Quasi-monochromatic of laser echo signals on transmittance of Mach–Zehnder interferometer for UV multi-longitudinal-mode high-spectral-resolution lidar

Abstract

The effective transmittance of Mach–Zehnder interferometer is a function of the complex degree of coherence in the construction of the UV multi-longitudinal-mode (MLM) high-spectral-resolution lidar (HSRL), in which the spectra of laser echo signals are broadened to 35 GHz and should be recognized as the quasi-monochromatic rather than the monochromatic. In this paper, we calculate the complex degree of coherence of the Mach–Zehnder interferometer (MZI) for the UV MLM laser and its corresponding Mie–Rayleigh backscattering signals. The results show that the phase of the complex degree of coherence for both quasi-monochromatic lights is zero, while their modulus of the complex degree of coherence are optimal when the optical path difference of MZI equals two times of laser optical cavity length. For the specified UV MLM laser with the linewidth of each longitudinal mode of 100 MHz, mode number of 80 and mode interval of 375 MHz within laser linewidth of 30 GHz, the optimal modulus of the complex degree of coherence has a fixed value of 0.776, while the optimal modulus of the complex degree of coherence for the laser echo signals is a function of the backscatter ratio and affects the effective transmittance for both Mie and Rayleigh channels of MZI, which are verified using the atmospheric modeling. Moreover, the relative standard uncertainties of the retrieved aerosol backscattering coefficient using the MLM HSRL are analyzed considering the systematic effect and random effect. The simulation results show that the presence of aerosol layers deteriorate the accuracy of the retrieved aerosol backscattering coefficient sharply, and the relative standard uncertainty produced by the modulus of the complex degree of coherence of Mie scattering spectra can be realized to less than 4% by improving the stability of the MLM laser.