Page 226 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Measurand Modulation

Measurand Modulation 219

10.3 Concentration Modulation
In the majority of cases we have no control over the absorption strength of the
unknown sample, and can only measure more or less sample as in Figs. 10.3 to
10.5. However, in the case of gas-phase samples, absorption can be modified
through pressure variation, and this can be a very effective technique to
improve sensitivity. The additional difficulty with gas sensing in the infrared is
that the signature of absorption lines is very complicated, but the lines them-
selves are narrow. Hence if a broadband source is used to cover the whole width
of the signature, even if the peak gas absorption strength is large, the reduc-
tion in integrated intensity received on a single detector will be tiny. Laser
spectroscopy gets around this by using a narrow-band source which can be
accurately tuned to an individual absorption line. If the line absorption is
strong, intensity modulation as the laser is tuned across the line will also be
significant. Pressure modulation gets around this by modulating the strength
of all absorption lines simultaneously.
The conceptually straightforward approach uses a reciprocating piston to
compress the unknown sample (Fig. 10.6). With suitable design of the piston
and transmission cuvette, compression ratios of 10 to 100:1 can be achieved,
and with it a similar variation in the absorption. Such systems for pressure-
modulation spectroscopy have been designed for sensitive gas sensing. The
mechanical complexity and perceived relative unreliability of conventional
piston systems can be overcome using acoustic resonant cells (Fig. 10.6b). These
are just “organ pipes” with the optical path arranged at an antinode of the
standing pressure wave. Excitation can be via piezoelectric or electromagnetic
actuators (loudspeakers), making use of the quality-factor (Q) of the resonance
to increase the magnitude of pressure variation for a fixed input power. It is a
pity that the incompressibility of liquids denies us this attractive modulation
technique.

(a) (b)
Transducer
Pressure
Organ wave
pipe

DP
Source Compressed DP
sample gas Large pressure
fluctuations
Figure 10.6 Pressure modulation of gas absorption using a recipro-
cating piston (a) and a resonant “organ-pipe” (b).

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