Investigations concerning the development of a fibre optic pressure sensor are reported. The phenomenon of light power attenuation in a periodically deformed graded index fibre has been studied theoretically. It appears that available theories are not suited to describe this attenuation process in the case of strong fibre deformations. We therefore suggest an approach which takes account of a correlation length in such strong attenuation processes. Experimental results, obtained with a newly developed test device, are given and are compared with this theory and with former results. It is concluded that the attenuation process is understood qualitatively, but that further research is needed to obtain a more detailed description of the observed behaviour.