Wavelength modulation spectroscopy with tunable diode lasers : a calibration-free approach to the recovery of absolute gas absorption line-shapes
Tunable diode laser spectroscopy (TDLS) has become the preferred option for industrial gas monitoring. TDLS with direct detection provides absolute measurement of a rotational / vibrational gas absorption line transmission function, facilitating the extraction of gas concentration (from line strength measurement). TDLS with wavelength modulation spectroscopy (WMS) enables AC detection of absorption line derivatives at frequencies where laser and 1/f noise is reduced. Coupled with lock-in detection, this provides a sensitivity improvement of up to 2 orders of magnitude. At fixed temperature and pressure, calibration to signals measured on a known gas composition has been used successfully to determine system scaling factors. However, demand has grown for gas monitoring in environments where the gas pressure is constantly varying and unknown. This introduces significant errors in the analysis as the primary system scaling factor is a function of linewidth, which is varying with the unknown pressure. Errors also arise from the inaccuracies in determining a number of instrument scaling factors, including the AM and FM characterisation of the laser. Pressure measurements may be made and the errors in concentration corrected, if the gas absorption linewidth can be accurately measured from the recovered signals and the instrument scaling factors can be accurately determined. However, the lack of accurate in-situ wavelength referencing schemes for use in the field, make linewidth measurement extremely difficult. Add to this the fact that conventional TDLS/WMS measurements are prone to systematic interference and the errors accumulated from inaccurate instrument scaling (noted above) and linewidth measurement, could determine a large final error on the derived concentration and/or pressure. This work reports the proposal, development and validation of both an in-fibre wavelength referencing scheme and a new technique for measuring the absolute absorption line transmission function using TDLS with WMS. Measuring the absolute absorption line transmission profile, as a function of the laser's wavelength scan across the absorption line, facilitates the extraction of the gas concentration and pressure via comparisons to theory (based on HITRAN data). Through novel signal processing techniques, the approach is free from systematic distortion and is absolute without the need for calibration. This new approach provides many of the benefits of TDLS/WMS, whilst offering the simplicity and accuracy of TDLS with direct detection. The promising results show that we have significantly advanced TDLS technology towards realising a stand-alone instrument for determining accurate gas composition measurements in harsh industrial environments.