Phase-Modulation-Amplifying Hollow-Core Fiber Photothermal Interferometry for Ultrasensitive Gas Sensing
Haihong Bao, Wei Jin, Yingzhen Hong, Hoi Lut Ho, Shoufei Gao, Yingying Wang
Abstract
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-Pérot 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.