Theoretical and Experimental Investigation of Fiber-Ring Laser Intracavity Photoacoustic Spectroscopy (FLI-PAS) for Acetylene Detection

An acetylene (C₂H₂) sensor was developed based on the fiber-ring laser intracavity photoacoustic spectroscopy (FLI-PAS) by incorporating a photoacoustic cell into the fiber-ring laser cavity to obtain the enhanced photoacoustic signal. A detailed theoretical model was constructed to describe the processes involved in FLI-PAS and to understand the factors affecting the sensor performance. Wavelength modulation spectroscopy with the second harmonic detection was implemented to increase the detection signal-to-noise ratio by using a custom-designed fiber Bragg grating wavelength modulator. We used this FLI-PAS sensor for C₂H₂ detection at 1531.59 nm. With the modulation amplitude and intracavitary power optimized, our sensor achieved a noise equivalent concentration of 36 ppbv at the 118-s integration time and a linear dynamic range of ~10⁵ . The limitations of the current FLI-PAS sensor were discussed in detail to explore feasible ways for further improvement in sensitivity and linear dynamic range.