Ultrasensitive sensing in air based on Sagnac interferometer working at group birefringence turning point

In this paper, a gas refractometer based on microfiber Sagnac interferometer is demonstrated, which can achieve an ultrahigh sensitivity when operating at the group birefringence turning point. We undertake a theoretical analysis and a simulated calculation to study the device characteristics and obtain the specific parameters of ellipticity and long axis of the elliptic microfiber for the group birefringence turning point. In the experiment, we obtain a positive sensitivity of 0.295 nm/KPa and a negative sensitivity of -0.219 nm/KPa during gas pressure and refractive index (RI) sensing, the obtained highest RI sensitivity can reach 160,938.9 nm/RIU. To further reveal its practical potential in gas detection, we conduct CO₂ gas concentration detection and the device also demonstrates ultrahigh sensitivity and good repeatability. Besides, temperature sensing is performed to explore its temperature response wherein it shows a sensitivity of 486.7 pm/ °C. These results show its potential for use in gas- and acoustic-sensing applications.