Photothermal Gas Sensing Using a Nanofiber Fabry–Pérot Cavity

We present a method to facilitate photothermal gas sensing by tapering an optical fiber to the nanoscale and then fusion-splicing two fiber Bragg gratings (FBGs) with the nanofiber to form a Fabry-Perot (F-P) cavity. This F-P cavity allows for high-resolution interferometric measurement of photothermal-induced phase variations while reducing the sensor footprint with its compact, in-line design. We used a 2-cm-long, 800-nm-diameter nanofiber to detect CH4 gas, achieving a noise equivalent concentration of 1.58 ppm. The normalized noise equivalent coefficient (NNEA) was <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.1\times 10^{-{8}}$ </tex-math></inline-formula> cm<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${}^{-{1}} \cdot $ </tex-math></inline-formula>W<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cdot $ </tex-math></inline-formula>Hz<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${}^{-{1}/{2}}$ </tex-math></inline-formula>. Compared to hollow core fibers, the nanofiber’s evanescent wave (EW) was directly exposed to the target gas, showing rapid gas exchange and a faster response time. Moreover, a strong photothermal signal was observed in the experiments owing to the high confinement of the evanescent field and the significant temperature coefficient of the fiber glass, exhibiting nearly 20-fold enhancement over hollow core fiber.