Optimization of calibration-free wavelength modulation spectroscopy technique for gas parameter measurement

The residual amplitude modulation (RAM) method, which is the simplest wavelength modulation spectroscopy (WMS) method for calibration-free measurement of gas concentration and pressure, can be optimized by selecting a specific modulation frequency of the laser diode. If the laser diode is modulated at its phase quadrature frequency (fπ/2) at which the phase difference, , between the intensity modulation (IM) and frequency modulation (FM) is 90°, the lineshape–bearing signal of interest is maximized for a given modulation index (m), instead of being scaled by . The 1650nm laser diode used in this study has a fπ/2 of 100kHz which is an order of magnitude lower than values reported in the literature and well within the range of low-cost electronics. This aspect of optimizing the RAM method has not yet been reported in the literature perhaps because the RAM method is a fairly recent development. This paper demonstrates time-varying calibration-free measurement of methane concentration and pressure with the laser diode modulated at its phase quadrature frequency. The time resolution of measurements is currently 10s which is suitable for many industrial processes. The time resolution can be improved by using dedicated data acquisition and processing hardware.