Hollow-core fiber-based photothermal spectroscopy for hydrogen detection

We present an optical hydrogen sensor based on photothermal spectroscopy with a hollow-core fiber, specifically targeting the 2121.8-nm quadrupole absorption line of hydrogen. Our experiments demonstrate the sensor's capability for detecting hydrogen gas at concentrations as low as 77 ppm, with a lock-in time constant of 1 second. The noise equivalent concentration goes down to 7.2 ppm, with an integration time of 1867 seconds. We have observed a counter-intuitive phenomenon that the photothermal signal decreases with increasing hydrogen concentration at higher concentration levels. Thermodynamics analysis suggests that it is the result of the interplay of molecular collisional relaxation, thermal conduction, and the thermo-optic properties of the gas medium within the hollow-core fiber.