Page 239 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Measurand Modulation

232 Chapter Ten

10.9 Summary
We have shown that the ultimate sensitivity of many optical instruments is
restricted to a poor value not by optical-system measurement noise, but by low-
frequency drifts in system gain. Source modulation does nothing to reduce these
errors. They can be orders of magnitude greater than expected from the elec-
tronic noise of a typical shot-noise limited detection system, which suggests that
the noise spectrum cannot be flat, and drift is not simply low-frequency Gauss-
ian noise. If the noise spectral density increases at low frequencies, the longer
we wait, the greater will be the error, and we can improve the ultimate sensi-
tivity (LOD) by modulating the optical measurand at sufficiently high fre-
quencies. The most promising techniques involve electronic modulation of the
physical measurand, ideally performed within the instrument at the point of
detection. However, to best use these systems, we will need a much better
understanding of the noise and drift performance over a wide range of
frequencies.

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.