Temperature-insensitive hydrogen sensor with polarization-maintaining photonic crystal fiber-based Sagnac interferometer

We demonstrated a temperature-insensitive hydrogen (H₂) sensor based on polarization-maintaining photonic crystal fiber (PM-PCF). The sensing element is a section of PM-PCF coated with Pd/Ag composite film and is incorporated into a single mode fiber Sagnac loop interferometer. The birefringence of PM-PCF is modulated by H₂ absorption that induces deformation of the Pd/Ag composite film, resulting in shift of the interference spectrum at the output of the Sagnac interferometer. A magnetron facing target sputtering technique with special Pd/Ag rectangular target structures is developed to deposit Pd/Ag composite film on the cylindrical surface of the PM-PCF and the atomic ratio of Pd/Ag composite film is controlled by setting appropriate volume ratio in rectangular target. An empirical linear relationship between atomic ratio and volume ratio was obtained and the H₂ sensing capacity with different atomic ratio of Pd/Ag composite films was investigated experimentally. The results showed that increasing the proportion of Pd in the composite film improves the H₂ detection sensitivity. The sensitivity can also be improved by using as longer length of Pd/Ag coated PM-PCF. With ∼100 mm long coated PM-PCF as the sensing element, we obtained ∼1.310 nm shift in the interference spectrum for H₂ concentration from 0 to 1%. The sensitivity coefficient is ∼131 pm% in the range of 1% to 4% H₂ concentration. The sensor has low temperature dependence and good repeatability.