Cryogenic Behaviour of Optoelectronic Devices for the Transmission of Analogue Signals via Fiber Optics

The transmission of analogue signals via fiber optics is a subject of interest for applications with detectors operating at cryogenic temperature, at ground or on space borne instruments. For those detectors, both thermal and galvanic decoupling shall be required. For a better understanding of the cryogenic performance of cooled analogue optical links, we have investigated the cryogenic performances of infra-red AlGaAs LEDs, a PIN photodiode, a Si photodiode and an avalanche photodiode (APD). Those devices are widely available on the market and have not been designed for low temperature operation. The high radiance AlGaAs LED has shown an enhanced responsivity when immersed in liquid nitrogen. In fact, the light output power measured with an Optical Spectrum Analyzer, increases by an order of magnitude at 77K and the light versus current characteristic becomes perfectly linear, even if the LED is driven with very low current. On the contrary, the performance of a Si photodiode and a PIN photodiode become degraded when these devices are operated immersed in LN. At 77K, the responsivity of a Si photodiode has shown a 46% reduction and a lower decrement, 23% , for a PIN photodiode. We have also verified the capabilities of a Si APD operated at cold: its responsivity at 77K is quite similar to that exhibited at 300K, but the reverse bias voltage required to get the same gain reduces by about 50 % .