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 (CO
₂
), ethylene (C
₂
H
₄
), and ethane (C
₂
H
₆
) are experimentally demonstrated using time-division multiplexing. A noise-equivalent concentration (NEC) of 57 parts-per-billion (ppb) for CO, 300 ppb for CO
₂
, 430 ppb for C
₂
H
₄
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, CO
₂
, C
₂
H
₄
and C
₂
H
₆
, 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.