Single-longitudinal-mode (SLM) direct Doppler wind lidar (DDWL) requires complex techniques of the seed injection, high precision frequency stability, and frequency locking to provide an output of the stable frequency SLM laser, resulting in a complex structure of DDWL. To reduce the technical difficulty and structural complexity of the excitation light source of DDWL, a multi-longitudinal mode (MLM) DDWL is proposed. In the MLM DDWL, a free-running MLM laser is directly used as an excitation light source and quadri-channel Mach-Zender interferometer (QMZI) with four periodic outputs is adopted as a spectral discriminator.

Firstly, for the typical Nd:YAG pulsed laser, the scattering spectra of atmospheric elastic echo excited by the MLM laser are analyzed which are coincident 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 overlapping degree is affected by the laser radiation linewidth, laser optical resonator length, laser center wavelength, and type of scattering particles. In addition, the scattering spectra of atmospheric elastic echo excited by each longitudinal mode of the MLM laser have 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 scattering spectra of atmospheric elastic echo excited by the MLM laser. The designed QMZI has four periodic output channels and the phase difference between 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 spectrum 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 ranging from –50 to 50 m/s, laser optical resonator lengths (L = 30 mm and 300 mm), and laser center wavelengths (λ = 1064, 532, and 355 nm). The SPCD files are fed to the QMZI simulation model as input signals. At the same time, the ray tracing on input signal is performed based on the principle of Monte Carlo simulation s, 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 laser center wavelengths λ = 1064 nm, λ = 532 nm, λ = 355 nm, the maximum detectable wind speeds of MLM DDWL are about 50, 30, and 20 m/s, and the wind measurement errors can be controlled within 2.5, 3.0, and 4.0 m/s, respectively. When the center wavelength of each laser is 532 nm, and the lengths of laser optical resonator are 30 mm and 300 mm, then the maximum detectable wind speeds of MLM DDWL are 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 longer the laser center wavelength and the shorter the laser optical resonator length, the larger the wind measurement range will be and the smaller the wind measurement error.