Free-spectral range tunable Fabry–Perot filter with superimposed fiber Bragg gratings

A novel free-spectral range (FSR) tunable Fabry–Perot filter (FPF) with two superimposed fiber Bragg gratings (SFBGs) is proposed and experimentally demonstrated. The SFBGs, initially uniform in period with slightly different Bragg wavelengths, were glued onto the lateral surface of a specially-designed cantilever beam and linearly chirped by beam bending-induced strain gradient. As a result, their reflection bands overlap and a grating-based FPF with a tunable FSR is formed. A continuously tunable FSR of 0.1–0.5 nm has been achieved in the experiment.