Wavelength-stabilization-based photoacoustic spectroscopy for methane detection

A compact and portable photoacoustic gas sensor was developed for sensitive methane (CH 4 ) detection at 1.6 µ m using a software-based wavelength stabilization scheme. A transmission-type photoacoustic cell was connected in series with a reference gas cell to measure the photoacoustic signal and the reference gas absorption for wavelength stabilization simultaneously. The central wavelength of the diode laser was locked to the target CH 4 line with a fluctuation of less than 10.6 MHz using a digital proportional–integral–derivative controller. The CH 4 sensor was designed to be insensitive to the incoherent external acoustic noise by the cumulative average of the demodulated photoacoustic signal by a digital lock-in amplifier. With an incident laser power of 6 mW, our CH 4 sensor achieved a minimum detection limit of 11.5 ppm at 10 s response time and an excellent linearity ( R 2 = 0.9999) in the concentration range of 400–6300 ppm.