Ppb-level methane detection based on a homemade Raman fiber amplifier and differential photoacoustic technology

A high-power near-infrared (NIR) wavelength-modulated differential photoacoustic spectroscopy (WM-DPAS) sensor for parts-per-billion (ppb) level methane (CH4) detection was reported by using a homemade Raman fiber optical amplifier (RFA). A 1653.7 nm distributed feedback (DFB) laser with a max output power of 21 mW was employed as a seed source to excite a high light power of 588-mW through the RFA, which greatly improved the sensor performance. A differential PA cell composing of two identical resonators with a length of 90 mm and a diameter of 6 mm was used to remove the background noise. Wavelength modulation spectroscopy (WMS) and 2nd harmonic (2f) demodulation techniques were applied for further improving the signal-to-noise ratio (SNR) of the PA signal. A 1σ measurement detection limit (MDL) of ~10 ppb for methane (CH4) detection was achieved with an integration time of 10 s. A long-term stability of the sensor was evaluated by an Allan deviation analysis which demonstrated the good stability of the PAS sensor. Finally, a measurement sensitivity of 0.17 ppb was achieved when the integration time was set to ~100 s.