Page 99 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Interlude: Alternative Circuits and Detection Techniques

92 Chapter Four

The methods of Fig. 4.10 are primarily static techniques, because the thermal
time constant of the sample heating process is rather long. However, if the
absorbed beam is modulated, it is possible to detect the modulation as sound.
The techniques are called photoacoustic detection photometry. The sound field
is still generated by expansion, so it is advantageous to use solvents with a high
expansion coefficient. In water this is maximized at 4°C. For gas detection con-
ventional electret or condenser microphones can be used. For liquids and solids
custom-built piezoelectric transducers are more common. The majority of pub-
lished work uses high-power pulsed lasers to excite the absorption, although it
is possible to obtain similar performance with continuously modulated sources,
including LEDs (see Hodgkinson, 1998). This broadens the range of wave-
lengths at which absorption can be measured without going to the complexity
and expense of tunable lasers. Using a microphone detector the technique is
dark-field, with zero signal at zero absorption. In principle, all these paramet-
ric techniques can deliver higher detection signal-to-noise than can direct detec-
tion. In practice, this is far from easy due to spurious absorption in optical
windows, so that equivalent performance can usually be obtained using high
stability and low-noise transmission measurement or the measurand
modulation techniques of Chap. 10.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.