Miniaturized Photoacoustic Trace Gas Sensing Using a Raman Fiber Amplifier

This paper presents the development of a Raman fiber amplifier optical source with a maximum output power of 1.1 W centered around 1651 nm, and its application in miniaturized 3D printed photoacoustic spectroscopy (PAS) trace gas sensing of methane. The Raman amplifier has been constructed using 4.5 km of dispersion shifted fiber, a 1651 nm DFB seed laser, and a commercial 4 W EDFA pump. The suppression of stimulated Brillouin scattering (SBS) using a high-frequency modulation of the seed laser is investigated for a range of frequencies, leading to an increase in the optical output power of the amplifier and reduction of its noise content. The amplifier output was used as the source for a miniature PAS sensor by applying a second modulation to the seed laser at the resonant frequency of 15.2 kHz of the miniature 3D printed gas cell. For the targeted methane absorption line at 6057 cm^-1, the sensor system performance and influence of the SBS suppression is characterized, leading to a detection limit (1σ) of 17 ppb methane for a signal acquisition time of 130 s, with a normalized noise equivalent absorption coefficient of 4.1 · 10^-9 cm^-1 W Hz^-1/2 for the system.