Hollow-core fiber-based mid-infrared photothermal spectroscopy for multi-component gas sensing

We report a mid-infrared (MIR) photothermal laser spectroscopy system for multi-component gas sensing with a single broadband anti-resonant hollow-core fiber gas cell. Three MIR pump lasers are used as the excitation sources to generate photothermal phase modulation and a near-infrared probe laser is employed to detect the photothermal phase modulation by using a Fabry-Perot interferometer configuration. Simultaneous detection of carbon monoxide (CO), carbon dioxide (CO2), ethylene (C2H4), and ethane (C2H6) are experimentally demonstrated using time-division multiplexing. A noise-equivalent concentration (NEC) of 57 parts-per-billion (ppb) for CO, 300 ppb for CO2, 430 ppb for C2H4 and 73 ppb for C2H6 are achieved with 1 s lock-in time constant. Allan deviation analysis reveals that the NEC can be reduced to 35, 120, 310, and 49 ppb for CO, CO2, C2H4 and C2H6, respectively, with an averaging time of 280 s. The dynamic range of the system is measured to be more than 4 orders of magnitude for all of the four gas components.