Currently, nanosecond pulsed 3 μm lasers are of interest for many scientific research and practical applications. For mid-infrared optical parametric oscillators (OPOs), pumping sources with longer wavelengths are desirable to reduce the quantum loss in the parametric conversion. Moreover, pumping sources with short pulse duration and high peak power can improve the conversion efficiency to the mid-infrared wavelength (3‒12 μm range) and obtain greater output power or energy. Another important application of nanosecond pulsed 3 μm lasers is related to the distinctive features of water and hydroxyapatite, i.e., extremely high absorption in the vicinity of the 3 μm wavelength range. Therefore, pulsed lasers in this wavelength range are widely employed for medical ablation surgery, particularly for dental and orthopedic applications. Further, lasers with high repetition rate can improve the ablation efficiency of hard tissue and speed up the treatment process. If the laser pulse duration is less than the thermal diffusion time, unnecessary thermal damage to the surrounding healthy tissue can be reduced. Therefore, it is a common endeavor to achieve a stable 3 μm laser output with a high peak power and short pulse duration at a high repetition rate.

Bulk LiNbO₃ crystals have excellent acousto-optical (AO) properties and can be used as an ideal AO medium, exhibiting higher transmission in the 3 μm wavelength range, lower acoustic attenuation coefficient (1 dB/cm @ 1 GHz), and higher damage threshold (>200 MW/cm²). More than 30 years ago, scientists attempted to use LiNbO₃ crystals to create an AO Q switch, but it failed to work at 3 μm wavelength. Recently, we innovated and developed a LiNbO₃-based AO Q switch, and its effectiveness was verified in our previous study. In this work, the output characteristics of a LiNbO₃ AO Q switch at a high repetition rate are investigated in an Er, Cr∶YSGG laser. Hence, the output characteristics of an AO Q-switched Er, Cr∶YSGG laser pumped by a flash lamp at a high repetition rate are studied. The thermal focal lensing effect in the gain medium is compensated using a plane-convex resonator (PCR), which significantly improves the beam quality and output capacity of the laser at a high repetition rate. A stable output of the LiNbO₃ AO Q switch in the Er, Cr∶YSGG laser is realized.

To effectively compensate for the thermal focal lensing effect, the thermal focal lengths of Er, Cr∶YSGG laser crystals are calculated theoretically. Because the thermal focal length of the laser crystal is related to many factors, the corresponding theoretical calculation cannot be completely accurate. Hence, the thermal focal length of the Er, Cr∶YSGG laser crystal is measured using the critical resonator stabilization method in a plane-parallel resonator at 100 Hz. The theoretical calculation and actual measurement results are presented in Fig.2. According to the design theory of the resonator with the embedded thermal lens, the curvature radius of the convex mirror in the plane-convex resonator should be -166.3 mm. Therefore, convex mirrors with curvature radii of -100, -150, and -200 mm are adopted as rear mirrors in the respective experiments to measure and collect laser pulse energy. It can be seen from Fig.3 that the compensation effect of the convex mirror with a curvature radius of -150 mm is better than those with -100 mm and -200 mm curvature radii.

To explore the influence of the reflectivity of the output coupler(OC)mirror on the output performance of the LiNbO₃ AO Q switch, the reflectivity is set at 60%, 70%, and 80% for experimental research in the plane-convex resonator, and the results are shown in Fig.4. The optimum reflectivity of the OC mirror of the LiNbO₃ AO Q-switched Er,Cr∶YSGG laser is 70%. To explore the influence of the thermal focal lensing effect on the output performance of the LiNbO₃ AO Q switch, a comparison experiment of Q-switching between the plane-parallel resonator and the plane-convex resonator is performed. It can be seen from Fig. 5 that the structure of the plane-convex resonator can improve the output performance of the LiNbO₃ AO Q-switched Er, Cr∶YSGG laser in a certain pump energy range. The diffraction efficiency is varied by changing the radio frequency driving power(RFDP) added to the Q switch, and the output performance of the laser is explored. As can be seen from Fig.6, when the repetition rate is 100 Hz, the maximum pulse energy and minimum pulse duration are 4.36 mJ and 76.8 ns, respectively, when the RFDP is 30 W. Moreover, thebeam qualityfactor (M²) and the stability of the LiNbO₃ AO Q-switched Er, Cr∶YSGG laser are examined. As shown in Fig.7, for the pulse energy of 4.36 mJ, the beam quality factor (MX2) in the X direction and the beam quality factor (MY2) in the Y direction are 8.3 and 7.7 at 100 Hz. Further, as shown in Fig.8, the degree of energy stability (standard deviation) of the laser is 2.55%.

The results show that the designed LiNbO₃ AO Q switch can realize nanosecond pulse output at a high repetition rate in a 2.79 μm Er, Cr∶YSGG laser. The plane-convex resonator structure can effectively compensate for the thermal lensing effect of the gain medium, optimize beam quality, and improve the output performance of the laser. Increasing the radio frequency driving power of the AO Q switch can increase the pulse energy and compress the pulse duration, thus improving the output performance of the LiNbO₃ AO Q-switched Er, Cr∶YSGG laser.

Ultrastable lasers have important applications in precision measurement, precision spectroscopy, and quantum information. However, it is difficult to meet the requirements of the aforementioned applications using lasers in the free-running state because of frequency jitter and drift caused by external environmental factors. Various active techniques for stabilizing the laser frequency have been proposed and implemented. Among them, the ultrastable cavity Pound-Drever-Hall (PDH) frequency stabilization technology has a high locking accuracy and it is mature with wide application. Single-frequency fiber lasers have undergone rapid development in recent years. Based on the fiber laser's characteristics of narrow linewidth and low noise, ultrastable lasers with superior performance are developed. However, according to current reports, the most widely used ultra-stable laser sources are DFB fiber lasers. Because the resonance cavity and the fiber grating cavity mirror of DFB fiber lasers are integrated into the same structure, the length of the laser resonance cavity and the fiber grating cavity mirror can be simultaneously controlled when used for frequency stabilization of an ultrastable cavity PDH, which is very beneficial for obtaining a long-term locked ultrastable laser. Compared with DFB fiber lasers, DBR fiber lasers have more practical value for achieving ultra-stable cavity PDH frequency stabilization because DBR fiber lasers do not require rare earth-doped fibers to exhibit photosensitivity, are easier to manufacture, and have more advantages in terms of the laser band, wavelength flexibility, and other aspects. However, when DBR fiber lasers are used for ultrastable cavity PDH frequency stabilization, owing to the independent active temperature control of the fiber resonant cavity, coordinating the frequency of the FBG center with the frequency of the ultrastable cavity mode locked by the PZT during the PDH frequency-locking process is difficult. This makes long-term locking of the frequency of the DBR fiber laser difficult due to degradation of the frequency locking owing to laser mode hopping, making it difficult to meet the requirements of special applications, such as quantum entanglement experiments. Therefore, further study is required to achieve long-term locking of DBR fiber lasers based on ultra-stable cavity PDH frequency stabilization.

A home-made 2 μm band DBR single-frequency fiber laser was used as the laser source. In order to quickly tune the frequency of the laser, a PZT that can be stretched axially along the fiber was pasted on the side of the laser resonator. The laser resonator was strictly insulated and equipped with an active temperature control device to reduce the influence of the external environment on the frequency stability of the laser so that it can meet the requirements of ultrastable cavity PDH frequency stabilization. A 1.0 μm band ultrastable cavity with an FSR of 1.5 GHz and a fineness of 15000 was used as the frequency reference, and the 1950 nm laser was locked to a transmission peak of the ultrastable cavity by using the ultrastable cavity PDH frequency stabilization scheme after using PPLN crystal frequency doubling. We experimentally confirmed that it is difficult to achieve long-term locking of ultrastable cavity PDH frequency stabilization based on a DBR single-frequency fiber laser. Therefore, herein, we propose and demonstrate a real-time temperature control scheme for DBR fiber resonators using a PZT feedback control signal to generate temperature control signals based on the frequency reference of the ultrastable cavity. This method first generates a temperature control signal by calculating and processing the PZT feedback voltage using a single-chip microcomputer and then realizes real-time temperature control based on an ultrastable cavity frequency reference for the DBR fiber resonator and its cavity mirror FBG through the temperature control signal, thereby resolving the issue of long-term locking of the DBR fiber laser.

The quality of the laser output characteristics is a determinant of whether the laser can be locked; therefore, we first tested the laser output characteristics. Herein, the laser temperature is set to change from 15 ℃ to 35 ℃, the laser wavelength is changed by 1.06 nm [Fig. 2(a)], and when the temperature is stable, the laser can ensure a single-longitudinal-mode operation. The results of single-longitudinal-mode operation measured using the F-P scanning interferometer are shown in Fig. 2(b). By applying triangular wave modulation signals of different frequencies and voltages to the PZT, the measured frequency tuning range of the laser is found to be 1.6 GHz@52 V and the response bandwidth is approximately 8 kHz [Fig. 2(c)]. To characterize the quality of the laser-locking results, the frequency noise is measured before and after laser locking. The measurement results [Fig. 4(a)] show that the laser frequency noise is decreased by to 3‒4 orders of magnitude compared with that before locking, reaching a minimum of 0.08 Hz²/Hz@18 kHz. Through indirect beat frequency measurements, the laser linewidth after locking is determined to reach 255 Hz [Fig. 4 (b)], the frequency jitter of the laser reaches approximately 7 kHz within 2 h, and frequency instability reaches 3.76×10^-13@1000 s (Fig. 5). Implementing a real-time temperature control scheme based on the frequency reference of the ultrastable cavity for the laser prevents the PZT voltage of the laser from reaching the voltage value at which the laser generates mode hopping [Fig. 7(a)]; thus, the laser will not lose its lock because of mode hopping. By monitoring the light intensity at the transmission port of the ultrastable cavity for 240 h using a photodetector followed by a digital multimeter, it is found that the transmitted light intensity remains relatively stable [Fig. 7(b)], which indicates that the developed laser achieves long-term locking.

We report a custom-built 2 μm band DBR fiber laser that can be used as an ultrastable laser source, in which frequency locking was achieved based on ultrastable cavity PDH frequency stabilization. The adiabatic constant-temperature packaging of the laser and built-in PZT with a frequency-tuning function meet the requirements of ultrastable cavity PDH frequency stabilization experiments. After frequency doubling using a PPLN crystal, the 1950 nm fiber laser successfully achieves frequency locking by using a ultrastable cavity with an FSR of 1.5 GHz, a fineness of 15000, and operation in 1 μm band as a frequency reference. DBR fiber lasers are difficult to lock in for a long time when implementing ultrastable cavity PDH frequency stabilization. We propose and demonstrate a scheme for using the PZT feedback control signal to trigger the generation of temperature control signals. Real-time temperature control is implemented based on an ultrastable cavity frequency reference for DBR fiber resonators to achieve long-term frequency locking of such DBR fiber lasers based on ultrastable cavity PDH frequency stabilization. The real-time temperature control scheme based on the frequency reference of the ultrastable cavity for DBR fiber resonators proposed herein provides an important reference point for realizing long-term ultrastable cavity PDH frequency stabilization of DBR fiber lasers.

Rubidium (Rb) atomic two-photon spectra have attracted great attention in connection with small atomic frequency standards due to their narrow linewidth, absence of Doppler background, and broadening characteristics. In the past few decades, extensive research has been conducted on Rb atomic two-photon spectroscopy. As early as the 1990s, F. Nez et al. measured the absolute frequency of the two-photon transition with an uncertainty of 1.3×10^-11. In 1994, Y. Millerioux et al. locked two lasers to the relevant hyperfine levels using Rb atomic two-photon transitions and achieved an instability of 3×10^-13 in the 2000 s. In 2000, J.E. Bernard et al. used a frequency-doubled 1556 nm laser to precisely measure the two-photon transition frequency with a stability of 4×10^-13 in 200 s. In 2020, Vincent Maurice et al. demonstrated a two-photon transition frequency standard on a micro-optical substrate using a miniature gas cell, achieving an instability of 2.9×10^-12 at 450 mW power for 1 s. In 2021, Zachary L. Newman et al. reported a two-photon frequency standard at NIST with an instability of 1.8×10^-13 in 100 s averaging time. The Rb two-photon optical frequency standard has the advantages of compactness and high precision, and with the support of micro-comb technology, it is expected to be adaptable to a wider range of application scenarios to become the next-generation high-performance atomic clock. Therefore, it is necessary to investigate this two-photon optical reference with compact volume and high performance.

We conducted a two-photon fluorescence spectroscopy experiment using a high-purity ⁸⁷Rb vapor cell and a 778.1 nm laser. The laser was generated by an external cavity diode laser (ECDL) and stabilized by direct current modulation. The laser was split into two beams by a polarization beam splitter (PBS) and coupled into single-mode polarization-maintaining fibers. One beam was used to excite the atoms in the vapor cell, which was heated to 110 ℃, and the other beam was used as a reference for the beat frequency measurement with an optical frequency comb. The fluorescence signal was detected by a photomultiplier tube (PMT) and amplified by a trans-impedance amplifier (TIA) and lock-in amplifier. The laser frequency was locked to the zero-crossing point of the error signal using a laser servo device. The experimental setup was fixed on an optical bench with no adjustable components so as to reduce the influence of optical alignment. We used a Glan-Taylor prism to maintain polarization, two focusing lenses to enhance the fluorescence signal, a high-reflectivity mirror, collecting lenses, a high-precision heating system, and an interference filter to optimize the fluorescence signal with a high signal-to-noise ratio and a magnetic shield to minimize the Zeeman effect.

We obtained the fluorescence spectra and error signals of the two-photon transition 5S_1/2-5D_5/2 at 420 nm in ⁸⁷Rb atoms using an external cavity diode laser (Fig.3). The laser frequency was scanned near the resonance and modulated by a sinusoidal current. We measured the dependence of the fluorescence on the laser power from 10 mW to 28.89 mW (Fig.4) and on the temperature of the Rb cell from 100 ℃ to 120 ℃. We determined the frequency shift coefficient of -7.11 kHz/mW (Fig.6), which shows a linear relationship between the optical power and the optical frequency shift over a range of optical power. We recorded the frequency distribution in two different situations (Fig.7) which shows that the beat frequency after locking is more stable than that before locking. Figure 8 illustrates the schematic diagram of the beam-focusing system. The alignment is shown near the focal point with the reflected light undeflected (left) and deflected by 0.005° (right). We tested the relation between the modulation width and the frequency shift (Fig.9). The Allan deviation of the beat frequency reached 1.50×10^-12 at an averaging time of 1 s and 2.88×10^-13 at 500 s (Fig.10).

A high-stability optical frequency reference based on the two-photon transition in ⁸⁷Rb is developed and characterized. The system parameters such as laser power, temperature of the ⁸⁷Rb cell, and modulation width are optimized for the locking performance. The key factors that limit the stability of the two-photon optical frequency reference are identified, including the signal-to-noise ratio of the spectrum, the internal modulation noise, the optical alignment of the counter-propagating beams, and the environmental sensitivity of the system structure. The two-photon optical frequency reference achieves a stability improvement of 1‒2 orders of magnitude over the conventional saturated absorption optical frequency reference and also reaches a high level among similar experimental schemes. To further reduce the frequency drift caused by environmental disturbances, future work can use low thermal expansion coefficient glass for the base and bracket of the optical components. Smaller Rb cell and optical elements are good ways to compress the optical path size. Ensuring a vacuum on the physical platform is another efficient way to decrease the influence of the environment. External modulation methods can also help to improve the system's performance.

Multi-wavelength Brillouin random fiber lasers (MBRFLs) are a new type of laser based on a random distributed feedback resonant cavity and the gain of the stimulated Brillouin scattering (SBS) effect. Because the SBS effect in MBRFLs have excellent properties, such as higher gain, lower threshold, narrower gain spectrum width, and higher sensitivity to environmental factors, it has been widely utilized in the study of fiber lasers. However, most studies are limited to non-polarization parameters, including wavelength, laser linewidth, intensity noise, and phase noise, and are rarely related to the polarization characteristics. In this study, we propose two novel orthogonal polarization interleaving multi-wavelength Brillouin random fiber lasers (OPI-MWBRFLs) that emit orthogonally polarized multi-wavelength light with single and double Brillouin frequency shift (BFS) intervals, based on the axial polarization pulling property of the SBS effect in polarization-maintaining fibers (PMFs). This system yields highly stable orthogonally polarized light, with an adjacent polarization extinction ratio as high as 33 dB. Compared with conventional MBRFLs, OPI-MWBRFLs can provide multi-wavelength lasing light with orthogonal polarizations between adjacent wavelengths, thus effectively eliminating inter-channel interference in dense wavelength division multiplexing (DWDM) systems with potential application in the fields of fiber sensing, optical fiber communication, and optical spectrum analysis.

In this study, we design two novel OPI-MWBRFLs that emit orthogonally polarized multi-wavelength light with single and double BFS intervals. First, based on the polarization vector propagation equation and the simplified intensity equation of the pump and signal lights of the SBS effect in PMFs, which theoretically indicate that the SBS effect in the PMF has an explicit axial polarization-pulling behavior. Second, we deduce the relationship between the traction direction and the state of polarization(SOP)of incident pump light, SBS gain, pump light polarization state, and pump light power. Finally, we realize a single BFS OPI-MWBRFL using a 3 km long PMF as the SBS gain medium, and demonstrate a double BFS OPI-MWBRFL by cascading a 21 km long single mode fiber(SMF)random cavity and a 3 km long PMF random cavity in the feedback loop of the single BFS OPI-MWBRFLs. In the double-BFS OPI-MWBRFL, we use a tunable laser (TLS) to output the pump light with a center wavelength of 1553.73 nm, then the pump light is adjusted by a polarization controller (PC1) and launched into 21 km long SMF through an ordinary SMF circulator (Cir1). In the random cavity, the SMF acts as a Brillouin gain medium and excitation of even-order Stokes light occurs in the opposite direction, of which 10% is output through the optical coupler (C2), and the rest is amplified by an erbium-doped fiber amplifier (EDFA1) and launched into a 3 km long PMF to stimulate higher-order Stokes light. Thus, by controlling the polarization state of the triggered Stokes light in the feedback loop, orthogonally polarized multi-wavelength lasers with single and double BFS intervals are output.

For single BFS OPI-MWBRFLs, the number of output Stokes light wavelengths is positively correlated with the pump optical power. The total output spectrum when the EDFA is in the range of 80‒170 mW is measured (Fig.3). When the output power of the EDFA is set to 80 mW and 100 mW, six and seven wavelengths, respectively, are observed with a space of 0.088 nm (Fig.2). Under these power settings, four odd-orders of Stokes lights resonating at sign (p^in⋅β^l)β^l are measured, and the even-order Stokes light resonating at -sign(p^in⋅β^l)β^l are measured. Further, the extinction ratio of the polarization component exceeds 33 dB [Fig.2(c)]. In contrast, a double BFS OPI-MWBRFL is demonstrated by cascading a 21 km long SMF random cavity and a 3 km long PMF random cavity in the feedback loop of a single BFS OPI-MWBRFL. An orthogonally polarized 4-wavelength lasing light with a double BFS interval is observed, and the adjacent polarization extinction ratio is as high as 34 dB (Fig.6). Finally, after monitoring and measuring the power fluctuation and wavelength shift of the output laser of both schemes for 1 h without any mechanical and temperature stabilization measures, we observe that the maximum power fluctuation and maximum wavelength shift are within 0.21 dB and 0.02 nm (Figs.4 and 7), respectively, indicating the good stability of the system.

In this study, two novel OPI-MWBRFLs are proposed and implemented based on the axial polarization pulling effect of the SBS in PMFs. First, we analyze and discuss the polarization-mode operating region of the PMF-BRFL system and the corresponding operating conditions. Second, two experimental systems that can output polarization multi-wavelength light are realized using different random laser cavities, and the polarization extinction ratio is higher than 33 dB. Finally, the polarization orthogonality of these systems is guaranteed by the natural nonlinear axial polarization pulling effect of the SBS in the PMFs, rather than by the artificial precise polarization control of the systems; thus, the two OPI-MWBRFLs exhibit excellent working stability in experiments in the absence of mechanical or temperature control. The results of these experiments are highly consistent with expectations and have broad application prospects in the fields of optical fiber sensing, DWDM optical fiber communication, and spectral detection.

Distributed Bragg reflector laser diodes (DBR-LDs) are widely used in pump sources, detectors, sensors, solar cells, and other applications because of their small size, long operating life, and high photoelectric conversion efficiency. With the development of modern technology and the demand for laser sources, higher requirements have been proposed for lateral modes of semiconductor lasers. The output of the fundamental lateral mode can be achieved by etching a narrow-ridge waveguide structure as this can limit the formation of higher-order lateral modes; however, it is difficult to further improve the maximum output power owing to the limitation of the narrow-ridge structure. Lasers, integrated by connecting a narrow-ridge waveguide to an optical amplifier, can obtain higher output power in the fundamental lateral mode. However, integrated devices are large, and the manufacturing process is complex. The method of etching microstructures on wide-ridge waveguide devices proposed in recent years ensures that the device overcomes lateral mode limitations and achieves excellent output performance. In addition, research on DBR devices has primarily focused on the spectral study of Bragg gratings. There has been less analysis of the influence of the Bragg grating on lateral mode distribution. In this study, a wide-ridge waveguide-based distributed Bragg reflector semiconductor laser with a combination grating structure (CDBR-LD) is designed and fabricated, and the influence of the combined grating structure on the modulation of lateral modes is investigated. The combination grating can modulate the spectral characteristics of the device and overcome higher-order lateral mode limitations.

The internal action of a semiconductor laser resonator with a combined grating structure is analyzed and calculated using a finite-difference time-domain method. Owing to the complex internal actions of the device, the internal process is divided into two parts, which are analyzed separately: the incident light and feedback light . The combined grating consists of hybrid and Bragg grating areas. Herein, the incident light refers to the light from the direction of the ridge waveguide to the hybrid grating area (Fig.2). The feedback light refers to the light from the Bragg grating area after the incident light is acted upon by the hybrid grating area (Fig.3). According to the distribution law of lateral modes, the energy of the fundamental lateral modes is concentrated in the central region, whereas that of the higher-order lateral mode is dispersed. The loss mechanism of each order of the lateral modes in the incident light and feedback light in the hybrid grating area is analyzed. The value of the narrowest width of the mixed grating region is W_G; the effect of W_G on the energy transmittance of each order lateral mode is compared (Fig.4). The ideal energy transmittance difference between the fundamental and the higher-order lateral modes is obtained with W_G of 15 μm. Therefore, the hybrid grating area in the combined grating structure can suppress the higher-order lateral modes of the device.

According to the analysis of the far-field spots of the device (Fig.5), spectra(Fig.6), and the output power characteristics (Fig.7), the far-field spot of the DBR-LD has significant spot-splitting as the injection current increases from 0.7 A to 1.0 A because of the strong mode competition caused by the higher-order lateral modes. The far-field spot-splitting effect of the CDBR-LD is significantly eliminated as the injection current increases from 0.7 A to 1.0 A because the loss of the higher-order lateral modes caused by the hybrid grating area reduces mode competition. This indicates that the combined grating structure can play a role in modulating the lateral modes of the DBR device. The DBR-LD has a red shift from 1031.87 nm to 1036.1 nm, and the full width at half maximum (FWHM) of the spectrum increases from 1.17 nm to 1.44 nm as the injection current varies from 0.35 A to 0.95 A. The CDBR-LD can maintain good spectral characteristics, which shows a red shift from 1031.25 nm to 1037.15 nm, and the FWHM of the spectrum increases from 0.5 nm to 0.61 nm. Moreover, the FWHM of the CDBR-LD spectrum is narrower than that of DBR-LD because CDBR-LD has a larger grating area. Finally, the DBR-LD exhibits a saturation output power of 406 mW at an injection current of 1.2 A with a slope efficiency of 0.333 mW/A. Additionally, the CDBR-LD exhibits a saturation output power of 433 mW at an injection current of 1.25 A with a slope efficiency of 0.337 mW/A.

A DBR semiconductor laser with a combined grating structure is proposed in this study. By etching a hybrid grating area on the front side of the Bragg grating area, the loss of higher-order lateral modes increases and weakens the mode competition, eliminating the far-field spot-splitting phenomenon in wide-ridge waveguide DBR semiconductor lasers. Subsequently, a CDBR-LD is fabricated and tested. The experimental results show that the far-field spot splitting of CDBR-LD is significantly reduced as the injection current increases from 0.7 A to 1.0 A. The FWHM of the CDBR-LD spectrum is narrower than that of the DBR-LD as the injection current increases from 0.35 A to 0.95 A. A minimal difference is observed between the output powers of the DBR-LD and the CDBR-LD at an injection current of 1.2 A. In addition, the waveguide and grating structure of the CDBR-LD are etched in one step using the ultraviolet lithography, which has the advantages of being a simple process with a low cost. Based on these results, it is expected that a DBR semiconductor laser with good lateral-mode characteristics can be obtained by optimizing the structure.

With the development of single-photon detection technology, human exploration of Earth and space has enhanced the study of Earth's surface changes, such as those in ice, terrain, and vegetation, and improved the understanding of the impact of glaciers on sea level change. These detections have led to new requirements for the light source of LiDAR technology, including lasers with high repetition rates, narrow pulse widths, and narrow linewidths for improved detection distance and accuracy, enabling better observations of changes in surface characteristics. Lasers with all-fiber structures are more compact and stable, which have higher photoelectric conversion efficiency and longer lifespan. The optical fiber structure is more conducive to the multi-beam ground detection. This is because a certain detection blind zone between the beams exists, and the detection of multiple beams reduces the distance interval between the beams, resulting in a certain degree of gridded high-precision detection. All-fiber lasers are expected to enable thousand-beam laser ground detection and direct ground measurement.

In this study, a continuous seed light of 62.6 mW is used, which is modulated into pulsed light by an electro-optical modulator through a rectangular pulse signal from a signal generator. First, the pulsed seed light is amplified by a gain optical fiber to obtain pulsed light with a wavelength of 1064.43 nm, a linewidth of 0.037 nm, and a peak power of approximately 9.33 W. Then, through a gain fiber for two-stage two-pass amplification, the pulsed light with a linewidth of 0.037 nm and a peak power of approximately 383.5 W is obtained. After the first two-stage amplification, an acousto-optic modulator (AOM) is connected to filter out the continuous wave components of the front stage and improve the contrast of the pulsed light. The third-stage amplification is done through a PLMA-YDF-15/130 double-clad gain fiber to obtain pulsed light with a linewidth of 0.046 nm and a peak power of 7.11 kW. The main amplification stage uses the PLMA-YDF-25/250 and photonic crystal fiber (PCF) for amplification effect comparison. The PCF amplified linewidth is smaller than that of the PLMA-YDF-25/250, with no spontaneous radiation, stimulated Raman scattering, or other nonlinear effects. It obtains a wavelength of 1064.44 nm, pulse energy of 298 μJ, and pulse width of 1.34 ns for lasers with a linewidth of 0.05 nm. The corresponding maximum peak power of the laser is approximately 223 kW. The temperature-matched lithium triborate (LBO) is used for the frequency doubling of fundamental frequency light at an energy of 298 μJ, resulting in a green light output of 155.5 μJ. The frequency doubling conversion efficiency is 52%, and a beam quality of Mx2=1.28 and My2=1.26 are also obtained.

To simplify the optical path and maintain the stability of the output, the forward amplification method is selected (Fig. 1). The main amplification stage uses the PLMA-YDF-25/250 and PCF for comparison. Under varying pump currents (Fig. 6), the former exhibits slightly higher optical conversion efficiency than the latter. At a current of 5.6 A, the PLMA-YDF-25/250 exhibits self-phase modulation effects, as shown by the spectral comparison (Fig. 7). Because the mode field area of the PCF is larger than that of PLMA-YDF-25/250, the threshold of nonlinear effects is increased, and other nonlinear effects, such as amplified spontaneous radiation and stimulated Raman scattering, are not observed at 298 μJ (Fig. 8). The optical path design uses a temperature-matched LBO crystal for frequency doubling on fundamental frequency light of 1064 nm, resulting in 155.5 μJ green light output with a frequency doubling conversion efficiency of 52% (Fig. 9).

In this study, a master oscillator power amplifier (MOPA) structure combined with photonic crystal fiber is used to obtain stable fundamental frequency light with a repetition rate of 10 kHz, a wavelength of 1064.44 nm, a linewidth of 0.05 nm, an energy of 298 μJ, and a peak power of approximately 223 kW. After the temperature-matched LBO frequency doubling, the resulting 155.5 μJ green light with a frequency doubling efficiency of 52% and beam quality of Mx2=1.28 and My2=1.26 can be used as a light source for space detection lidar.

Mid-infrared optical frequency combs are widely used in precision spectroscopy, optical frequency metrology, instrument calibration, and other fields. Fiber-type dual-arm structure difference frequency generation (DFG) mid-infrared combs based on mode-locked fiber lasers are currently the primary technology for generating mid-infrared combs. The spectral tuning range and spectral bandwidth are two key indicators of DFG mid-infrared combs. The spectral tuning range is ensured by the wide tuning range of the fundamental frequency pulse, and the spectral bandwidth is associated with the crystal phase-matching acceptance bandwidth and the spectral width of the fundamental frequency pulse. Generally, the fundamental frequency pump pulse is generated by directly amplifying the oscillator output pulse, whereas the fundamental frequency signal pulse is obtained by amplifying and compressing the output pulse of the oscillator and then pumping a highly nonlinear fiber (HNLF) to generate long-wave frequency shift solitons. Although many reports on wide-tunable DFG mid-infrared combs exist, the bandwidth of two-color fundamental frequency pulses is narrow, owing to the limitation of the gain bandwidth of fiber amplifiers, and thus limits the bandwidth of the generated DFG mid-infrared combs. Therefore, the generation of a fundamental frequency pulse with a wider spectrum to obtain DFG mid-infrared combs with larger bandwidths and tuning ranges as well as the design and development of a practical light source device requires further research.

A fully polarization-maintaining 9-cavity fiber laser was used as the pulse source, and the repetition frequency was locked to the rubidium atomic clock through a servo feedback loop. The output of the oscillator was filtered and shaped and further divided into two paths using an optical coupler (OC) after erbium-doped fiber amplification (EDFA-1). It was then amplified by self-similarity fiber amplifiers EDFA-2 and EDFA-3. The EDFA-3 output pulse after being compressed serves as fundamental frequency pump pulse, the EDFA-2 output pulse after being compressed was used to pump HNLF to generate a supercontinuum (SC), and the frequency-shifted solitons were extracted as the fundamental frequency signal pulse. The two-color fundamental frequency pulses were output through the collimator (Co) collimation space, and the polarization state was adjusted by half-wave plates. The mirrors (M) of M1 and M2 were added to the collimator-2 output port to form a time delay line for adjusting the time synchronization of the two-color fundamental frequency pulses. After the two-color fundamental frequency pulses were combined by a dichroic mirror (DM), they were focused on a GaSe crystal by a lens (L1) with a 40 mm focal length to generate a DFG mid-infrared comb. The comb output by L2 collimation after the fundamental frequency light was filtered by a long pass filter (LPF) (Fig. 1). The integration and packaging of the optical combs were performed using a photoelectric separation method.

The average power of the fundamental frequency pump pulse is 485 mW, the center wavelength is 1.57 μm [Fig. 4(b)], and the pulse width is 45 fs [Fig. 4(a)]. The central wavelength of the fundamental frequency signal pulse is 1.85 μm, and the bandwidth is 250 nm [Fig. 5(a)]. The optical comb system was integrated and packaged by photoelectric separation packaging, and a prototype was prepared (Fig. 6). The measured center wavelength of the difference frequency light was continuously tuned in the 8.0‒10.5 μm range. The bandwidth of each tuning band obtained is greater than 1 μm, and the bandwidth of the 9.5 μm band reaches 2.43 μm, indicating that the wider fundamental frequency signal pulse expands the spectral tuning range and bandwidth of the DFG comb. The average power of each tuning band is greater than 240 μW, and the average power of the band with an 8 μm central wavelength reaches 470 μW [Fig. 7(a)]. The average power fluctuation is less than 1.5%, indicating that the power stability of the optical comb is excellent [Fig. 7(b)].

We independently designed and developed a stable broadband and wide tuning range DFG infrared comb. The fiber link was designed with full polarization-maintaining fiber. By locking the repetition frequency of the pulse source and using technologies such as self-similar fiber amplification, soliton compression, and SC generation, the two-color fundamental frequency pulses with center wavelengths of approximately 1.57 μm and 1.85 μm were obtained. An adjustable time delay line was used to precisely control the time synchronization of the two-color fundamental frequency pulses, and the spatial overlap of the two-color fundamental frequency pulses was strictly regulated. Using a GaSe nonlinear variable frequency crystal, the DFG mid-infrared comb output was obtained through the DFG process. The integrated and packaged instrumented mid-infrared comb has a spectral coverage of 7‒13 μm and a maximum spectral bandwidth of 2.43 μm. The design and development of the DFG mid-infrared optical comb offers a base for the development of optical combs for practical applications such as wavelength calibration and multi-component gas detection.

Atmospheric water vapor has a significant impact on the greenhouse effect, water cycle, weather phenomena, atmospheric physical and chemical reactions, and air quality; therefore, the detection of atmospheric water vapor profiles is crucial. Differential absorption lidar (DIAL) is a high-precision, high-spatiotemporal-resolution atmospheric water vapor detection system with important application prospects for airborne and satellite platforms. The absorptivity values of light with wavelengths in the vicinity of 940 nm, 935 nm/936 nm, 942 nm/943 nm, and 944 nm are high in water vapor and are less affected by interference from other gases, making it suitable for lidar emission light sources. Nd∶GSAG crystals exhibit excellent radiation resistance and are therefore suitable for use in space environments. It can directly generate the laser with wavelength of 942 nm pumped by laser diode (LD) and has advantages such as high efficiency and stability, long lifespan, light weight, and small volume. It is suitable for use on airborne and satellite platforms. Differential absorption lidar for water vapor detection requires high wavelength stability, linewidth, and spectral purity of the emitted laser. Therefore, further research on the spectral and laser performances of Nd∶GSAG, as an excellent 942 nm laser working material, is warranted. In addition, reducing the doping concentration of Nd³⁺ in garnet laser crystals is expected to increase the fluorescence lifetime, reduce the thermal lensing effect, and improve the laser beam quality. Therefore, optimizing the doping concentration of Nd∶GSAG is expected to improve its 942 nm laser efficiency and beam quality.

According to the stoichiometric ratio of Nd_0.045Gd_2.955Sc₂Al₃O₁₂, the raw materials Gd₂O₃, Sc₂O₃, Al₂O₃, and Nd₂O₃ are weighed, evenly mixed, pressed into circular blocks, and calcined in a muffle furnace to obtain polycrystalline raw materials. Finally, a single-crystal furnace is used to grow crystals with a size of 50 mm×70 mm, and the laser ablation (LA)-inductively coupled plasma mass spectrometry (ICP-MS) is used to measure the crystal composition. Single-crystal rocking (XRC) and X-ray powder diffraction (XRD) tests are performed on the crystals using an X-ray diffractometer. The transmittance spectra are measured using an ultraviolet(UV)/visible/near-infrared spectrophotometer. The fluorescence lifetime and emission spectra are obtained using a steady-state/transient fluorescence spectrometer, wherein the fluorescence lifetime is excited by an optical parametric oscillator and the fluorescence emission spectrum is excited by an 808 nm fiber coupled laser. The pump source in the laser experiment is an 808 nm fiber-coupled laser, and the resonant cavity is a 10-mm long flat cavity. The dimensions of the laser gain medium are 2 mm×2 mm× 6 mm.

The crystal formula is Nd_0.025Gd_2.64Sc_1.79Al_3.28O_11.60, in which the Nd³⁺ doping atomic fraction is 0.94%. Further, the full width at half maximum (FWHM) of the XRC curve is 0.019°, and the XRD peak is consistent with that in the standard card ICSD78052. At the strongest absorption peak of 808.5 nm, the absorption coefficient is 3.79 cm^-1, the absorption cross section is 3.41×10^-20 cm², and the FWHM of the absorption peak is 3.23 nm, which is better than that (2.79 nm) of Nd∶YAG crystal with doping atomic fraction of 0.6%. Moreover, 1060 nm is the strongest emission wavelength excited at 808 nm, with emission cross-sections of 5.62×10^-20 cm² and 2.33×10^-20 cm² at 1060 nm and 942 nm, respectively. The fluorescence lifetime is 275 μs, which is 22 μs longer than that of Nd∶GSAG crystal with doping atomic fraction of 1.20%. The FWHM of the spectrum of the 942 nm laser is 0.53 nm, with a maximum output power of 0.54 W, a conversion efficiency of 5.6%, a slope efficiency of 9.1%, and a laser threshold of 3.35 W. At a laser power of 0.4 W, the beam quality factors Mx2 and My2 in the horizontal and vertical directions are 2.72 and 3.45, respectively. The waist diameter is small and the waist diameters d_x and d_y in the horizontal and vertical directionsare 0.1048 mm and 0.1185 mm, respectively. All indicators are better than those of the 466 nm laser of Nd∶YAG crystal with doping atomic fraction of 0.6%.

The grown Nd∶GSAG crystal with doping atomic fraction of 0.94% has good crystal quality. The Nd doping increases the cell parameters and crystal density. At 808.5 nm, the absorption coefficient of the Nd∶GSAG crystal with doping atomic fraction of 0.94% is less than that of Nd∶GSAG with doping atomic fraction of 1.20%, and the thermal lensing effect can be reduced by increasing the crystal length. The FWHM of the absorption peak is greater than that of Nd∶YAG, which has a lower requirement for a pump source. The fluorescence lifetime and emission cross-section at 942 nm are better than those of high-concentration crystals; the grown crystal is therefore more conducive to 942 nm laser output and energy storage. The maximum laser output power, optical conversion efficiency, slope efficiency, laser threshold, and beam quality of the 942 nm laser are superior to those of the 946 nm laser of Nd∶YAG crystal with doping atomic fraction of 0.6%. The 942 nm waist diameter and laser spectral FWHM are the smallest among those of the four wavelengths (i.e., 942 nm, 946 nm, 1060 nm, 1064 nm), indicating good monochromaticity. The results indicate that the Nd∶GSAG crystals with low doping concentrations exhibit excellent laser performances at 942 nm.

The features of deep-ultraviolet lasers are high single-photon energy, short wavelength, and easy absorption by materials. They are widely used in high-density optical data storage, high-resolution optical microscopy, material processing, spectral analysis, scientific research, and medical sterilization and diagnostic equipment. Currently, most deep-ultraviolet lasers are obtained by two or more nonlinear frequency conversions of near-infrared lasers; however, the efficiency of this method is generally low. In recent years, rare-earth ions (Pr³⁺) that can emit a visible laser directly at room temperature have attracted considerable attention. Its emission wavelengths span over the blue (485 nm), green (523 nm), orange (604 and 607 nm), and red (640, 698, and 721 nm) regions. The appearance of Pr³⁺ also makes it possible to obtain a deep-ultraviolet laser through a single nonlinear frequency conversion. Polarized emission spectra of the Pr³⁺∶LiYF₄ crystal were measured at room temperature. In addition to the standard transition wavelengths, a weak fluorescence spectrum of the ³P₀→³H₅ transition was observed in the tested fluorescence lines at 546 nm. Recently, in our experimental group, we used a double-end-pumping Pr³⁺∶LiYF₄ crystal for frequency-doubling of the weak spectral line with a β-BaB₂O₄ (BBO) crystal and obtained a continuous deep-ultraviolet laser at 273 nm with a power of 128 mW. Compared to ordinary solid-state lasers, single-frequency lasers have the advantages of excellent stability, narrow spectral lines, and good coherence. This study added a mode selection element to explore the 273 nm deep-ultraviolet laser further. A single longitudinal mode deep-ultraviolet laser with a center wavelength of 272.93515 nm was successfully obtained, and the maximum output power was 32 mW. This study is essential for measuring the content of the antidepressant sertraline hydrochloride.

The absorption properties of polarized Pr³⁺∶LiYF₄ crystals were studied. The absorption efficiency of the Pr³⁺∶LiYF₄ crystal at 444 nm wavelength for π polarization was measured (~94%), and the absorption efficiencies at two wavelengths for σ polarization were compared. The absorption efficiency at 441 nm (~79.5%) was higher than that at 444 nm (~53%). The absorption of laser by the Pr³⁺∶LiYF₄ crystal has polarization characteristics; thus, two laser diodes (LDs) of different wavelengths were combined by polarization as the pumping source. As a result, the entire pump power can be improved, and the polarization characteristics of the pump can be retained such that the absorption efficiency of the crystal correspondingly improves. Therefore, two LDs with an output power of 3.5 W, 444 nm in π-polarization direction and 441 nm in σ-polarization direction were used as the pump source; the length of Pr³⁺∶LiYF₄ crystal was 7 mm and that of the BBO crystal was 5 mm for intracavity frequency doubling. A V-shaped folded cavity was designed (Fig. 4). The beam waist radius (69 um) in Pr³⁺∶LiYF₄ crystal was designed to be small to ensure absorption efficiency. In contrast, the beam waist radius (102 μm) in BBO crystal was designed to be relatively large to reduce the power density of deep-ultraviolet laser and damage to the crystal (Fig. 5). Simultaneously, two different Fabry-Perot (F-P) etalon combinations were used to select the longitudinal mode. Two F-P etalons, with thickness of 0.5 mm and 1.2 mm and reflectivity of 60% and 70%, respectively, were selected with an incident angle of 0.25°. According to the cavity length, the longitudinal mode interval was calculated to be 2.34 GHz (0.00233 nm). The transmittance curves of the etalon sets were simulated for different longitudinal modes when the beam was incident at a small angle. When one of the longitudinal modes had the maximum transmittance (T=100%), the adjacent longitudinal mode exhibited a single-transmission loss of approximately 20% (Fig. 6). Under the condition that the longitudinal mode in the resonant cavity oscillates and is lost multiple times, only a single longitudinal mode at T=100% can initiate the oscillation, thereby ensuring a single longitudinal mode output of the laser.

Without any mode selector in the cavity, a deep-ultraviolet laser at 273 nm with an output power of 85 mW was obtained, and the measured results were multiple longitudinal modes. After adding two etalons, the single longitudinal mode 273 nm laser spectrum was measured using a wavelength meter (High Finesse WS7). The wavelength was single, and there was no adjacent longitudinal mode. The center wavelength was 272.93515 nm, the spectral linewidth was less than 80 fm (Fig. 7), and the wavelength stability was measured for two hours at a wavelength variation of 4.5 pm(Fig. 8). The output power of a single longitudinal mode deep-ultraviolet laser at 273 nm was measured using a power meter (Coherent FieldMaxII-TO). The maximum output power of a single longitudinal mode deep-ultraviolet laser at 273 nm (32 mW) was obtained when the combined output power of the two LDs at 441 and 444 nm was 6240 mW. The curve of the laser output characteristics was fitted to the experimental results. The output power of the single longitudinal mode deep-ultraviolet laser at 273 nm increases with increasing pump power. The slope also tends to increase potentially owing to the gradual adjustment of the LD wavelength to the absorption peak of the Pr³⁺∶LiYF₄ crystal. As the pump power continues to increase, the LD wavelength gradually deviates from the absorption peak of the crystal, the thermal lens effect of the crystal intensifies, and the slope of the curve gradually flattens (Fig. 9). We used a coherent power meter to test the stability of the maximum power of a single longitudinal mode 273 nm deep-ultraviolet laser. The root-mean-square (RMS) of the power stability was 0.717% after 1 h of continuous testing (Fig. 9). We measured the far-field beam shape using a beam profile analyzer (Spiricon BM-USB-SP928-IOS), which was a long strip owing to the walk-off effect of the BBO crystal. The beam quality (M² factor) was measured as 2.29 in the X- and 2.21 in the Y- direction using a beam quality analyzer (Thorlabs BP209-VIS/M) (Fig. 10).

In this study, a simple and effective V-shaped folded cavity was designed using a Pr³⁺∶LiYF₄ crystal made in China as the laser gain medium, and a π-polarized laser with a center wavelength of 444 nm and a σ-polarized laser with a center wavelength of 441 nm were used as the pump sources. Two different F-P etalons were used to select the longitudinal mode in the cavity, and the BBO crystal doubled the fundamental frequency of 546 nm to realize the stable operation of a 273 nm single longitudinal mode deep-ultraviolet laser. The measured center wavelength is 272.93515 nm, the maximum output power is 32 mW, and the RMS power stability is 0.717% in a 1 h continuous measurement. The pump source selected in this experiment matches the absorption peak of the crystal well, maximizes the absorption efficiency, and improves the laser output power. In future, we plan to continue to optimize the resonator, increase the power of the injected pump light, and further improve the output power of the 273 nm single longitudinal mode deep-ultraviolet laser.

To improve the long-term stability of a laser gyro, a real-time loss measurement system for space triaxial laser gyro mirrors exposed to He-Ne discharge plasma is designed. The loss-change process of the mirror in plasma is experimentally studied. The influence of low- and high-temperature environments on the variation law of loss is studied. Combined with the gas discharge fluid model, the discharge characteristics of the He-Ne plasma in the cavity of the laser gyro are simulated, and the energy and distribution of electrons and ions are obtained. The loss change mechanism in the plasma environment is discussed. The research results play an important role in further improving the stability of laser gyro mirrors under the action of plasma.

Considering that the cavity ring-down and resonant measurements are both based on a passive cavity, the loss change process of a mirror in plasma cannot be measured. Therefore, a real-time loss measurement system for space triaxial laser gyro mirrors exposed to He-Ne discharge plasma was designed based on the characteristics of the orthogonal optical paths of three resonators and shared mirrors. For example, channels Ⅰ and Ⅱ shared concave mirror 3 and plane mirror 1 (Fig. 1), and the loss of channel Ⅰ was monitored using the cavity ring-down method (Fig. 2). It was found that the loss of channel Ⅰ increased when channel Ⅱ was powered on. Since concave mirror 3 is in the discharge path, the increase in loss was caused by the action of the plasma on concave mirror 3. Based on this method, the loss of concave mirror 3 before and after plasma action in the cavity was monitored. The results showed that the loss increased rapidly and tended to be stable during discharge. Once the power supply was turned off, the loss decreased dramatically, flattened out, and finally dropped to the initial value in the subsequent natural standing process. Furthermore, the variation law of the loss under low- and high-temperature conditions after power failure was studied (Table 1). The experiments showed that high temperature had a positive effect on reducing the incremental portion of loss caused by the plasma, but low temperature did not.

The loss of the mirror increases under the action of the plasma in the cavity; therefore, it is necessary to deeply analyze the parameters of the electrons and ions in the plasma, especially the energy and distribution of these particles located at the mirror. A gas-discharge fluid model is constructed in combination with the structure of the laser gyro. The simulation results show that the energies of the electrons and He⁺ are the highest at the inner surface of the cathode (Fig. 3). During the discharge process, the energy range of electron is 6.6‒10.5 eV (Fig. 4) on the mirror, and when the discharge reaches equilibrium, the energy range of electron is 2.1‒3 eV on the mirror. The electron energy is higher than the binding energies of SiO₂ and Ta₂O₅ in the discharge process, and the electron energy is equivalent to the defect absorption peak when the discharge reaches equilibrium. Therefore, the electrons will produce more defects in the mirror, leading to changes in its reflection and loss characteristics. In general, a high temperature can be applied to the mirror to eliminate defects and impurities in the mirror, such as electrons and holes, to reduce the loss. Therefore, the loss is reduced after the high-temperature experiments. However, the heat treatment commonly used for the mirror is several hundred degrees Celsius, and most defects and impurities in the mirror cannot obtain enough energy to be completely eliminated at 75 ℃, so the loss reduction is limited.

In this study, an accurate and effective real-time measurement system for space triaxial laser gyro mirrors exposed to He-Ne discharge plasma is designed. The variation law of the loss before and after plasma action is studied. The corresponding experiments are designed based on the loss-change phenomenon. It is found that high temperatures have a positive effect on the loss recovery. Finally, the energy and distribution of the electrons and ions on the surface of the mirror are simulated. The simulation results show that the energy of the electrons is high enough to cause numerous defects in the mirror. Therefore, the stability of the laser gyro mirror in the plasma can be improved by reducing the electron energy on the surface of the mirror and enhancing its anti-electron damage ability, thereby improving the long-term stability of the laser gyro.