Broadband tunable all-optical 40 Gbit/s NRZ-to-RZ format conversion exploiting SHG+DFG in LiNbO3 waveguides in a MZI configuration

All-optical 40Gbit/s NRZ-to-RZ format conversion scheme with broad signal conversion bandwidth is presented based on the cascaded second-harmonic generation and difference-frequency generation (SHG+DFG) in a periodically poled lithium niobate (PPLN) waveguide. The considered device is a Mach-Zehnder interferometer (MZI). The conversion mechanism relies on the combination of amplification and nonlinear phase shift induced on the signal field during SHG+DFG processes, which making the MZI get unbalanced. The conversion process is numerically calculated with a good performance achieved when ignoring group-velocity mismatching, showing that NRZ-to-RZ format conversion can be successfully realized with wavelength unchanged. The nonlinear phase shift induced on the signal field is analyzed in detail. It is found that large values for nonlinear phase shift can be achieved when quasi-phase-matching (QPM) for SHG but appropriately phase mismatched for the DFG process. Finally, the effect of the nonlinear phase shift induced on the signal field on broadening the signal conversion bandwidth is further discussed. Approximately, 94nm conversion bandwidth can be achieved by keeping pump-wavelength unchanged while tuning signal wavelength, which provides the possibility of realizing a broadband tunable NRZ-to-RZ format conversion.

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