Based on the time-domain simulation model of coherent wind measurement, it is deduced that the detected atmospheric backscattered spectrum is a two-dimensional convolution of the real atmospheric information, and the instrument point spread function. The spatial resolution can be improved by applying the image deconvolution algorithm to the inversion of the Coherent Wind Lidar (CWL) signal. The effect of the deconvolution algorithm in improving spatial resolution is compared in simulation and experiment. The results show that the spatial resolution can be improved effectively by using the deconvolution algorithm when the transmitted pulse waveform is known.
Spaceborne lidar for ocean vertical profile detection requires a narrow linewidth pulsed laser source with a wavelength of 486.134 nm, which coincides with the H-β Fraunhofer absorption line. An injection-seeded optical parametric oscillator (OPO) is developed to meet this requirement. The 355 nm ultraviolet (UV) laser obtained from a 1064.4 nm Nd:YAG nonplanar ring oscillator (NPRO) after multi-stage cascade amplification and frequency tripling is used as the pump source for the OPO. The OPO is a single-resonant three-mirror ring cavity structure with a critical phase-matched β-barium borate (BBO) as the nonlinear crystal. A 486.134 nm continuous-wave (CW) single-frequency laser obtained by frequency doubling of a 972.268 nm laser diode (LD) with a linewidth of less than 100 kHz is injected into the OPO as a seed source. With an incident 355 nm pump energy of 58.4 mJ, a signal pulse energy of 23.8 mJ and pulse width of 5.05 ns is obtained, with the pump to signal conversion efficiency of 40.7%. The central wavelength of the signal pulse is 486.134 nm, and the corresponding spectral linewidth is 0.017 nm, with the measured wavelength stability over 30 minutes of less than 11 MHz.
In this paper, the effects of turbulence intensity and transmit-receive matching angle residuals on detection performance of spaceborne coherent wind lidar were studied and analyzed. The antenna efficiency equation was derived and simulated in the target plane by Monte Carlo and Backpropagated local oscillator (BPLO) methods. Normalized CNR was defined as a measure. The antenna aperture corresponding to the maximum normalized CNR was considered optimal. We simulated the optimal aperture with different mismatch angles of 0 µrad, 2 µrad, 4 µrad, and 6 µrad under weak, intermediate, and strong turbulence intensities respectively. From the simulation results, it is concluded that:as the turbulence intensity and angular residual increase, the coherence length and the optimal antenna aperture decreases. Besides, under strong turbulence, the effect of mismatch angle on the normalized CNR is weakened and the appropriate range of antenna aperture is narrow. The optimal antenna aperture is about 400 mm under the condition of weak or intermediate turbulence.
A compact, discrete path Nd:YAG Innoslab amplifier system was presented. At a repetition rate of 12.7 W amplified output power was achieved successfully with the seed power of 2.7 W. The corresponding extraction efficiency was 14.2%.
To solve the small-optical aperture and stroke problem of existing fast steering mirror (FSM), we focus on the design of FSM driven by piezoelectric ceramics in space laser communication and lidar systems. The structure design of the FSM and the theoretical analysis of the piezoelectric actuators are carried out. The special structure and installation process of the mirror assembly are designed to ensure the accuracy of the mirror surface. Theoretical calculation and simulation analysis are conducted to evaluate the mirror’s output angle. The dynamic model of the FSM is established to analyze the stiffness. The operating principle and characteristic analysis results of FSM are verified by experimental tests. The results show that the FSM can provide a mechanical excursion angle of ±2.35 mrad, the closed-loop linearity of X axis and Y axis are 0.71% and 0.67%, respectively, and the closed-loop bandwidth of the FSM is 28 Hz. The surface shape accuracy of the mirror after installation can reach 1 / 50λ.
With the wide application of spaceborne lidar, 2 μm laser with high repetition rate and high energy has become an important candidate for coherent detection lidar. Conductively cooling is recognized as the critical technology for high energy, 2 μm lasers. The structure and thermal design of a totally conductively cooled, diode side-pumped, 2 μm laser amplifier is introduced in the paper. The amplifier consists of a 20-mm-long Tm: Ho: YLF crystal pumped by 2-banks of 3-radially arranged diode lasers (LD). Through the research and analysis of the structure and thermal coupling of the amplifier head, the conductively cooling scheme satisfying the need of the application in the space environment is obtained. The peak power consumption of LD is 200 W and the average heat consumption is 23.76 W at 10 Hz. When the coolant temperature is 17°C, the stable temperature of the crystal center is about 30°C, which achieves the result of 2.6 times of laser energy amplification. The experimental data matches the result very well.
A reliable, high-energy, and efficient 2 μm laser is a key component in the development of a coherent Doppler wind detection lidar. A theoretical and experimental analysis of (Tm, Ho) co-doped laser amplifiers is presented. Considering the influence of energy transfer, upconversion, and ground-state depletion, the amplified pulse energy as a function of input pulse energy can be predicted at different temperatures. To validate the simulated results, a set of conductively cooled, end-pumped (Tm, Ho):LuLiF, and side-pumped (Tm, Ho):YLF amplifiers have been constructed. The theoretical performance is found to be in good agreement with the experimental results in both end-pumped and side-pumped amplifiers.
The characteristics and capability of a homemade all-fiber 1.54-μm pulsed coherent Doppler lidar (CDL) were validated in field experiments by comparing the detection results with a collocated lidar and sounding balloons. With the range gate of 30 m and temporal resolution of 16 s at velocity–azimuth display mode, the detection capability of the CDL ranged from 0.1 to 5 km, and the time sequence and height position of this CDL were calibrated by the collocated lidar. In the intercomparison experiments with sounding balloons, the discrepancy of 30-s averaged measurement results of horizontal wind speed and wind direction was nearly 0.7 m / s and 5.3 deg, respectively. The good agreement achieved in such a short averaged time period was a convincing case of intercomparison experiments between CDL and sounding balloon. The CDL system demonstrated good reliability and operational stability in field experiments.
Airborne integrated path differential absorption (IPDA) lidar system is an important instrument to verify the performance and data inversion methods of future space-borne lidar systems for atmospheric CO2 measurement. A ground vertical path validation experiment of atmospheric CO2 measurement by an airborne double-pulsed 1.57-μm IPDA lidar has been implemented. The experiment was carried out and temperature, pressure and humidity profiles of Local Meteorological Station at almost the same time are adopted. Backscattering signals from clouds at altitudes of nearly 5 km were received. To avoid the influence of stray light from mirrors, the energy monitoring signal was delayed through the 200 m multimode fiber. But it is interfered by the aerosol scattering echo signals. Inversely, considering the stray light as monitoring signal, the inversion result of XCO2 is pretty good. Six methods are studied and compared to reduce the bias and improve the CO2 column-averaged dry-air mixing ratio (XCO2) accuracy. The “PIM, AVD” and “PIM, AVX” methods are more effective when clouds are acted as hard target. The mean value of lidar measured XCO2 calculated by “PIM, AVD” and “PIM, AVX” methods is 409.63 ppm. The average value of in-situ instrument UGGA is 411.05 ppm over the same period. The bias between IPDA lidar and UGGA is -1.42 ppm. With averaging 148 shots, the standard deviation of XCO2 of the IPDA lidar system is 3.68 ppm.
Space-borne integrated path differential absorption (IPDA) lidar for global observation of methane (CH4) requires a tunable single-longitudinal mode (SLM) pulsed laser source at 1645 nm, which coincides with appropriate absorption line of CH4 molecules. To meet this application, a pulsed injection-seeded optical parametric oscillator (OPO) using potassium titanyle arsenate (KTA) as the nonlinear crystal is developed. The OPO set-up is a four-mirror stable ring cavity with two pieces of 15-mm-long KTA crystal in critical phase-matching cut for wavelengths around 1645 nm. A single frequency Nd:YAG master oscillator power amplifier (MOPA) laser at 1064 nm serves as the pump. A distributed feedback (DFB) fiber laser with a linewidth of 3 MHz is used for injection of the OPO. To insure successful injection seeding process and enough frequency stability, a cavity-length control method based on the optical heterodyne technique is applied on the OPO cavity. Root-mean-square (RMS) of the frequency variation of the signal pulse compared to the seed laser is measured to be 9.9 MHz, and the Allan deviation is less than 0.25 MHz for averaging time of more than 10 s. With 11 mJ pump pulse input at 50 Hz repetition rate, a signal pulse energy of 1.8 mJ is obtained. The pulse width of this OPO is 15 ns and corresponding linewidth is 45 MHz.
A compact single-frequency master oscillator power amplifier laser system composed of three-stage thulium-doped fiber amplifiers was developed. At a repetition rate of 10 Hz, >100-μJ pulse energy at 2050.5-nm wavelength, with ∼431-ns pulse width, was achieved successfully. The pulse profile could be actively controlled by adjusting the drive signal of an acoustic-optical modulator. This all-fiber laser system could be utilized as a seeder laser for a solid-state power amplifier system.
The high spectral resolution lidar (HSRL) technique employs a narrow spectral filter to separate the aerosol and molecular scattering components from the echo signals and therefore can retrieve the aerosol optical properties and lidar ratio (i.e., the extinction-to-backscatter ratio) profiles directly, which is different from the traditional Mie lidar with assumed lidar ratio. Accurate aerosol profiles measurement are useful for air quality monitoring. In this paper, a spaceborne HSRL lidar system simulation model based iodine vapor cell filter was presented. According to three different atmosphere aerosol distribution models and the uncertainties of atmosphere temperature and pressure, the signal to noise ratio (SNR) and the relative errors profiles of the backscattering coefficients of this lidar was simulated theoretically in daytime and nighttime. The result shows that the errors of aerosol backscattering coefficients are smaller in the aerosols dense area than in the sparse area. As altitude increases, the relative error of backscattering coefficient is increased. The relative backscattering coefficient error is within 16.5% below 5 km with 30 m range resolution and 10 km horizontal resolution.
A single-longitudinal-mode (SLM) double-pulse injection-seeded neodymium-doped yttrium aluminium garnet (Nd:YAG) laser was established utilizing an RbTiOPO4 electro-optic crystal to modulate the optical path of the slave resonator for generating a resonance condition. The Q-switcher was fired twice during every pump period. This enabled the laser to emit a pair of SLM laser pulses with a time separation of 200 μs. Each pulse had a pulse energy of 13 mJ at 50-Hz repetition rate, pulse duration of 20±0.5 ns, and linewidth of 30±0.3 MHz (within 2 min). The beam quality factor of M2 was <1.22. A frequency jitter of 1.4 MHz was obtained within 2 min.
An all‐fiber pulsed coherent Doppler LIDAR (CDL) system is described. It uses a fiber laser as a light source at a 1.54‐μm wavelength, producing 200 μJ pulses at 10 kHz. The local oscillator signal is mixed with the backscattered light (of different frequency) in the fiber. The atmospheric wind speed is determined through the fast Fourier transform applied to the difference frequency signal acquired by an analog‐to‐digital converter card. This system was used to measure the atmospheric wind above the upper‐air meteorological observatory in Rongcheng (37.10°N, 122.25°E) of China between January 7 and 19, 2015. The CDL data are compared with sounding‐ and pilot‐balloon measurements to assess the CDL performance. The results show that the correlation coefficient of the different wind‐speed measurements is 0.93 and their discrepancy 0.64 m/s; the correlation coefficient for wind‐direction values is 0.92 and their discrepancy 5.8 deg. A time serial of the wind field, which benefits the understanding of atmospheric dynamics, is presented after the comparisons between data from CDL and balloons. The CDL system has a compact structure and demonstrates good stability, reliability, and a potential for application to wind‐field measurements in the atmospheric boundary layer.
A single-mode single frequency eye-safe pulsed all fiber laser based on master oscillator power amplification structure is presented. This laser is composed of a narrow linewidth distributed laser diode seed laser and two-stage cascade amplifiers. 0.8 m longitudinally gradient strained erbium/ytterbium co-doped polarization-maintaining fiber with a core diameter of 10 μm is used as the gain fiber and two acoustic-optics modulators are adopted to enhance pulse extinction ratio. A peak power of 160 W and a pulse width of 200 ns at 10 kHz repetition rate are achieved with transform-limited linewidth and diffraction-limited beam quality. This laser will be employed in a compact short range coherent Doppler wind lidar.
Recent progress in the research of a diode pumped, single-frequency 355nm laser for direct-detection wind lidar is
presented. An injection seeded Nd:YAG laser was designed and built. A 'delay-ramp-fire' technique is used to achieve
single-longitudinal-mode and stable energy. In this technique, stable time relation between the resonance peak and the
pump pulse is achieved by feedback controlling the delay time between the pump pulse and the ramp voltage. The resulting
single frequency pulses are amplified and frequency tripled. This laser operates at 100Hz and provides 30mJ/pulse of
single-frequency 355 nm output with M2 value of <1.5. The frequency stability of the injection seeded Nd:YAG laser was
investigated. The piezo hysteresis is found to be the main reason to cause the frequency unstability. In an environment
avoiding high frequency vibration the frequency stability is determined by the motion linearity and ramping speed of the
piezo actuator. A modified approach is proposed to improve the frequency stability of an injection seeded laser.
KEYWORDS: Fizeau interferometers, LIDAR, Doppler effect, Sensors, Interferometers, Reflectivity, Laser stabilization, Signal detection, Wind energy, Signal to noise ratio
Fringe technique is preferred to edge technique of wind measurement in troposphere for a direct-detect Doppler wind lidar. However, most fringe-technique based Doppler lidar systems have been developed to date are based on conventional Fabry-perot interferometer. The purpose of this paper is to introduce our development of fringe-technique lidar based on Fizeau interferometer in which the signal can be detected more conveniently using commercial linear detector. The pre-development of the lidar system is described including interferometer's optimum design, the frequency stabilization of Fizeau interferometer and the choice of multi-anode detector. In additional, the wind error of the system is simulated with taking account of Rayleigh noise. Results shows that the wind error can be less than 0.56m/s under 5 km with 30s integral time.
A direct-detection Doppler lidar for planetary boundary layer wind field measurement utilizing multi-beam Fizeau interferometer (MFI) is proposed. Fringe imaging and edge techniques are popular methods of incoherent wind speed measurement. In boundary layer, where aerosol backscattering is strong, it is good to measure wind velocity with fringe imaging technique. Fabry-Perot interferometers (FPI) are standard instruments in former incoherent lidar. Their performance is restricted from detector channels. However, linear fringes of MFI can be measured directly by linear detector, for example, charge coupled device (CCD). The MFI consists of two flat plates which assembled much like a FPI, but wedged by a small angle. Through three PZTs fixed on one plate, the plates spacing can be tuned and the wedge angle can be adjusted. The physical properties of the MFI are discussed in this paper. The factors affecting the measurement of Doppler frequency shift and the correction methods are analyzed and presented. A set of practical system parameters is proposed. The numerical simulation of system performance is implemented. Under different parameters of MFI, error of horizontal wind speed is compared in boundary layer. It shows that the error can be less than 1m/s using the optimized parameters of MFI.
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