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Optical Sources, Detectors, and Systems by Robert H. Kingston

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Chapter 4
The Ideal Photon Detector and Noise
Limitations on Optical Signal
Measurement
In the previous chapters we have treated the theory and behavior of
the two significant types of optical or infrared source, first, the blackbody
or a diffuse reflector, and second the laser. We now consider the opti-
mum means of detection for the radiation from the sources, realizing that
our detector, with rare exceptions will be situated at the focus of an
optical receiver which in its simplest form is a lens. Historically, the first
radiation sensors were thermal detectors, that is, devices whose temper-
ature was a function of the incident optical power, such temperature
then being electrically sensed to determine the power level. Actually,
Herschel, in an early experiment that established the existence of infrared
radiation, used a simple glass thermometer with a blackened reservoir
bulb.
Current thermal detectors are commonly used where sensitivity
and speed of response are not critical and they depend on a temperature-
induced voltage (thermocouple), resistance (bolometer), or dielectric
polarization (pyroelectric)
(Kingston,
1978,
Ch.
7 ). A distinct advantage
of the thermal detector is its broad wavelength response. As long as the
"blackening" of the element produces appreciable absorption at the
wavelength of interest, the detector will respond efficiently. Of course,
the detector size must be many wavelengths so that it encompasses the
diffraction-limited focal spot of the optical (or infrared) collection system.
In this text, we are interested in the ultimate performance of optical
detection systems and will limit our detailed discussions to the "photon"
detector, a device that ideally extracts the maximum amount of inform-
ation available from the incident radiation. Thus far, we have treated in
detail both thermal sources as well as coherent or laser sources. Any of
these sources produces radiation by either spontaneous or stimulated
downward
energy transitions which release packets of energy, hv, to the
electromagnetic field. We now consider the absorption process and in
particular a process where an upward transition of an electron absorbs a
quantum of energy and we are able to sense the presence of the electron
73

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