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 FSR 2

中文翻译：

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基于新型双折射和游标效应的光子晶体光纤Sagnac干涉仪的传感特性

本文研究了基于平面相位双折射，零组双折射和游标效应的光子晶体光纤（PCF）Sagnac干涉仪的传感特性。对温度敏感的液体填充到PCF的气孔中。我们实现了平相双折射，灵敏度可达到4.9 nm /°C和3.5 nm /°C。我们发现了一个新现象，即随着环境的变化，在零组双折射波长附近会出现更多的干涉条纹。获得了-19.9 nm /°C和28.6 nm /°C的高灵敏度传感器。我们还演示了基于PCF的两个级联Sagnac干涉仪可以实现Vernier效果。灵敏度为1 nm /°C（单个Sagnac干涉仪）和10.4 nm /°C（级联Sagnac干涉仪），灵敏度提高了十倍以上。灵敏度与自由光谱范围（FSR）1和FSR 2之差成反比