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

## TL;DR

A new gas sensor using nanofibers detects gases like acetylene with high sensitivity and stability for environmental and industrial use.

## Contribution

A novel polarization-mode photothermal interferometry technique in a single-mode nanofiber coupler significantly improves gas detection sensitivity.

## Key findings

- The sensor achieved an acetylene detection limit of 6 ppb.
- It maintained stability below ±1.2% over 30 hours.
- The design uses standard fused directional coupler technology for cost-effectiveness.

## Abstract

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.