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.
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.
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.
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.
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.