Proposal and simulation of all-optical NRZ-to-RZ format conversion using cascaded sum- and difference-frequency...

All-optical 40 Gbit/s format conversion from nonreturn-to-zero (NRZ) to return-to-zero (RZ) is proposed and simulated for the first time, using the cascaded sum- and difference-frequency generation (SFG+DFG) in a periodically poled lithium niobate (PPLN) waveguide incorporated in a Sagnac interferometer structure. Simultaneous single-to-triple channel NRZ-to-RZ format conversion is achieved. Both optical spectra and eye diagrams exhibit impressive conversion performance. The duty cycle, pulse width ratio, Q-factor and extinction ratio (ER) of the converted RZ are analyzed. It is found that flexible NRZ-to-RZ format conversion can be implemented with great tunability, i.e. both input NRZ signal wavelength and converted RZ wavelength can be tuned in a wide wavelength range (>60 nm).