Free spectral range variations of grating-based Fabry–Perot filters photowritten in optical fibers

Abstract

We fabricated grating-based Fabry–Perot (FP) interferometers by photowriting a pair of Bragg gratings with a spacing of a few millimeters in a germanosilicate fiber. The measured free spectral range (FSR) of the FP filters is found to vary with the frequency in the spectral range of the filters. Relative FSR variation reaches values close to 10%. An analytical model is presented and compared with a coupled-mode theory in a matrix approach. The computations and the experimental results demonstrate that the main FSR variation (10%) is due to the spectral modification of the reflection coefficients of the two gratings. The relative FSR variation that is due to the propagation effective-index modification is found to be close to 10−5. A comparison between a grating-based FP filter photowritten in a fiber and a bulk FP filter is presented.