Heterodyne interferometric photothermal spectroscopy for gas detection in a hollow-core fiber

Abstract

Photothermal spectroscopy with the optical pump-probe configuration has been used for sensitive gas detection in hollow-core fibers. In this study, we demonstrate a new method of heterodyne interferometric photothermal gas sensing with a simplified sensor design and enhanced sensor stability. As a proof-of-principle, we used an interband cascade pump laser at 3.6 μm for nitrous oxide (N2O) detection in a hollow-core anti-resonant fiber (HC-ARF). A Mach-Zehnder interferometer (MZI) with a 1.5 μm near-infrared probe laser was implemented to measure the photothermal-induced refractive index change inside the HC-ARF. The probe laser was split into two beams, one transmitted through the HC-ARF and one frequency-shifted by using an acoustic-optic modulator. Both beams were recombined to generate a beat note signal, which was then demodulated by two cascaded lock-in amplifiers to extract the photothermal signal. Based on the first harmonic detection of wavelength modulation, we achieved a normalized noise equivalent absorption (NNEA) coefficient of 7.7×10−9 cm-1WHz-1/2. Compared to conventional photothermal gas sensors with the MZI configuration, our new detection scheme eliminates the sophisticated opto-mechanical stabilization system and exhibits the intrinsic immunity to the power fluctuation of the probe laser.