Camera for laser beam profiling from 1.0 to 2.0 microns wavelength with an indium gallium arsenide based focal plane array

Abstract

Extended wavelength InGaAs material is ideal for laser beam profiling applications from 1 micron to 2 microns wavelength. We report on a focal plane array and camera designed specifically for this application. The format of the camera is 320 x 256 pixels on a 25 micron pitch, and the operation is snapshot exposure with a 16 ms exposure time. The camera may be triggered for synchronization with laser pulses and has a 60 Hz maximum readout rate. Two challenges are encountered with extended wavelength InGaAs material compared to lattice matched material. The first is lower quantum efficiency at the shorter wavelengths due to transitional buffer layers that absorb at the shorter wavelengths. The second is the larger dark current caused by lattice mismatch between the InP substrate and the absorption layers. Neither challenge is a problem for laser beam profiling, since a large energy or power is available from the source. To accommodate the dark current, a gate modulated (GMOD) readout circuit is used, where the continuously variable capacity is increased to several million electrons. Both CW and pulsed illumination linearity are good, allowing accurate profiling. The temperature of the focal plane array is held near room temperature with a thermoelectric cooler for stability. To provide a corrected image, nonuniformity corrections for offset and gain are stored in the camera.