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 ONFgassensors 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 super mode 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 wave guide 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 2cm-long over coupled ONF coupler, we achieved an acetylene detection limit of 6pp band an instability below ±1.2% over 30h. 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.