Results and Discussions Based on the three beam expansion methods proposed in this study, the experimental prototype expanded the transmitted beam to 5 mrad in the elevated direction under the condition of beam expansion by a cylindrical lens (Fig. 6). Simultaneously, in the defocus beam expanding mode, to realize coherent detection of the laser echo signal based on the all-fiber optical path, a fiber collimator with a high-order phase beam expander is used to bring the field-of-view of the 3 mrad laser echo signal into a single-mode polarization-maintaining fiber with a core diameter of 9.5 μm (Fig. 3). The echo signal is further received by the optical fiber collimator array with a high-order phase spherical lens, the overlapping field-of-view of 0.5 mrad is realized, and the total receiving field-of-view of the system is 5.5 mrad (Fig. 4). In the case of beam expansion, the chimney at 1.1 km and the high-reflectivity target at 5.4 km are detected. After multipulse coherent accumulation and self-focusing processing, the signal-to-noise ratio is greater than 30 dB (Figs. 12 and 13), which satisfies the requirements of high-resolution imaging.

To ensure a detection range, the receiving telescope of lidar should adopt a larger aperture to receive more echo energy. When receiving with a single-element detector, increasing the receiving aperture typically reduces the receiving beam width. Array detectors are commonly used to receive wide field-of-view echo signals to achieve a wide receiving beam and form a large observation width. Considering an optical system with a 100 mm aperture and 480 mm focal length as an example, it is assumed that a single-element detector with a photosensitive surface size of 9.5 μm is used for reception. For a central wavelength of 1.55 μm, the corresponding receiving beam width is approximately 20 μrad, which is close to the diffraction limit. The size of the detector array required to cover a receiving field-of-view of 3 mrad is 150×150, resulting in a sharp increase in the number of LiDAR channels with a coherent detection system and an extremely complex technical implementation of the system. The size of photosensitive surface of the single-element detector increases to approximately 1 mm. In principle, one channel can realize the reception of a wide field-of-view laser echo signal.

Receiving beam broadening can realize the function of a detector with a large photosensitive surface. Receiving beam broadening can be realized at the primary lens or the feed, and the method of realizing receiving beam broadening at the feed can include the common aperture function of the primary lens. Following references [5-6] and the feed beam expansion method, three receiving beam broadening methods, including defocus beam expansion, cylindrical lens beam expansion, and wavelength-conversion beam expansion based on a membrane diffractive lens, are proposed. A simulation calculation was performed. Some verification results are provided in combination with the development of the actual system. In addition, considering the problem of receiving gain reduction caused by beam expansion, the working distance equation of LiDAR in the case of beam expansion is also provided in this paper, and the analysis and verification are conducted based on experimental data.

The LiDAR prototype developed in this study utilized the lightweight and thin characteristics of the membrane diffractive lens to realize the lightweight and large aperture of the receiving system. Combined with the receiving beam expander system, the targets at 1.1 and 5.4 km were detected using the receiving beam expander system. These results indicate that the proposed method is effective. Under the condition of a large receiving aperture, combined with transceiver beam broadening, it can not only reduce the number of receiving channels but also ensure the imaging resolution and long-distance detection signal-to-noise ratio, as well as obtain a large instantaneous observation width. It is of great significance to continue relevant research, which is expected to meet the requirements of long-distance, wide-range, high-resolution imaging. The decrease in the receiving gain caused by the receiving beam expansion can be compensated by adding amplifiers to the electronics. At present, electronic amplifiers with a gain of 40-50 dB are very common, and a reasonable design of the system parameters is of great significance.