Multi-longitudinal-mode (MLM) high-spectral-resolution lidar (HSRL) is a novel laser remote sensing technique for realizing fine detection of aerosol optical properties. The Mach-Zehnder interferometer (MZI) with a periodic transmittance is selected as the spectral discriminator for directly separating aerosol Mie scattering and molecular Rayleigh scattering spectra excited by the MLM laser. In principle, the design of the MZI for the application of the MLM-HSRL should meet two conditions. One is that the optical path difference of the MZI is twice the laser cavity length, and the other is that the optical path difference of the MZI is an integer multiple of the laser wavelength. The laser elastic echo scattering signal received by the MLM-HSRL has Gaussian transmission characteristics consistent with the laser beam, which makes the incident light beam on the MZI have a divergence angle. The divergence angle of the incident light beam leads to a deviation in the optical path difference of the MZI, which makes the optical path difference of the MZI fail to equal an integer multiple of the laser wavelength and then makes the discrimination capability of the MZI worse. As the divergence angle of the incident light beam cannot be eliminated, a field-widening technique for the MZI with a large optical path difference based on compensated glasses is proposed to reduce the influence of the divergence angle of the incident beam on the discrimination capability of the MZI.

In this study, a field-widening technique for the MZI with a large optical path difference is studied for the application of the MLM-HSRL for realizing fine detection of aerosol optical characteristics. First, the required design parameters of the MZI with a large optical path difference and an inversion method of aerosol optical properties in the MLM-HSRL system are analyzed. Second, the mathematical relationship between the divergence angle of the incident light beam and the effective transmittance of the MZI with a large optical path difference is established, and the maximum allowed divergence angle for the MZI (O_PD=1000 mm) is calculated. Third, a field-widening technique for the MZI with a large optical path difference based on compensated glasses is proposed. The principle of the field-widening technique is explained, and the mathematical model between the optical path difference of the MZI with the field-widening technique and the divergence angle of the incident light beam is established. According to such analysis, compensated glasses are selected, and their length is calculated. Fourth, the theoretical modeling and simulation verification of the proposed field-widening technique are carried out.

The discrimination capability of the MLM-HSRL system is affected by the divergence angle generated by the Gaussian transmission distribution of the laser elastic echo scattering signal. Theoretical analysis shows that the maximum allowed divergence angle of the MZI (O_PD=1000 mm) is no more than 0.4 mrad, so as to ensure an excellent discrimination capability of the MZI with a large optical path difference (Fig. 4). A field-widening optical path of the MZI with a large optical path difference based on compensated glasses is shown in Fig. 5. Compensated glasses are chosen to be the HK9LGT glass, whose refractive index (standard state @532.0 nm) is 1.517, and the coefficient of thermal expansion is 7.6×10^-6. The total length of the required compensated glasses is 1165.767 mm. In addition, the transmittance of the compensated glasses decreases as the length increases, and the transmittance decreases by 0.2% when the length increases by 10 mm (Table 1). The theoretical analysis shows that the allowed divergence angle of the MZI with a large optical path difference (O_PD=1000 mm) should be less than 25.6 mrad after the field widening (Fig. 6). Zemax simulation results show that the effective transmittance of T_aa is lower than 0.7, and positive discrimination effect cannot be achieved when the divergence angle is greater than 0.4 mrad before the field widening. The effective transmittance of T_aa is 0.825-0.793, and MZI has excellent discrimination capacity (Fig. 9) when the divergence angle is 0-5 mrad after the field widening.

MZI, as a spectral discriminator, is the core device in the MLM-HSRL system. The Gaussian transmission distribution of the laser elastic echo scattering signal makes the incident light beam on the MZI with a large optical path difference inevitably have a divergence angle. In this paper, the influence of the divergence angle on the discrimination capability of the MZI with a large optical path difference is analyzed in detail, and the maximum allowed divergence angle of the MZI (O_PD=1000 mm) is 0.4 mrad. In order to reduce the influence of the divergence angle on the transmittance of the MZI, a field-widening technique for the MZI with a large optical path difference based on compensated glasses is proposed. The theoretical analysis results show that the maximum allowed divergence angle of the MZI (O_PD=1000 mm) is 25.6 mrad after the field widening. The proposed field-widening technique enlarges the allowed divergence angle range of the system by nearly 50 times. The simulation results show that the effective transmittance of T_aa decreases rapidly with the increase in the divergence angle, and the discrimination capability of the MZI becomes worse at a divergence angle greater than 0.4 mrad before the field widening, while T_aa decreases slightly with the increase in the divergence angle, and the discrimination capability of the MZI is positive at a divergence angle ranging from 0 to 5 mrad after the field widening. The proposed field-widening technique can extend the received field angle of the MZI with a large optical path difference and improve the discrimination capability of the MZI with a large optical path difference.