Phase-Sensitive Detection Electronics for Wavelength Modulation Spectroscopy Experiments

Wavelength modulation spectroscopy (WMS) is a non-intrusive detection technique, which enables the measurement of several parameters such as pressure, concentration, and temperature of a species of interest by analyzing the latter's absorption lineshape profile. The method employs a synchronous modulation and detection apparatus to increase the signal to noise ratio and hence the sensitivity of the measurement. Phase-sensitive detection and demodulation of the signal is performed at the modulation frequency and its harmonics. Research at ODU demonstrates that each particular measurement can be performed optimally at an appropriate harmonic of the modulation frequency. The most suitable harmonic depends on many factors, including the particular noise environment in which the measurement will be performed, as well as the type of measurement and therefore the nature of the information to be extracted. This work describes the design and implementation of electronic circuits used to perform detection at several harmonics simultaneously. Such simultaneous detection in real-time improves the sensitivity and the precision of the measurements. The detection channels were constructed, tested, and successfully incorporated into the current ODU wavelength modulation spectroscopy apparatus, allowing three channels, operating at different harmonics to perform measurements of a species absorption lineshape profile simultaneously.