Gas sensing with mode-phase-difference photothermal spectroscopy assisted by a long period grating in a dual-mode negative-curvature hollow-core optical fiber

Abstract

We demonstrate sensitive gas detection with mode-phase-difference photothermal spectroscopy assisted by a long period grating (LPG) inscribed on a dual-mode negative-curvature hollow-core fiber (NC-HCF). The LPG is inscribed using a pulsed CO2 laser, which enables pump propagation in the fundamental LP01 mode to achieve maximum photothermal phase modulation while exciting both the LP01 and LP11 modes at the probe wavelength to form a dual-mode interferometer for detection of the phase difference. With a 1533 nm pump and a 1620 nm probe, a noise equivalent concentration of ∼2.2 ppb acetylene is achieved with an 85-cm-long NC-HCF gas cell and 1 s lock-in time constant.