A modified ladar system with a Geiger mode APD to remove a dead time problem

Abstract

A laser radar (LADAR) system with a Geiger mode avalanche photodiode (GAPD) is used extensively due to its high detection sensitivity. However, this system requires a certain amount of time to receive subsequent signals after detecting the previous one. This dead time, usually 10 ns to 10 μs, is determined by the material composition of the detector and the design of the quenching circuits. Therefore, when we measure objects in close proximity to other objects along the optical axis using the LADAR system with GAPD, it is difficult to separate them clearly owing to the dead time problem. One example for that is a case of hidden objects behind partially transparent blinds. In this paper, we suggested a modified LADAR system with GAPD to remove the dead time problem by adopting an additional linear mode avalanche photodiode (LAPD) as a complementary detector. Because the LAPD does not have dead time while still maintaining relatively low detection sensitivity, the proposed system can measure an object placed within the dead time with high detection sensitivity. Light is emitted from the pulsed laser of a light source and is delivered into a fast photodiode to generate a start signal. Most of laser pulses are directed onto the target and scattered from the surfaces of targets. The scattered light in the field-of-view of the system is divided by a polarizing beam splitter, after which it becomes incident to two different types of APDs, the GAPD and the LAPD. The GAPD receives the signals from the target with high sensitivity, and the signals scattered in the dead time zone are then detected by the LAPD. The obtained signals are analyzed at the same time. In this way, the signals scattered from objects placed within the dead time can be distinguished clearly.