ABSTRACT
Photothermal interferometry is a highly sensitive spectroscopic technique for trace gas detection. However, the performance of the state-of-the-art laser spectroscopic sensors is still insufficient for some high-precision applications. Here, we demonstrate optical phase-modulation amplification by operating a dual-mode optical fiber interferometer at destructive interference for ultrasensitive carbon dioxide detection. With a 50 cm long dual-mode hollow-core fiber, amplification of photothermal phase modulation by a factor of nearly 20 is achieved, which enables carbon dioxide detection down to 1 parts-per-billion with a dynamic range of over 7 orders of magnitude. This technique could be readily used to improve the sensitivity of phase modulation-based sensors with a compact and simple configuration.
ABSTRACT
We demonstrate a compact all-fiber oxygen sensor using photothermal interferometry with a short length (4.3 cm) of hollow-core negative curvature fibers. The hollow-core fiber has double transmission windows covering both visible and near-infrared wavelength regions. Absorption of a pump laser beam at 760 nm produces photothermal phase modulation and a probe Fabry-Perot interferometer operating at 1550 nm is used to detect the phase modulation. With wavelength modulation and first harmonic detection, a limit of detection down to 54 parts per million (ppm) with a 600-s averaging time is achieved, corresponding to a normalized equivalent absorption of 7.7 × 10-8 cm-1. The oxygen sensor has great potential for in situ detection applications.
ABSTRACT
We study photothermal phase modulation in gas-filled hollow-core optical fibers with differential structural dimensions and attempt to develop highly sensitive practical gas sensors with an in-line Fabry-Perot interferometer for detection of the phase modulation. Analytical formulations based on a hollow-capillary model are developed to estimate the amplitude of photothermal phase modulation at low modulation frequencies as well as the -3â dB roll-off frequency, which provide a guide for the selection of hollow-core fibers and the pump modulation frequencies to maximize photothermal phase modulation. Numerical simulation with the capillary model and experiments with two types of hollow-core fibers support the analytical formulations. Further experiments with an Fabry-Perot interferometer made of 5.5-cm-long anti-resonant hollow-core fiber demonstrated ultra-sensitive gas detection with a noise-equivalent-absorption coefficient of 2.3×10-9 cm-1, unprecedented dynamic range of 4.3×106 and <2.5% instability over a period of 24 hours.
