The different choice of modulation and algorithms exerts influence on the final imaging result. For modulation devices, liquid crystal spatial light modulator (LC-SLM), digital micromirror device (DMD) and light-emitting diode (LED) array are mainly introduced. Five metrics including programmable, modulation speed, structured detection, grayscale modulation and price are introduced to compare the performance of different popular devices. The pros and cons of different devices are detailed. For sampling schemes, random patterns, Hadamard patterns, Fourier patterns and wavelet patterns are introduced. A simulation is performed to compare the sampling efficiencies of different sampling schemes. For reconstruction algorithms, three categories of algorithms, i.e., non-iterative algorithms, iterative algorithms and deep learning-based algorithms are introduced. Five metrics including running speed, undersampling, applicability, robustness and reconstruction are introduced to compare the performance of different popular reconstruction algorithms for single pixel imaging.

For modulation devices, DMD is the most popular one and outperforms other devices due to its fast modulation speed and programmable characteristics. However, only grayscale patterns can be loaded on DMD and the modulation speed can still be improved further. LC-SLM is an alternative to DMD, taking the advantages of price and ability of grayscale modulation. However, the modulation speed of LC-SLM limits its practical application. LED arrays provide cheaper price and faster modulation speed, so it is a better choice in the future.

For sampling schemes, a simulation is performed to compare the performance of different sampling bases under different sampling rates (Fig. 4). The numerical metric shows that Fourier basis performs the best under all sampling rates. Hadamard basis outperforms wavelet basis under high sampling rates while wavelet basis is better under low sampling rates. Random pattern performs the worst since it is not an orthogonal sampling basis.

For reconstruction algorithms, there exists a trade-off between the non-iterative, iterative and deep leaning-based algorithms. The performances of different algorithms are compared (Fig. 5). Non-iterative algorithms require less computation compared with other algorithms, but the reconstruction quality and robustness are the worst. Iterative algorithms recover good quality of image, even in a low sampling rate. The deep learning-based algorithms require tremendous training dataset and training time in advance, so that in reconstruction, the running speed and reconstruction quality of deep learning-based algorithms are the best.

In this paper, the history and principle of single pixel imaging are briefly reviewed. Different modulation devices, sampling strategies and reconstruction algorithms are compared in different aspects. The performances of different strategies are analyzed and compared in detail. Finally, the future development and application of single pixel imaging are discussed. Our work shows that the optimal combination of modulation schemes, sampling strategies and reconstruction algorithms should be analyzed and selected to achieve perfect and efficient imaging.

A whispering gallery mode resonator (WGMR) is an optical device that can confine light in both spatial and temporal dimensions with total reflection. Owing to its high-Q and small mode-volume characteristics, a WGMR is a powerful and reliable platform for various photonic applications such as microlasers, optomechanics, biosensing, laser stabilization, and nonlinear optics.

The WGMR-based self-injection locking technology uses resonant backscattering inside the resonator to purify the spectrum, which significantly reduces the laser linewidth and has been extensively used for laser stabilization. Because no electronic feedback control is required in this scheme and a WGMR can achieve a Q-factor of over 10⁹ with a micron-scale, the technique can achieve a sub-hertz linewidth with a compact size. In addition, the WGMR with a wide transparency window can be integrated with lasers operating in various wavelength bands to achieve narrow-linewidth lasing.

In a high-Q WGMR, the photon’s lifespan can be prolonged from nanoseconds to microseconds, which improves light-matter interaction, making WGMRs well-suited for the acquisition of ultralow-threshold nonlinear optical effects. Numerous nonlinear effects, including second-harmonic generation, four-wave mixing (FWM), stimulated Raman scattering, and stimulated Brillouin scattering, have been theoretically investigated and experimentally demonstrated in WGMRs. In particular, optical frequency combs (OFCs), namely microcombs, based on four-wave mixing in a WGMR have attracted substantial attention and have been generated in many integrated phonic platforms. However, the conventional generation of microcomb requires narrow-linewidth pumps, which is challenging for on-chip integration. The WGMR-based self-injection locking is a viable alternative scheme for integrated microcomb.

The study introduces important parameters of WGMRs, followed by the recent research progress on self-injection locked laser based on the WGMR, nonlinear optical effects, and high-level integration of microcombs using self-injection locking.

First, four key parameters of WGMRs are summarized, namely resonance wavelength, free spectral range (FSR), quality factor, and mode volume.

WGMRs are excellent candidates for achieving a substantial linewidth reduction owing to their high-Q property. In 2015, researchers presented a 1550-nm semiconductor laser with sub-hertz instantaneous linewidth, which is stabilized by WGMR-based self-injection locking technique. The typical structure of a narrow-linewidth self-injection-locked laser with a high-Q WGMR is presented in Fig. 1. The scheme utilizes resonant Rayleigh scattering in the resonator, due to the internal and surface inhomogeneities. In backscattered light, a fraction of the light is reflected to the laser with the frequency of the selected high-Q WGMR. Owing to the fast optical feedback, a notable depression in laser frequency noise is achieved, which corresponds to the linewidth of the laser. In 2017, researchers proposed a theoretical model for a WGMR-based self-injection locking method, as shown in Fig. 2. The analysis indicated that the degree of linewidth reduction scales with the square of Q. Moreover, a detailed theoretical model illustrated the importance of five structural parameters that may improve linewidth reduction. The five parameters were the backscattering efficiency, the phase delay of feedback light, laser-microresonator frequency detuning, coupling regimes, and the optical path length between the laser and resonator. Self-injection locking is based on the WGMR technology and has been widely employed in lasers from UV to mid-infrared bands, as shown in Table 1. However, the linewidths of a self-injection locked laser in the UV and mid-IR bands require further improvement.

The nonlinear characteristics of high-Q WGMRs have been systematically investigated. The threshold power of the nonlinear effect is proportional to the mode volume and inversely proportional to the square of Q. Therefore, a laser with low output power, ranging from microwatts to milliwatts, can generate nonlinear effects inside a WGMR. Moreover, high-performance photonic applications have been obtained based on an integrated nonlinear WGMR, as shown in Fig. 5. In recent years, FWM-based microcombs have attracted increasing interest, and researchers have demonstrated microcombs on various nonlinear material platforms, such as Si₃N_4,silica, Hydex, silicon, AlN, SiC, and AlGaAs, with a full list provided in Table 2. Owing to low propagating loss, wide transparency windows, and a CMOS-compatible foundry(CMOS, complementary metal oxide semiconductor), WGMRs based on the Si₃N₄ platform have been extensively investigated. In addition, WGMRs based on AlGaAs have achieved significant improvements in Q factors in recent years. AlGaAs with high refractive and high nonlinear refractive indices is another ideal platform for obtaining an integrated microcomb.

The generation of microcombs pumped by on-chip lasers is both of paramount importance and full of challenges. The WGMR-based self-injection locking technique can achieve a fully chip-based micro-comb. In this case, the WGMR provides optical feedback to narrow the linewidth of the laser and serves as nonlinear material to generate microcomb. To date, a considerable number of studies have demonstrated fully chip-based OFCs based on laser diodes and high-Q WGMRs, as shown in Fig. 6. To determine the nonlinear dynamics of self-injection locking, researchers have investigated the self-injection locking process by considering nonlinear interactions.

A WGMR with a high Q and small volume is an excellent candidate for an ultranarrow linewidth laser with low-threshold nonlinear effect excitation. Furthermore, a high level of integration of microcombs can be achieved by combining both the self-injection locking method and low-threshold FWM. Finally, further developments can be made in self-injection-locking parameter optimization and Q-factor improvements.

The rapid development of the modern optoelectronic information industry has increased the requirements for semiconductors in optoelectronic devices. Low-dimensional semiconductor materials with smaller sizes, adjustable performance, and greater integration flexibility are the basis for developing a new generation of nano-optoelectronic devices and systems. Quantum dots occupy an important position in low-dimensional quantum structure materials owing to their unique structural and physical properties, which make them a promising, low-dimensional structural material for the next generation of optoelectronic devices. However, the performance of quantum dot-based devices has not matched the theoretical expectations of the material because quantum dots are difficult to fabricate at a high standard. As quantum dots are very small, the number of hot carriers is increased, which significantly affects the modulation speed and results in a decrease of the carrier collection efficiency of the device.

In a quantum well, another well-known low-dimensional structure, the carriers are constrained only in the direction of the well width, and the density of the state presents a ladder-like distribution, which is significantly larger than that of quantum dots. Thus, a quantum well is more suitable for the collection and storage of carriers than quantum dots. This makes it possible to study and develop a composite quantum structure that combines quantum dots and wells. As a new type of semiconductor low-dimensional quantum structure, the indium-based well-dot composite quantum structure inherits the advantages of both traditional quantum dots and wells while overcoming their inherent constraints due to its composite nature as well as the available control over the structure. Furthermore, well-dot composite quantum structures can be used to implement energy-band engineering more effectively, improve the physical and optical properties of traditional low-dimensional semiconductor materials, and expand optoelectronic device applications.

This paper describes in detail the structural properties, optical characteristics, and application prospects of low-dimensional indium-based well-dot composite quantum structures. The content covers three types of structures: an indium-based well-dot tunnel-coupled quantum structure, a dots-in-a-well quantum structure, and a self-assembled indium-rich cluster composite quantum structure. For an indium-based well-dot tunnel-coupled composite quantum structure, the well and dots are generally separated by a spacer. The coupling degree between the well and dots is dependent on the material, component, and thickness of the spacer. The flexibility of this approach makes it possible to achieve better device performance.

For a dots-in-a-well composite quantum structure, the larger absorption cross section of the quantum well increases the well’s ability to capture carriers, which limits their presence around the quantum dots. Thus, this type of composite quantum structure exhibits the significant fluorescence enhancement phenomenon. Because a quantum well can form a potential barrier, the probability of carriers escaping from the dots is reduced in this composite quantum structure.

A self-assembled indium-rich cluster composite quantum structure is constructed based on the unique indium-rich cluster effect which occurs during the epitaxial growth of the InGaAs/GaAs materials. The material has an irregularly strained quantum structure with different bandgaps and researches on this structure reveal that it possesses outstanding optical characteristics that have not been achieved by other quantum structures. The ultra-broad and uniform gain spectra in bipolarization, from this structure, indicate an opportunity for the development of high-performance semiconductor lasers, such as polarized dual-wavelength laser diodes, polarization-independent semiconductor optical amplifier devices, and ultra-broadband tunable semiconductor lasers with uniform spectral output power for all tunable wavelengths.

In conclusion, owing to the introduction of tunnel injection technology, well-dot tunnel-coupled composite quantum structures significantly improve the performance of optical devices, resulting in higher gains, lower currents, better temperature stability, higher speed modulation performance, and a lower linewidth enhancement factor. The structures show good application prospects in the development of high-performance semiconductor lasers and semiconductor optical amplifiers. In addition, the dots-in-a-well composite quantum structure effectively overcomes the inherent limitations of quantum dots, improves the density of quantum dots, enhances the carrier capture ability of the structure, and significantly improves optical performance. This composite structure shows great application value for optimizing the performances of quantum dot lasers, infrared detectors, solar cells, and other devices. Furthermore, the self-assembled indium-rich cluster composite quantum structure exhibits excellent optical properties, such as ultra-wide and uniform gain, as well as bimodal spontaneous emission and absorption. The structure has shown broad development prospects for the realization of a new generation of ultra-wide tunable semiconductor lasers with uniform spectral power, polarized dual-wavelength lasers, and polarized independent semiconductor optical amplifiers. Simultaneously, as the lasing wavelength tuning range of the composite structure is in the terahertz band, it can be developed into a new tunable terahertz source in the future. In addition, we believe that in the future, by using an InGaAs indium-rich cluster composite quantum material as a new activation medium, combined with second-harmonic technology, we can achieve continuous high-power and wide-tuning short-wave laser output that is difficult for wide band gap materials. Because each well-dot composite structure has its own unique advantages and disadvantages, the structure can be selected according to the desired device’s performance requirements, to maximize the advantages of the individual structure.

Optical frequency comb (OFC), often dubbed as "the ruler in the frequency domain" and a "bridge" between microwave and optical frequencies, plays an important role in time and frequency metrologies. Among these OFC technologies, the one leveraging two OFCs with slightly different comb tooth spacings, that is, dual-comb technology, has been the most thoroughly studied one in recent years. It has been utilized in various applications such as absorption spectroscopy, absolute distance ranging, pump-probe measurement, and radio-frequency spectrum measurement, as it can realize high-resolution and broadband optical measurements. Based on the conventional optical frequency comb technology, dual optical combs are generated using two independent mode-locked lasers with slightly different frequencies. However, the need for complicated feedback control and laser systems to maintain mutual coherence between the dual combs could be a key bottleneck for this technology when moving towards on-site detection and a broader application range. The generation of high-quality dual-optical frequency combs with low complexity has become a popular topic in dual-comb technology research.

The single-cavity dual-comb technology that realizes high-coherence dual-optical frequency comb generation with one laser has become a prominent research direction in current optical frequency comb technology. This has significantly contributed to the advancement of low-complexity dual-comb technology. This study reviews the development of single-cavity dual-comb technology from a broad perspective, specifically focusing on the single-cavity dual-comb fiber lasers that have been extensively explored over the past decade. Various technical pathways to implement single-cavity dual-comb sources and their characteristics are summarized, as well as the new directions for further developing this technology.

Studies on ultrafast lasers have traditionally focused on achieving shorter pulse width, higher power, and lower noise. Ensuring the generation of a single pulse train in the laser cavity is the preferred choice for realizing high-quality mode-locked lasers. However, for dual-comb sources, the mutual stability between two pulse sequences, instead of the stability of a single optical comb, is the key performance target. Over the past decade, to develop dual-comb lasers with good mutual coherence, researchers worldwide have designed various lasers based on the concept of multiplexed mode-locked laser (M²L²), an idea introduced by our group. Because both combs share a laser cavity, M²L² can realize good passive mutual coherence of the output combs without active stabilization. Thus far, four types of multiplexing methods, namely, wavelength-, directional-, polarization-, and pulse shape multiplexing, corresponding to different physical dimensions of optical pulses, have been studied to generate dual combs from a single cavity. These are illustrated in Fig. 1, and their corresponding performances are summarized in Table 1.

The advantages of the single-cavity dual-comb (SCDC) technique pave the way for realizing low-complexity dual-comb systems. Dual-comb measurement techniques can be divided into two categories: time-domain and frequency-domain measurements, which impose different requirements on the coherence and frequency stability of dual combs. Because the single-cavity dual-comb laser is significantly different from the traditional fully referenced dual-comb source in terms of power, spectral width, repetition rate difference, coherence, and stability, it is necessary to validate the applicability of SCDC lasers to existing dual-comb applications. All major dual-comb applications, such as optical spectroscopy, terahertz spectroscopy, ranging, and pump-probe measurements, have been demonstrated using SCDC lasers (Fig. 6).

Although the shared cavity design of SCDC sources yields considerable advantages in terms of overall system complexity and cost, there could be inevitable periodic collisions between two ultrashort pulses in such a laser cavity. This is fundamentally different from undisturbed pulse propagation in conventional single-pulse-train mode-locked lasers. Thus, this could be an interesting research subject, as well as a potential engineering challenge. Therefore, several studies have recently been conducted on pulse interaction in SCDC lasers with different multiplexing methods (Fig. 11).

Moreover, novel measurement schemes leveraging more than two combs have shown new capabilities required by certain applications. However, such light sources based on frequency-stabilized combs are considerably complex and expensive. Therefore, single-cavity multi-comb technology could be an attractive alternative solution. The concept of a multidimensional multiplexed mode-locked laser (M³L²) has been proposed to realize single-cavity triple-comb and multi-comb generation. Several demonstrations, such as dead-band-free high-resolution microwave frequency measurement, real-time absolute distance measurement with increased ambiguity range, and dynamic spectroscopic characterization for fast spectral variations based on the single-cavity tri-comb laser and quad-comb laser, have been demonstrated (Figs. 12 and 13).

CDC technology, owing to its unique advantages in system complexity, power consumption, and cost, has grown extensively over the past decades. From an often-overlooked phenomenon in the labs, it has been transformed into a major driving force with the potential to propel dual-comb technology into out-of-lab applications. Owing to the unique multiplexed optical cavity structure and intracavity dual-comb pulse dynamics, related studies span various topics, from fundamental soliton physics to engineering solutions. However, some technical challenges remain to be overcome before it can become truly successful in real-world applications. The trade-off and balance between these performance parameters in SCDC systems, such as the tunability and stability of the repetition frequency difference, pulse energy, and mutual interaction, and the associated intriguing physics behind them, could further motivate the academia to conduct innovative investigations.

Fiber-comb filters demonstrate significant application potentials in ultrafast optics, optical information processing, fiber sensing, laser processing, and laser guidance. As a class of fiber-laser control devices, filters play an irreplaceable role in laser generation, soliton formation, and pulse-waveform control, which are essential components of tunable, narrow-bandwidth, and multi-wavelength fiber lasers. Therefore, studying the effect of filtering on pulse generation and transmission and manufacturing new filter devices are valuable.

In the experiments, first, a Mach-Zehnder interferometer (MZI) is developed by connecting two arms with two 3 dB couplers through fiber splicing. The fabricated MZI is then incorporated into a laser cavity. The laser cavity adopts a hybrid cavity mode-locking method that combined nonlinear polarization rotation (NPR) and few-layer black phosphorus (BP). Second, the ultrafine fiber is extracted from the standard single-mode fiber (SMF) via the flame brush technique, the tapered fiber is knotted, and the fabricated micro-knot resonator (MKR) is obtained. The fabricated MKR is then incorporated into the laser cavity. The laser cavity is mode-locked using a real saturable absorber.

Based on the MZI multi-wavelength mode-locked fiber laser, when the pump power reaches 270 mW, a multi-wavelength continuous laser output is generated (Fig. 4). To obtain different pulse states, we increase the pump power to 327 mW and adjust the polarization controller (PC) to realize single-wavelength locking (the central wavelength is adjustable) (Fig. 5). The NPR suppresses mode competition and introduces the filtering effect. The modulation depth of the sample MZI is large; thus, multi-wavelength mode-locked laser sequences are realized at room temperature. A dual-wavelength mode-locked laser is obtained by increasing the power to 400 mW (Fig. 6). Three-wavelength mode-locked lasers with different distance intervals are obtained by further increasing the power to 420 mW (Fig. 7). The interval between two adjacent wavelengths is an integer multiple of the free spectral range. By further increasing the power to 440 mW, four-wavelength mode-locked laser is obtained (Fig. 7).

In the passive harmonic mode-locked fiber laser based on MKR, by increasing the pump power to 60 mW, we observe bound-state mode-locking, accompanied by a gradual accumulation of nonlinearity in the cavity (Fig. 9). Under the same pump power, in the absence of MKR, the pulse shape output by the laser cavity is observed to be a typical soliton pulse (Fig. 10). In a numerical simulation, researchers found that the addition of a narrow-band filter to the resonator limits the pulse bandwidth, the filtering effect leads to pulse splitting, and the splitting threshold of the soliton can be further reduced to generate more pulses. Under the combined action of dispersion, nonlinear effects, filter effects, and other factors in the passive mode-locked fiber laser cavity, multiple pulses are obtained and transmitted back and forth in the resonator simultaneously. Each pulse is evenly distributed at equal intervals. The phases between the pulses are matched, thus achieving passive harmonic mode-locking. The fundamental repetition frequency of the laser cavity is 8.6 MHz. When the pump power is increased to 100 mW, the 8th-order harmonic mode-locking is generated. By increasing the power and adjusting the PC, the 24th-order harmonic mode-locking is obtained. The fundamental repetition frequency in this case is 206.4 MHz (Fig. 11).

A theoretical model to further study the effects of the filter on the laser is established. The propagation path of the pulse in the resonator cavity is simulated, and the influence of various devices in the cavity on the pulse is considered. The pulse circulated in the cavity for one circle is used as the input signal for the next cycle until the pulsed light field reaches a stable self-consistent state. An evident modulation phenomenon is observed in the spectrum (Fig. 12), and stable double pulses can be observed in the time domain. In this case, the formation of multiple pulses is mainly a result of the spectral filtering effect.

Filtering effects have a significant impact on pulse formation, waveform regulation, and dynamic transmission. In this study, a multifunctional thulium-doped fiber laser with an all-fiber MZI as a compact comb filter is demonstrated to achieve a multi-wavelength continuous wave (CW) laser output and multi-wavelength mode-locked laser pulse output. A single-wavelength mode-locked laser in the spectral range can be used to tune the output and simultaneously realize four-wavelength mode-locked pulses. An all-fiber MKR is fabricated using the tapering technique. Bound solitons are obtained in the cavity of an erbium-doped fiber laser, and the mode-locked state is realized by switching from the fundamental frequency state to harmonic mode-locking. By adjusting the power and polarization state in the resonator, the 24th-order harmonic mode-locking is achieved. The pulse shape is still bound-state soliton. The experimental results and numerical analysis reveal that the filtering effect has a significant influence on the pulse shape.

Laser precision processing exhibits an accuracy that ranges from microns to tens of microns and provides advantages such as high processing accuracy, application to a wide range of materials, and minor thermal damage. In 1986, Dr. Mourou and Strickland used microwave amplification technology to amplify and compress laser-chirped pulses and achieved results better than those of traditional methods such as Q-switching and mode-locking, leading to the rapid development of laser precision processing technology.

Because of the increasing demand for lightweight components and high-value-added products in critical industries such as aerospace, automobile, electronics, and medical devices, laser precision processing technology has become essential in precise machining, surface finishing, high-performance joining, and functional surfaces for drag reduction, stealth, and anti-icing.

In this paper, we summarize our current work on laser precision processing in recent years. The work includes laser precision polishing, metal-composite high-strength joining, one-stop fabrication of functional surfaces, ultrafast laser drilling on bones, and ultrafast laser-assisted silicon wafer manufacturing.

The laser precision polishing technique offers an adaptable, accurate, and environmentally friendly solution for enhancing the surface quality of additive-manufactured metallic components. The technique can considerably reduce the surface roughness and average grain size of the original laser-additive-manufactured metallic alloys. Moreover, porosity can be reduced in the laser-polished layer. Laser precision polishing can substantially improve mechanical properties, such as microhardness, residual stress, tensile strength, and fatigue performance, and reduce corrosion resistance. Magnetic-field-assisted polishing methods are also proposed to improve the uniformity in surface quality and performance.

The high-strength laser-joining method is proposed to produce hybrid joints of titanium alloys and carbon-fiber-reinforced polymers (CFRPs). Before the laser-joining process is implemented, a surface-texturing treatment is employed on the metal surface to improve joint strength through the formation of interlock structures between the titanium alloy and CFRP. This method effectively eliminates micropores in the joints, thereby increasing the fracture strength of the joints to up to 60 MPa.

The one-stop fabrication method for functional micro/nano surfaces is proposed to achieve excellent anti-icing properties on laser-fabricated hierarchical structures consisting of micropillars and nanoparticles, with water droplets retaining their liquid state for over 8 h at -8.5±0.5 ℃. Distinct micro/nano hybrid structures, including regular laser-induced periodic surface structures (LIPSSs), semi-continuous nano-bumps, and nanoscale-to-microscale protrusions, are induced on a tungsten-carbide-cobalt (WC-Co) alloy by applying the laser precision machining technology, thus significantly reducing the optical reflectance at the laser-treated surfaces in the visible wavelength range.

We demonstrate the feasibility of ultrafast laser drilling in vitro large-size holes in an animal bone with high efficiency and minimal collateral damage. The laser precision machining technology is utilized to drill millimeter-scale holes in the bone under different cooling conditions, including gas- and water-assisted processes. A 4 mm hole with a smooth and clean surface is successfully drilled, and the highest removal rate of 0.99 mm³/s is achieved. The bone and bone marrow are distinguished by using a real-time monitoring system based on real-time spectral responses during laser drilling.

We employ laser precision processing for wafer thinning and grinding. The wafer thickness is reduced from 199 to 102 μm, and compressive stress is achieved at the laser-machined surface with the depth of the heat-affected zone (HAZ) being less than 1 μm. Laser grinding of silicon wafer effectively eliminates damages such as micro-cracks, micropores, and saw-marks on the surfaces and reduces surface roughness. Laser thinning and grinding technologies have limited influence on the electrical properties of the silicon wafer.

In this paper, we review the recent achievements in the research on laser precision technologies conducted by our group. We show that laser precision processing is a promising method for practical applications in the aerospace industry, medical devices, and integrated circuits, mainly because of its multiple functions, high flexibility, high precision, and applicability to various materials. Although industrial mass production may still be challenging, emerging technologies, including online monitoring systems, ultra-precision operation platforms, and high-power laser sources, have potential to improve the quality, efficiency, and reliability of laser precision processing.

Metal additive manufacturing (MAM) processes can directly produce fully dense near net shape components, which are widely used in aerospace, medical, and defense applications. Due to its unique fabrication benefits, additive manufacturing has become one of the fastest-growing and most-active research directions worldwide. However, MAM is carried out under extreme thermodynamic conditions that involve metal melting and solidification, interactions among different elements, and generation of thermal stresses. Hence, various internal defects, such as lack of fusion porosity, pores, cracks, and internal stresses, will inevitably be generated during the MAM process. Normally, defects and residual stresses will significantly affect the quality and mechanical properties of the MAMed components. To eliminate the defects and control the residual stresses, researchers have been focusing on the kinetic behavior of the molten pool, formation mechanism of defects and unstable solid-state phase transformations, and the evolution of the residual stresses. It is wildly recognized that in situ characterization of the defect formation mechanisms in the molten pool and monitoring of the residual stress changes during the MAM process is very challenging. Because the traditional measurement techniques, such as X-ray detection, X-ray diffraction, and ultrasonic detection, can only analyze defects and residual stresses after the components have been manufactured, it is necessary to find a technique capable of performing in situ analysis during the MAM process.

The rapidly developing synchrotron radiation and neutron diffraction-based characterization technologies have proven to be some of the most effective methods for in situ analysis of defect formation mechanisms, crack initiation, phase transformation, and stress evolution during the AM process. This paper reviews the principles of synchrotron radiation and neutron diffraction technologies and their advantages and practical applications in AM, and summarizes the recent progress and future prospects of their applications in AM.

Over the past decade, with the rapid development of characterization techniques based on synchrotron radiation and neutron diffraction, a large volume of research has been carried out to investigate the formation mechanisms and distribution of internal stresses during the AM process (Tables 1 and 2). The synchrotron radiation-based characterization methods can broadly be divided into the following three different types: synchrotron X-ray imaging, synchrotron X-ray diffraction, and synchrotron computed tomography. The synchrotron X-ray imaging can characterize in situ the formation process of internal three-dimensional defects in materials and analyze in situ the molten pool dynamics. The synchrotron X-ray diffraction can be used to analyze the internal stress states and phase transformation processes in materials, and works with the tensile test to dynamically analyze in situ the internal dislocation density of parts. The synchrotron computed tomography can reconstruct three-dimensional models of additively manufactured components to analyze surface defects, and can assess the impact of internal defects during the service process of components using in situ mechanical tests. The neutron diffraction technologies can be divided into non-in-situ neutron diffraction, in situ neuron diffraction, and electrically neutral nuclear scattering techniques. In addition to examining the macroscopic residual stresses in additive components, the characterization based on the neutron diffraction technologies can also measure the metal texture, crystal lattice parameter changes, strain, grain size, density of dislocations, and other parameters. It can also detect the concentration and the location of light elements, such as hydrogen and lithium, in the crystalline structure. Using the synchrotron X-ray imaging, Qu of the University of Wisconsin-Madison, has discovered that nanoparticles can be adopted to eliminate all types of large spatters by simultaneously stabilizing molten pool fluctuations and controlling liquid droplet coalescence. They have also demonstrated that the control of laser powder bed interaction instabilities by TiC nanoparticles is feasible, which has led to the elimination of large spatters and printing of lean-defect samples with good consistency and enhanced properties (Figure 2). Beese of the Pennsylvania State University and Oak Ridge National Laboratory performed in situ neutron diffraction studies of lattice strain evolution and offered a new perspective on the understanding of dislocation-solute interactions and their impact on work-hardening behavior in high-temperature alloys. These observations can pave the way for a fundamental understanding of the abnormal increase in strength at elevated temperatures commonly observed in a wide range of high-temperature structural alloys and may have important implications for tailoring thermomechanical properties using microstructure control in MAM.

Although the characterization techniques based on synchrotron radiation and neutron diffraction have been widely used in the AM process, further development is still needed to expand their applications in AM along the following directions. In situ detection techniques still need improvements, and the temperature field, velocity field, cooling rate, and solidification parameters must be considered in real-time models to reduce the internal defects and improve the quality of additively manufactured components. In addition, in situ inspection of the deposition processes should combine high-resolution and ultrafast synchrotron X-ray imaging, high-speed light photography, and infrared thermometry to develop in situ AM characterization techniques with higher resolution and contrast sensitivity. Furthermore, molten pool dynamics models should be established, which can then guide the design and optimization of AM process parameters. Meanwhile, taking the advantage of the synchrotron radiation and neutron-based X-ray diffraction techniques, the internal stress distribution and microstructure evolution during metal forming and servicing processes can be analyzed. Moreover, the advantages of adopting the synchrotron radiation and neutron-based techniques for measuring the three-dimensional stress field at the crack tip can help to establish the elastoplastic nonlinear micromechanical models of additively manufactured components, which can be used to analyze the fatigue and fractures caused by the multiscale stress field variations.

Spin-exchange relaxation-free (SERF) comagnetometers have been widely applied in fundamental physics exploration, including the fifth force measurement, dark matter detection (axion and axion-like particles) and CPT (charge conjugation, parity inversion, time reversal) and Lorentz symmetry violations. Besides, it has an internationally recognized development potential in inertial navigation. SERF comagnetometers contains alkali metal electronic spin ensemble and noble gas nuclear spin ensemble. The study of the responses to external excitations inevitably involves the coupling of two ensembles. A general response model for K-³He comagnetometer has been introduced to characterize the dynamics of the hybrid pumping K-Rb-²¹Ne comagnetometer. However, there are significant differences between the two kinds of spin ensembles combinations. The unexplored comparison of dynamics between the two spin ensembles combinations would present fruitful discoveries. In this paper, we establish a complete model for K-Rb-²¹Ne comagnetometer and compare the responses of K-³He and K-Rb-²¹Ne comagnetometers. The influencing factors of the response rate and the amplitude are quantified. This research sheds light on the study of the difference between different spin ensembles combinations for the improvement of the dynamic performance and sensitivity of comagnetometers, and is expected to promote the development of inertial navigation and fundamental physics exploration.

In this paper, transient and steady-state response models are established from the Bloch equations that describe the coupling of electronic and nuclear spins. In the K-Rb-²¹Ne comagnetometer, the electronic effective magnetic field cannot be ignored, so the spin exchange relaxation rate is considered in the model. Based on the established model, the influence of various influencing factors on the dynamic response is analyzed. For the transient response, the main influencing factor is the bias magnetic field. The dynamic responses to the magnetic field, angular velocity and anomalous field at the compensation point and the strong coupling point are compared through experiments and simulations. For the steady-state response, the relationship between response coefficients and bias magnetic field for different input signals is simulated. In order to further study the influencing factors of the angular velocity response strength, we analyze the variation of the angular velocity response coefficient with electronic spin polarization and relaxation rate at the self-compensation point.

The dynamic response model of coupled ensembles in the SERF comagnetometer is established, and the influencing factors of the dynamic response are quantitatively analyzed with simulation and experiments. The effects of bias magnetic field, coupled spin ensemble polarization, electronic spin relaxation rate and other factors on the steady response signal are clarified. We find that there is a significant difference of 75 times in the dynamic response rate between the strong coupling point and the self-compensation point (Fig. 3). Besides, the response strengths to different input signals are simulated under various bias magnetic fields. At the self-compensation point, the responses of comagnetometer to the anomalous field and inertial rotation are maximized, while the sensitivity to the magnetic field is minimized (Fig. 4). It is further analyzed that there is an optimal polarization at the self-compensation point, which makes the angular velocity response coefficient the highest (Fig. 5). This optimal point is related to the atomic species and the electronic spin relaxation rate, at which the angular velocity response coefficient can be doubled by reducing the electronic spin relaxation rate. This paper clarifies that the sensitivity and dynamic performance of SERF comagnetometers can be further improved by optimizing the electronic spin polarization and the relaxation rate, which is expected to expand its application in inertial navigation and fundamental physics exploration.

In the present study, a complete coupled ensemble dynamic response model of the SERF comagnetometer is established. The dynamics of K-Rb-³He system is studied via simulation, and the dynamics of K-Rb-²¹Ne system is experimentally studied to verify the correctness of the established model. The dynamic response rate is found to be significantly affected by the bias magnetic field. Based on the experimental conditions such as the experimental temperature and optical power in this paper, the response rate at the strong coupling point is 75 times faster than that at the self-compensation point. The influencing factors of the steady-state response strength are quantitatively analyzed by simulation. The steady-state response is mainly affected by the electronic spin polarization and the electronic spin relaxation rate. There is an optimal polarization which maximizes the angular velocity response coefficient. At the same time, the decrease of the electronic spin relaxation rate can increase the angular velocity response coefficient of the K-Rb-³He system from 0.26 to 0.51, and that of the K-Rb-²¹Ne system from 1.48 to 2.31. Therefore, by reducing the electronic spin relaxation rate and optimizing the electronic spin polarization, the response coefficient can be improved, thereby improving the sensitivity of inertial measurement, providing a good foundation for the development of fundamental physics exploration and inertial navigation.

Combustion involves very complicated physical and chemical reactions of fuels. Chemical reactions transform the fuel energy into thermal energy, accompanied by high temperature as well as combustion products at high pressure. The thermal energy then drives mechanical devices for mechanical movements and greatly promotes the industrial development. However, the combustion efficiency and the working temperature range of the fuel determine the performance and service life of the combustion equipment. Also, the combustion process inevitably generates carbon oxides, nitrogen oxides and other pollutants, which can seriously damage human health and the global environment. It is essential to explore the reactions of complicated combustion fields and reveal their states in real time for combustion optimization and intrinsic exploitations.

The distributions of temperature field and gas component concentration inside the combustion reveal the combustion performance more intuitively. The transient changes of the flame temperature directly reflect the stability of the combustion process, and are closely related to the combustion efficiency, gas pollutant emission and unburned carbon loss. The gas component concentration distribution is also an important indicator of the fuel combustion efficiency and combustion cleanliness. For the combustion reaction mechanism and combustion performance improvement, the online monitoring of temperature and gas component concentration is the prerequisite. However, these reactions often occur in harsh environments with high temperatures and pressures, and the confined layout of the measurement space poses a serious challenge to these measurements.

With the development and innovation of lasers, laser spectroscopy has been widely used in combustion monitoring and turns to be one of the important tools for combustion diagnosis. The continuous vibration in the combustor, the radiation from the violent fluctuation of the flame, and the high-speed turbulence of the flow all bring great distortions into the detection of optical intensities. Meanwhile, the actual combustion process changes very drastically, and the flame parameters, such as temperature, gas fraction concentration, and flow rate, are non-uniformly distributed in the confined space. If only the projections along a single laser path are measured, the spatial resolution along the path is missing and it fails to reveal the distribution along the path. For multi-dimensional imaging of gas parameters in the combustion field, absorption data from multiple laser paths across the region of interest are used to reconstruct the distributions inside by tomographic techniques.

In recent decades, laser absorption spectroscopy (LAS) has been widely used in combustion diagnosis, benefiting from the development of low-cost and easy-to-use distributed feedback laser diodes. As a non-contact method with high sensitivity and rapid response, LAS has been combined with computed tomography (CT) methods for cross sectional imaging by using spectral data from multiple laser paths at multiple angles. In this way, real time visualizations of flame temperature and gas component concentration distributions are realized for postprocessing of the combustion reaction mechanism. LAS is also a preferable technology in complicated combustion diagnosis due to its advantages of simple structure and good environment adaptability.

Laser absorption spectroscopy tomography and its application in monitoring of dynamic and complex combustion field are reviewed. Firstly, the measurement principles of common LAS methods, including direct absorption spectroscopy (DAS), wavelength modulation spectroscopy (WMS) and amplitude modulation spectroscopy (AMS), are briefed. The application of these measurement techniques to the intrinsic parameter monitoring in combustion field along a single laser path is also described. Secondly, the state-of-the-art of optical sensing module and circuit module is illustrated for LAS tomography instruments. For combustion field of interest in different cases, tomogrpahic sensors in terms of moving scanning sensors and fixed angle sensors are compared for specific applications. Also, data acquisition systems for the tomographic images are included, such as systems of high frame rate raw signals at high speed for a short period of time and systems at a low frame rate for a long period of monitoring time. Then the principle and development of LAS imaging technology are introduced. Image reconstruction methods, e.g., analytical method, iterative method and nonlinear method are presented to monitor the intrinsic parameters of complex combustion field. These methods have unique advantages in certain applications, such as fast solution speed or high solving accuracy. Finally, the specific applications of LAS tomography in laboratory flames and harsh field experiments are briefed.

Laser absorption spectroscopy has made great strides in spectral acquisition methods, data acquisition systems, image reconstruction algorithms and other key techniques, and has got progress in the application of combustion field parameters monitoring in both laboratory and industrial sites. However, there still exist urgent needs for further developments and thorough investigations, including but not limited to the development of wide spectrum laser sources, spectral data acquisition in extreme environments, image reconstruction models in cases of very few angular projections, new image reconstruction methods incorporating combustion models, and sensor systems suitable for ultra-high dynamics, etc. Further in-depth studies are expected to meet the increasing demand for onsite applications over wide temperature ranges, high velocity dynamics and multi-component distributions.

A spin-exchange relaxation-free (SERF) atomic co-magnetometer achieves angular velocity measurement by determining the probe laser's optical rotation angle. The detection system's polarization error, as a direct interference quantity, is coupled with the output signal and leads to the signal drifting, which sufficiently decreases the inertial measurement system's long-term stability. In the SERF atomic co-magnetometer detection system, the polarization error primarily originates from the non-desirable elliptical polarization component and polarization azimuth error. Most studies focus on the analysis and suppression of the non-desirable elliptical polarization component, but the polarization azimuth error caused by the residual rotation angle has not been widely researched. Thus, the signal drift caused by this polarization error needs to be resolved immediately, and the accuracy of the SERF atomic co-magnetometer needs to be improved. In this study, a polarization error suppression approach based on the optimization of polarization fluctuation sensitivity coefficient is suggested. The accuracy of the atomic co-magnetometer is enhanced by modifying the probe laser wavelength corresponding to the scale coefficient's maximum value and decreasing the atomic vapor cell's temperature. Furthermore, this approach does not add additional devices that can introduce other noises and is simpler to scale down.

In this study, by optimizing the polarization fluctuation sensitivity coefficient, the polarization error of the SERF atomic co-magnetometer is suppressed. First, the equation for the output signal of the SERF atomic co-magnetometer under ideal conditions is modified, considering that the actual atomic spin precession detection system will introduce inevitable residual optical rotation angle error due to the imperfect manufacturing process of optical components or the changes of the ambient temperature and mechanical stress. The influence of the spurious angular velocity caused by the residual optical rotation angle on the co-magnetometer drift is examined according to the modified output signal expression, and the polarization fluctuation sensitivity coefficient is obtained. This coefficient is related to the probe laser wavelength and the atomic cell's temperature. These two parameters are further adjusted to the position where the co-magnetometer is least sensitive to polarization fluctuations, therefore realizing the polarization error suppression. Furthermore, an SERF atomic co-magnetometer is constructed and the suggested approach is experimentally confirmed. The probe laser's wavelength is varied by adjusting the laser source's temperature control parameters. The atomic cell's temperature is controlled using a non-magnetic electric heating system combined with a proportion integration differentiation (PID) closed-loop controller. Under different conditions, the scale coefficient and static drift tests of the atomic co-magnetometer are conducted. Finally, the Allan standard deviation is applied to examine the test findings.

The spurious angular velocity caused by the residual optical rotation angle introduces polarization error to the SERF atomic co-magnetometer. The correlation between the residual optical rotation angle and spurious angular velocity is expressed by the polarization fluctuation sensitivity coefficient. When the scale coefficient of the co-magnetometer is the maximum, the polarization fluctuation sensitivity coefficient has the minimum value [Fig. 2(a)]. Thus, the co-magnetometer is the least sensitive to polarization fluctuation at that time. The scale coefficient varying with the proble laser wavelength has a Lorentzian lineshape (Fig. 4) and peaks at one wavelength detuned from the atomic resonance wavelength. Furthermore, with the increase of atomic cell temperature, the polarization fluctuation coefficient increases monotonically [Fig. 2(b)]. The polarization error is also suppressed by appropriately reducing the atomic cell temperature under the condition that the atomic co-magnetometer is maintained at the SERF region. The drift error analysis of the test data of the SERF atomic co-magnetometer is conducted by Allan variance, and the bias instability is decreased from 0.012 (°)/h to 0.008 (°)/h (Fig.6). The polarization error of the co-magnetometer is efficiently suppressed.

In this study, the influence of the polarization error of the SERF atomic co-magnetometer detection system on the output signal drift is examined. And we quantitatively investigate the co-magnetometer drift caused by the residual optical rotation angle using the polarization fluctuation sensitivity coefficient. This polarization fluctuation sensitivity coefficient converts the change of unit residual optical rotation angle into the resulting spurious angular velocity's change. A parameter optimization method is then suggested to reduce the polarization fluctuation sensitivity coefficient by adjusting the probe laser wavelength and atomic vapor cell temperature, which can minimize the effect of the polarization error caused by the residual optical rotation angle on the co-magnetometer drift. The experimental confirmation is performed on the designed SERF atomic co-magnetometer. The finding demonstrates that the polarization error of the SERF atomic co-magnetometer is suppressed by modifying the probe laser wavelength corresponding to the scale coefficient's maximum value and suitably reducing the atomic vapor cell's temperature. The drift error analysis of the SERF atomic co-magnetometer test data is performed by Allan variance, and the bias instability is reduced from 0.012 (°)/h to 0.008 (°)/h, confirming the effectiveness of the suggested polarization error suppression method.

Planetary seismology is a new discipline for imaging the internal structure of planetary objects, and it shows the law of planetary motions and determines whether they are habitable. The complete internal structure of planetary objects cannot be obtained using conventional seismometers, which can only measure three translational components in the geological movements. Thus, three rotational components in the geological movements must also be measured. Nowadays, the rotational components are frequently measured using gyroscopes. Large ring laser gyroscopes are employed to achieve ultra-high precision measurement, although they can only function at a fixed site. The fiber optic rotational seismometer (FORS), which is based on the fiber optic gyroscope (FOG), is insensitive to the translation motion and has the benefits of low noise, high sensitivity, and portability, which is ideally adequate for measuring three rotational components in planetary seismology. High precision is required for the FORS, but as a civil system, low cost is also required. The biggest challenge for a practical high-precision FORS is to satisfy the demands of high performance and low cost simultaneously. In this study, the error mechanism of depolarized FOGs is shown and a noise suppression technique is proposed. Based on the differentially depolarized FOG, a three-axis high-precision seismometer (BHFORS-3C) is developed with self-noise smaller than 4.5 nrad·s^-1·Hz－12. Long-term seismic observations demonstrate that BHFORS-3C is ready for field application and offers crucial support for the observation of 6-component planetary seismology and the accurate analysis of seismic activities.

The output spectrum of Sagnac interferometer assembly (SIA) in a depolarized FOG is the modulation of the original input spectrum. The conventional analysis approach obtains the modulated output spectrum model by calculating the transfer function of the SIA. The depolarized SIA consisted of several polarization-maintaining fibers (PMFs) and single-mode fiber (SMF) coil is a common polarization interferometer. The modulated output spectrum is actually the polarization interference spectrum, and the output spectrum model can be obtained using the Jones matrix approach. The modulated spectrum is easily affected by the environment and becomes unstable, and the fluctuation of the modulation spectrum has a considerable effect on the noise and drifts of depolarized FOGs. In this study, a spectral modulation suppression technique is proposed. A phase modulator is inserted in a PMF of the depolarizers and additional high-frequency phase modulation is applied. The spectral modulation can be efficiently suppressed, and so are the noises and drifts.

We previously demonstrated that a multifunctional integrated optical circuit can function well over a wide bandwidth (Fig. 5) and proposed a differential FOG (DFOG), which consists of two equivalent interferometric FOGs sharing a single SIA and driven by two broadband sources with various wavelengths. The DFOG has good common-mode error rejection capability and the errors because of temperature and magnetic field (Fig. 6) can be efficiently suppressed. In this study, we propose a differential depolarized FORS (DD-FORS) (Fig. 7) based on the DFOG and the depolarized FOG using additional high-frequency phase modulation. A 3-axis high-precision seismometer is developed (Fig. 8) with self-noise smaller than 4.5 nrad·s^-1·Hz－12 (Fig. 10). Long-term seismic observations have been conducted in Huainan, Lijiang, and Beijing (Fig. 9), and the result shows the reliability and portability of BHFORS-3C. The long-term formal observations in Lijiang demonstrate that BHFORS-3C has achieved reliable measurements of remote strong earthquakes and near-field weak earthquakes.

In the current study, the output spectrum model of SIA in a depolarized FOG is built based on the polarization interference principle, and the error mechanism of depolarized FOGs is shown. A noise suppression technique using additional high-frequency phase modulation in SIA is proposed and verified. A high-precision FORS based on the differential depolarized FOG is proposed and a 3-axis high-precision seismometer which has the characteristics of low cost, low noise, and adaptability to the environment is developed. The long-term observations in several places demonstrate the high reliability and portability of BHFORS-3C. Considerable observational data have been recorded and preliminary verification of the rotational seismic model has been conducted. Our study offers a practical high-precision 3-component seismic rotation observation instrument for planetary seismology.

Spin exchange relaxation free (SERF) atomic inertial measurement instruments have become important in paving the direction for the potential development of ultra-high-precision inertial measurement instruments by virtue of their ultra-high theoretical accuracy and easy integration. The realization of the SERF state requires a stable and uniform atomic density, which is closely related to temperature.Therefore, it is necessary to accurately measure the temperature and its gradient inside a cell. Among the various existing methods for temperature measurement, such as the platinum resistance method, the fiber grating method, the technique of infrared temperature measurement, and the spectral absorption method, only the last one can accurately measure the temperature inside the cell. However, all existing research on the spectral absorption method focuses on the accuracy of temperature measurement and the analysis of influencing factors. Thus, relevant research on the temperature gradient that exists inside a cell is lacking. Therefore, in this study, the spectral absorption method is proposed to measure the temperature field inside the alkali metal cell of the serf inertial measurement system. A longitudinal temperature gradient is obtained inside the cell by the continuous control of the laser orientation in the incident chamber. This method can accurately evaluate the cell temperature and the temperature gradient of high-precision quantum sensors. It provides important technical support for optimizing the design of electric heating without a magnetic system or an oven structure.

In this study, the spectroscopic absorption is used to measure the temperature inside a cell. First, the relationship between the temperature and the ratio of the intensity of linearly polarized light before and after passing through the gas cell is theoretically analyzed. Simultaneously, the influence of the frequency of the linearly polarized light on the accuracy of temperature measurement is also analyzed, and the linearly polarized light frequency under the highest temperature sensitivity is obtained. Subsequently, an experimental platform is built. The temperature at the center of the cell is measured and compared with that of platinum resistor at the monitoring point. After that, the temperatures at five points at equal interval in the longitudinal direction of the cell are also measured by moving the light source on the stage, and the results thus obtained are compared with the temperature simulation results to evaluate the accuracy of the method.

When the parameters of the cell are determined through theoretical analysis, it is observed that the ratio of the intensity of light before and after entering the cell is affected jointly by the cell temperature as well as the frequency of the linearly polarized light. The ratio decreases with an increase in cell temperature (Fig. 1). When the frequency of the linearly polarized light is close to the transition frequency at the D1 line of the Rb atom, the ratio increases with the temperature change rate of the gas cell, thus indicating that it is more sensitive to the temperature change (Fig. 2). At the optimum laser frequency, the temperature at the center of the cell is measured using an experimental setup. The result obtained is less than that at the oven walls, and this difference increases as the temperature increases (Fig. 6). The study shows a loss in heat conduction from the oven to the cell, and this loss increases as the oven temperature is increased, which is consistent with the actual situation. Subsequently, four points are selected at equal interval in the longitudinal direction of the cell, and the test results show that the longitudinal temperature gradient of the cell is 0.744 ℃/mm (Fig. 7). For comparison, the simulation results at the same location indicate that the temperature gradient of the cell is 0.725 ℃/mm (Table 1). The two results are comparable with the difference lying in the allowable error range, thus showing the accuracy of this method in evaluating temperature gradients.

In this study, an online method for measuring the temperature and its gradient in alkali metal cells based on spectral absorption is proposed. This method can obtain accurate temperature field information using the intensity ratio of the linearly polarized light before and after entering the cell in the SERF inertial measurement instrument. At the optimal laser frequency, the temperature gradient of the cell in the current heating structure is 0.744 ℃/mm. This result is similar to the simulation result, which shows the accuracy of this method. Thus, this method is suitable for the accurate evaluations of the cell temperature and the temperature gradient of high-precision quantum sensors, including SERF inertial measurement instruments, and provides important technical support for the optimization of the design of electric heating without a magnetic system or oven structure.

Interferometric fiber optic gyroscopes (IFOGs) are angular velocity measurement sensors based on the Sagnac effect. With the unique advantages of high performance, high sensitivity, anti-irradiation and high reliability as a solid-state technology with no moving parts, IFOGs have become the significant devices of inertial navigation and orbit and attitude control systems on satellites and spacecrafts.

In recent years, satellite technology has developed rapidly, especially in low-orbit, miniaturized, and high-throughput satellite technology. According to the Union Concerned Scientists (UCS) satellite database, as of January 1, 2022, 4852 satellites were in orbit around the Earth, of which 511 belonged to China. The development of satellite technology has put forward new requirements for IFOGs in space. On the one hand, it is a high-reliability technology. In the satellite fault analysis, the failure rate of the orbit and attitude control subsystems is always at a high level. As the core component of the orbit and attitude control subsystems, in the design process of the IFOGs, ensuring reliability should always be put in the first place. From high-intensity vibration during launch to galactic cosmic radiation, high-energy particles, thermal vacuum, etc. after entering orbit, the complex and harsh space working environments require that IFOGs must be fully prepared in terms of resisting external disturbances and improving environmental adaptability. Especially in the high orbit, high inclination, and deep space operation environment, the energy, dose rate, and cumulative radiation dose of space particles are greatly increased. It is significant to improve the anti-irradiation performance of IFOG components. On the other hand, it is a miniaturization technology. While ensuring lifetime and performance, the device with lower power consumption, lighter weight, and lower cost is always a better choice. Resources will be further limited when applied to small satellites especially. Therefore, research on miniaturization is necessary to enhance the competitiveness of IFOGs in low or medium precision applications.

At present, the IFOG products of industrial sectors in Europe and the US have been applied in the space field in batches. The companies including iXblue in France, Northrop Grumman and Honeywell in the US, and Optolink and Fizoptika in Russia have rich experience in the development and application of spacial IFOGs. With the continuous miniaturization and environmental adaptability improvement, IFOG products are increasingly used in various small spacecrafts. In recent years, with the development of related technology, the level of domestic IFOGs has conjointly been rapidly improved, which play roles in a growing number of space missions, such as Tiangong-1, Shenzhou series spacecrafts, Chang’e series of lunar probes and several in-orbit satellites. After years of research, Beihang University starting from the two routes of high-reliability and miniaturization has successively made breakthroughs in key technologies such as structural configuration optimization, detection technology innovation, functional density improvement, and components upgrading of IFOGs for space. Several spacial IFOGs developed by Beihang University are working stably on more than 60 in-orbit satellites.

This paper reviews several key technologies of IFOGs for space applications developed by Beihang University. The dual-light-source four-axis IFOG solution is first introduced (Fig. 3), and the in-orbit fault diagnosis technology of IFOGs is described (Fig. 4). This is based on the desire for high reliability and downsizing. It has significant advantages over the conventional six-independent-axis redundancy approach in terms of weight, dimension, and power consumption while achieving double backup. The configuration of miniature three-axis IFOGs and the simplest three-axis configuration made possible by time-division multiplexing (TDM) technology are introduced in order to further realize downsizing (Fig. 5). Including source backup, the reliability can be increased. Additionally, an IFOG design based on polarization-maintaining photonic crystal fiber (PM-PCF) is demonstrated to further enhance the anti-irradiation performance. The typical IFOG products for space developed by Beihang University are presented in Table 3.

The IFOGs should be highly reliable due to the hostile space environment. The dual-light-source four-axis arrangement that was presented offers double backup for all of the IFOG components, considerably increasing reliability. The in-orbit fault diagnosis approach is typically applicable to multi-axis IFOGs, which enhances the IFOG’s environmental adaptability while avoiding additional costs associated with all-digital implementation. In order to prevent resource waste brought on by excessive redundancy, a more focused redundant backup is designed for space applications with relatively mild operating environments based on the failure modes and failure rate of components in space. The simplest scheme of the three-axis IFOG could be achieved by the TDM technology. The mass, volume, power consumption, and cost will be significantly decreased, and the device shrinking makes it more appropriate for tiny spacecraft like small satellites. The introduction of the PCF scheme demonstrates an important development trend for IFOGs: improved anti-irradiation properties due to low radiation induced attenuation (RIA) of PCF.

The lightweight type-aware light detection and ranging (LiDAR) is an active three-dimensional (3D) optical imaging technology used for environment perception. Mounted on platforms such as vehicles and aircrafts, the type-aware LiDAR performs as the eye to capture and process environmental information, providing real-time and accurate 3D data for target detection, identification and decision making. Compared with the traditional surveying and mapping LiDAR, the type-aware LiDAR has the advantages of smaller volume, larger amount of data, higher transmission rate and higher-resolution 3D imaging. As the demand of 3D sensing grows, the lightweight type-aware LiDAR technology becomes one of the hotspots in the future development. The type-aware LiDAR is mainly applied in the fields of aerospace exploration and autonomous driving. To meet the requirements of miniaturization and intelligence, the key technologies of type-aware LiDAR are developing toward lightweight system design. In this paper, the current key technologies and typical applications of lightweight type-aware LiDAR are summarized, and the development trend of key technologies is forecasted.

Firstly, the integration-level LiDAR has been greatly contributed by the development of fiber and semiconductor lasers, single photon detector, and micro-electro-mechanical system (MEMS) and optical phased array (OPA) laser scanner. Fiber laser has been employed for aerospace application with adjustable repetition rate of 5-50 kHz. With an inherently safer wavelength for autonomous driving application, vertical cavity surface emitting laser (VCSEL) of 1550 nm has better carrier confinement than semiconductor laser of 905 nm. Single photon avalanche diode (SPAD) array with hundreds of pixels has been developed for 3D imaging on femtosecond magnitude. The 4 pixel×4 pixel silicon photomultiplier (SiPM) has been developed to capture light intensity information, whose single pixel is composed of 100 SPADs (Fig. 1). To facilitate integration, scan unit of type-aware LiDAR is gradually developing from the traditional multi-beam mechanical scanning type to MEMS, OPA, and flash LiDAR (Table 1), and the ranging system is developing toward system on chip (SoC) technology.

Secondly, the ranging performance of LiDAR has a significant influence on the quality of point cloud, which is elaborated from two aspects of ranging system and ranging algorithm. The ranging system can be categorized as time-to-digital converter (TDC) and analogue-to-digital converter (ADC) according to the implementation schemes of time discrimination. In order to miniaturize the ranging system, the TDC/ADC hybrid SoC is designed to achieve long-range detection (Fig. 4). Toshiba has developed a 40-channel LiDAR SoC with a ranging error less than 0.25% and a distance ranging from 25 m to 225 m. Subsequently, the ranging algorithms for saturated, weak, and multiple echo waveforms are summarized separately. The selection of algorithms depends on the signal situation and application condition. With the recent developments, SoC is becoming the mainstream form of ranging system and the ranging algorithms need to evolve for processing more complicated signal.

Thirdly, the research status of pointing error correction of type-aware LiDAR is introduced, including the correction of scanning mechanism internal errors and system installation errors. The scanning mechanism internal errors can be eliminated by correction function obtained from mechanism analysis (Fig. 6), which is elaborated in various scanning mechanism of prism, MEMS and OPA. The system installation error is reduced by modeling the difference between the actual laser path and the ideal path, as well as adopting model optimization methods include network method, two-face method and length-consistency method. The future research of the pointing error correction needs more generalization and process standardization.

Fourthly, lightweight type-aware LiDAR has been employed in versatile applications. For autonomous driving application, low cost and potential for integration are required (Fig. 9). Mechanical (ranging 100-500 m, accuracy 2-7 cm) and hybrid solid-state scanning systems (ranging 150-500 m, accuracy up to 1 cm) are currently the prevailing types of LiDAR loaded on autonomous vehicles. As for space application, the flash LiDAR is commonly utilized in space rendezvous and hazard avoidance tasks due to its high directivity, high resolution and high precision (Table 3 and Table 4).

Lightweight type-aware LiDAR has become the development frontier and research hotspot in the field of intelligent sensors. In the past decade, hybrid-solid MEMS LiDAR as a mature product has sprung up. As the integrated chip technology fast grows, SoC technology perhaps becomes a mainstream solution to performance improvements of lightweight type-aware LiDAR with compactness, high resolution and high speed. In the future, the on-chip technology integrated with signal processing algorithm, state-solid scanning mechanism combined with universal correction method, and mutual promotion of technologies between civil and aerospace applications, will motivate the development of lightweight type-aware LiDAR.

Single-photon LiDAR is widely combined with emerging imaging technology fields such as low-light detection, ultra-long-range detection, artificial intelligence (AI), and computational imaging, producing remarkable research results. Conventional linear detection LiDAR can be divided into different categories of scanning detection, direct detection, coherent detection and non-scanning detection. LiDAR systems based on the time-of-flight (ToF) technique are capable of ranging detection of remote targets by directly interpreting the time difference between the outbound and return laser pulses. Initially, researchers tended to achieve long ranging distance by increasing the laser output power and the aperture of transceiver system. Adopting these methods, the weight and power consumption of the LiDAR system are further increased. Besides, the pursuit of high signal-to-noise ratio (SNR) echo signals in conventional LiDAR resulted in low system frequency and weak detection ability of dynamic targets. In long-distance ranging, conventional LiDAR systems were susceptible to extreme conditions such as dust and fog weather and lost effectiveness.

With the advancement of single-photon detector (SPD) and precision electronic-timing technology, LiDAR based on time-correlated single photon counting (TCSPC) appeared, which has become a new way to solve the above problems. Compared with the conventional LiDAR systems, TCSPC LiDAR systems have the characteristics of higher sensitivity, higher depth accuracy, shorter acquisition time and higher photon efficiency. By time accumulation of the single echo and photon statistics, TCSPC LiDAR does not rely on single pulse measurement results, so that it does not emphasize on high SNR of single detection pulse and high laser power. TCSPC LiDAR with the detection sensitivity reaching the single-photon level is called single-photon LiDAR. In the single-photon LiDAR system, only one photon can be detected and tagged, which achieves the theoretical detection limit. This new photon statistics scheme emphasizes on the full employment of the limited echo photon information, thus improving the photon utilization rate while maintaining high sensitivity. In improving working distance and detection efficiency of the LiDAR system, TCSPC has incomparable advantages over the traditional technology.

Many corresponding advances have been achieved in single-photon LiDAR systems, but they still face a series of challenges in noise suppression and performance improvement such as working range and depth accuracy. This review aims to act as an introduction to the topics of ToF-based single-photon LiDAR for the general reader and to provide a brief introduction to the current technologies available.

The process of accumulating discrete echoes in single-photon LiDAR can be regarded as targets acquisition through laser pulses, which means that a few echo photons contain abundant target information. Firstly, the ToF ranging method, TCSPC technique and fundamental principle of single-photon LiDAR are summarized. Then, the single-photon LiDAR system components are introduced, including the laser, optical transceiver system, single-photon detector, TCSPC module, and the control and data processing unit. Performances of lasers applied in typical single-photon LiDAR are listed (Table 1). The main abilities of single-photon LiDAR are ranging distance and detection accuracy. In order to achieve high-precision long-distance three-dimensional imaging of single-photon LiDAR, one method is to optimize the hardware part. The traditional way is to promote transmission laser power and increase the efficiency of optical transceiver system by expanding the receiving aperture and suppressing the noise of the optical transceiver structure. The main parameters of single-photon detectors include dark count rates (DCRs), dead time and detection efficiency.

Image reconstruction algorithms were designed for solving the case of low echo photons in single-photon LiDAR. For example, the first-photon imaging algorithm was adopted to improve the utilization efficiency of echo photons. According to the detection probability model of echo photons in the first-photon imaging algorithm, the spatial structure and reflectivity of the three-dimensional scene are acquired, and the target information is fully reconstructed with high quality. Before imaging depth recovery, noise suppressed or removed cannot be neglected. Mainstream algorithms, such as sparse Poisson intensity reconstruction algorithm (SPIRAL), optimize the regularization term in different scenes to achieve the optimal reconstructed image. In the end, the main development history of single-photon LiDAR is introduced (Fig. 8), and the technical applications of single-photon LiDAR in long-distance detection imaging and autonomous vehicles are discussed. At present, the farthest imaging and ranging working distance of single-photon LiDAR has been expanded from sub-kilometer level (2013) to 200 km (2021), and the imaging accuracy has reached millimeter level from sub-decimeter level. It should also be pointed out that the single-photon LiDAR was developed in the direction of real-time target recognition. Single-photon LiDARs are capable of dynamic targets tracking and transient images recording, which is expected to be applied in the field of autonomous vehicles and space fragment monitoring.

Single-photon LiDAR has gained extensive attention and produced remarkable research in long-range detection imaging because of its single-photon detection and tagging ability. At present, the research on single-photon LiDAR mainly demonstrates the imaging principle and reconstruction algorithm in the laboratory, and more research is needed to verify the effect and adaptability of few-photon algorithms in the actual environment. In summary, single-photon LiDAR still needs in-depth and detailed explorations to further promote the system performance and optimize new reconstruction algorithms.

When marine targets on the sea surface are detected or continuously monitored using mid-infrared sensors, the results are negatively impacted by the radiance due to sun glint, which can cause the sensors to be saturated and target signals to be obscured. Polarization detection technology has been relied upon in the past to solve this problem owing to the polarization properties of sun glint. The traditional single-polarizer system for suppressing sun glint depends on the geometric position of the sun and the sensor. The system can only be successfully applied when the polarization degree of sun glint is obvious or when the sun glint is not strong. In certain situations, the residual sun-glint radiance is greater than the sensor-saturation response. In contrast, the dual-polarizer system, in which the polarizer has a fixed direction, achieves good contrast enhancement; the scenario applicability, however, is still limited. Furthermore, the polarization directional reflectance tends to vary across different observation scenes, which restricts the scope of the aforementioned methods. Therefore, to solve the original problem, in this study, based on the analysis of the mid-infrared polarization properties of sun glint on the sea surface, a mid-infrared switchable dual-polarizer detection system is designed. For different detection scenes, two work modes, both combined with an image-processing algorithm, are selected to realize the extraction of ship targets covered in sun glint.

In this study, to deal with a variety of sun-glint scenes, a mid-infrared switchable dual-polarizer detection system (Fig. 2) that can simultaneously suppress the s- and p-polarized components of sun glint is proposed (Fig. 3). Polarizer 1 was placed in front of the sensor and polarizer 2 on the rotatable support. A single-polarizer was used when the rotatable support was empty. We established whether one or two polarizers should be used based on the type and amount of sun glint, and accordingly named the scenarios as single-polarizer mode and dual-polarizer mode, respectively. The amount of sun-glint radiance can be calculated theoretically using Cox and Munk’s "slope" probability model and Fresnel’s law. The polarized type of sun-glint radiance is related to the sea surface conditions of the sensor points, the observation geometric conditions, and the atmospheric transmittance of the path. The degree of polarization of sun glint is always positive in the mid-infrared wavelength band and it reaches a maximum of 100% as the observation angle approaches the Brewster angle. At this stage, the sun glint is dominated by the s-polarized component, which can be suppressed through the single-polarizer mode. However, a p-polarized component exists in sun glint in most viewing cases, and the intensities of the p- and s-polarized components are nearly equal to the approximate horizontal observation angle. At this stage, a dual-polarizer was utilized to suppress the residual p-polarized component. Because the working mode of single/dual-polarizers can switch flexibly, the proposed polarization detection system can significantly reduce sun glint and enhance the contrast of target images for a variety of sun-glint scenes.

Two groups of sun-glint scenes in "the lake in Zizhuyuan" and "the off shore in Changdao, Yantai city" are selected for experiments, so as to clarify the scene adaptability under different polarization working modes.

The image obtained through the single-polarizer mode can effectively highlight the obscured target (a big boat with size 2.8 m×1.5 m and a small boat with size 1.2 m×0.8 m); a similar contrast enhancement occurs in the dual-polarizer mode (Fig. 6). For the small-boat target, the relative contrast in the single-polarizer mode is 0.61, but clutter bright spots appear around the target owing to the effect of wind and waves in the dual-polarizer mode, and the relative contrast drops to 0.18 (Table 2). Therefore, for long-distance near Brewster angle detection of small targets, the single-polarizer mode should be adopted. The dual-polarizer mode is adopted to further suppress the p-polarized component for a short distance with an approximate horizontal observation angle to improve the relative contrast of the target (Fig. 8). Compared with the single-polarizer mode, the relative contrast of the target is increased from 0.15 to 0.68 (Table 3). For the best angle of the two polarizers, the sun-glint radiance can be calculated according to MODTRAN and Cox and Munk’s models; the theoretical calculation can subsequently be carried out by comparing the contrast function.

Based on the polarization properties of sun glint on the sea surface, a switchable mid-infrared detection system is proposed for a variety of detection scenes. Two polarization working modes are formulated to suppress the sun glint by calculating the polarized type and amount of sun-glint radiance. The single-polarizer mode is utilized when the s-polarized component dominates, whereas the dual-polarizer mode is used when the residual p-polarized component is greater than the sensor-saturation response after single-polarizer suppression. The suppressed image is then processed to realize effective ship target detection in sun glint. Therefore, the proposed mid-infrared dual-polarization detection system can significantly enhance the contrast of target images and effectively improve the detection ability for various sun-glint scenes.

Terahertz (THz) science is one of the technological frontier fields of significant research in the world, and THz technology has crucial applications in aerospace, national security, communication radar, quantum information, material science, biomedicine and other fields. The THz electromagnetic wave is located between microwave and infrared, and its spectrum width is about thirty times greater than that of microwave and millimeter waves. It is a strategic frequency resource that various countries are scrambling for, and enormous demands occur in both military and civil applications. However, this frequency band, which connects electronics and photonics, has not been fully exploited and utilized. The THz frequency band has many unique characteristics, such as the time-resolving ability with narrow pulse widths at the picosecond level, the ability to penetrate paper and clothing, the spectral properties of many matters, and the low photon energy. These unique properties give THz waves many important applications, such as nondestructive testing, communication radar, security checks and anti-terrorist, and biomedicine. However, the key factor which hinders the development of THz science and applications is the lack of high-performance THz sources, core devices, and system integration. Among them, the lack of high-efficiency, high-beam quality, and high-stability strong-field THz radiation sources is the focus and difficulty of the current solution.

The methods of generating THz radiation include mainly the electrical and optical methods. This paper mainly discusses the optical methods to generate THz radiation. Optical methods for generating THz radiation include femtosecond laser excitation of nonlinear crystals, photoconductive antennas, plasmas, and so on, which are widely used in sensing imaging and communication. However, the low efficiency and low energy of the current THz radiation source directly limit the nonlinear effects of THz-matter interaction, novel quantum matter state regulation, electron acceleration, biomedicine and other multifaceted frontier scientific and applied research. Therefore, researchers in related fields are working to improve the performance of THz sources and to further promote the development of THz technology.

After the presentation of the tilted pulse front technique, lithium niobate crystals with large nonlinear coefficients, mature manufacturing processes, and high destruction thresholds are expected to realize the generation of high-energy strong-field THz radiation through femtosecond laser action. Currently, pumping lithium niobate crystals by femtosecond laser based on the tilted pulse front technique is still one of the effective ways to generate high-energy strong-field THz radiation. Therefore, it is crucial and necessary to summarize the relevant research on lithium niobate strong-field THz sources to promote the development of this field.

In this paper, the study on lithium niobate strong-field THz sources is summarized as follows. Firstly, the development of strong-field THz generation based on femtosecond-laser-pumped lithium niobate crystals is reviewed in five stages (Fig. 1). Shen’s research group at the University of California generated the world’s first THz pulsed radiation from lithium niobate crystals by laser pulses. Hebling’s group at the University of Pécs, Hungary, proposed the tilted pulse front technique to solve the phase mismatch between near-infrared (NIR) light and THz in lithium niobate crystals. Then, the development of strong-field THz generation via tilted pulse front technique based on femtosecond-laser-pumped lithium niobate crystals has been initiated.

Secondly, the principle of lithium niobate tilted pulse front is described in four aspects: the history of the tilted pulse front theory of lithium niobate (Fig. 2), the key factors to be considered in the theoretical model (Fig. 3), the phase matching and pulse front tilt angle (Fig. 4), and the main methods of model calculation, respectively. Guidance is provided for generating high-energy strong-field THz sources in the future by summarizing the historical evolution of the theoretical model for generating strong-field THz based on the tilted pulse front technique and the mechanism of radiation efficiency saturation of lithium niobate THz strong sources.

Thirdly, the generation of lithium niobate single-period strong-field THz is described. A typical optical path diagram (Fig. 5) based on lithium niobate to generate a single-period strong-field THz and the composition of the tilted pulse front device are introduced.

Fourthly, the lithium niobate multi-period strong-field THz generation is described. Two methods to generate multi-period strong-field THz are introduced: the one based on the tilted pulse front technology of lithium niobate (Fig. 6) and the one based on the quasi-phase matching of periodically polarized lithium niobate (Fig. 7). Lithium niobate crystals are one of the most popular materials for generating strong-field THz, both for single-period and multi-period.

Fifthly, the applications of lithium niobate strong-field THz are discussed, which are presented in three aspects: strong-field THz-matter interactions (Fig. 9), strong-field THz electron acceleration and manipulation (Fig. 10), and strong-field THz biological effects (Fig. 11). These applications demonstrate the advantages of this type of strong-field THz sources and raise the need for higher strong-field THz sources.

Finally, the strong-field THz sources and applications at Beihang University are summarized and the results of Beihang University and its collaborative team in this field are introduced (Fig. 12). We are looking forward to the unprecedented new challenges and opportunities that extreme THz science and applications and their multidisciplinary intersection will bring in the future.

Pumping lithium niobate crystals by femtosecond laser via tilted pulse front technique is one of the effective ways to generate strong-field THz. In summary, lithium niobate crystals are one of the popular materials for generating strong-field THz sources, and lithium niobate strong-field THz sources have played an important role in the applications and studies such as the strong-field THz interactions with matter, electron acceleration and manipulation, and biomedicine. In order to promote better development in strong-field THz sources, the study of lithium niobate strong-field THz source still needs to be deeply explored from the aspects of theoretical basis, structure, and application scenarios.

Terahertz spectroscopy, which mainly includes terahertz time-domain spectroscopy (THz-TDS) and terahertz frequency-domain spectroscopy (THz-FDS), is an important research field of terahertz science and technology. THz-TDS based on the excitation and detection of femtosecond ultrashort laser pulses has been widely used in the study of spectroscopic properties of substances owing to its wide range of operating frequency (0.2-6.0 THz) and high signal-to-noise ratio. However, the resolution of THz-TDS is affected by its detection time window. The oscillatory disturbances generated in the system limit its frequency resolution to GHz, and the mechanical scanning speed is slow. In addition, THz-TDS obtains time-domain signals, and the spectrum of the entire frequency band of the substance must be achieved through a Fourier transform; moreover, it cannot arbitrarily select a certain range of frequencies for measurement. These factors have limited the spectral analysis performance and application of THz-TDS to some extent. Continuous-wave THz-FDS based on coherent detection is another developing terahertz spectroscopy technology with the advantages of high frequency resolution, simple structure, and tunability of the output frequency. Therefore, THz-FDS has gradually become a powerful tool for research on high-Q terahertz devices. With respect to system structure, the THz-FDS system can be divided into reflection type (measuring the reflection spectrum) and transmission type (measuring the transmission spectrum) schemes. In this paper, we mainly solve the construction and performance optimization of transmission and reflection THz-FDS systems and systematically study the data processing methods to extract material electromagnetic parameters.

There are different data processing methods for both transmission and reflection THz-FDS structures. For different sample states, such as gas, liquid, and solid, we designed appropriate bearing structures and data processing methods. For the transmission THz-FDS, we compared the three methods of fitting, finding extrema, and the Hilbert transform to extract the electromagnetic parameters of samples. Finally, we selected the Hilbert transform method combined with time-domain zeroing to eliminate the Fabry-Pérot (FP) effect, and the frequency-domain resolution was approximately 14 MHz. In addition, solid samples should be sufficiently thin to avoid FP effect, and gas samples should be sealed in a container to prevent leakage. The measurement of liquid samples was more complicated because the multiple reflections and transmissions between the container cell and the liquid caused the FP effect; we used the transmission matrix method to eliminate this effect. For the reflection THz-FDS, we designed a liquid container using high-resistivity silicon and measured liquids using a self-referenced calibration method to avoid beam offset. Furthermore, we used the singly subtractive Kramers-Kronig (SSKK) method to correct for phase errors that arose during solid sample replacement.

In the study of transmission THz-FDS, we measure the transmission spectrum of the atmospheric environment in the frequency range of 0.05-2.00 THz under the condition of relative humidity of 7.5% and temperature of 24.8 ℃. Approximately 16 water vapor absorption peaks can be clearly observed (Fig. 5). In addition, we demonstrate the relationship between different water vapor concentrations and terahertz absorption intensity at three frequency points of 0.558 THz, 0.753 THz, and 0.989 THz. The results show that with an increase in the water vapor concentration, the terahertz transmittance of each frequency point presents an apparent downward trend, that is, the absorption of water vapor to the terahertz wave gradually increases (Fig. 6). We then determine the refractive index and permittivity of lactose monohydrate via the Hilbert transform, finding extrema, and fitting methods. The average refractive index is approximately 1.7, and the average real part of the permittivity is approximately 3.2. All three methods accurately exhibit the absorption peaks of lactose monohydrate at three respective frequencies of approximately 0.53 THz, 1.20 THz, and 1.37 THz, which are in good agreement with previously reported results (Fig. 7). In the process of liquid measurement, we design a sample cell using high-density polyethylene (HDPE) and utilize the transfer matrix method to eliminate the FP oscillations generated by multiple reflections and transmissions between the liquid sample and the sample cell wall. We measure the refractive indices of non-polar liquid cyclohexane and n-hexane, which are consistent with previous test results (Fig. 9). In the study of reflection THz-FDS, we use the self-referenced method to extract the accurate permittivities of methanol and ethanol, which are consistent with the fitting results of the triple Debye model (Fig. 12). Finally, we use the SSKK method to eliminate the phase error and obtain the refractive index and absorption constant of the solid-doped silicon. The measurement and computational analysis data are essentially consistent with previous research results (Fig. 14).

As a key research direction of terahertz science and technology, terahertz spectroscopy has mainly developed two spectral systems, namely THz-TDS and THz-FDS. Between them, THz-FDS has technical advantages such as high frequency resolution, simple structure, and frequency tunability, which compensate for some defects in THz-TDS research. In this study, the system structure and data processing principle of transmission and reflection THz-FDS are discussed in detail. The transmission THz-FDS built by our research group is used to study the high-resolution terahertz spectral data of water vapor samples in the atmosphere. The transmission coefficient and refractive index of lactose monohydrate obtained by the extrema method and the Hilbert transform method are compared in detail. In addition, the transfer matrix is used to eliminate the FP resonance in the process of measuring liquid samples by the transmission method, and the accurate refractive indices of non-polar liquids n-hexane and cyclohexane are thus obtained. Furthermore, in research on continuous-wave reflection THz-FDS, the permittivities of methanol and ethanol in the terahertz frequency band are accurately obtained using the self-reference method. The SSKK method is utilized to extract the refractive index and absorption coefficient of solid-doped silicon. The above research results comprehensively verify the effectiveness of our continuous-wave THz-FDS system and corresponding data processing methods, thereby laying a good foundation for the development and application of terahertz frequency-domain spectroscopy.