Hollow-core fiber assisted high sensitivity spectroscopic gas sensing

Abstract

Among various gas sensing technologies, laser absorption spectroscopy (LAS) stands out due to its advantages in sensitivity and selectivity. Hollow-core fiber (HCF) has been proven to be an efficient platform for light-gas interactions. Although HCF-based LAS was first demonstrated in 2004, its sensitivity long remained at the tens of parts per million (ppm) level. In 2015, the first HCF-based gas sensor with parts-per-billion (ppb) level sensitivity was demonstrated using photothermal spectroscopy (PTS) with near-infrared (NIR) pump. It is well known that gas molecules have stronger absorption in the mid-infrared (MIR) due to fundamental vibrational absorption. Thus, we employ a MIR pump to access the strongest absorption lines for high sensitivity but still use a NIR probe for cost-effective high-performance fiber-optic circuits. We demonstrate the use of a HCF Fabry-Perot interferometer (FPI) to detect photothermal (PT) phase modulation. A 14-cm-long anti-resonant HCF is used and ethane detection with a limit of detection (LOD) of 2.6 ppb with 410 s averaging time is achieved. Through noise analysis, we have concluded that the FPI structure requires a probe laser with narrow linewidth. Apart from the single-component gas detection, we also demonstrate simultaneous detection of multiple gases with FPI-PTS using a single HCF gas cell. Our system utilizes three pump lasers with wavelengths of 1.39 μm, 2.00 μm, and 4.60 μm, corresponding to the absorption lines of H2O, CO2, and CO. These lasers were frequency-division multiplexed (FDM) to generate PT phase modulations simultaneously at different frequencies. A common probe FPI operating at 1.55 μm is used to detect the phase modulations. This simple system combines multiple pump beams from NIR to MIR for the most efficient interaction among the gas sample, the pump beams, and the probe beam in the same gas cell, minimizing the usage of expensive MIR components. Phase noise is dominant due to the long optical path difference (OPD) between the interference beams in FPI structure. We then demonstrate the use of an in-fiber dual-mode interferometer (DMI) as the probe. The DMI has a much smaller OPD, and hence, smaller phase noise level. A gas sensing method named MIR reflective DMI-PTS is then demonstrated. With a 1.8-m-long double-pass HCF gas cell and a 3.27 μm pump source, a noise-equivalent absorption coefficient (NEA) of 2.16×10-9 cm-1 is achieved. According to the noise analysis, this system is near shot-noise limited after using a balanced detector. For gases with no strong optical absorption but Raman active, Raman spectroscopy offers a solution for tracing them. We demonstrate cavity-enhanced stimulated Raman gain spectroscopy (CE-SRGS) for hydrogen detection. The combination of a high-finesse cavity with a HCF has enabled us to demonstrate high sensitivity hydrogen detection with a short sensing length. With the Stokes light locked to an 8-cm-long HCF cavity of 300 finesses, the enhancement factor of 100 has been achieved. Targeting the Q1(1) transition, we achieved a LOD of 62 ppm using 1 s time constant at 1 atm, making our results comparable with the best existing hydrogen sensors.