Research On All-Optical Modulation Format Conversion Based On Fiber Nonlinearity

Recently, much research on advanced modulation formats other than traditional non-return-to-zero and return-to-zero has been done, such as phase-shift-keying. They have different dispersion and nonlinearity performances. In future optical network, different modulation formats may be selectively used depending on the network size and the bit rate. As a result, it needs a modulation format converter to connect different kinds of modulation formats in edge nodes. All-optical modulation format converter can realize modulation format convertion in optical field and avoid O-E-O conversion. So far, lots of all-optical modulation format convertions using semiconductor optical amplifier and nonlinear fiber have been reported.This paper is mainly about the theory research on the modulation format convertion technology based on the cross phase modulation in nonlinear fiber. The main content includes:(1) On the basis of coupled nonlinear Schr(o)dinger equations(NLSE) including the loss, group velocity dispersion, self phase modulation, and cross phase modulation, theoretical model of all-optical modulation format convertion based on nonlinear fiber is given. The 40 Gbit/s NRZ-OOK to RZ-BPSK and RZ-QPSK is numerical simulated by solving the NLSE with the split-step Fourier method. And the affection to the modulation format convertion output induced by the polarization mismatch, the walk-off effect and initial pulse sharp is also studied. The output is influrenced by polarization mismatch. And the results show that the plateau shape of NRZ pulse guarantees to generate chirp-free phase change at the center portion of the converted BPSK pulse.(2) Considering the input state of polarization of the NRZ-OOK signal, a new scheme of polarization intensitive NRZ-OOK to RZ-BPSK modulation format convertion is presented. The 40 Gbit/s format convertion is numerical simulated and the simulation result is given in different input states of polarization of the NRZ-OOK signal. The results show that the phase of the converted BPSK signal is hardly influrenced by the input state of polarization in this scheme when the input state of polarization of the NRZ signal is scrambled.