Ultrastable Laser Wavelength Locking Technique Optimized for WMS-Based Methane Detection

Abstract

Fixed wavelength modulation spectroscopy (FWMS) featured as that wavelength is modulated around a constant value instead of a dynamic scanning ramp in the standard wavelength modulation spectroscopy (WMS). To eliminate the influence of center wavelength drift on absorption measurement in the FWMS system, a software-based proportional–integral–derivative (PID) controller is developed to lock the laser wavelength at methane line 1653.72 nm for CH4 detection in this study. Moreover, two crucial parameters of PID, gain and integral time, are optimized to improve the laser wavelength locking performance. As a result, the laser wavelength is locked to the target CH4 line with a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1{\sigma }$ </tex-math></inline-formula> fluctuation less than 406 kHz (better than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${{{3.7} \times }}{{{10}}}^{{-{3}}}{\text {pm}}$ </tex-math></inline-formula> ) with a robust 100% locking success rate, which is the best result reported so far to our knowledge. Based on the wavelength locked FWMS CH4 sensor, linearity is tested to be <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}^{{2}}$ </tex-math></inline-formula> = 0.998 in a low concentration range of 0–1000 ppm. The detection sensitivity is 52 ppb at 0.1 s and could be improved to 36 ppb if the integration time is extended to 1.4 s. Compared with standard WMS, CH4 detection stability is improved by five times.