Linear-mode photon counting with the noiseless gain HgCdTe e-APD

A linear mode photon counting FPA using HgCdTe MWIR cutoff e-APDs has been designed, fabricated, and characterized. The broad spectral range (0.4 μm to 4.3 μm) is unique among photon counters, making this a "first of its kind" system spanning the visible to the MWIR. The low excess noise ((F(M) ≈ 1) of the e-APDs allows for robust photon detection while operating at a stable linear avalanche gain in the range of 500 to 1000. The ROIC design included a very high gain-bandwidth product RTIA (3x10¹¹ Ohm-Hz) and a 4 ns output digital pulse width comparator. The ROIC had 16 high bandwidth analog and 16 LVDS digital outputs. The 2x8 array was integrated into an LN2 Dewar with a custom LCC and daughter board design that preserved high bandwidth analog and digital signal integrity. The 2x8 e-APD arrays were fabricated on 4.3 μm cutoff HgCdTe and operated at 84 K. The measured dark currents were approximately 1 pA at 13 V bias where the measured APD gain was 500. This translates to a predicted dark current induced dark count rate of less than 20 KHz. Single photon detection was achieved with a photon pulse SNR of 13.7 above the amplifier noise floor. A photon detection efficiency of 50% was measured at a background limited false event rate (FER) of about 1 MHz. The measured jitter was in the range of 550 ps to 800 ps. The demonstrated minimum time between distinguishable events was less than 10 ns.