在周期性极化铌酸锂波导中的波长转换与光谱分析

实验研究了在准相位匹配LiNbO₃波导中级联的倍频和差频波长转换，通过对转换频谱分析和波形对比分析，讨论了在脉冲泵浦情况下走离效应对光脉冲之间波长转换的影响：由于波导的色散性能，不同波长的光脉冲在波导中传播具有不同的群速度，处在0.8 μm光波段的倍频光脉冲比处在1.55 μm光波段的泵浦光脉冲的传播速度慢，导致了倍频光脉冲与泵浦光脉冲在传播的过程中发生走离；因此倍频脉冲在频域被压缩而在时域被展宽，并遗传给差频过程使得转换脉冲的谱宽变窄、脉宽增加、占空比变大，表明脉冲光泵浦波长转换对信号频谱及波形不是严格透明的转换。实验同时实现了一对多通道的波长转换，表明连续控制光的线宽对转换信号的信噪比有较大的影响。

Wavelength Conversion and Spectral Analysis in Periodically polarized Lithium Niobate Waveguide

Wavelength conversion exploiting cascaded second harmonic and difference frequency generation (c(SHG/DFG)) in periodically polarized LiNbO3 (PPLN) waveguides was experimentally researched. While wavelength converter was pumped with a pulsed wave, the pump pulse can be used to carry the information and wavelength conversion occurs between the pump wave and converted wave, thus wavelength conversion transferring the information from the pump wave to the converted waves includes two processes of second order nonlinear reaction: the first wavelength conversion from pump wave to SH wave occurs with SHG process, and the second wavelength conversion from SH wave to converted wave occurs with DFG process. In the first process the group velocities mismatching (GVM) for pulses at different wavelengths due to material property load the temporal walk-off between pump pulse and SH pulse located in the 1.5 microm band and in the 0.8 microm band, respectively, so that SH pulse slowly propagates compared with pump pulse, and SH pulse width is broadened along propagation length. As a result, in the second process the converted DF pulse generates waveform distortion owing to the broadening of SH pulse in the first process. Both the waveform and the spectrum of converted pulse in our experimental results testify to the fact that SH pulse possesses a narrow spectral width, which is consistent with a long SH pulse, and the spectral width of converted DF pulse is compressed but its temporal width is broadened correspondingly. Therefore the influence of walk-off between pulses demonstrates that the pulsed pumping wavelength conversion is disadvantageous to the transparence of the data format. However, pulsed pumping wavelength conversion also presents great potential that can be applied in future optical networks. Tunable wavelength conversion can be easily implemented by changing the wavelength of control CW, and single-to-multiple channel wavelength conversion can be realized by increasing the number of the CW lasing pump channels. This is very important and it enhances the flexibility in the management of the multi-channel WDM network. Finally, a tunable and single-to-dual channel wavelength converter based on the scheme of pulsed pumping wavelength conversion achieved by our experiment setup, and two channel converted pulses simultaneously replicate the bit rate carried on pump pulses. It is pointed out that the quality such the signal-to-noise ratio of converted pulse is affected by spectral width of control CW.