Optical power budget and device time-constant considerations in undersea laser-based sensor design

Abstract

The required dynamic range of a practical undersea laser-based sensor is limited by its range-to-target. The dynamic range of the sensor needs to accommodate the variations in target reflectivity and specularity, water scattering and absorption, and mode of operation of the sensor at the target range. For example, the same sensor may be used to sense a diffuse flat black target and a shiny metallic mirror at normal incidence. Similarly, the same sensor may be deployed both in murky coastal waters and in clear deep oceanic waters. Since a detector used in a sensor has a limited (intrascene) dynamic range of perhaps 40 dB, methods of mitigating the required dynamic range at the sensor must be employed. Further, the required time scales of each specific dynamic range absorber must be considered. Therefore, a systematic optical budget approach considering time scales is employed.After considering maximum laser power out and minimum detectable power, the remaining available dynamic range is allocated to specific devices based on time responses matched to requirements. Devices are discussed and classified according to their parameters, and a final recommended system design is presented. The final system consists ofa continuously variable neutral density filter wheel, and a galvanometrically scanned variable neutral density filter wheel. Other devices and their merits are discussed.

Laser scanning imaging system for underwater robotics vehicle

“…The two approximations are reasonable for many target and application scenarios. The first order LIDAR equation is (Leatham, 1991 The LIDAR equation is widely used in atmospheric and underwater researches, for example, to calculate the first order transmission, and to estimate the power budget in laser-based sensor design. However, it still has some obvious limitations.…”