Single-longitudinal-mode (SLM) direct Doppler wind lidar (DDWL) needs the complex techniques of the seed injection as well as high precision frequency stabilization and frequency locking to provide an output of the stable frequency SLM laser, resulting in the complicated construction of the DDWL. To reduce the technical difficulty and structural complexity of the excitation light source of DDWL, a multi-longitudinal mode (MLM) DDWL is proposed. The MLM DDWL directly employs the free-running MLM laser as the excitation light source and uses the quadri-channel Mach-Zender interferometer (QMZI) with four periodic outputs as the spectral discriminator.
Firstly, atmospheric elastic echo scattering spectra excited by the MLM laser are analyzed for the typical Nd: YAG pulsed laser, which presents a coincidence distribution with the longitudinal modes of the MLM laser. The peaks of atmospheric elastic echo scattering spectra excited by the MLM laser overlap with each other. The overlap degree is influenced by the laser radiation linewidth, laser optical resonator length, laser center wavelength, and scattering particle type. In addition, atmospheric elastic echo scattering spectra excited by each longitudinal mode of the MLM laser has the Doppler frequency shift introduced by atmospheric wind. Therefore, it is necessary to select an optical interferometer with the periodic transmittance curve as the spectral discriminator of MLM DDWL.
Subsequently, a QMZI is designed as the spectral discriminator to achieve high-precision measurement for the Doppler frequency shift of atmospheric elastic echo scattering spectra excited by the MLM laser. The designed QMZI has four periodic output channels and the phase difference of adjacent channels is π/2. The mathematical model of the transmittance function of the QMZI is established. The effective transmittance of the QMZI for atmospheric elastic echo scattering spectra excited by the MLM laser is analyzed based on the partial coherence theory of quasi-monochromatic light interference and the polarization effect of light. On this basis, the data inversion algorithm of MLM DDWL is constructed.
Finally, the simulation experiments of wind measurement are carried out. The QMZI simulation model is built by the non-sequential mode of Zemax optical simulation software. The atmospheric elastic echo scattering spectra excited by the MLM laser are configured by the SPCD files of Zemax optical simulation software under different theoretical wind speeds (from -50 m/s to 50 m/s), laser optical resonator lengths (L=30 mm, L=300 mm), and laser center wavelengths (λ=1064 nm, λ=532 nm, λ=355 nm). The SPCD files are fed to the QMZI simulation model as input signals. At the same time, the ray tracing based on the principle of Monte Carlo simulation is performed for the input signals, and the output signals of the four channels of the QMZI simulation model are recorded to retrieve the atmospheric wind information. The simulation results show that the proposed MLM DDWL can achieve high-precision measurement of atmospheric wind information. With the laser optical resonator length of 300 mm and different laser center wavelengths (λ=1064 nm, λ=532 nm, λ=355 nm), the maximum detectable wind speed of MLM DDWL is about 50 m/s, 30 m/s, and 20 m/s, and the wind measurement errors can be controlled within 2.5 m/s, 3.0 m/s, and 4.0 m/s, respectively. With the laser center wavelengths of 532 nm and laser optical resonator lengths (L=30 mm, L=300 mm), the maximum detectable wind speed of MLM DDWL is about 50 m/s and 30 m/s, and the wind measurement errors can be controlled within 2.0 m/s and 3.0 m/s, respectively. Therefore, the larger the laser center wavelength and the smaller the laser optical resonator length, the larger the wind measurement range and the smaller the wind measurement error.