The gain spectrum nonuniformity is one of the most important problems in using erbium-doped fiber amplifiers (EDFA) in wavelength-division-multiplexing transmission systems. Several techniques for the gain spectrum flattening with the use of passive filters have been proposed: mechanical deformation of the fiber to couple fundamental and leaky modes [1], etching of the grating at the surface of a D-fiber [2], utilization of a twin-core fiber filter [3], application of photoinduced fiber Bragg grating [4], application of a Fourier filter based on the incorporation of the glass plate [5] or a tapered fiber filter [6].
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