Wavelength Continuously Tunable All-Fiber Flat-Top Comb Filter Based on a Dual-Pass Mach-Zehnder Interferometer

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 he 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 he 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 re-configurable wavelength division multiplexing networks.