Abstract

We present a wavelength continuously tunable all-fiber flat-top comb filter. The proposed comb filter is based on a dual-pass Mach–Zehnder interferometer (MZI), constructed with two couplers and a polarization controller (PC), along with a polarizer at the input port. While the interference is created from the optical path difference provided by the two cascaded couplers, the dual-pass structure is based on the feedback of a fiber loop mirror, formed by the second coupler with the PC inserted in the loop, which serves as a reflectivity-variable mirror. The coupling ratio of the first coupler is determined by our optimized results on characterizing a basic dual-pass MZI without a PC. By using Jones matrix formulation, we theoretically analyzed the proposed comb filter. To achieve flat-top spectra, both amplitude condition and phase condition have to be satisfied. Our simulation results revealed that by changing the state of the PC, tunable flat-top spectra can be achieved, and moreover, the flat-top spectra can be continuously adjusted within one free-spectral-range with the help of a rotatable polarizer. The theoretical prediction has been verified by our experimental results. The unique tuning property makes the proposed filter an excellent candidate for applications in dense reconfigurable wavelength division multiplexing networks.

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