The theoretical study and the experimental realization of an ultranarrow bandpass filter, joining a fiber Bragg grating and a dielectric mirror directly deposited at the extremity of the fiber tip, is presented. The features of such a filter are in very good accordance with the results of theoretical simulations.

Fabry-Perot bandpass filters made of mirrors with both high- and low-Dn (refractive-index modulation) have simultaneously a broad rejection band and a narrow passband. The higher Dn's are obtained with multilayer mirrors and the lower with Bragg gratings (BG's). Implementation of a sampling calculation technique based on the characteristic matrix formalism used for interference coatings allows for simulation of hybrid filters constructed from multilayer mirrors and BG's. The possible defects of hybrid filters are extensively analyzed. Bandpass filters made purely of both high- and low-Dn BG's are also simulated. All these filters are useful for wavelength division multiplexing applications.

A rigorous modal three-dimensional calculation method is used to analyze the diffraction and absorption losses in Fabry-Perot (FP) multilayer interferential filters between both single-mode input and output fibers. With an ordinary personal computer, this method affords accurate calculation of the absorption losses and the diffraction into the cladding modes of both fibers as well as the reflectance into the input fiber. The calculation results show that absorption losses and diffraction are troublesome for high-finesse FP filters.

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.

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