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 mu m 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 mu m wavelength yielded a noise equivalent absorption of 2.25 center dot 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.

Automated summary

A novel photothermal gas sensor configuration is demonstrated in this work. By analyzing the phase shift of the self-heterodyne beatnote signal of a mode-locked laser, the gas refractive index modulation caused by an auxiliary laser excitation is efficiently detected. IQ demodulation combined with Wavelength Modulation Spectroscopy-based signal analysis is used to optimize and simplify spectroscopic data acquisition and analysis. A proof-of-concept experiment for carbon dioxide detection at 2 µm wavelength shows 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 with the appropriate excitation source to induce the photothermal effect.