Grating writing model for materials with nonlinear photosensitive response

This paper Presents a model for the growth of grating devices in materials with a nonlinear photosensitive response to incident radiation. We derive an expression for the written refractive index modulation in the material in terms of this photosensitive response and the incident fluence profile along the device. We investigate the validity of the model experimentally using gratings written in two optical fibers with very different photosensitivity response functions. These gratings are characterized both spatially and spectrally, through direct measurements of the refractive index modulation and the analysis of the spectral filter response. The results show good agreement with the model. We extend the analysis to show how to compensate for the nonlinear material response in order to achieve a desired grating design in a given material. This compensation method is used to improve the performance of a dispersion flattened grating device to display both high bandwidth utilization and low group delay ripple over the reflection band.