Humidity-Induced Effects on Polymer Planar Bragg Gratings

The increasing dissemination of polymer materials in optics demands for a thorough understanding of their optical properties and their dependencies on ambient conditions. Here, we report on the influence of humidity on polymer planar Bragg gratings (PPBGs) in bulk polymethylmethacrylate (PMMA). We identify the contributions of both the humidity-induced variation of the grating period and the refractive index, which in turn results in a spectral shift of the Bragg wavelength. It is demonstrated that humidity saturated PPBGs show a change of the grating period as well as a rise of the refractive index resulting in a strong red shift of the reflection wavelength of about 4.24 nm. Furthermore, Bragg gratings are written into humidity saturated PMMA. After dehydration of the PMMA, the Bragg wavelength shows a permanent blue shift of 4.19 nm as compared with conventionally written Bragg gratings in dry PMMA.