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

摘要

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.