Reduction of the diffraction in strongly resonant thin-film Fabry-Perot filters between single-mode fibers

Simulations show that the cumbersome diffraction losses occurring in narrow bandpass thin-film Fabry-Perot filters (TF-FP filters) inserted between two narrow-core optical fibers are strongly reduced by introducing into the filter a pair of film materials with high refraction index contrast. They are even further reduced by transforming the optically resonant layers of the filter into gradient index photo-refractive resonant lenses. This permits the design of compact TF-FP filters between fibers with passbands as narrow as those of TF-FP filters deposited on bulk substrates.