Ultrasensitive Gas Detection via Polarization-Mode Photothermal Interferometry in a Single-Mode Nanofiber Coupler.

Optical nanofibers (ONF) have emerged as versatile platforms for studying light-gas interactions at the micro/nanoscale, yet existing ONF gas sensors remain limited in detection sensitivity. Here, we report a polarization-mode photothermal interferometry technique that precisely measures the gas absorption-induced phase difference between two polarization states of the symmetric supermode of a single-mode ONF coupler. The high power density and large evanescent field associated with the ONF coupler enhance the efficiency of photothermal phase modulation, while the strong waveguide birefringence and noise-immune differential phase detection confer environmental immunity, jointly yielding an order-of-magnitude enhancement in the signal-to-noise ratio. With a 2 cm-long overcoupled ONF coupler, we achieved an acetylene detection limit of 6 ppb and an instability below ± 1.2% over 30 h. This compact ONF gas sensor, based on standard fused directional coupler technology, provides a promising route toward cost-effective and high-performance solutions for environmental monitoring and industrial applications.