Off-Axis Four-Reflection Optical Structure for Lightweight Single-Band Bathymetric LiDAR

A traditional bathymetric light detection and ranging (B-LiDAR) has disadvantages such as large volume, heavy weight, necessity for airport and runway, and high cost for operation. For these reasons, this article presents an off-axis four-reflection optical structure for single-band (532 nm) B-LiDAR carried on unmanned aerial vehicle (UAV). This optical system fully considers characteristics of the laser echo energy under different water conditions, which relate to the optical system parameters, such as peak power of laser emission, field of view (FOV), and receiver aperture area. The proposed optical system designs the objective lens, which is composed of one avalanche photodiode (APD) detector and two photomultipliers (PMTs) detectors, the primary mirror, the second mirror, the plane mirror, and the third mirror, two split field mirrors that separate the echo signals from shallow water, medium water, and deep water, respectively. This proposed optical system was verified in a laboratory tank, swimming pool, Lijiang river, lake, and the Qiaogang Sea Bay. It is found that the maximum water depth measured can reach 25.0 m with an error of less than 0.1 m on average. The dimension and weight of this LiDAR reach $90\times 160\times90$ mm and 10.25 kg, respectively, which is the lightest and smallest B-LiDAR worldwide.