Optical integrated aperture imaging involves an array of objects in a certain form based on multiple small aperture sub-mirrors arranged in a certain form. Its imaging resolution can be equivalent to that of a large-aperture lens, which is widely used in astronomy, medicine, commerce, military scientific research, and other fields. Resolving the problems of loading error and environmental engineering requires independent static or dynamic phase compensatory adjustment for each sub-mirror to meet the phase modulation accuracy requirement of 0.1λ (λ is the wavelength of incident light)or the higher requirement. Liquid optical devices have potential application prospects owing to their compact structure, light weight, and low price, and they do not require mechanical devices. A piezoelectric ceramic tube has the advantages of high sensitivity, good linearity, strong integration, and easy control. Furthermore, the tube can be filled with clear liquid and its length is adjustable, providing a new approach for the preparation of liquid optical phase modulators. Much research has been conducted on magnesium alloy liquid optical devices and good results have been achieved. However, further research is necessary to make practical engineering applications possible and new liquid optical phase modulators must be developed.

Transparent liquid is filled into the cavity of a piezoelectric ceramic tube. The length of the liquid produces a micrometer-level displacement change because of the inverse piezoelectric effect. Then, encapsulation and illumination are performed in the cavity length direction and the optical phase produces little change. A piezoelectric ceramic tube with an inner diameter of 15 mm, outer diameter of 20 mm, and height of 12 mm is filled with methyl silicone oil and encapsulated with a gasket, an upper cover sheet, and an lower cover sheet. Finally, a Michelson interferometer is used to observe and analyze the accuracy, range, response time, and other performance characteristics of the phaser.

First, integer fringes are detected using fixed-line grayscale values. Alignment gray value detection involves making a horizontal reference line in the middle of the interference image and recording all gray values on the reference line. These gray values can reflect the intensity change trend of the interference fringes. Then, the curve is detected and recorded once at a certain voltage interval, and a series of curves of the relationship between the gray values and the position of the fringes are obtained. The differences between the positions of the dark and bright fringes are compared; as a result, the phase shift can reflect the relationship between the shift of phase and the voltage (Fig. 6). The interference fringes are recorded, and an extreme value of the grayscale curve appears every 7.0 V interval, which can be understood as a first-order shift of the interference fringe, indicating that the optical path is adjusted by λ/2 (Fig. 7). Moreover, the fixed-line grayscale values are used to detect the fractional fringes, and the interference fringes can move in one direction under the driving voltage. The fractional counting of interference fringe can be achieved by comparing the movement of the fringes in a single cycle and calculating the movement of a fringe relative to the previous position (Fig. 8). The variation curve of the optical path of phase modulator with the applied voltage measured at 0-28 V shows the good linear relationship (Fig. 9). The measured phaser response time is 7 ms (Fig. 10).

To meet the high-precision requirements and achieve a large adjustment range, a liquid optical phase modulator is constructed by filling the transparent liquid into a piezoelectric ceramic tube. Optical phase adjustment is performed through a small electric displacement generated by the piezoelectric effect in the light-transmitting direction. The piezoelectric ceramic tube filled with transparent liquid is used as the core structure of transmissive optical phase modulator. The optical phase modulation devices in this study are not only compact and significantly reduced in cost but also have high precision and a wide range. An experiment is conducted using the fixed-line grayscale values and a Michelson interferometer with a wavelength of 632.8 nm. Under a voltage of 0-28 V, the phase modulator can reach a modulation range of 0-4π and an accuracy of λ/36. When the applied voltage is 150 V, the modulation range can be expanded to 20π. This meets the requirements of optical synthetic aperture subaperture phase modulation.

Visible light communication (VLC) needs to take into account both lighting and communication, so multi-light-source distribution is required, and optical multiple input multiple output (OMIMO) technology emerges. The traditional OMIMO system improves the channel capacity by activating all antennas to transmit information, resulting in strong co-channel interference between channels and poor information reliability. Optical spatial modulation (OSM), as a new type of OMIMO technology, activates one transmit antenna at every moment to avoid co-channel interference between channels. At the same time, since the transmitting antenna itself carries some bit information, the transmission rate of the system is improved. However, OSM must meet the requirement that the number of transmit antennas is an integral power of 2, and only one antenna is used at the same time, which limits its application. On the basis of the OSM system, the optical generalized spatial modulation (OGSM) system transmits information by activating multiple antennas. At the same time, all activated antennas can transmit the same or different data information, thereby further improving the transmission rate. But the antenna selection of OGSM is random, so we propose a VLC multiple input multiple output (MIMO) system scheme based on OGSM. The scheme improves the transmission rate by activating multiple transmitting antennas and combining with modulation methods such as multiple phase shift keying (MPSK) system; at the same time, a norm-based antenna selection algorithm is introduced, which greatly reduces the computation complexity.

In order to analyze the performance of OGSM system, we combine OGSM system with OMIMO system. At the transmitter, the number of activated transmit antennas is used to carry the constellation modulated transmission symbols to construct a constellation mapping table; at the same time, in order to improve the bit error performance, a norm-based antenna selection algorithm is introduced in combination with the channel characteristics. The system selects the sub-channel with the largest channel norm to transmit information in turn, and then selects the optimal antenna combination. At the receiving end, in order to obtain the antenna combination and modulation symbols, the maximum likelihood (ML) detection algorithm is used to obtain the Euclidean distance. The theoretical bit error rate (BER) of the OGSM system is analyzed through the segmented boundary theory. Furthermore, the spectral efficiency, transmission rate and complexity of the OGSM system under different conditions are analyzed (Table 2).

For reliability, we use a norm-based antenna selection algorithm to improve the bit error performance. When the number of active antennas N_a at the transmitting end is 2, the modulation method is binary phase shift keying (BPSK), and the transmission rate is 4 bpcu, it can be obtained that for the BER of 10^-4, using the norm-based antenna selection algorithm, and for 2, 3, and 4 receiving antennas, the average signal-to-noise ratio (SNR) is improved by 3.1 dB (Fig. 2). When the modulation method is BPSK, the number of active antennas N_a at the transmitting end is 2, and the number of receiving antennas N_r is 1 and 2 respectively, for the BER of 10^-4, the SNR of 4 receiving antennas scheme is reduced by 8.4 dB in comparison with the 3 receiving antennas scheme (Fig. 3). When the number of active antennas N_a at the transmitting end is 2, the number of receiving antennas N_r is 2, and the modulation methods are BPSK, 4-pulse amplitude modulation (4PAM), quadrate amplitude modulation (QAM), and quadrature phase shift keying (QPSK), for the BER of 10^-3, compared with BPSK, the transmission rates of the other three modulation methods are all increased by 2 bpcu, and the SNRs are increased by 0.7 dB, 3.2 dB, and 5.1 dB, respectively (Fig. 4). For effectiveness, with the increase of the modulation order and the number of activated transmit antennas, the transmission rate and spectral efficiency will increase (Table 2).

Aiming at the disadvantage that OSM can only activate one antenna at the same time, an OGSM scheme is proposed. This scheme significantly improves the transmission rate of the system by changing the transmitting antenna number from one to multiple and by using modulation methods such as MPSK. At the same time, using the norm-based antenna selection algorithm, the system gets better bit error performance than those using the traditional random antenna selection algorithm. The main conclusions include: by increasing the number of receiving antennas, the bit error performance will be improved; when the number of activated transmitting antennas is constant, by using different modulation methods, such as BPSK, QPSK, 4PAM, and QAM, with the increase of the modulation order, the transmission rate of the system will increase, but the bit error performance will be impaired.

Free-space optical communication has a high communication rate, large capacity, good confidentiality, small size, light weight, low power consumption, and strong anti-interference ability. An instantaneous deployment or setup of a mobile platform communication system is possible. The invention of high-power light sources overcomes the issue of optical signal attenuation in mild weather. Atmospheric turbulence can result in beam drift, beam expansion, light intensity flicker, angle of arrival fluctuation, and other influences, especially at a distance of 1 km or more, which will increase the bit error rate of the link, degrading the communication performance. For an free-space optical communication system, adaptive optical compensation, coherent reception, mode diversity reception, and other technologies have been studied to reduce the impact of atmospheric turbulence. More optical signals can be coupled into the fiber as unrelated signals, and the orthogonality of the mode can conduct diversity reception using multiaperture and multimode fiber for signal acquisition. In this study, we first introduce the turbulent atmospheric channel and the Gamma-Gamma model. We derive the electric field distribution, coupling efficiency, and normalized cutoff frequency of the few-mode fiber receiving under weak conductance. Second, we establish the simulation system of 100 Gbit/s DP-QPSK free-space optical transmission with mode diversity reception and use the LMS-MIMO (multiple input multiple output based on least mean square) equalization algorithm for digital processing. We hope that our experimental results can be helpful for the study of using mode diversity receiver technology in atmospheric turbulence.

In this study, an experimental system of 100 Gbit/s DP-QPSK free-space optical transmission with mode diversity was set up. First, the optical carrier passed through the atmospheric channel and was received using few-mode fiber. Using independent transmission of signals in different modes, the few-mode fiber could transmit more signals. Additionally, the mode multiplexed signals transmitted in the few-mode fiber were damaged by mode coupling, differential group delay, and so on. The equalization of different mode signals at the receiving end was equivalent to a multiple input and multiple output system. The equalization of signals at the receiving end was MIMO equalization. Therefore, the digital single processing (DSP) part adopts a butterfly filter to construct the LMS-MIMO algorithm for dynamic equalization.

The system’s performance was simulated and verified, and the performance of single-mode fiber receiving and multimode fiber spatial diversity receiving at the same transmitting power was compared. First, the LMS algorithm was used to determine the training sequence length of the MIMO algorithm under weak, medium, and strong turbulence receiving conditions of multimode and single-mode fiber. Secondly, under the same turbulence intensity, the bit error rate of single-mode fiber reception and few-mode fiber reception and processing is compared. The experimental results show that few-mode fiber reception can achieve the same level of bit error rate under low optical signal-to-noise ratio compensation condition. Figs. 6-9 show the constellation diagram under the condition of medium turbulence atmosphere; Figs. 10-13 show the constellation diagram under the condition of strong turbulence atmosphere. As can be seen from the figure, single-mode optical fiber reception has a good compensation effect on atmospheric turbulence effect. The constellation map becomes fuzzy with the increase of turbulence intensity due to increased atmospheric scattering intensity, beam offset, and other effects. After balancing with MIMO algorithm, the clear constellation map was basically restored. Further, we compared the performance of single-mode fiber receiving and multimode fiber receiving under weak, medium, and strong turbulence conditions. In the case of weak, medium, and strong turbulence, the optical signal-to-noise ratio (OSNR) compensation costs of the three modes were 1.8 dB, 2.5 dB, and 3.0 dB (Fig. 14-16), respectively, indicating that the LMS-MIMO algorithm can compensate for signal damage well under different turbulence intensities.

In this study, a free-space optical transmission system based on MIMO mode diversity coherent reception is proposed. The experimental setup of 100 Gbit/s DP-QPSK free-space optical transmission with mode diversity is established by simulation, and the performance of the scheme with less mode diversity and the scheme with single-mode diversity is compared. Simulation results show that the system’s performance is improved by 1.8 dB, 2.5 dB, and 3.0 dB, respectively, under weak, medium, and strong turbulence conditions. It shows that the LMS-MIMO algorithm can compensate for signal damage well under different turbulence intensities. This work verifies that the receiving effect of few-mode fiber is better than that of single-mode fiber. To develop and innovate the atmospheric turbulence system, the ideal number of reception modes under various turbulence intensities will be investigated in the future, along with improved MIMO equalization and introducing machine learning methods.

Recently, with the rapid increase in mobile devices and applications, how to meet people’s increasing demand for wireless data has become a hotly debated issue. Due to its many inherent benefits, including license-free spectrum, low cost, high security, strong antielectromagnetic interference capability, and high-speed short-range wireless communication, the visible light communication (VLC) system has received a lot of attention. The performance of the VLC system will be significantly impacted by non-line-of-sight, and the radio frequency system must also take into account the restricted spectrum resources despite having the advantage of wide coverage. Therefore, combining the characteristics of the RF system and the VLC system, a heterogeneous VLC/RF hybrid network is proposed.

An energy efficiency (EE) maximization-based resilient resource allocation algorithm is proposed to address imperfect channel state information (CSI) and low system EE for a heterogeneous VLC/radio frequency (RF) network with channel uncertainty. Considering the constraints of the maximum transmit power of the VLC access point (AP) and the RF AP, the transmit time of each user, the system band, the quality-of-service requirement of VLC and RF users, and the rate outage probability constraint with random channel uncertainty, a robust efficiency maximization resource allocation problem is established under random channel uncertainties based on rate outage probability. Using Dinkelbach’s method, the original optimization problem based on the outage probability is converted into an equivalent deterministic one, which is transformed into two convex optimization subproblems using the alternating optimization algorithm. This thesis proposed an EE maximization algorithm based on alternate iteration.

The performance of the proposed strategy for the heterogeneous VLC/RF hybrid network is simulated and examined using MATLAB. The higher the maximum transmission power of VLC AP, the higher the system’s EE (Fig. 3). As the RF/VLC subsystem estimation error increases, the outage probability increases. Because the larger the estimation error, the larger the difference between the actual value of the channel gain and the estimated value, and the probability of system user interruption increases. Moreover, the rate-maximum robust algorithm has the lowest outage probability, followed by the proposed robust resource allocation algorithm, while the power-minimum robust algorithm has the highest outage probability over threshold. The power-minimum algorithm is 16.7% more robust than the algorithm proposed in this thesis (Fig. 4). In the RF subsystem, EE increases with the channel estimation error. Additionally, the EE increases with the increase of RF channel estimation error under the same VLC channel estimation error condition (Fig. 5). Under the same VLC outage probability threshold condition, the system EE also decreases with the increase in the RF outage probability threshold. Simultaneously, under the condition of the same RF interruption probability threshold, if the VLC interruption probability threshold increases, the EE of the system will also decrease (Fig. 6). With the increase in the user’s minimum demand rate, the system EE of the four algorithms decreases slowly. The system EE is not affected by the rate demand constraint because the user’s minimum demand rate is small. However, as the minimum required rate increases, the system will use a larger transmission power to meet the user rate. At this time, the power consumption ratio is greater than the rate increase ratio, thus resulting in a decrease in the overall EE of the system (Fig. 7).

This thesis studies a heterogeneous VLC/RF network based on imperfect CSI. Under the conditions of maximum transmission power, system time slot, bandwidth, and minimum required rate, this thesis analyzes the transmission power of the VLC and RF systems and the transmission time of the VLC system. To maximize the EE of the system, a joint optimization with the RF system’s bandwidth allocation problem was performed. A corresponding model was created for the proposed optimization problem, and the objective function was then transformed into the form of parameter subtraction using the Dinkelbach method. The problem was then solved using the alternating optimization algorithm. The problem is decomposed into two subproblems; the locally optimal solution is obtained based on the Lagrangian duality principle, and the complexity of the proposed algorithm is analyzed. Simulations verify that the proposed algorithm has high EE and good robustness.

Transmission networks are facing explosive growth in data traffic owing to the rapid developments in data centers and cloud computing services. Optical networks with high speeds, large capacities, and high-cost performances are now in urgent demand. Telecom operators and equipment manufacturers are actively promoting the application and deployment of 400G technologies. To further develop and utilize the transmission capacity of a single fiber and approach the limit of Shannon’s theorem, the next research direction of optical transmission is prioritizing further improvements in the capacity of a single channel, compressing the transmission channel interval, and increasing the number of channels. However, increasing the number of multiplexed channels leads to a serious nonlinear effect, which is inversely proportional to the effective area of the fiber. Therefore, the transmission bandwidth and nonlinear effect of the fiber can be improved and reduced, respectively, by increasing the effective area of the fiber. Consequently, the development of next-generation G.655 and G.656 fibers, particularly for broadband transmission DWDM systems, has important practical application value.

In this experiment, outside vapor deposition (OVD) was used to fabricate a non-zero dispersion-shifted fiber (Fig. 3). According to the transmission principle of optical fibers, G.655 must increase the refractive index difference (RID) of the 1st core layer Δn₁ to increase the waveguide dispersion and shift the zero dispersion wavelength to 1500 nm, but the increase in Δn₁ leads to a decrease in the fiber core radius. However, a core radius that is extremely small results in an increase in the dispersion and nonlinear effects caused by high power density. Therefore, the profile structure selected in this experiment is a triangular core and ring structure with a central depression (Fig. 2). First, by decreasing the RID and gradually increasing the thickness of the 2nd/3rd core radii, the mold field diameter (MFD), zero-dispersion wavelength, and other parameters of the optical fiber were measured and analyzed based on this structure (Table 1). Second, in Table 3, by increasing the 1st core radius alone, the influences of the 1st core radius on the optical fiber parameters were investigated.

This study investigates optical fibers’ optical properties, geometric parameters, and dispersion performance by varying the refractive index profile structure and core radius (Fig. 4). The refractive index profile structure can significantly affect the cutoff wavelength, effective area, and dispersion values. In the optimized profile-type study, when the core radius of R₁, R₂, and R₃ are approximately 2.3, 5.6, and 8.2 μm and the RID of each layer is 0.52%, 0.04%, and 0.14%, respectively, the MFD can reach 9.6 μm, and the dispersion value ranges from 1.6 to 9.5 ps/(nm·km) in the C+ L band. Furthermore, the results indicate that the core radius of the fiber has an important influence on the MFD and zero-dispersion wavelength (Table 3). The decrease in the 1st core radius is proven beneficial for enlarging the MFD, shortening the cutoff wavelength, extending the zero-dispersion wavelength, increasing the zero-dispersion slope, and lowering the dispersion (Fig. 5). The spectral loss diagram, dispersion curve, and macrobending loss diagram of the designed optical fibers were comprehensively evaluated (Fig. 6). The test results indicate that the attenuation values are 0.296 dB/km, 0.195 dB/km, and 0.203 dB/km at 1383, 1550, and 1625 nm, respectively, and the dispersion is within the standard of the G.655.D fiber. The macrobending losses at 1550 and 1625 nm are lower than 0.027 dB and 0.045 dB, respectively, demonstrating a good bending resistance.

In this study, a non-zero dispersion-shifted fiber with a triangular core and annular structure was designed and fabricated using OVD. Based on this structure, the fiber exhibited a shorter cutoff wavelength, lower attenuation, larger MFD, and lower dispersion at 1550 nm. The results show that the MFD is 72 μm², and the dispersion value is 1.6-9.5 ps/(nm·km) in 1530-1625 nm. The attenuation at 1550, 1383, and 1625 nm excelled that of the ITU-T G.655.D standard. Overall, the proposed profile structure realizes a translation of the zero-dispersion wavelength, low macrobending loss, and large effective area, which is suitable for DWDM applications in the C+ L band. In long-distance optical fiber communication, nonlinear effects, such as FWM and XPM, can be suppressed well, and the cost of dispersion management can be reduced, which has important practical application value.

800 nm diode lasers with high power and high beam quality are the recommended laser sources for long-distance laser illumination and laser wireless energy transfer applications. However, due to the restrictions of material gain and epitaxial structure, 800 nm diode lasers have the drawbacks of relatively low power and poor beam quality. Spectral beam combining (SBC) is one of the most dependable beam combining techniques for high-power diode lasers to achieve high beam quality and high brightness. SBC in an external cavity feedback configuration has been implemented with success in the 800-900 nm band. In order to increase the SBC power, it is necessary to further increase the numbers of laser units, but this will lead to the increase of the size of optical components and the length of optical path, which brings great difficulties for installation and adjustment. The physical size of the SBC laser can be lowered by adding the relay imaging structure, but the issue of the huge size of optical elements also exists. Especially for the SBC source with the combining number of over 10 diode laser arrays (LDAs), the output pointing of each LDA is susceptible to be inconsistent due to packaging, micro lens assembly, and other factors. Moreover, the conventional relay imaging structure cannot be optimized for each LDA, and the SBC performance will be significantly reduced, including the power, beam quality, and efficiency.

An SBC structure with separated reflection relay imaging is suggested (Fig. 1). The large-size relay imaging lens is decomposed into a combination of multiple small-size cylindrical mirrors with the same focal length as that of the large-size lens. Each cylindrical mirror matches a specific LDA, and its optical axis aligns with the corresponding LDA emission direction. By modifying the cylindrical mirror, the separated relay imaging structure can compensate for the pointing inconsistency and optical aberration to a certain extent, achieving good SBC performances.

12 LDAs with the front facet reflectivity of <0.5% are employed for SBC. Each LDA consists of 19 transverse electric (TE) polarization emitters with a filling factor of 20%, a pitch of 500 μm, a cavity length of 2 mm, and a slow axis divergence angle of 8°. The focal lengths f₁, f₂, and f₃ of the cylindrical lenses T₁, T₂, and T₃ are 200 mm, 20 mm, and 190 mm, respectively. The equivalent focal length of the transformation lens is 1.9 m. Compared with the conventional SBC structure, the physical distance between the front facet of LDA and the grating decreases from 3.8 m to 0.82 m. The cylinder lens T₁ is composed of 12 tiny cylinder mirrors with an aperture of 10 mm and a spatial period of 11 mm in the SBC direction. The negative first-order transmission grating is used as the dispersive element with the period (Λ) of 1765 line/mm, wavelength of 800 nm, and Littrow angle of 44.93°. The negative first-order diffraction efficiency is >90% in the range of 770-830 nm for S polarized light. After SBC, the threshold current is reduced from 15 A to 7 A with the help of external feedback. Driven by 50 A current and 21.2 V voltage, the continuous wave (CW) power, the electro-optic conversion efficiency, the overall slope efficiency, and the average slope efficiency of each LDA are 442.9 W, 41.8%, 10.8 W/A, and 0.9 W/A, respectively (Fig. 2). The whole spectral range is from 777.12 nm to 811.28 nm and the spectral width is 34.16 nm (Fig. 3). 12 spaced spectral bands can be observed, matching 12 LDAs. The relative intensity of each spectral band is relatively consistent, and the height difference is no more than 20%, indicating that each LDA achieves good spectral locking and resonance. After being focused by an aspherical lens with a focal length of 18.75 mm, the intensity distributions before and after the focus are measured and assessed using the second-order moment method. After fitting, the whole beam parameter product is 4.00 mm×mrad (Fig. 6), close to that of a single emitter of 3.50 mm×mrad.

Aiming at the SBC of a large number of laser units, an SBC structure based on separated reflection relay imaging is demonstrated. The huge-size optical element is miniaturized and divided into several small-size elements. SBC of 12 LDAs with 228 emitters is realized by a single external cavity, with the CW power of 442.9 W, the electro-optic conversion efficiency of 41.8%, and the beam parameter product of 4.00 mm×mrad. This offers a viable technical method for the miniaturization of the SBC structure. The next step is to directly multiply the laser power through polarization beam combination, and the fiber coupling technology will be combined to realize the 800 nm power output of kilowatt-level with the core diameter of 50-100 μm, convenient for real applications.

Benefiting from their unique structures, Innoslab lasers have achieved high power and high beam quality output. However, there is still a lot of harmful heat in the laser crystal under the high-power pumping. To obtain an amplification output with high beam quality, the heat in the laser crystal during the laser amplification process must be effectively removed. An indium layer is added between the laser crystal and the heat sink before amplification. During this process, defects such as voids are inevitably introduced, which will inhibit the heat dissipation. Heat is accumulated in the crystal because of defects in the indium layer, resulting in wavefront distortion that leads to poor beam quality and even the crystal fracture due to inhomogeneous thermal stress distribution. Therefore, it is necessary to analyze the effects of defects characteristics on the temperature distribution inside the crystal. In this paper, the influences of the sizes and positions of the indium layer defects and the distance between two adjacent defects on the crystal temperature distribution are quantitatively investigated.

In this paper, the effects of the sizes and positions of the indium layer defects and distance between two adjacent defects on the crystal temperature distribution are studied by the finite element method using the COMSOL software. Firstly, the heat source distribution model is obtained based on the distribution of pump light and the absorption characteristics of the crystal, and the temperature distribution inside the crystal is got through the steady-state heat conduction equation. After that, the temperature distribution of the crystal without defects in the indium layer is obtained. Then, circular defects with different sizes, positions and spacings are introduced into the ideal indium layer to obtain the temperature distribution of the laser crystal with defects in the indium layer. The influences of indium layer defects on crystal temperature distribution are obtained by comparing the temperature distribution of the crystal without and with defects in the indium layer.

The temperature of the crystal in the pumping direction is low in the middle and high at the sides of the crystal without defects in the indium layer, and the highest temperature point of the crystal is near the incident surface of the pump light on the crystal. With a defect inside the indium layer, the temperature rises at the position where the defect is located. As the defect size is increased, the temperature increment value increases, and the highest temperature point shifts from the incident surface of the pump light to the location near the defect (Fig. 5). The effects of the defect on the temperature distribution of the crystal with the defect above the high-temperature region of the crystal are more significant than those of the defect above the low-temperature region (Fig. 6). The influences of two adjacent defects on the crystal temperature distribution are coupled when the distance between the two defects is small, resulting in more significant effects of the defects on the crystal (Figs. 7 and 8).

In this paper, the temperature distribution of crystal with and without defects in the indium layer is simulated, and the distribution characteristics are qualitatively analyzed using the heat conduction equation under steady-state conditions by establishing the heat source distribution model of laser crystal and introducing defects into the indium layer. The highest temperature of the crystal surface obtained by simulation without defects in the indium layer is relatively consistent with the experimental results. At the same time, the influences of the defect size on the temperature distribution in the X, Y and Z directions of the crystal, and the influences of the defect positions and the distance between two adjacent defects on the temperature distribution in the X direction of the crystal are quantitatively analyzed. It is found that the larger the defect size is, the larger the temperature increment value and temperature rise area caused by the defect are, and the highest temperature point in the X direction of the crystal shifts from the crystal end face to the location near the center of the defect when the defect size is larger than 0.65 mm. The influences of the defect in the high-temperature region on the crystal temperature distribution are more significant than those of the defect in the low-temperature region. The effects of two defects that are far apart on the crystal temperature distribution are independent of each other. However, when the defects are close to each other, their effects on the heat dissipation of the crystal are coupled, resulting in more significant effects of the defects on the crystal temperature distribution. The work in this paper is useful in deciding whether or not to consider the influences of indium layer defects on the experiment during the laser design and laser crystal installation, which is of guiding significance for laser design and experiments.

Due to the rapid development of fiber laser technology, tunable fiber lasers have become an important development direction of fiber lasers. Their flexible wavelength tuning characteristics are highly valuable in optical communication, optical sensing, and spectral synthesis. However, to achieve a periodic and stable wavelength tuning operation, tunable fiber lasers often use electrically driven piezoelectric ceramics, heaters, fiber gratings or tunable sweeping filters, and other sweeping devices, which results in a complex structure and reduced output performance of the fiber laser, significantly limiting its development and practical applications.

In recent years, a new type of tunable fiber laser based on the self-sweeping effect has gained the interest of researchers. Compared with general tunable fiber lasers, the self-sweeping fiber laser can achieve spontaneous, stable, and periodic wavelength tunability without using complex tuning elements or electrically driven drives. The phenomenon of the self-sweeping effect was first reported in 1962 when a periodic shift of wavelength was observed in a ruby laser. Half a century later, based on the excellent waveguide medium of optical fiber, researchers in Russia realized the first ytterbium-doped (Yb-doped) self-sweeping fiber laser. Thereafter, based on the typical Fabry-Perot linear cavity structure, researchers observed the self-scanning effect in different bands using different gain media. These bands have greatly facilitated the application of self-sweeping fiber lasers in various research fields and self-sweeping fiber lasers have become one of the research hotspots. In this study, we design a linear-cavity-controlled Yb-doped self-sweeping fiber laser based on a circulator, which is easy to implement, inexpensive, and flexible in controlling the self-sweeping characteristics, such as self-sweeping range and self-sweeping rate, potentially expanding the practical applications of self-sweeping fiber lasers.

In this study, a Yb-doped self-sweeping fiber laser based on a circulator is built based on the previous Fabry-Perot linear cavity structure using a circulator instead of a flat-cut port of the fiber as the reflecting end of the resonant cavity. The resonant cavity of the laser consists of a fiber loop mirror and a circulator. The 3-port and 1-port of the circulator are fused together to form a ring.The laser enters from the 2-port, passes through the 3-port, and returns to the Yb-doped fiber for amplification from the 1-port. The flat-cut port of the fiber can provide 3.5% energy of the Fresnel reflected light, whereas the remaining 96.5% energy of the laser is transmitted out of the cavity, resulting in large losses and a high self-sweeping threshold. Although the circulator has some insertion loss, it can significantly reduce the self-sweeping threshold compared with the fiber flat-cut port. Additionally, since the self-sweeping range and self-sweeping rate are dependent on the output power, the output power of the laser can be changed to regulate the self-sweeping range and self-sweeping rate by adjusting the intracavity loss. By introducing a mechanically variable optical attenuator in the circulator, the self-sweep range and self-sweep speed of the laser can be adjusted flexibly by changing the intracavity loss.

When the laser is operated with self-sweeping mode, its self-sweeping threshold is only 22.5 mW, the output slope efficiency is 10.65%, and a self-sweeping range of 1065.4217-1071.4225 nm (approximately 6 nm) and a self-sweeping rate of 0.56-8.83 nm/s are obtained, which can be observed in the range of 12.08-115.2 kHz (Fig. 4). The optical signal-to-noise ratios (OSNRs) of the laser output spectra are all greater than 40 dB, with a maximum value of 53.18 dB, indicating a good output performance (Fig. 5). A summary of the characteristics of the self-sweeping law reveals that the self-sweeping rate and average pulse repetition frequency are consistent as a function of the output power, i.e., both increase with the increase of output power and are linearly proportional to the square of the output power. After the introduction of a mechanically variable optical attenuator, the experimental results obtained by adjusting the optical attenuator to change the intracavity loss show that with the increase of intracavity loss, the self-sweeping band range gradually drifts toward the short wavelength direction, the self-sweeping rate gradually becomes slower, and the self-sweeping range gradually becomes narrower. The sweeping range in the whole process can cover 1056.5773-1067.4093 nm, approximately 10.83 nm (Fig. 6). Therefore, using a variable optical attenuator to continuously change the resonant cavity loss is an effective technical method for achieving flexible output control of the self-sweeping fiber laser.

In this study, a linear cavity Yb-doped self-sweeping fiber laser that produces a normal self-sweeping effect is realized using a circulator as the reflecting end of the resonant cavity rather than a flat-cut port of the fiber. The cavity structure is simple, easy to implement, and inexpensive. The self-sweeping threshold of the laser is only 22.5 mW, the output slope efficiency is 10.65%, and a self-sweeping range of 1065.4217-1071.4225 nm (approximately 6 nm) and a self-sweeping rate of 0.56-8.83 nm/s with an average pulse repetition frequency of 12.08-115.2 kHz are obtained. The OSNRs of the laser output spectra are all greater than 40 dB, with a maximum of 53.18 dB, which indicates good output performance. When a mechanically variable optical attenuator is introduced into the circulator, the self-sweeping characteristics, such as self-sweeping range and self-sweeping rate, can be adjusted by adjusting the variable optical attenuator to change the cavity loss and extend the sweeping range to 10.83 nm. Our experiments provide a simple, effective, stable, and low-cost method for self-sweeping modulation, potentially facilitating the practical application of self-sweeping fiber lasers in spectral measurement, analysis, and other laser applications.

High repetition tunable femtosecond light sources are widely used in time-resolved spectroscopy, nonlinear spectroscopy, and bioimaging, and other fields. Optical parametric oscillators (OPO) and optical parametric amplifiers (OPA) are the most frequent equipment to generate tunable femtosecond pulses and the high pump energy in single pass amplification. The threshold of OPA is limited by the minimum pulse energy to create a supercontinuum seed. OPA is usually applied in low repetition and high-energy laser systems. For high repetition system, a high average power pump laser source is required. OPO is applied in high repetition systems, where the threshold is much lower. However, the complex cavity and poor stability constrain its application. OPA with an extremely low threshold is a possible solution for a stable high repetition tunable ultrafast laser source. In earlier studies, various techniques were used to lower the OPA threshold. These techniques fall into two categories: lowering the threshold of seed generation through supercontinuum generation in highly nonlinear optical fibers, and replacing the gain medium with crystals with high nonlinear coefficients like PPLN. The cost of these OPAs is much higher, and the stability is also poor. The article reports a femtosecond OPA design with an extremely low threshold and excellent stability. The threshold of our setup is only 300 nJ at 1030 nm and the long-time stability for 6 h is approximately 0.22%. Our design is possible to replace femtosecond OPOs.

To reduce the OPA threshold, a YVO₄crystal, the nonlinear refractive index of which is much higher than that of common crystals, is used as the medium of supercontinuum generation. The calculated threshold of the YVO₄crystal is 0.54 MW at 1030 nm. Details of the setup of OPA are shown in Fig. 1. The pump laser source is a femtosecond fiber laser with a central wavelength of 1030 nm, a repetition rate of 1 MHz, an average power of 10 W, and a pulse width of 300 fs. The 0.3 W power is used to pump the OPA. The pump laser is split by a half wave plate and a thin film polarizer. Approximately, the 180 nJ energy is focused in the YVO₄crystal to generate the supercontinuum seed. The residue pulse is frequency doubled in a barium borate crystal (polar angle θ=23.4°, azimuth angle φ=0°, and thickness t=1 mm) to generate the pump pulse and the conversion efficiency is approximately 50%. The pump pulse at 515 nm and the supercontinuum seed are combined by a dichromic mirror and focused into another barium borate crystal (θ=22.3°, φ=0°, and t=5 mm). A wavelength separator separates the amplified signal and idler pulse, and the reflected signal is compressed by a pair of Brewster angle prisms.

The supercontinuous white light is generated in the YVO₄crystal using 1030 nm light with energy of about 180 nJ. The spectrum of supercontinuum white light generated in the YVO₄ crystal is shown in Fig. 3. The amplified signal is observed when the delay between the seed and pump pulse is carefully adjusted. The maximum output energy of the signal is 18.5 nJ at 690 nm. The transmission efficiency of the compressor is 93%. The compressed pulse width at 690 nm is 59 fs and the minimum pulse width is 35 fs at 870 nm. The wavelength of tunable pulses is 650-950 nm, which is limited by the spectrum of the supercontinuum seed. The time-bandwidth of the output signal is far from the Fourier transfer limit because the higher order dispersion is not compensated. The compressed pulse width is shorter than the pulse width of the reported OPO. The spectra of output signals are shown in Fig. 5. The energy and the minimum compressed pulse width are shown in Fig. 6. Also, the 6 h stability test reveals an excellent result. The power stability is 0.22%. This design's framework is straightforward, the optical distance is less, and the seed is only amplified once. These are the primary causes of the output power's stability. Due to the single-stage architecture, the input pump pulse energy of this OPA could not be so high. Herein, the highest pump pulse energy is 10 μJ, which is limited by the fiber laser. Under the highest pump energy, the output of the OPA is still stable.

In this article, a new design of single-stage OPA is reported. A YVO₄ crystal is used for supercontinuum seed generation to achieve an extremely low threshold, and the threshold is down to 300 nJ at 1030 nm. The tunable output is acheived at 650-950 nm, and the minimum compressed pulse width is shorter than 35 fs. This design also exhibits excellent long-time stability, and the stability of the output power for 6 h is only 0.22%. It is possible to replace OPO with a pump laser at the repetition rate of dozens of MHz. This OPA design has great potential in time-resolved spectroscopy, nonlinear spectroscopy, bioimaging, etc.

With recent developments, the power of lasers has continuously improved, which has caused the inevitable increase in demand for laser output modules and heat dissipation, increasing the burden in terms of volume and weight as well as the serious problems of laser thermal effect on laser beam quality. Compared with traditional lasers, direct liquid-cooled solid-state lasers use a gain medium to directly contact the cooling fluid for efficient heat exchange, which greatly reduces the laser’s cooling system weight. Scaling and amplification of the output power can be achieved by stacking the slices in the gain module, and researchers favor its excellent characteristics. Therefore, a comprehensive optimization of the design of direct liquid-cooled solid-state lasers is required to discharge as much waste heat as possible and improve the output beam quality.

For the direct liquid-cooled thin-disk selected in this study, the gain module was composed of multiple thin-disk arrays, and the pumping mode of double-side pumping was used. The pump beam enters the pump sheet array along the side of the gain sheet. The distance between them was approximately 0.3 mm, forming a typical microchannel structure, in which the specially treated laser cooling fluid flows at a high speed and removes waste heat in the thin disk. The laser was incident on the gain medium along the Brewster angle to reduce interface loss. The gain medium material was Nd∶YAG crystal, and the cooling fluid was heavy water. As the working conditions of each gain sheet were the same, the optical path difference generated by the gain medium and half-flow field of the microchannel on both sides was selected as the research object. The temperature of the sheet was calculated by Fourier’s law, the flow field of the microchannel was calculated using the Navier-Stokes equation, the SST (Shear Stress Transport) model was selected as the solution model, and the stress and deformation field of the gain medium were calculated using related technology. Finally, the obtained data was introduced to the established optical path difference model, and the wavefront aberration calculation of the gain medium was performed under the corresponding working conditions.

The results show that with the increase of the microchannel height and gain medium thickness, the optical path difference (OPD) increases (Figs. 4 and 5); however, with the increase of the Reynolds number of the cooling fluid, the OPD decreases (Fig. 6), but the optimization efficiency also decreases. The design range of the single-medium thermal power density Q is determined by two aspects: (1) when the total power remains unchanged, the larger the Q, the fewer the gain medium sheets quantity required, which benefits beam quality control; (2) a larger Q is likely to cause breakage of the gain medium and cavitation in the cooling liquid. The OPD increases with the increase of Q (Fig. 7). As the computational complexity increases exponentially when the exhaustive method is used to optimize multiple variables, meanwhile, the dispersion of variables by the exhaustive method can also lead to losing the optimal solution, this study uses genetic algorithm to optimize the design of the OPD. Compared with the OPD root-mean-square (RMS) value of 3.73 μm and peak-valley (PV) value of 7.24 μm under the pre-optimized design (Fig. 3), the optimized design OPD RMS value is 3.27 μm and the PV value is 6.11 μm (Fig. 9), which are optimized by 12.3% and 15.6%, respectively (Fig. 8).

In this study, using a larger microchannel height did not improve the heat transfer effect; on the contrary, the OPD caused by the cooling liquid accounted for the main part. Therefore, the height of the microchannel should be minimized while designing the laser. Increasing the thickness of the gain medium reduces its surface area, deteriorates the heat transfer, and produces a more inhomogeneous optical path difference distribution. Therefore, the thickness of the gain medium should be minimized in the actual design. When the Reynolds number of the cooling fluid increases to a certain extent, improving only the convective heat transfer thermal resistance between the microchannel and gain medium has a minimal effect on the overall thermal resistance. Therefore, as the Reynolds number of the cooling fluid increases, the heat transfer effect improves and the OPD decreases, but the optimization efficiency also decreases. A higher thermal power results in a more uneven optical path difference distribution. However, as the thermal power is affected by the pump light, optimizing this variable in the actual design must still consider the output power. As a smaller Q typically requires more gain slices, the beam passes through too many complicated paths, which is detrimental to beam quality. Therefore, the actual optimization of Q should be designed according to actual needs.

An unprocessed, on-axis Si substrate has a single-layer atomic step structure on its surface. The epitaxial growth of III-V materials on substrates results in the high-energy planar defect called antiphase domain (APD). The APD reduces the minority carrier lifetime in devices, degrading the performance of devices. Placing an on-axis Si substrate in the hydrogen environment for high-temperature annealing can promote the transformation of single-layer atomic steps into double-layer atomic steps and suppress APD generation at the GaAs/Si interface. However, the molecular beam epitaxy (MBE) technology cannot take hydrogen as annealing environment. Existing experimental methods involve changing the experimental process, which is unique and difficult to reproduce, to promote the annihilation of APD in GaAs materials. However, the APD annihilation mechanism remains unclear. In this study, the formation energy of APD propagating along the {110}, {111}, and {112} planes in GaAs materials at different temperatures is calculated using the first principle to explore the APD annihilation mechanism. The most stable propagating plane of the APD changes from {110} to {112} when the temperature exceeds 660 K. A 1.4-μm thick GaAs epitaxial layer is grown on an on-axis Si (001) substrate using the MBE technology. The results demonstrate that the APD density on the GaAs surface decreases and the annihilation probability of the APD increases with an increase in the growth temperature. At high growth temperatures, the APD can easily be twisted to the {112} plane and annihilate.

Aiming at the phenomenon of APD kink and annihilation in on-axis GaAs/Si(001) epitaxial materials, this paper presents the detailed exploration and analysis of theoretical simulations and experiments, respectively. According to the different propagation planes of the APD in GaAs, APD models propagating along the {110}, {111}, and {112} planes are established. The APD formation energy on these three propagation surfaces and their variation trends with temperature are obtained using the first principle. Experimentally, GaAs epitaxial layer is grown on an on-axis Si (001) substrate based on the MBE technology using a three-step method. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) are used to characterize the surfaces and cross sections of the samples.

The APD formation energy on different propagation planes varies with temperatures. At 0-660 K, the APD formation energy on the {110} propagation plane is the lowest, and at 660-1500 K, the APD formation energy on the {112} propagation plane is the lowest (Fig. 3). In the range of 450-600 ℃, the higher the growth temperature, the lower the APD density (Fig. 5) and the higher the APD annihilation degree in the sample (Fig. 6), which is consistent with the calculated results.

In this study, the formation energy of APDs propagating along the {110}, {111}, and {112} planes in GaAs is calculated based on the first principle, and the effect of growth temperature on the APD formation energy in vacuum environment is determined. The theoretical results demonstrate that, at 0 K, the formation energy of the APD propagating along the {110} plane is the lowest; in the temperature range of 0-660 K, the formation energy of the APD propagating along the {110} plane is the lowest; and in the temperature range of 660-1500 K, the APD propagating along the {112} plane has the lowest formation energy. The experimental results demonstrate that when the temperature increases from 450 ℃ to 600 ℃, the APD density decreases by 42%, and in the sample with a growth temperature of 500 ℃, more APDs along the {112} plane are found to meet other APDs and annihilate. A higher growth temperature promotes the kink of the APD to the {112} plane and then the APD annihilates in the GaAs material with a certain thickness, which is consistent with the theoretical calculation results. In this study, based on the first principle and MBE technology, the propagation characteristics of APD in on-axis GaAs/Si (001) materials are theoretically analyzed and experimentally verified. The results have a guiding significance for the experimental process exploration of growing high quality APD-free GaAs materials on on-axis Si (001) substrates by MBE technology and promote the research on high-performance on-axis silicon-based lasers by MBE technology.

Hybrid unstable resonators have been widely used in slab solid-state lasers and gas laser systems in recent years due to their high extraction efficiency and high beam quality. So far, hybrid unstable resonators have not been used in DF gas flow chemical laser devices to achieve high-power, high-beam quality mid-infrared continuous wave (CW) laser output. In addition, the beam quality in the direction of stable resonator is always poor and hard to handle, and the physical process of far-field spot from high-order transverse mode oscillation to single fundamental transverse mode oscillation should be studied theoretically and experimentally.

A kind of off-axis hybrid unstable resonator with Z-fold in the flow direction is proposed and studied. The Z-folded off-axis hybrid unstable resonator is a flat concave stable resonator in the gas flow direction, and Z-folded to reduce the gain size in the direction of the stable resonator. In the height direction, it is a positive branch confocal unstable resonator, as shown in Fig. 1. The magnification in the direction of unstable cavity is 1.2. The gain medium is generated by the DF gain generator driven by combustion. Nitrogen is used as diluent, and the static pressure of the gain air flow is generally less than 666.7 Pa. The gain size is about 30 mm in the air flow direction and 40 mm in the height direction, and the gain length of the laser single pass is 125 mm, while after Z-folding the gain length becomes 375 mm. The virtual assembly diagram of the gain generator module and the optical cavity module is shown in Fig. 2. Figure 3 shows the schematic diagram of the monitoring optical path, with which the output power, far-field spot, output spectrum and near-field spot are monitored. We calculate the oscillatory self-reproducing modes of Z-folded off-axis hybrid unstable cavities by means of fast Fourier transform based on the diffraction theory of plane-wave angular spectra. In the calculation, the seed beam used for the optical cavity oscillation is a rectangular parallel plane wave conforming to the cross section of the gain medium (30 mm×45 mm). There are four mirrors in the optical cavity, and the seed beam oscillates back and forth between the concave spherical mirror 4 and the convex cylindrical mirror 1. After the gain saturation, a self-reproducing mode with stable intensity and phase distribution is formed. The calculation process is shown in Eqs. (1)-(10).

The mid-infrared CW laser output with average power of 550 W, beam quality β value of 1.8 (the β value is 1.5 in the stable resonator direction and 1.9 in the unstable resonator direction), and spectral coverage of 3680-4089 nm is realized in a miniaturized DF gas flow chemical laser device (size of gain medium: 30 mm×40 mm×125 mm), as shown in Figs. 4-6. The calculation results of the far-field spot are shown in Fig. 4. Figures 4(a), 4(b) and 4(c) respectively correspond to the gain size in the direction of the air flow, which is determined by the rectangular apertures in front of the concave spherical mirror 4, being limited to 10 mm, 8 mm, and 6.5 mm. High-order transverse mode oscillations appear in the X direction for the far-field light spots in Figs. 4(a) and 4(b). Figure 5 shows the experimental results of the far-field spots. When the gain size in the air flow direction is limited to 10 mm and 8 mm, the intensity distributions of the near-field spot (10 mm×6.6 mm, 8 mm×6.6 mm) and the far-field spot in the experiment have the characteristics of three bright spots and two bright spots, respectively. The bright spot appears as a Hermitian-Gaussian high-order transverse mode, and the beam quality β values of the far-field spots in the air flow direction are 4.2 and 3.4, respectively. As the gain size in the air flow direction is limited to 6.5 mm, we experimentally obtained a high beam quality CW laser output with a beam quality β value of 1.8, and the beam quality in the air flow direction, i.e., the direction of the stable cavity, is 1.5.

In order to achieve high-beam quality and high-power mid-infrared laser output based on a miniaturized DF gain generator (gain volume is 30 mm×40 mm×125 mm), we propose and study a Z-folded off-axis hybrid unstable resonator in the gas flow direction as the optical resonator of DF chemical laser device, and the beam quality in the gas flow direction of the resonator is studied theoretically and experimentally. Finally, mid-infrared CW laser output with an average power of 550 W, a high beam quality β value of 1.8 (the β value is 1.5 in the stable cavity direction and 1.9 in the unstable resonator direction), and a spectral coverage of 3680-4089 nm is achieved.

High peak power and continuous tuning are main characteristics of transversely excited atmospheric-pressure [TE(A)] CO₂ lasers which have great potential applications in laser spectroscopy, laser chemistry, laser medicine, laser manufacturing, military and so on. Unlike those working at a pressure around 1 atm, tailing phenomenon is mostly eliminated in multi-atmospheric-pressure CO₂ lasers and the pulse width is compressed from hundreds nanoseconds to tens nanoseconds level, so that the total interactive efficiency of the abovementioned applications is further improved. Compared with other pulse witdh compressing techniques such as mode-locking, Q-switching, plasma switching, etc., another advantage of multi-atmospheric-pressure CO₂ laser is the integration of narrow pulse width and continuous tuning. Such a laser can achieve accurate output of arbitrary wavelength in the 9-11 μm range. A typical application is proved to realize stimulated Raman scattering and excite isotope molecules, which may be an alternative high efficiency solution of present huge separation facilities.

The theory consists of the continuously tunable theory and multifrequency dynamic model. The continuously tunable theory means that as the working pressure increases, the discrete spectral lines are gradually broadened until they overlaps each other, so the output becomes continuously tunable. The multifrequency dynamic model can describe the progress of pulsed CO₂ laser at high pressure. In this model, changing the electricity, gas and optical parameters can result in different output laser pulse waveforms, which can be further used to analyze the output characteristics such as energy, pulse width, power and wavelength. In experiment, a multi-atmospheric-pressure pulsed CO₂ laser was set up. The laser mainly consisted of the energy injection unit, the main discharge unit and the resonant cavity. A high voltage DC power supply (≤32 kV) and a triple Marx circuit were used for power supply and pulse modulation in the energy injection circuit. The pre-ionization structures and main discharge electrodes were arranged inside the chamber. The chamber was filled with gas mixture of carbon dioxide, nitrogen and helium. The resonant cavity adopted a plane-concave cavity type, which contained an output coupling window (the coupling mirror made of Ge) and a diffraction grating. The grating was driven by a server motor working at the Littrow angle (with the grating constant of 1/150 mm) to realize a tunable output. By substituting the optimal parameters obtained from the experiment into the theoretical model and analyzing the output pulse characteristics, it is found that the theoretical results are consistent with the experimental values. Thus the experiment is proven reasonable and correct.

The output properties of transversely excited multi-atmospheric-pressure CO₂ lasers are studied in this paper. Under a working pressure of 7 atm, an output with 590 mJ pulse energy and corresponding 35.7 ns pulse width is obtained. It has been reported in 2015 that von Bergmann et al. generated short pulses around 250 mJ and 60-150 ns under the pressure of 1-10 atm. In comparison with former studies, we raised the pulse energy record by nearly 100% and compressed the minimum pulse width record by nearly 50%. Furthermore, a continuously tunable property was observed at a lower pressure of 7 atm. More than 30 mJ average output was detected every 5.86×10^-4 μm from line 10R(32) to line 10R(26). However, the band range measured in experiment was reduced since the loss was bigger than the gain at the edge of four bands as the working pressure rose (Fig. 6). Meanwhile, the modulation degree of the R band is smaller than that of the P band, leading to the smoother output distribution of the R band (Fig. 7). The whole data acquired under lower pressure perform obvious continuous tuning properties and growing tendencies.

A transversely excited multi-atmospheric-pressure CO₂ laser is studied. A gas mixture (VCO2∶VN2∶VHe＝2∶1∶16) at 7 atm was adopted in the cavity. The discharging voltage was 72 kV and the total injection energy was 19.88 J. The resonant cavity contained a Ge coupling window with the reflective index of 36% and a diffraction grating with the grating constant of 1/150 mm driven by a servo motor. The total tuning range of CO₂ laser in the 9.2-10.8 μm band is about 1.43 μm with four bands (9R, 9P, 10R, 10P). New records were achieved at line 10P(20) (10.59 μm) with the corresponding maximum energy of 590 mJ and the minimum pulse width of 35.7 ns. The estimated pulse peak power was about 16 MW. Obvious continuously tunable properties were measured from line 10R(32) to line 10R(26). In the future, output properties under higher working pressures will be studied and the coupling efficiency will be optimized. In combination with the study of multi-atmospheric-pressure CO₂ laser amplifier, an oscillator-amplifier system is in the plan, which may be helpful to the research progress of laser isotope separation.

The 2.1 μm holmium (Ho) laser has important applications in biomedicine, infrared optoelectronic antagonism, polymer material processing, and mid-far infrared nonlinear frequency conversion. Compared with mainstream in-band pumped Ho lasers, the intracavity pumped Ho laser can achieve efficient room-temperature Ho laser output based on the compact pump structure of conventional 800 nm laser diode (LD) without additional high-performance 1.9 μm thulium-doped (Tm) all-solid-state or fiber laser pump sources. In this Ho laser structure, thulium-doped and Ho-doped gain media are placed together in the resonant cavity, and 1.9 μm laser is generated in the resonant cavity under the pumping of Tm medium by conventional LD, and the Ho-doped medium is pumped in the same band. Compared with Tm and Ho co-doped lasers with conversion loss, this mechanism has a higher LD-Ho conversion efficiency at room temperature because there is no Tm laser leakage from the cavity during laser operation. In this study, by using Tm∶YLF crystal with negative thermal lens effect to alleviate the combined thermal lens effect of the Ho∶YVO₄ laser pumped in the cavity, the highest Ho laser output power of 2052 nm is 3.3 W, the slant efficiency is 14.5%, and the LD-Ho laser photoconversion efficiency is 11%. The acavity-pumped Ho vanadate laser has the highest laser power and laser efficiency.

Polarization absorption spectra of Tm∶YLF crystal sample with Tm atomic fraction of 3% and Ho∶YVO₄ crystal sample with Ho atomic fraction of 0.6% are measured using a ultraviolet-vision-near-infrared spectrophotometer. It is used to evaluate the spectral overlap between the absorption bands of the Tm∶YLF laser and the Ho∶YVO₄ crystal in the cavity (Fig. 1). The designed intracavity pumped Ho∶YVO₄ laser adopts a c-cut Tm∶YLF crystal with a size of 3 mm×3 mm×14 mm and an a-cut Ho∶YVO₄ crystal with a size of 3 mm×3 mm×4 mm (Fig. 2). The 2 μm laser power is measured using a pyroelectric power meter. The laser wavelength is measured using a mid-infrared spectrometer. The beam quality of the Ho laser pumped into the cavity is measured using a beam quality (M²)analyzer.

A Tm∶YLF laser output of 11.38 W is obtained at 92 nm LD incident power of 30 W with a slope efficiency of 44.3% and light-to-light conversion efficiency of 37.9%. The laser center wavelength measured at the highest Tm laser power is 1909.7 nm, corresponding to the fluorescence emission band (σ polarization) along the a-axis of Tm∶YLF crystal (Fig. 3). At a pump power of 8.5 W under the wavelength of 792 nm, the laser oscillates and achieves the highest Ho∶YVO₄ laser output of 3.3 W, and the corresponding LD-Ho laser optical conversion efficiency reaches 11% [Fig. 4(a)]. The fitting results show that the slope efficiency of the Ho laser reaches 14.5%, which is significantly higher than that of the Tm∶ YAP-pumping Ho∶YVO₄ laser (10.4%). The leaky 1909 nm Tm∶YLF laser is detected near the threshold pumping power (8.5-9.8 W), and then the Tm laser signal disappears and Ho∶YVO₄ laser starts to vibrate [Fig. 4(b)]. In the process of increasing the output power, the output wavelength of the Ho laser is stable at (2052.2±0.5)nm, and no residual Tm laser signal is detected. This phenomenon can be interpreted as follows: when the Ho laser starts to vibrate near the threshold value, the energy consumed by the Tm laser in the cavity is limited, and the Tm laser signal can be observed. With an increase in Ho laser power, the energy consumed by the Tm laser increases. At this point, the Tm laser enters a new steady state, and the gain generated by LD pumping is mainly balanced with the resonant absorption loss of the Ho∶YVO₄ crystal. Compared to the strong Ho laser signal, the Tm laser leakage signal is so weak that it is drowned in spectral noise. The beam quality in the horizontal and vertical directions at the highest Ho laser power is 1.33 and 1.46, respectively (Fig. 5).

Tm∶YLF laser intracavity pumped Ho∶YVO₄ laser with a compact structure and direct pumping by a conventional laser diode is reported. To achieve efficient overlap of the absorption band between the Tm∶YLF laser and Ho∶YVO₄ crystal, a Tm∶YLF crystal cut along the c-axis is used to achieve the σ-polarized Tm laser output near 1909 nm, which is verified by the Ho laser pumped into the cavity. In the Tm∶YLF laser experiment, a maximum output of 11.3 W with 1910 nm Tm laser is achieved, with a corresponding slope efficiency of 44.3% and light-to-light conversion efficiency of 37.9%. The highest 2052 nm laser output power of 3.3 W is achieved with the intracavity pumped Ho∶YVO₄ laser. Owing to the weak thermal lens effect of the Tm∶YLF crystal, intracavity pumped Ho∶YVO₄ laser output is guaranteed with high beam quality. The above-mentioned results indicate that the room-temperature Ho laser can be pumped at the watt level from the Ho vanadate-doped crystal by the direct pumping of a conventional LD.

Phase retarders are vital optical devices used in various optical polarization applications. Phase retarders are constructed using uniaxial birefringent crystals called wave plates. In theory, all birefringent crystals can be used to manufacture wave plates. However, the corresponding thickness of a zero-order 1/4 wave plate in the visible spectrum is only a dozen microns, even if it is constructed from a quartz crystal with low birefringence, making manufacturing difficult. Therefore, the plates are often manufactured with a thickness exceeding 300 μm, called multi-order wave plates. The retardation of a multi-order wave plate is significantly influenced by the temperature of the application environment, which is unfavorable in the natural environment. The design principle and structure of a thick-unit zero-order wave plate that overcome the difficulty in manufacturing zero-order wave plates and the adverse effects of ambient temperature on the retardation of multi-order wave plates are proposed in this study. The design principle and performance parameters of thick-unit zero-order wave plates are analyzed to realize the production of multi-order wave plates, with a retardation accuracy higher than those of the existing zero-order wave plates, whose retardation is less sensitive to temperature than those for multi-order wave plates with the same thickness.

First, the relationship between the optical axis angle and the thickness of the thick-unit zero-order wave plate was determined based on the optical properties of uniaxial birefringent crystals after obtaining the required retardation for the designed wavelength using equations of refraction and the refractive index of the e-ray [Eqs. (9) and (10)]. Second, the effects of the wave plate thickness accuracy and temperature variation on the phase retardation of thick-unit zero-order wave plates were analyzed. The proposed design was compared with a conventional design in an actual design case. Third, experimental samples were fabricated using optical quartz crystals in the proposed design, and their phase retardation was examined using normalized polarization measurements (Fig. 5).

The fundamental design equations [Eqs. (9) and (10)] of a thick-unit zero-order wave plate can be used to design zero-order wave plates with a selectable thickness for any uniaxial birefringent crystal (Fig. 3). Furthermore, the effects of the thickness accuracy and temperature variation on the phase retardation are investigated based on a design case using optical quartz crystals. The results are as follows: 1) A thickness difference of 1 μm has an influence of less than 0.3° on the phase retardation of the thick-unit zero-order wave plate, which is less than 6% of that of the conventionally designed zero-order wave plate or the multi-order wave plate with the same thickness difference (Fig. 4). This indicates that, by using the same manufacturing technology, the thick-unit zero-order wave plate design can achieve higher precision of phase retardation. 2) The effect of temperature variations on the phase retardation of the thick-unit zero-order wave plate is similar to that for the conventional zero-order wave plate and is only 1/30 of that for a conventional multi-order wave plate (Table 1). 3) Tests on the samples verified the feasibility of designing and manufacturing thick-unit zero-order wave plates. When the ambient temperature is varied within a range of ±5 °C, the variation in the phase retardation is within 0.15° (Table 2).

In this study, a design principle and method for thick-unit wave plates are proposed and any uniaxial birefringent crystal can be used to manufacture a zero-order wave plate with a selectable thickness. This overcomes the manufacturing difficulty in conventionally designed zero-order wave plates owing to the minimal thickness and adverse effects of temperature variations on the phase retardation of multi-order wave plates. Given the significant advantages of highly precise retardation, stable temperature, flexible thickness, and ease of manufacturing, the proposed design provides a novel approach for designing and fabricating high-performance zero-order wave plates.

Spherical lens plays an important role in the optical system, and spherical lens cannot be manufactured without optical testing. The existing apparatus for measuring the structural parameters of spherical lens include: noncontact spherometer, coordinate measurement machine, and Abbey refractometers. However, these low-efficiency measuring apparatus can only measure one specific parameter at a time. Although the laser differential confocal interferometer can realize high-precision measurement for the full parameters of a lens, it is very expensive, and the complicated measurement optical path makes the installation and adjustment time-consuming. Phase measuring deflectometry (PMD) requires a simple device, is economical, and has a high dynamic range, which makes it popular. However, there is ambiguity among the structural parameters, refractive index, and thickness in the PMD measurement process. In this study, a method for measuring the structural parameters of a single lens by transmission deflectometry is proposed. This method can be used to measure the structural and postural parameters of lenses simultaneously. The measurement system is simple and does not require accurate alignment. We believe that the proposed method provides a novel way for the full-parameter measurement of spherical lenses. Moreover, it is also feasible for aspheric lenses.

Based on the principle of PMD, a structural parameter measuring method of a single lens by transmission deflectometry was proposed. First, the single lens model was built in the coordinate system in the form of a polynomial, which was derived in detail in this study. Then, the models of the measurement system and lens were established in the software based on the calibration data from the experiment. From the reverse Hartman test perspective, rays were considered to emit from the pinhole of the camera and intersect the screen after being refracted by the lens. The difference between the intersection coordinates measured in the experiment and that traced in the software was used to construct the objective function. Furthermore, as the deflection magnitude of the emitted light was caused by the front and back surfaces of the lens being tested, it was difficult to obtain the correct results with only one camera. Therefore, a dual-camera device was employed to address this issue. Finally, by optimizing the structural and postural parameters of the lens in the model using the least squares algorithm, the real value could be obtained.

In this study, a double-convex lens was simulated and experimentally tested. In the numerical simulation, the parameters of the lens were preset (Table 1), and the method proposed in this study was used for measurement. The simulation results show that the fitted values are basically the same as the preset values, and the relative errors are smaller than 4×10^-4%. The effect of the calibration errors is shown in the simulation, and the results (Table 2) show that the relative errors are smaller than 0.3% and 2% for the structural and postural parameters, respectively. In the experiment, the results (Table 3) show that the relative error of the lens structural parameter is smaller than 1%. As the lens postural parameters in the global coordinates are difficult to calibrate accurately, we verify the sensitivity of the proposed method by measuring the variation of the lens postural parameters. The results (Fig. 8) show that the proposed method has good measurement accuracy for pose parameters.

In this study, the structural parameter measuring method of a single lens by transmission deflectometry is proposed. The numerical simulation results verify the feasibility of this method. The measurement accuracy of the proposed method is evaluated by measuring a double-convex lens in the experiment. The proposed method can measure multiple parameters simultaneously without requiring accurate adjustment. Moreover, the measurement cost is considerably lower compared with other existing methods. This new method facilitates full-parameter measurement for a single lens. Moreover, as the proposed method uses mathematical polynomials to construct the lens model and completes the lens parameter measurement by optimizing the polynomial coefficients, it is theoretically feasible for measuring other surfaces with higher degrees of freedom, such as aspheric surfaces.

A laser tracing measurement system is a type of portable three-dimensional coordinate measurement system. To achieve high-precision tracing and measurement of a spatially dynamic target, it is necessary for the laser tracing measurement system to detect the relative change of target position accurately in real time. It is important to study the high-speed servo control method of the driving motor, which controls the pitch axis and rotation axis motion in the tracing measurement system. The proportional-integral (PI) control is the most common method for controlling servo motors, but the dynamic control accuracy effect of PI control is poor. The PI control method is more suitable for a static or stable model parameter system. Although the laser tracing measurement system is a follow-up system, the traditional PI control method has big challenges in meeting the system control requirements, especially in dynamic response speed, speed steady-state error, and anti-interference ability. A current predictive control (CPC) method based on a nonlinear extended state observer (NESO) is proposed herein. The CPC method is used to improve the dynamic response speed of the system. The improved NESO is used to eliminate the interference of nonlinear disturbances and improve the stability and robustness of the laser tracing control system. It can realize high-precision tracing control of a laser tracing system.

A laser tracing measurement system uses a permanent magnet synchronous motor to drive the movement of a two-degree-of-freedom rotary mechanism. The control system adopts a current-speed-position three-closed-loop system. To improve the dynamic response speed of the laser tracing system, a CPC algorithm that has an advantage in terms of the dynamic response speed is introduced into the current loop. However, the accuracy of the mathematical model of the system affects the accuracy of the CPC algorithm. When the model is mismatched, the predictive control performance is affected. To eliminate the influence of nonlinear disturbance on the CPC control algorithm during motor operation, an NESO is introduced into the current loop to observe the disturbance and then compensate for it back to CPC to improve the robustness of the system. To verify the control performance of the proposed CPC-NESO, MATLAB/Simulink is used for simulation experiments, and a multimotor control platform based on semiphysical simulation is used for real experiments (Fig. 7). The anti-interference ability of the control method is verified by adding an external load to the system. The control effect of the proposed method is evaluated by comparing the PI control method and the system control standard.

Laser tracing control systems must track 1-m/s linear speed motion with a cat's eye reflector within 1 m, and the output stability of the motor must reach ±0.01 r/min. In the simulation results (Figs. 4-6), when the improved NESO control method is compared with the PI control method, the steady-state error is reduced by 50%, improving the stability of the motor. The response capability of the laser tracing measurement system is improved. In the case of external interference, the speed is less affected. The real experimental results agree well with the simulation results (Fig. 8), demonstrating that, under the same speed overshoot, the improved NESO control method has a smaller steady-state error, faster response, and more stable speed response than the PI method. The anti-interference ability of the system is improved, and the control accuracy better meets the control requirements of a laser control system.

A permanent magnet synchronous motor control system in a laser tracing measurement system requires a fast response, high precision, and high stability. Therefore, a laser tracing measurement control model based on an improved NESO is established. The experimental results show that, when the motor speed is 955 r/min, the steady-state error of speed is 1.7 r/min, and the speed decreases by 1.85% when the 0.1 N·m load is added to the motor after the speed becomes stable. Experimental results show that, compared with the traditional PI control method, under the same speed overshoot, the method proposed in this study results in a smaller steady-state error, a faster response speed, a more stable speed response, and an improved anti-interference capability of the system. The proposed method can satisfy the fast response, high steady-state accuracy, and high robustness control requirements of permanent magnet synchronous motors in laser tracing control systems.

The FELiChEM is an infrared free electron laser (FEL) facility, currently under construction, and it consists of two oscillators that generate middle- and far-infrared lasers covering the spectral range of 2.5-200.0 μm. Each oscillator includes an undulator, a pair of gold-plated spherical mirrors with adjustable poses, a resonant cavity composed of a vacuum chamber, and three POP-IN detectors. The magnetic axis of the undulator, optical axis of the resonator, and electron beam propagation axis must be aligned with high precision to achieve saturated lasing. To monitor the beam current, the reflected light from the spherical mirrors must pass through three 1-mm holes on the POP-IN detectors controlled by stepper motors via a collimating telescope. According to physical calculations, the transverse inclination of the resonator mirror and the transverse off-axis deviation should be less than 50 μrad and 0.1 mm, respectively, ensuring that the three POP-IN operating points are coaxial with the electron beam centerline and the coaxial accuracy is 0.15 mm.

Through the analysis of relevant engineering experience and actual measurement conditions, we analyze and install the two oscillators based on a laser tracker, photoelectric autocollimator, and collimating telescope. Initially, a laser tracker is used to install major equipment, such as magnets, undulators, vacuum chambers, and mirror supports, based on the installation control network. The installation accuracy is better than 0.15 mm and 0.1 mm in the beam and transverse directions, respectively. In accelerator physics, the transverse direction refers to two directions perpendicular to the beam direction. After this equipment is installed, a laser tracker and photoelectric autocollimator are used to install and adjust a gold-plated spherical mirror. First, based on the laser tracker, the photoelectric autocollimator is positioned in the space coordinate system with high precision to ensure that the axis of the photoelectric autocollimator is coaxial with the optical axis of the FEL, and the coaxial accuracy is better than 0.1 mm. Then, based on the photoelectric autocollimator, the postures of the upstream and downstream mirrors are adjusted to be within 10″. Based on the same principle, a laser tracker and collimating telescope are used to collimate and locate the three POP-IN detectors. First, two reference target points are located on the optical axis of the FEL based on the laser tracker and installation control network. The two target points must be adjusted with high precision, and the internal coincidence accuracy with the optical axis of the FEL must reach 0.03 mm. Taking the above two target points as a reference, two high-precision reticle target balls are used to adjust the height of the optical axis center of the collimating telescope to the coaxial laser optical axis, and the coaxial accuracy is better than 0.1 mm. The three POP-IN detectors are pushed into the vacuum chamber successively to reach the working point, and a collimating telescope is used to adjust them. The coaxial measurement accuracy of a single POP-IN is better than 0.05 mm. The total coaxial accuracy is better than 0.15 mm. During the operation of the infrared FEL, the radiation is relatively large. To ensure real-time monitoring and adjustment of the resonator mirror, a set of laser online alignment systems is added. The adjustable aperture is shaped and then sampled by two flat mirrors, and the light spot emitted by the flat mirror is analyzed to determine the coincidence of the incident light and outgoing light. The device has a detection accuracy of ±30 μm for the spot center position.

The requirements for the alignment and positioning of electron guns, accelerator tubes, magnets, and other equipment during device installation are not stringent. The alignment is completed using a laser tracker and 1.5-inch reflective target balls, with a transverse positioning precision and beam direction positioning accuracy better than 0.1 mm and 0.15 mm, respectively. A combination of a laser tracker and photoelectric autocollimator completes the installation and adjustment of the middle- and far-infrared oscillators (Fig. 3). A combination of a laser tracker and collimating telescope completes the installation and adjustment of the POP-IN detectors (Fig. 5). Real-time monitoring of the oscillator mirror is possible during the FEL alignment process. A set of laser online alignment systems is added (Figs. 4 and 6), which has been proved to be stable and reliable through offline and online debugging.

By referring to the engineering experience and technical solutions of relevant scientific research institutions, a technological solution combining a laser tracker, photoelectric autocollimator, and a collimating telescope is selected based on the reliability of the specific project implementation and the needs of the infrared FEL project. This scheme includes offline calibration experiments and on-site installation. The smooth output of an infrared FEL device demonstrates that the scheme is feasible and reliable.

Narrowband photodetectors are widely applied in color image recognition, machine vision, biological sensing, image processing, and other applications. A conventional narrowband photodetector is a combination of a broadband photodetector and a series of optical filters, which significantly increases the device cost, architectural complexity, and energy loss. Therefore, to circumvent this issue, several filter-free approaches are established, including utilization of materials with narrowband absorption, charge collection narrowing (CCN) concept, plasmonic effect, and perovskite/polymer synergetic layers. Among these approaches, perovskite(PVSK) narrowband optoelectronic devices based on metal nanoparticles (MNP) are a topic of significant interest in the fields of solar cells and photodetectors, which incorporate enhanced light via the surface plasmon resonance (SPR) of MNP into the excellent optoelectronic properties of perovskite (high absorption coefficient, long carrier separation distance, high carrier mobility, and adjustable optical broadband). However, the quality of the perovskite film degrades when MNP is integrated with the device that is in direct contact with the perovskite film. One of the ways to solve this problem is to introduce an intermediate layer between PVSK and MNP for film quality. In this study, the polymethylmethacrylate (PMMA) membrane is introduced between perovskite and silver island films, and a new structure of a narrowband enhanced perovskite photodetector is proposed. The device performance can be improved by varying the perovskite precursor solution temperature and thickness of the perovskite.

The narrowband-enhanced perovskite photodetector is fabricated as follows. First, the thin silver film is deposited on a glass substrate via physical vapor deposition, and the silver nanoisland film (Ag NF) is prepared via annealing at 300 ℃ for 30 min. The average diameter of the Ag NPs is approximately 80 nm. Subsequently, the PMMA intermediate layer is spun on the surface of Ag NF, the perovskite film is spun on the PMMA via spin-coating, and silver electrodes with a thickness of 100 nm are deposited on the perovskite film via thermal evaporation. The narrowband-enhanced perovskite photodetector has a planar structure of SiO₂/Ag NF/PMMA/perovskite. The thickness of the PMMA interlayer is controlled by varying the spin-coating speed and the concentration of the PMMA solution. The grain size and thickness of the perovskite films are governed by the perovskite precursor solution temperature and spin-coating speed, respectively.

The influence of PMMA thickness on the narrowband bandwidth is investigated in the experiment. The central wavelength of the narrowband response spectrum is approximately 490 nm. The lowest transmissivity decreases with the addition of the PMMA film. The transmissivity first decreases and then increases as the thickness of PMMA increases, where destructive interference occurs via the PMMA film (Fig. 4). The effect of the perovskite precursor solution temperature on the grain size of perovskite films is investigated. The scanning electron microscope (SEM) images (Fig. 5) show that the grain size of the perovskite first increases and then decreases with increasing temperatures, and the maximum grain diameter is ~5 μm at 60 ℃, which has the higher photocurrent. Moreover, the thickness of the perovskite film is analyzed under the optimization experimental conditions (temperature of 60 ℃, thickness of 278 nm). When the spin-coating speed is 5000 r/min, the maximum photocurrent and large photocurrent gain are obtained (Fig. 6). The performance of the device at optimal parameters (Fig. 7) is characterized. Subsequently, this performance is compared with those of the reported filter-free narrowband photodetectors, as summarized in Table 1.

Herein, a narrowband perovskite photodetector based on the surface plasmon resonance of the silver island film is prepared, and the narrowband of the photodetector is enhanced by introducing thin film between the perovskite and silver island films. The grain size of the perovskite films is governed by the perovskite precursor solution temperature, and the light propagation path is controlled by the thickness of the perovskite to improve the narrowband response. The effects of the PMMA film thickness, precursor solution temperature, and perovskite thickness on the performance of the detector are investigated. The as-prepared photodetector with optimized parameters exhibits a narrow photo-response peak centered at approximately 490 nm with a full width at half maximum of 110 nm, as well as a high responsivity of 0.6 A·W^-1 at an applied bias voltage of 2 V, high external quantum efficiency of 159%, normalized detectivity of 2.73×10¹³ cm·Hz^1/2·W^-1, and response time of 247 ms(rise time)/266 ms(fall time). The detector presented in this paper has a simple fabrication process, particularly in the control method, and exhibits high performance, thus presenting a new strategy for the fabrication of narrowband photodetectors.

Particle-fluid two-phase flows exist widely in energy, industry, environment, and other fields. Thus, it is important to simultaneously determine the parameters of two-phase flow, such as the particle size distribution, volume fraction, and flow velocity. For example, in a thermal power plant, the particle size distribution of pulverized coal and its feed flow are critical issues in ensuring combustion efficiency and reducing pollutant emissions. In mineral processing and feed manufacturing, real-time measurement of these parameters is important for maintaining stable conditions to ensure the quality of products. It is also important in other applications, such as pipeline transportation using air as the carrier medium, in which real-time monitoring of flow velocity is necessary. With the development of science and technology, different online measurement techniques have been proposed and used to measure the parameters of particles in a two-phase flow, including ultrasonic spectrometry, image methods, and digital holography.However, some techniques are limited in their applications because their measurements can be affected by poor environmental conditions. In principle, ultrasonic spectrometry requires several particle and dispersion parameters, which are usually unavailable. Digital holographic technology employs a complicated optical setup that can be easily contaminated by dust. In addition, this technique is suitable only for measuring low particle volume fractions. Therefore, there is an urgent need to develop a simple measurement setup for real-time and online (or inline) measurements of the particle parameters of two-phase flows.

Transmission fluctuation correlation spectrometry (TFCS) is an optical measurement technique that utilizes the autocorrelation characteristics of transmission fluctuation signals of a narrow light beam to obtain information on particle size distribution and volume fraction. In this study, a simple beam-splitting setup was used, which produced two parallel narrow beams, allowing the detection of two channels of transmission signals for further correlation of the transmission fluctuations. Compared with a method that uses only one light beam, this optical setup can measure two channels of transmission fluctuation signals; therefore, cross-correlation spectra of these signals may be obtained, from which particle velocities can be extracted. In addition, the auto-correlation spectra of the transmission fluctuations of either or both channels can be calculated to obtain information on the particle size distribution and volume fraction.

Experiments are performed using spherical/non-spherical particles dispersed in water. The nominal diameters of the samples range from 200 to 900 μm. In the first part of the measurements (Fig. 2), the suspension of the particles is driven by a pump (BT-800) and circulates in a closed system at a constant flow velocity of approximately 1.15 m/s in the measuring zone, which is measured using a laser velocimeter (TM680). The He-Ne laser beam is expanded and collimated at a diameter of approximately 8 mm. The light beam propagates along the direction normal to the particle flow, and a bi-element photodiode (S4204) is employed to measure the transmitted signals. Two pinholes are installed in front of the photodiode. The diameter of the pinholes is 850 μm and the distance between their centers is 1.4 mm. The signals are amplified and recorded using a multichannel A/D card (PCI-50612). The sampling time for each measurement is 4.096 s, and the sampling frequency is 125 kHz. The second part of the measurements uses the same optical setup; however, the particles are fed by a vibrator (Fig. 7). Under gravity, the particles pass through the measurement zone at a constant velocity. The cross-correlation spectra of the transmission signals of the channels and the autocorrelation spectra of the transmission signals of a single channel are calculated using the fast Fourier transform (FFT). The delay time of the cross-correlation spectrum corresponding to the peak of the spectrum is used to extract the velocity of the particles. The results (Table 1) agree with those obtained using a laser velocimeter. The relative errors range from -8.8% to 0.95%. The particle size distribution and volume fraction are extracted from the autocorrelation spectrum. A modified Chahine iteration algorithm is used for the inverse calculation. The mean particle sizes x₅₀ (Table 2) of the measured samples agree with the nominal diameters. All samples are also measured using a laser particle analyzer (Bettersize2600). It is found that the mean particle sizes x₅₀ measured using the proposed TFCS (Table 2) agree well with those of the laser particle analyzer (Table 3); the relative error range from 0.1% to 5%. However, the sizes of x₁₀ and x₉₀ obtained using the TFCS (Fig. 5 and Table 2) differ significantly from those of the laser particle analyzer (Table 3). This difference is caused mainly by the distribution of the velocity of the particles when they pass through the measurement zone owing to the flow condition and the pulse caused by the pump. The measured particle volume fractions are compared with those based on the known weights of the samples (Fig. 6). A good agreement is achieved between the two sets of values for spherical particles [Fig. 6 (a)]. However, for non-spherical particles, the volume fractions obtained using the TFCS are higher than those based on the weights of the samples [Fig. 6(b)], which means that shape correction is required. All measurements are repeated several times, and the results for particle velocity, particle size distribution, and volume fraction show good repeatability. Data processing, including computation of the spectra and inversion, can be completed within 3-5 s. Thus, the proposed measurement is useful for real-time applications.

This study introduces a simple optical setup for transmission fluctuation correlation spectrometry, which can be used to simultaneously measure particle velocity, particle size distribution, and volume fraction. Two parallel narrow light beams are produced to measure transmission fluctuations and obtain autocorrelation of the signals of a single beam and cross-correlation of the signals of two beams. The particle velocity is obtained from the transmission cross-correlation spectrum, and the particle size distribution and volume fraction are deconvoluted from the autocorrelation spectrum.

The measurements are implemented using both spherical and nonspherical particles, and the TFCS results are compared with those obtained using other methods. The results show good agreement and repeatability. The measurement and subsequent data processing can be completed within 10 s. Therefore, the proposed setup is promising for use in real-time and online applications in particle-fluid two-phase flows.

Terrestrial laser scanning (TLS) technology provides an efficient and accurate method for obtaining three-dimensional tree data. The separation of branches and leaves using single-tree point clouds is required when extracting tree structure parameters or determining the above-ground biomass. Additionally the separation of branches and leaves using TLS data enhances the ecological applicability of TLS data . Therefore, an efficient method for separating branches and leaves using tree point clouds can improve the application range of TLS. Existing branch and leaf separation methods either require precise calibration of lidar instruments to obtain intensity data for branch and leaf separation or use supervised classification methods, which require a considerable amount of manual intervention to select training data, and retraining is required for trees from different environments or different tree species. They are not universal. To tackle these issues such as poor separation results, low separation efficiency, and low automation in the current TLS point cloud tree branch and leaf separation, this study further optimizes the unsupervised classification method based on geometric features and proposes a branch and leaf separation method combining the shortest path analysis algorithm and graph segmentation algorithm.

The branch and leaf separation algorithm proposed in this study uses geometric features and structural analysis to classify the point cloud of a single tree into different components. First, a graph segmentation algorithm is used based on point geometric features and point cloud density. Subsequently, according to the three-dimensional coordinate vector P_i(x_i, y_i, z_i) of the point i in the point cloud, the algorithm constructs the covariance matrix of its neighborhood, calculates the three eigenvalues (λ₁, λ₂, and λ₃) of the corresponding point and the corresponding features vectors (e₁, e₂, and e₃) according to this matrix, and combines with the local point cloud density to carry out the coarse separation of branches and leaves. Second, it is the shortest path detection.The Dijkstra algorithm is used to calculate the shortest path from the lowest point in the tree point cloud to the remaining points. Then the skeleton of branches and trunks is extracted according to the growth structure of the tree, and the points of branches and trunks are extracted based on the skeleton of branches and trunks to realize the coarse separation of branches and leaves. Finally, the results of two coarse separations are combined to achieve the final fine separation of branches and leaves.

This study employs three trees with different point spacings and 16 trees with different data quality from different tree species to perform quantitative and qualitative experiments to test the branch-leaf separation ability and robustness of the proposed method. First, three trees with different point spacings are separated 20 times using a pseudorandom method to select parameter values. Although the input parameters are modified several times by the pseudorandom method, the accuracy rate of each branch and leaf separation result of each tree is above 0.92. The branch points of the tree are extracted and the standard deviation of each evaluation index is observed to be below 0.01 (Table 3). Furthermore, using point cloud data of 16 trees with different data quality from different tree species for branch and leaf separation, the accuracy of branch and leaf separation of trees with missing data is not high, but the accuracies of branch and leaf separation of all trees are above 0.9 (Table 4). This indicates that the method proposed in this study has high branch and leaf separation ability and has good robustness. Moreover, the TLS separation method and the LeWos method are used to separate the branches and leaves of the three trees and the separation abilities are compared. The branch and leaf separation accuracies of the TLS separation method, the LeWos method, and the proposed method are compared (Tables 5-7). The classification indexes of this method are better than those of the TLS separation method and the LeWos method. Specifically, when the branches and leaves of the medium and small trees are separated, the evaluation indexes of the proposed method are significantly higher than those of the TLS separation method. Although the TLS separation method can thoroughly separate the tree trunk from the larger branches, it is easy to classify leaf points close to branches as branch points (Fig. 13). These tiny leaves affect the spatial structure of the dots, making it difficult to separate them. The evaluation indexes for the branch and leaf separation results of the LeWos method are good, but it is prone to misclassification when facing the buttress structure of trees. Some point clouds of some buttress structures are classified as leaf points and small branches of trees cannot be effectively separated (Fig. 14). The method proposed in this study, combined with the shortest path analysis algorithm, can effectively distinguish small leaves and extract branches to avoid the interference of leaves. The experimental results reveal that the proposed method has strong branch and leaf separation ability and high robustness.

This study proposes a branch and leaf separation method that combines the shortest path analysis algorithm and the graph segmentation algorithm using tree point clouds. The feasibility and robustness of the proposed method are verified by specific experiments. The experimental results indicate that this method can realize high-precision branch and leaf separation for trees with different spacings. The classification accuracies on three types of data reach 0.9697, 0.9469 and 0.9560, respectively, and the kappa coefficients are 0.8475, 0.8547 and 0.8925, respectively. The branch and leaf separation results obtained by the method in this study serve as references for the subsequent application of single tree analysis.

Nitrogen dioxide (NO₂) is a crucial environmental pollutant. NO₂ generated from automobile exhaust is a critical source of pollution, which has severely affected human health and influenced the quality of urban air and global climate change. The photoacoustic spectroscopy (PAS) based on the photoacoustic effect is a critical technology for trace gas detection owing to its benefits of zero background noise and high detection sensitivity. This study suggests a gas sensor based on PAS with a diode laser at 444 nm as the light source. The NO₂ concentration in the atmosphere is monitored. NO₂ concentrations emitted from various types of automobiles are compared, and the photolysis of NO₂ is investigated. We hope that the findings of the study can offer a significant reference for the improvement of urban air quality and play a critical role in the comprehensive regulation of urban air pollution.

A diode laser with a central wavelength of 444 nm is employed as the excitation source, and an optical power meter behind the photoacoustic cell is employed to monitor the laser power’s stability. The optical chopper can modulate the optical intensity at the photoacoustic cell’s resonance frequency, and a reference signal (a square wave signal) is input to the lock-in amplifier. A microphone with a sensitivity of 45 mV/Pa is installed in the middle of the photoacoustic cell to detect the photoacoustic signal. The detected signal is amplified using a preamplifier and demodulated using a lock-in amplifier, and then collected and processed using the data acquisition card and a computer with LabVIEW software. The vacuum pump, pressure controller, and mass flow controller are employed to regulate the pressure and flow of gas in the photoacoustic cell. A filter is installed in front of the gas inlet to prevent the interference of aerosols.

The photoacoustic signal intensity increases as the pressure and volume fraction of NO₂ increase (Fig. 3). The resonance frequency increases with the increase in pressure since both the gas density and speed of sound increase as the pressure increases (Table 1). The resonance frequency slightly decreases with the increase in the volume fraction of NO₂ since the heat capacity ratio and molar mass of gas change with the volume fraction’s variation (Table 2). There is a good linear relationship between the signal-to-noise ratio and optical power, and the system does not achieve the saturation absorption state [Fig. 4(a)]. When the chopper runs at the photoacoustic cell’s resonance frequency, the air near the window of the photoacoustic cell flows, and the optical power into the photoacoustic cell and the photoacoustic signal intensity slightly change. When the distance between the optical chopper and the photoacoustic cell’s window is 50 mm, the maximum signal is obtained [Fig. 4(b)]. The part of NO₂ is dissolved using water vapor, and the vibrational-translational relaxation rate is also influenced, leading to an inverse ratio between photoacoustic signal and relative humidity [Figs. 4(c) and (d)]. It takes some time for the signal to be stable as the pressure changes (Fig. 5). The Allan variance analysis exhibits that the sensor detection limit is 1×10^-9 with an integration time of 100 s (Fig. 6). The change characteristic of NO₂ concentration in two days from 8: 00 a.m. to 6: 00 p.m. is determined and the trend is consistent with the monitoring data from the Jining Environmental Monitoring Center (Fig. 7). NO₂ concentrations in exhaust from five types of automobiles are detected (Fig. 8). At low speed, it can be deduced that at least 150 g NO₂ will be released if the automobile is driven 100 km and the photolysis rate of NO₂ is ~50% from 12: 00 a.m. to 2: 00 p.m. (Fig. 8).

A gas sensor based on photoacoustic spectroscopy is suggested. The 444-nm blue laser is employed as the light source to achieve NO₂ detection in the atmosphere and automobile exhaust, and NO₂ photolysis is examined. The relationship between pressure and resonance frequency and that between the volume fraction of NO₂ and resonance frequency are investigated. The linear relationship between signal-to-noise ratio and optical power is studied. The influence of the position of an optical chopper and relative humidity on the photoacoustic signal is examined. The detection limit of 1×10^-9 with an integration time of 100 s is obtained using Allan variance analysis. Atmospheric NO₂ is monitored for two days, and the result has a good consistency with the data from the Jining Environmental Monitoring Center. Finally, the photolysis of NO₂ emitted from automobiles is also investigated. This offers high sensitivity sensor for NO₂ detection in practical applications.

3D reconstruction of space targets can provide prior structural information for space services, which is a key technology for improving system autonomy. Conventional 3D reconstruction methods rely on handcrafted features to recover the 3D structure of objects by dense matching. Therefore, affected by the symmetrical structure and non-Lambert imaging of spatial targets, conventional 3D reconstruction methods often suffer from mismatching and insufficient matches of feature points, resulting in a low reconstruction accuracy. In recent years, with continuous developments in deep learning technology, convolution neural networks (CNNs) have been widely used in computer vision. Compared with the handcrafted features used by conventional 3D reconstruction methods, the deep features extracted by CNNs can introduce high-level semantics of images for more robust matching. Inspired by this, a 3D reconstruction method based on MVSNet for space targets is proposed. This algorithm organically applies CNNs with different structures to improve the accuracy and completeness of 3D reconstruction. We hope that our basic strategy and findings will be beneficial to the 3D reconstruction of space targets.

The space-target 3D-reconstruction algorithm model (Fig. 1) is described as follows. First, in view of the imaging characteristics of a space target, the influence of the geometric structure and material of the model on reconstruction results is analyzed, and a multi-view acquisition system for space targets based on the Blender platform is designed. Subsequently, deep visual image features are fully extracted via multi-scale convolution based on MVSNet. The coder and decoder are then used to gather and regularize the spatial context information for stereo matching, which effectively avoids the heavy dependence of conventional methods on the feature points in the reconstructions of low-textured, reflective, and repetitive texture regions. Finally, the residual network is used to solve the boundary smoothing problem caused by the multiple convolutions to further improve the reconstruction results. The model is tested on both the DTU dataset and our self-collected space-target dataset. Its performance is compared with those of VisualSFM, COLMAP, and SurfaceNet through both qualitative and quantitative evaluations. The running time of the proposed algorithm and other methods is also measured to verify the efficiency improvement. In addition, the influence of different numbers of matching views on the accuracy of the reconstructed model is studied to discuss the most appropriate settings.

The proposed method outperforms conventional 3D reconstruction methods in handling low-textured, specular, and reflective regions, which can completely restore typical structures, such as satellite cabins and roofs (Figs. 6 and 7). The mean accuracy, mean completeness, and overall errors of the proposed algorithm are 0.449, 0.379, and 0.414 mm, respectively. The proposed algorithm has the best accuracies among all four compared algorithms (Table 1). In particular, the accuracy of the proposed method is 20% higher than that of the advanced open source software COLMAP. The running time study shows that our method is faster with an average time cost of approximately 230 s for reconstructing one scan (Table 2). The running speed of the proposed method is 100 times faster than that of COLMAP and 160 times faster than that of SurfaceNet. In addition, the performance study on different numbers of matching views shows that more views result in a better performance (Table 3). However, the accuracy improvement is the greatest when three matching views are used for 3D reconstruction. Thus, the matching view number is set as 3 for model training. In general, the proposed model is optimal in terms of reconstruction accuracy and speed.

This study proposes a method for the 3D reconstruction of spatial targets based on the MVSNet deep learning network. First, deep visual image features are fully extracted by the multi-scale 2D convolution, and then spatial context information for stereo matching is fully gathered and regularized through the skip connection between the coding and decoding paths. Subsequently, the matching cost is converted into the depth value probability using the SoftMax function, and the expectation is calculated as the initial estimation value. Finally, the final depth estimation map is obtained by strengthening the edge semantic information through the residual network. Experimental results show that the accuracy of the proposed method is 20% higher than that of the advanced open source software COLMAP. Moreover, the running speed is 100 times faster than that of COLMAP and 160 times faster than that of SurfaceNet. In general, this model can effectively introduce the high-level semantics of images for more robust matching and has the low system running time, which can provide a technical reference for space operation automations and further promote the application of 3D reconstruction in this field.

Lithium-ion batteries are widely used in electronic equipment, electric vehicles, and other applications owing to their advantages of high specific energy, high power, long cycle life, and little or no pollution. Using lithium-ion batteries is a good strategy to achieve the "double carbon" goal. However, abusing lithium-ion batteries may trigger a thermal runaway event, and the burning of toxic and highly flammable gases emitted from thermal runaway can cause severe fires or explosions. Analyzing the gas production behavior of lithium-ion batteries during thermal runaway is of great significance. However, the traditional technology used for detecting pyrolysis gas from lithium-ion batteries struggles to meet the needs of online detection because of many shortcomings, such as a long detection cycle, strong cross-interference, easy saturation, and the inability to detect homonuclear diatomic molecules. The basic understanding of the risk of gas release during the thermal abuse of batteries is still limited. Therefore, an effective technology that can perform a high-resolution online in-situ analysis of battery pyrolysis gas is urgently needed.

In this work, a laser Raman spectroscopy technique is used to rapidly detect the changes in the composition and concentration of the main gases released from the thermal runaway of abused lithium-ion batteries . Hence, an online analysis of the risk of pyrolysis gases of lithium-ion batteries is possible. The pretreatment methods by discrete wavelet transform (DWT) and adaptive iterative re-weighted penalized least squares (airPLS) are used to denoise and deduct the background of the spectral data, respectively. Partial least squares (PLS) is used to establish a quantitative model for the target gas, which can accurately and stably analyze the information on pyrolysis gas from lithium-ion batteries.

First, the Raman spectra of a battery with state of charge (SOC) of 100% during thermal runaway is compared with those of standard reference gases (Fig. 3), and the main gases released by battery pyrolysis are determined to be CO₂, CO, H₂, CH₄, C₂H₄, and C₃H₆. CH₄, C₂H₄, and C₃H₆ have the problems of intensive and overlapping characteristic peaks, especially in the 2502-3444 cm^-1 range. Second, to eliminate the spectral noise and baseline interference on the effective spectral information, this study uses DWT and the airPLS algorithm, respectively. The grid search strategy is used to optimize the DWT parameters, which is determined by the root mean square error of prediction (RMSEP) of the PLS model (Fig. 4). The different DWT parameters have great influence on the model, and the optimal DWT parameters from different gases varies. In addition, to quantify the composition of the unknown pyrolysis gas from the battery, the variables most relevant to the measured gas concentration are screened out in the spectral library by the classical PLS method to establish a multiple regression model (Table 4). In the prediction results on the validation set, the minimum value of predictive correlation coefficient (R²) is 0.937, and the maximum value of RMSEP is only 0.452%, indicating that the PLS model accurately extracts the concentration of the pyrolysis gas from the battery. Finally, the laser Raman spectroscopy technology is used in the thermal abuse experiment of a lithium-ion battery, and the surface temperature, voltage, internal pressure of the abused device, and gas data are recorded in real time (Fig. 6). The thermal abuse process of the battery is divided into three stages. The gas composition and concentration change significantly across the three stages. The gas composition and concentration remain unchanged after the thermal runaway ends, and the maximum error of gas concentration change is no more than 0.29% (Table 5). The gas concentration and its relationship with the SOC are consistent with the results obtained by different gas detection technologies in previous literature. These results show that laser gas Raman spectroscopy technology can effectively and stably analyze the pyrolysis gas from a battery.

The main components of the pyrolysis gas from a lithium-ion battery are determined as CO₂, CO, H₂, CH₄, C₂H₄, and C₃H₆ using Raman spectroscopy analysis. The above six gases and N₂ and O₂ from the air can accurately reflect the temperature and stage of the thermal runaway of a lithium-ion battery, and effectively evaluate the risk of thermal runaway of lithium-ion batteries. The thermal runaway gas components of a lithium-ion battery are analyzed online using a Raman spectroscopy system, and the spectral data of the gas samples are quantitatively analyzed using a spectral preprocessing algorithm and PLS model. The results show that the Raman spectroscopy can analyze the dynamic changes of the main pyrolysis gases from a lithium-ion battery and air components in seconds. The maximum root mean square error of the spectral model is no more than 0.45%, and the minimum root mean square error is only 0.04%, which effectively meets the requirements of the online analysis of typical gas products in the thermal runaway process of a lithium-ion battery. Therefore, the Raman spectroscopy is expected to become an effective technique for the online in-situ study of thermal runaway release gases from lithium-ion batteries and accurately assess the explosion risk of lithium-ion batteries.