Evaluation and optimization of NIR HgCdTe avalanche photodiode arrays for adaptive optics and interferometry

The performance of the current high speed near infrared HgCdTe sensors operating in fringe trackers, wavefront sensors and tip-tilt sensors is severely limited by the noise of the silicon readout interface circuit (ROIC), even if state-of-the- art CMOS designs are used. A major improvement can only be achieved by the amplification of the photoelectron signal directly at the point of absorption by means of avalanche gain inside the infrared pixel. Unlike silicon, HgCdTe offers noiseless avalanche gain. This has been verified with the LPE grown 320x256 pixel λ c =2.5 μm HgCdTe eAPD arrays from SELEX both on a prototype ROIC called SWALLOW and on a newly developed ROIC, specifically designed for AO applications, called SAPHIRA. The novel features of the new SAPHIRA ROIC, which has 32 parallel video channels operating at 5 MHz, will be described, together with the new high speed NGC data acquisition system. Performance results will be discussed for both ROICs. The LPE material on the SWALLOW prototype was excellent and allowed operation at an APD gain as high as 33. Unfortunately, the LPE material of the first devices on the SAPHIRA ROIC suffers from problems which are now understood. However, due to the excellent performance of the SAPHIRA ROIC even with the limitations of present HgCdTe material, it is possible with simple double correlated sampling to detect test patterns with signal levels of 1 electron. An outlook will be given on further developments of heterojunctions grown by MOVPE, which eventually may replace eAPD arrays grown by LPE.