Photothermal Gas Detection Using a Mode-Locked Laser Signal Readout

In this work, a novel configuration of a photothermal gas sensor is demonstrated. The measured gas sample is delivered to a gas cell placed inside the linear cavity of a 1.55 μm mode-locked fiber laser. The gas refractive index modulation resulting from the excitation by an auxiliary continuous wave laser is efficiently probed by analyzing the phase shift of the self-heterodyne beatnote signal of the mode-locked laser. IQ demodulation combined with Wavelength Modulation Spectroscopy-based signal analysis was employed to optimize and simplify the spectroscopic data acquisition and analysis. A proof-of-concept experiment with detection of carbon dioxide at 2 μm wavelength yielded a noise equivalent absorption of 2.25·10⁻⁷ for 1000 s integration time. The proposed sensor configuration is versatile and can be used to probe any gas molecule, provided an appropriate excitation source is used to induce the photothermal effect.