Input Amplifiers for Optical PCM Receivers

The factors which limit the performance of an optical receiver are optical quantum noise, leakage noise of the detector, thermal noise introduced by the detector load resistor, and various forms of noise introduced by the input amplifier. If an avalanche detector is used, the leakage noise has two components--one which is gain independent and the other which is gain dependent. In addition, the gain process introduces a signal-dependent noise. In high-speed pulse-code-modulation (PCM) receivers, input-amplifier noise plays an important role in the determination of system performance. If leakage current is negligible, it can be shown 12 that without avalanche gain the required signal power to achieve a given error probability varies as the square root of the thermal noise power and with optimum avalanche gain with the sixth root of the thermal noise power. Furthermore, the optimal avalanche gain varies as the cube root of the thermal noise power.* * These results assume an excess noise coefficient of 0.5. 1771