We have developed and experimentally investigated a long-range 1.645 µm coherent Doppler wind lidar (CDWL) system. A compact 1.645 µm single-frequency Er:YAG laser is utilized as the laser transmitter. The impact of laser transmitter parameters on wind detection was assessed using the figure of merit (FOM) concept. To enhance the measurement efficiency, the influence of wave aberrations on the heterodyne efficiency was analyzed. A Galilean telescope with an optical aperture of 100 mm is designed as the optical antenna based on the analysis. The line of sight (LOS) detection range exceeds 30.42 km with a data rate of 1 Hz at an elevation angle of 3.5°. To evaluate the effectiveness of the CDWL, comparison experiments were conducted between the 1.645 µm CDWL and a calibrated 1.55 µm CDWL, revealing a correlation coefficient of 0.9816 for the whole detection path in the wind velocity measurement.

1.6 µm high-order vortex modes carrying orbital angular momentums (OAMs) play significant roles in long-range Doppler lidars and other remote sensing. Amplification of 1.6 µm high-order vortex modes is an important way to provide high-power laser sources for such lidars and also enable the weak echo signal to be amplified so that it can be analyzed. In this work, we propose a four-pass Er:YAG vortex master-oscillator-power-amplification (MOPA) system to amplify 1.6 µm high-order vortex modes. In the proof-of-concept experiments, 1.6 µm single OAM mode (l = 3) is amplified successfully and the gain ranging from 1.88 to 2.36 is achieved. Multiplexed OAM mode (l=±3) is also amplified with favorable results. This work addresses the issue as the low gain of Er:YAG vortex MOPA, which provides a feasible path for 1.6 µm high-order vortex modes amplification.

A 1645-nm single-frequency vortex beam with narrow linewidth from an Er:YAG nonplanar ring oscillator (NPRO) using an annular pump beam is demonstrated. The pump beam from a 1532-nm fiber laser is shaped to an annular beam by an axicon. The Er:YAG NPRO generates a 1.96-W single-frequency vortex beam under a pump power of 13 W. The linewidth of the 1645-nm vortex laser is measured as 6 kHz. This work provides a convenient way of single-frequency vortex beam generation.

All-solid-state single-frequency pulse lasers have been widely used in coherent laser detection, laser remote sensing, laser ranging, and other fields due to their advantages of narrow line width, long coherent length, high stability, and compact structure. In recent years, in the field of coherent laser detection, the demands for wind field measurement, aerosol detection, and coherent imaging are increasing, and single-frequency lasers are the key devices of lidar. Among them, the 1 064 nm single-frequency pulsed laser based on Nd:YAG crystal can not only be directly used as the light source of lidar for wind field and aerosol detection, but also can generate single-frequency laser output of other wavelengths through nonlinear frequency conversions such as frequency doubling, sum frequency, and optical parametric oscillation. In this paper, an LD-pumped single-frequency Nd:YAG master oscillator and power amplifier system with a wavelength of 1 064 nm has been developed. Nd:YAG crystal has a high absorption at 808 nm. The crystal absorbs a large amount of the pump light, which will induce the thermal lensing effect and decrease the quality and stability of the beam. The thermal effect is more serious when the crystal is pumped continuously. Therefore, it is very important to decrease the thermal effect. In this paper, the steady-state heat transfer model of the Nd:YAG rod is studied, and three Nd:YAG rod models with different parameters are built for comparative analysis. It is shown from the simulated results that the temperature of the Nd:YAG rod can be decreased by low doping concentration and end-face bonding. In the experiment, a six-mirror ring cavity is used as the oscillator. A Faraday polarizer, a polarizer, and a half-wave plate are inserted into the cavity to eliminate the spatial hole-burning effect and obtain a unidirectional single-frequency laser output. The laser pulse is obtained by an acousto-optic Q-switch. A pulse output with an energy of 2.18 mJ and a pulse width of 63.2 ns is obtained at the repetition rate of 25 Hz, and its single-frequency characteristic is validated by monitoring the waveform of the output pulse. In order to achieve a higher energy output, a power amplification system is established after the oscillator. The pulsed laser with an energy of 1.85 mJ incidents in the amplifier after being shaped by a lens. An LD side-pumped Nd:YAG module is used in the power amplification system. After the amplification, a laser output of 15.85 mJ with a pulse width of 62.7 ns is obtained with the gain of about 8.6 times. The single-frequency ring laser oscillation and power amplification system has potential applications in lidar and optical parametric oscillators.

激光测速雷达较传统大气数据系统在测量精度方面有显著的优势.为了能够在稳定流场下实现对比测试,将激光测速雷达和高精度超声波风速风向仪一同放置于风塔200 m高度进行长时间对比.以10 min、1 min、1 s、0.1 s为尺度对两者测量的真空速(风速)和侧滑角(风向)进行分析.数据表明激光测速雷达的测量精度和超声波风速风向仪(速度精度0.18 m/s、风向2°)是同级别的.

An Er, Yb:glass slab amplifier pumped by laser diode (LD) array is designed in this paper, which adopts a multi-bounce structure to improve extraction efficiency. It is expected to achieve the laser output with greater energy in the 1. 5 mu m band, and the laser amplification gain can reach 35. 29. Based on the steady-state heat conduction theory, the thermal effect of the Er, Yb : glass slab amplifier is analyzed and the thermo-mechanical coupling model is established. The thermodynamic characteristics of the slab medium at different parameters are compared by the finite element method. The results indicate that the thermal effect can be effectively alleviated by increasing the width-thickness ratio of the slab and reducing the power density of the pump light. Based on the analysis, a method for wavefront distortion compensation is proposed.

Coherent Doppler wind lidar has made significant progress in wind profile measurement because of its high temporal and spatial resolution. All-fiber Coherent Doppler wind lidar has caused most interest in mobile wind detection due to its excellent stability and reliability. A theoretical model was analyzed based on the signal-to-noise ratio (SNR) function, the SNR of continuous-wave and pulsed lidar with different focusing distances was evaluated. A 1550 nm all-fiber mobile Doppler wind lidar was designed for wind profile measurement. The system consists of a narrow-linewidth seed laser, a single-mode fiber amplifier, transceiver optics, and a data processing system. In order to verify the performance of the mobile laser lidar, a velocity calibration experiment was carried out on a moving car with a speed of 10-30 m/s in December 2020. The wind velocity of laser lidar and ultrasonic anemometers was measured and compared with a different elevation angle of laser. Results show good agreement between both measurements. The correlation coefficient is great than 0.96, and the standard deviation of velocity is less than 0.79 m/s.

Coherent Doppler wind lidar (CDWL) is used to measure wind velocity distribution by using laser pulses. However, the echo signal is easily affected by atmospheric turbulence, which could decrease the effective detection range of CDWL. In this paper, a variation modal decomposition based on honey badger algorithm (VMD-HBA) is proposed and demonstrated. Compared with conventional VMD-based methods, the proposed method utilizes a newly developed HBA to obtain the optimal VMD parameters by iterating the spectrum fitness function. In addition, the Correlation Euclidean distance is applied to identify the relevant mode and used to reconstruct the signal. The simulation results show that the denoising performance of VMD-HBA is superior to other available denoising methods. Experimentally, this combined method was successfully realized to process the actual lidar echo signal. Under harsh detection conditions, the effective detection range of the homemade CDWL system is extended from 13.41 km to 20.61 km.

A 2 μm single-frequency, all-solid-state laser is one of the preferred light sources for coherent laser wind lidar and differential absorption lidar. In order to obtain 2 μm single frequency pulse laser with the high energy and hundred nanoseconds pulse width, the Ho:YAG non-planar ring cavity laser pumped by Tm:YLF solid state laser was used as the seed source to design and develop an injection-seeding single frequency Q-switched Ho:YAG pulse laser. A 2090 nm single-frequency pulse laser with an average pulse energy of 18.51 mJ and pulse width of 110.9 ns was obtained at a repetition rate of 200 Hz. The beam quality M2 factors of the output laser are 1.16 in the X direction and 1.25 in the Y direction, and the pulse spectrum width is 4.05 MHz.

Coherent wind lidar is suitable for multiple scenarios. In this paper, a 1.55μm portable wind lidar with 10kHz 10μJ-level 200ns-pulsewidth output was developed. The theoretical model was established based on signal-to-noise ratio (SNR) function and coherent detection principle. The influence of linewidths and pulse widths of single frequency pulsed laser on the wind velocity measurement was studied and verified in comparison experiments. The blind area of the lidar was also analyzed. Solved from spectrum and SNR distribution in range bins, detection range of the lidar is an important detection parameter. The portable wind lidar reached the detection range to 2km with the inclination of 0°, and the range to 1.5km with the inclination of 90°.

A wavelength tunable single-longitudinal-mode (SLM) Er:YAG ring laser around 1.6 µm is demonstrated. By using an acousto-optic modulator (AOM) to force unidirectional operation, up to 10.4 W and 8.7 W SLM laser output power are obtained at 1645.22 nm and 1617.33 nm, with corresponding slope efficiencies of 45% and 40%, respectively. Besides, stable dual-wavelength operation at both 1645 nm and 1617 nm is also achieved with the maximum power of 9.1 W. By rotating the birefringent filter (BRF) in the ring cavity, the wavelength could be tuned from 1616.77 nm to 1617.51 nm and 1644.51 nm to 1646.12 nm. The line width is measured to be 125 kHz at 1617 nm and 131 kHz at 1645 nm via the time-delayed self-heterodyne method. As far as we know, 8.7 W is the highest continuous-wave SLM output power at 1617 nm.

A denoising method based on singular value decomposition (SVD) and variational mode decomposition (VMD) is proposed for wind lidar. Utilizing the covariance matrix based lidar signal simulation model, the performance of VMD, SVD, and VMD-SVD is evaluated. The results show that the VMD-SVD method is of better performance, and the output signal-to-noise ratio (SNR) is about 12 dB at the input SNR of -9dB. The actual lidar signals processing is performed with this combined denoising method, and the detection range and wind speed at pulse accumulation numbers of 50,100, and 300 are compared. We set the wind speed resulting from noisy signal with pulse accumulation number of 300 as the reference wind speed, and the mean value and standard deviation of wind differences are analyzed. The results show that the denoising method can not only increase the detection range while ensuring the accuracy of wind speed estimation but also achieve the same detection distance with fewer pulse accumulations, thereby improving the temporal resolution. For the pulse accumulation number of 50, the detection range is extended to 24 km from 18.45 km, and the standard deviation of speed difference is 0.88 m/s; for the same detection range, the temporal resolution is increased by about 6 times.

A1645nm injection-seeded Q-switched Er:YAG ceramic laser pumped by a 1532 nm fiber laser with changeable pulse repetition frequency (PRF) used for Coherent wind measurement Lidar is demonstrated. Single-frequency operation of Er:YAG laser is achieved by injection seeding technique. A ‘M-shape’ ring cavity is utilized to eliminate the effects of spatial hole burning. The laser delivered single-frequency pulses with energy ranging from 6.6 to 10.2 mJ. The corresponding pulse width and PRF varied between 179 ns-271 ns and 300 Hz-1 kHz, respectively. And the line width at 300 Hz is measured to be 2.82 MHz. The measured M2 factors are 1.51 and 1.54 in x and y directions, respectively.

Solid-state single-frequency lasers around 1.6 µm are ideal sources for coherent Doppler wind lidars (CDWLs). A CDWL system with 1645 nm sing-frequency, injection-seeded Er:YAG ceramic laser is demonstrated. The Er:YAG laser based on an "M-shaped" ring resonator operates at pulse repetition frequencies (PRFs) of 300-1000 Hz at room temperature. The maximum single-frequency output energy is 10.1 mJ with a pulse width of 179 ns at 300 Hz. The 1645 nm Er:YAG laser is first used in a long-range CDWL system, and a line of sight (LOS) wind velocity up to 25 km is detected with 90 m range resolution in 0.5 s observation. To verify the reliability of the measurement results, the relationship between detection range, pulse energy, and accumulated numbers is also demonstrated.

Long-range wind sensing using coherent Doppler lidar is attractive in many fields such as wind shear warning, aerosol detection and aircraft wake vortex detection. Recently, single frequency, all-solid-state laser around 1.6 mu m has caused great interests for its eye-safety and high pulse energy. Velocity accuracy which is one of the key factors of wind lidar systems needs to be calibrated. The 1645 nm eye-safe coherent Doppler wind lidar based on injection-seeded technique consists of laser systems, transceiver optics, and data processing systems is developed. The average power of the laser pulses is 2.6 W with a pulse width of 190 ns at a repetition rate of 300 Hz. The accuracy of velocity measured by the lidar system is calibrated with a velocity calibrator based on a servo motor with a maximum speed of 3000 r/min and a rotating disk with a diameter of 300 mm. A real-time display software based on the LabVIEW platform is designed to get the velocity results and signal to noise ratio (SNR) from the FPGA acquisition module, and the central frequency correction algorithm is used to eliminate frequency jitter of the laser. To calibrate the wind velocity near 0 m/s, a nonmoving hard target at the range of 1.2 km with an elevation angle of about 0.5 deg is measured. Results show that a velocity accuracy (standard deviation of the measurement errors) of 0.38 m/s in the range +/- 40 m/s and the accuracy of zero velocity is 0.16 m/s.

A novel Er:YAG laser system operating at 1645 nm with high pulse-repetition-frequency (PRF) of kHz level is demonstrated. A ring cavity with double gain medium end-pumped by two fiber lasers is utilized to obtain high pulse energy. A novel ‘triple-reflection’ configuration on a piezoelectric actuator (PZT) is adopted to achieve high-repetition-rate at 3-kHz operation with the ramp-fire locking method. Single frequency pulses with maximum average power of 18.3 W at 3 kHz are obtained, and the pulse duration time is 318 ns. The full line width at half maximum (FWHM) of the pulses measured by the heterodyne technique is 1.71 MHz at 3 kHz. To the best of our knowledge, this is the highest PRF single-frequency laser pulses achieved based Er:YAG gain medium.

We present an injection-seeding Q-switched 1645 nm Er:YAG ceramic laser with high frequency stability and energy stability by combining the injection-seeding technique and Pound-Drever-Hall technique. 10.31 mJ single-frequency pulses with optimal frequency stability (525 kHz) and relative energy stability (0.52%) at a high pulse repetition rate of 1 kHz are obtained. To the best of our knowledge, this is the highest frequency stability and energy stability so far in a high pulse repetition frequency, large pulse energy, single-frequency Q-switched Er-doped solid laser. This single frequency laser with high stability provides an excellent light source for a coherent lidar system.

介绍了相干激光测风雷达系统中用转盘进行速度标定的方法,在实验室搭建了激光雷达速度标定装置.采用1.645μm种子注入单频脉冲激光器,在激光中心频率相对种子光移频量约62MHz、转盘直径300mm、转盘速度-47.12～47.12m/s可调、激光出射方向和转盘线速度方向夹角为36°时,采用相干探测的方法测出了多普勒频移并反演出了测量速度;数据处理过程中分别采用最大值频谱估计法和频谱对齐法计算多普勒频移和转盘速度.实验结果显示,频谱对齐法相对于最大值频谱估计法能够提高36.3％的测速精度,因此在激光雷达测风系统中采用频谱对齐法相对于常用的最大值频谱估计法测量的风速精度更高,这对提高相干激光雷达测风的风速精度提供了实验依据和参考标准,验证了频谱对齐法在激光雷达测风的数据处理中的可行性.

A high pulse repetition frequency (PRF) single-frequency Er:YAG ceramic ring laser is demonstrated. A double-ceramics ring cavity both end-pumped by a 1532-nm fiber laser is also used to increase the pulse energy. The maximum single-frequency output energies are 21.0 mJ, 18.3 mJ, and 14.4 mJ at the PRF of 500 Hz, 750 Hz, and 1 kHz, respectively. Correspondingly, the pulse widths are 93.2 ns, 106.8 ns, and 130 ns. To the best of our knowledge, this is the highest energy at high PRFs obtained from a single-frequency injection-seeded Er:YAG ceramic laser.