In this work, a reflective optical fiber bundle sensor with double paths compensation has been realized to measure the response characteristics of the Pd–Y film when loading/unloading hydrogen in various concentrations. Fig. 2 shows the schematic diagram of the gas sensor system. A 7 mW high-power red LED is utilized to provide computer-controlled light pulse through a coupled opti-cal fiber bundle. The light is split in two beams as testing signal and reference signal by the fiber bundle, and then projected to two chambers separately. The reflected signals from the two chambers are collected by their respective silicon-photo diode. After a noise-compensated algorithm, the signal can be expressed as follows: