Gas Detection via Absorption-Induced Phase Shift Retrieved by Phase Demodulation in Intensity-Modulated Absorption...

The detection sensitivity of laser absorption spectroscopy is inherently constrained by intensity noise, making conventional amplitude-demodulation techniques based on direct optical power measurements susceptible to laser instability. To overcome this limitation, a novel gas sensing strategy was proposed, in which gas concentration information was encoded in the optical phase rather than intensity, enabling intrinsic immunity to intensity noise. Specifically, the laser intensity was modulated and transmitted through the target gas. By demodulating the phase of the first-harmonic (1f) component of the transmitted signal, the phase shift induced by gas absorption was extracted. This phase variation, arising from the absorption-associated dispersive response of the medium, enables accurate quantification of the target gas concentration. Acetylene (C2H2) was selected for experimental validation. The system performance across a wide dynamic range was investigated using a 20 cm single-pass cell and a high-finesse optical resonator of CEAS for high- and low-concentration regimes, respectively. The system achieved detection limits of 0.9 ppm and 18.6 ppb in the high- and low-concentration regimes, respectively, with sensitivity improvements of 7.1-fold and 3.6-fold compared with conventional intensity demodulation schemes. These results demonstrate that the proposed approach delivers superior stability and sensitivity, providing a robust solution for high-sensitivity trace gas detection.