Effect of the Pd–Au thin film thickness uniformity on the performance of an optical fiber hydrogen sensor

Thin alloy film of Pd and Au, formed by simultaneous electron-beam and thermal evaporation techniques, respectively, is used in the design of an optical fiber hydrogen sensor. The sensor consists of a multimode fiber (MMF) in which a short section of single mode fiber (SMF), coated with the Pd–Au thin film, is inserted. Due to core diameter mismatch, the SMF cladding guides light, allowing the interaction between the sensing layer and the guided light. When the sensor is exposed to hydrogen, the Pd–Au layer refractive index diminishes and causes attenuation changes on the transmitted light. Several samples with different layer thickness uniformity were fabricated and tested in a very simple experimental set-up. We have observed that the sensor signal change is dependant on layer thickness uniformity, since the effective interaction length between the evanescent field and the sensing layer is increased. By contrast, such uniformity practically has no influence on the time response of the sensor. The resulting Pd–Au film can detect 4% hydrogen with a response time of 15 s.