COUGAR: a liquid nitrogen cooled InGaAs camera for astronomy and electro-luminescence

Abstract

A SWIR FPA was designed and manufactured with 640*512 pixels, 20 µm pitch and InGaAs detectors for electroluminescence characterization and astronomical applications in the [0.9 -1.55 µm] range. The FPA is mounted in a liquid nitrogen dewar and is operated by a low noise frontend electronics.One of the biggest problem in designing sensors and cameras for electro-luminescence measurements is the autoillumination of the detectors by the readout circuit. Besides of proper shielding of the detectors, the ROIC shall be optimized for minimal electrical activity during the integration time of the very-weak signals coming from the circuit under test. For this reason a SFD (or Source Follower per Detector) architecture (like in the Hawaii sensor) was selected, resulting in a background limited performance of the detector.The pixel has a (somewhat arbitrary) full well capacity of 400 000 e -and a sensitivity of 2.17 µV/e -.The dark signal is app. 1 e-/pixel/sec and with the appropriate Fowler sampling the dark noise lowers below 5 erms . The power consumption of the circuit is limited 2 mW, allowing more than 24 hours of operation on less than 1 l of liquid nitrogen. The FPA is equipped with 4 outputs (optional readout on one single channel) and is capable of achieving 3 frames per second. Due to the non-destructive readout it is possible to determine in a dynamic way the optimal integration time for each observation.The Cougar camera is equipped with ultra-low noise power supply and bias lines; the electronics contain also a 24 bit AD converter to fully exploit the sensitivity of the FPA and the camera.

InGaAs detectors and FPA’s for a large span of applications: design and material considerations

“…The generated light is wave-guided in the substrate and tends to escape from the sides of the chip. For those applications the integration time is very long and hence the detectors need to be cooled to 200 K or even 77K; under these assumptions, the detector shunt resistance becomes very high and hence the SFD stage seems to be the best alternative, such as demonstrated by the Virgo [9], Hawaii [15] and Cougar device [16]. In this interface circuit only one detector interface is active at the time instead of all matrix elements in parallel.…”