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 ${{\rm CO}_2}$ laser, which enables pump propagation in the fundamental ${{\rm LP}_{01}}$ mode to achieve maximum photothermal phase modulation while exciting both the ${{\rm LP}_{01}}$ and ${{\rm LP}_{11}}$ 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 ${\sim}2.2$ ppb acetylene is achieved with an 85-cm-long NC-HCF gas cell and 1 s lock-in time constant.
© 2020 Optical Society of America
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