Improved performance of a cooled photoreceiver for high speedcommunication

An alternative method of improving photoreceiver sensitivity in a high-bit-rate optical transmission system is described. HEMTs have recently become commercially available and are known to generate mainly thermal noise and to exhibit transconductance enhancement at cryogenic temperatures. This suggests that a cooled photoreceiver with a HEMT in the first stage of the front-end amplifier could have an improved sensitivity with respect to that of room temperature operation. The DC transconductance variation between room temperature and 40 K for HEMTs of various technologies is presented. The signal-to-noise (S/N) ratios of a photoreceiver with a first-stage HEMT working at room temperature and 45 K are compared. The S/N improvement is currently about 4 dB (optical). It is found that HEMT noise generated in the frequency range considered does not seem to be of pure thermal origin. The results confirm that the proportion of MOCVD (metalorganic chemical vapor deposited) HEMTs exhibiting electrical characteristic degradation at low temperature is far less for MBE (molecular-beam epitaxial) HEMTs