TABLE OF CONTENTS In Chap. 3 the fundamental noise contributions to our signal, shot noise of currents and thermal noise of resistors, were discussed. These noise sources in general have no particular spectral character, delivering the same power per unit bandwidth up to a very high frequency. However, the typical noise power spectrum seen in a real photoreceiver in a real environment is far from being as smooth as the quantum analysis leads us to expect. First, we have seen how the frequency-dependent networks of electronic amplifiers and other components connected to the photodiode can significantly modify the noise density as a function of frequency. Further, the majority of semiconductor devices and even many processes exhibit a 1/ f character, which greatly increases noise density at low frequencies, typically below about 500 Hz. The voltage and current noise spectral densities of both discrete transistors and opamps show this 1/ f character, with a corner frequency in this region.
Many man-made sources of electrical and optical interference are present at the output of an optical receiver. At 50/60Hz, 100/120Hz, and 150/180Hz there are often very strong electrical interference signals from line-voltage wiring and power supply transformers. They can usually be greatly suppressed, if not totally eliminated, by good electrostatic and/or ferromagnetic screening of receivers, through diligent bypassing of power supply and signal leads entering the receiver and through the avoidance of ground loops. In addition, many other natural interfering light sources find their way to our detectors that vary...
