Page 100 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Source: Photodetection and Measurement

Chapter
5

System Noise and Synchronous Detection

5.1 Introduction

In Chap. 3 the fundamental noise contributions to our signal, shot noise of cur-
rents 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 compo-
nents 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 500Hz. The voltage and current noise
spectral densities of both discrete transistors and opamps show this 1/f charac-
ter, 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 in
intensity at a low frequency. These include sunlight (1/24 hour frequency), sun-
light modulated by clouds (ª1/minutes frequency) and flashing television and
computer screens (25 to 120Hz frame rate, 15 to 150kHz line rate).
All this suggests that measurements should be made, if possible, far away
from the low-frequency region 50 to 500Hz. Electrical interference can be fairly
quiet in the 3 to 50Hz region, but 1/f noise is large and the low frequencies
make measurements done there rather slow. Shifting the measurement of

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