Optical fiber hydrogen sensor based on core diameter mismatch and annealed Pd–Au thin films

We report on optical fiber hydrogen sensors based on absorption changes of evanescent fields caused by a Pd or Pd/Au alloy thin film. The sensor consists of a small piece of standard single-mode fiber (SMF) coated with a palladium or Pd/Au thin film sandwiched between two multimode fibers (MMFs). Due to core diameter mismatch the SMF cladding guides light. When the device is exposed to hydrogen the layer refractive index diminishes and causes attenuation changes of the evanescent fields. Such attenuation changes can be easily detected with a simple transmission measuring setup. Robust sensors with time responses of 15 s and good sensitivity suitable for the detection of hydrogen concentration below the critical 4% are demonstrated.