Wavelength Conversion Of Pulsed Waves In PPLN-Based Waveguide

All-optical wavelength converters (AOWCs) will play key roles in future DWDM and WDMA optical communication networks. Wavelength conversion exploiting cascaded second-harmonic and difference-frequency generation (cSHG/DFG) in periodically poled LiNbO3 (PPLN) waveguides has manifested its potential as one of the best solutions: in addition to high wavelength conversion efficiency, negligible spontaneous emission noise and broad conversion bandwidth, it can achieve a flexible wavelength conversion such as a tunable wavelength conversion, a single-to-multiple channel wavelength conversion and a multi-wavelength simultaneous wavelength conversion, which increases the flexibility in the management of the multi-channel WDM network. Theoretical and experimental research on cSHG/DFG-based all-optical wavelength conversion technology is presented in this paper. The main contents are as follows:(1) The important applications of AOWCs in future optical communication systems and several usual types of AOWCs are described. The principles and latest research progresses of DFG-AOWC are introduced.(2) The nonlinear couple-mode equations describing the optical wavelength conversion processes in quasi-phase matching (QPM) waveguides are derived based on optical waveguide, nonlinear optics and QPM theories. The expression of converted power and efficiencey in both SHG and DFG processes are addressed under the assumptions of plane wave approximation and slowly varying amplitudes, respectively. Furthermore, the nonlinear coupled-mode equations of wavelength conversion between pulses are derived when the group velocity and group-velocity dispersion (GVD) of pulses are considered.(3) The influence of walk-off between pulses due to group-velocity mismatching in wavelength conversion are analyzed in both frequency and time domain. The broadening of second-harmonic pulse, the converted waveform distortion with peak offset and the intraband crosstalk in pulsed-pumping wavelength-conversion are discussed with the change of waveguide dimensions and pulse parameter. And then a dual-direction wave-injected scheme is proposed to avoid waveform distortion and an advancing pump-injected scheme is demonstrated to minish intraband crosstalk. A novel scheme of utilizing the property of walk-off effect to produce several pulses in one-pulse-period are also introduced and researched.(4) The relationships of the conversion efficiency and conversion bandwidth with the waveguide dimensions, pump and signal power, pump and signal wavelength, pulse parameter and delaying between pulses etc. are numerically calculated and analyzed, and theoretical research on the mechanism of improving pump-wavelength tolerance with pulse pumping is presented as well.(5) The parametric-process and the photo-refractive-index-change in AOWC converters injected with high power are theoretically analyzed. The energy oscillates between pump, signal, second-harmonic and difference-frequency wave are demonstrated, and the expression of the optimum-waveguide-length is obtained. The potential application of pulse reshaping is pointed out as well.(6) The cSHG/DFG optical wavelength converters based on a periodically poled PPLN waveguide is experimentally realized. A tunable and single-to-dual channel wavelength converter pumping with picosecond pulses at a repetition rate of 40GHz is implemented, and the waveform distortion of converted pulse is illustrated in our experiments.