Phase-Modulation-Amplifying Hollow-Core Fiber Photothermal Interferometry for Ultrasensitive Gas Sensing

Photothermal interferometry (PTI) with hollow core fibers (HCFs) have enabled highly sensitive spectroscopic gas sensors in an all-fiber format. Here we report remarkable improvement in the limit of detection of HCF-PTI, in terms of noise equivalent concentration (NEC), by exploiting the optical-phase-modulation amplifying (OPMA) effect of an HCF resonating cavity. By locking the wavelength of a 1550 nm probe laser to the resonance of a 10-cm-long HCF Fabry-Prot cavity with a finesse of ~700, OPMA of more than two orders of magnitude is achieved, which enables ultra-sensitive gas detection with large dynamic range. With 1654 nm, 1532 nm, and 761 nm pump lasers, we demonstrate detection of methane, acetylene, and oxygen with noise-equivalent-concentration of 15 parts-per-trillion (ppt), 2.7 ppt, and 0.56 parts-per-million (ppm), respectively. Further improvement in NEC is possible by use of a higher finesse cavity with a longer length of HCF. Extension of the technique to other gases, other types of phase or dispersion modulation-based sensors, and other optical resonating cavities is straightforward.