Abstract
In this paper, the sensing characteristics of a photonic crystal fiber (PCF) Sagnac interferometer based on flat-phase birefringence, zero-group birefringence and the Vernier effect have been studied. The temperature-sensitive liquid is filled into the air holes of the PCF. We realize flat-phase birefringence, by which the sensitivity can reach 4.9 nm/°C and 3.5 nm/°C. We find a new phenomenon in that there are more interference fringes appearing near the zero-group-birefringence wavelength with the environment changing. Highly sensitive sensors with −19.9 nm/°C and 28.6 nm/°C are obtained. We also demonstrate that two cascaded Sagnac interferometers based on PCF can realize the Vernier effect. The sensitivities are 1 nm/°C (single Sagnac interferometer) and 10.4 nm/°C (cascaded Sagnac interferometers), and the sensitivity is improved by more than 10 times. The sensitivity is inversely proportional to the difference between free spectral range (FSR)1 and FSR2, and the highest sensitivity reaches 23.2 nm/°C (−58 500 nm/RIU, resolution: 3.419× 10−7 RIU) by optimizing the PCF length in Sagnac interferometer 2. The average sensitivity based on two cascaded PCF Sagnac interferometers is up to −309 957 nm/RIU and the resolution is 6.453× 10−8 RIU as the refractive index of the analyte varies from 1.33 to 1.331, which is very competitive in the field of chemical sensing.
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