Range-intensity profile of a SWIR laser gated-viewing system for varying temporal response of in-pixel amplification

A laser gated-viewing system primarily provides range-gated 2D images without any range resolution within the range gate. However, it is possible to extract 3D information by the combination of two images with appropriately overlapping range gates. This approach uses the fact that the range-intensity profile can be basically described by the convolution of the temporal functions of the reflected laser pulse and the detector sensitivity. Thus, this profile consists of a rising and falling edge with a plateau region in between. We have previously shown ([1]) how to generate these two required images from only one laser pulse by correlated double sampling realized in the read-out integrated circuit of the gated-viewing camera. Because there is a marginal temporal difference between the capturing times of the reset and signal level images this approach is notably suited for 3D imaging in dynamic scenarios with fast moving objects. The length of the slope of the range-intensity profile limits the distance range in which 3D reconstruction is possible. In another previous work ([2]), we have extended this 3D distance range by refining the original reconstruction algorithm and using an illumination laser with significantly longer pulse duration. The gated-viewing camera that we use for the experiments is sensitive in the short-wave infrared spectral band and has a focal plane array with 640 x 512 avalanche photodiodes with a pixel pitch of 15 mm. In this paper, we experimentally investigate the influence of the temporal detector behavior on the 3D distance range. Increasing the detector response time by reducing the current of the in-pixel capacitive transimpedance amplifier gives a further important extension of the mean slope length of the range-intensity profile by a factor of approximately 3. As an unexpected outcome, the length of the slope depends linearly on signal strength. This dependency has been finally taken into account in an exemplary 3D reconstruction showing reasonable range results with respect to the signal strength.