Optical fiber sensor have shown a great potential to become chemical sensors and have appropriate properties as long term stability, immune to electromagnetic radiation and multiplexing capability. In this work we present a way of measure the hydrogen present with just optic fiber as sensing device. The hydrogen enters the fiber following a diffusion behaviour. This hydrogen generates absorption peaks at several frequencies. We study that frequencies and determine witch ones are appropriate to use for a hydrogen sensor. We use the data collected to do a model of the fiber and simulate and validate that model with the experiments. We conclude the analysis with the advantages of that kind of sensor in a nuclear waste repository.