Introduction
Hardware Installation and Maintenance Manual 1
–7
Figure 1–3 Typical normalized absorption signal from a
laser diode absorption spectrometer
Note that contamination of the mirrors results solely in lower overall signal.
However, by tuning the laser off-resonance as well as on-resonance and
normalizing the data, the technique self calibrates every scan resulting in
measurements that are unaffected by mirror contamination.
Wavelength modulation spectroscopy (WMS) signal detection
SpectraSensors takes the fundamental absorption spectroscopy concept a step
further by using a sophisticated signal detection technique called wavelength
modulation spectroscopy (WMS). When employing WMS, the laser drive
current is modulated with a kHz sine wave as the laser is rapidly tuned. A lock-
in amplifier is then used to detect the harmonic component of the signal that
is at twice the modulation frequency (2f), as shown in Figure 1–4. This phase-
sensitive detection enables the filtering of low-frequency noise caused by
turbulence in the sample gas, temperature and/or pressure fluctuations, low-
frequency noise in the laser beam or thermal noise in the detector.
1.0
0.99
0.98
0.97
0.96
0.95
Normalized Absorption Signal
Wavelength [a.u.]
Wavelength [a.u.]
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