Optical circuits for equalizing group delay dispersion of optical fibers

Optical circuits are synthesized for equalizing the group delay dispersion of single-mode fibers. The transfer function of the equalizing circuits are given by Chebyshev polynomials of the second kind. The various realization methods for the group delay equalizer are shown, including periodic structures using birefringent crystals, birefringent fibers, and Mach-Zehnder interferometric planar optical circuits. An optical equalizer employing TiO₂ birefringent crystals was fabricated and evaluated by using an optical network analyzer, which operates by making modulation-envelope phase and amplitude measurements while scanning the optical carrier frequency. The measured optical equalizer characteristics show excellent agreement with the simulation analysis. The effectiveness of the equalizer for substantial reduction of the dispersion penalty for a 10 Gb/s signal transmitted over 30 km of normal dispersion fiber was demonstrated. The periodicity of the equalizer results in periodic dispersion-free bands, and hence, the equalizer is suitable for use in future multichannel FDM systems