| Second-order nonlinear optical frequency conversion is widely used in many applications,such as spectroscopy,ultrashort pulse second harmonic generation?SHG?,signal processing and optical communications.Phase matching and group-velocity matching?GVM?should be satisfied simultaneously in order to achieve broadband frequency conversion.Quasi-phase matching?QPM?is a simple and effective way to realize phase matching,and 5?mol%MgO-doped lithium niobate?5?mol%MgO:LN?is used as an example to illustrate how to realize the GVM condition.As for SHG,simulation shows that the acceptance bandwidth are 53.9 nm and 166.4 nm respectively for Type-I?o+o?e?and Type-0?e+e?e?interaction of QPM in a 10-mm long 5?mol%MgO-doped periodically poled lithium niobate?5?mol%MgO:PP LN?crystal with the temperature set to 20?°C.Similarly,as for difference frequency generation?DFG?,the acceptance bandwidth are 282.3 nm and 35.4 nm around 3.403?m with the signal and pump wavelength fixed at 1.550?m and 1.064?m respectively for Type-0?e+e?e?interaction of QPM.The tuning of the temperature to the broadband frequency conversion is also discussed in the paper.To further broaden the acceptance bandwidth of the frequency conversion,a method which using a genetic algorithm by adjusting the positions and quantities of the fundamental wavelengths appropriately is presented.The reduced effective nonlinear coefficient dreff???which can scale the conversion efficiency of the frequency conversion is also discussed in the paper.The simulation results show that the acceptance bandwidth of SHG is approximately 243.3?nm for type-I?o+o?e?interaction around the wavelength of zero-group-velocity dispersion in crystal,which is enhanced by 4.5 times with the efficiency decrease to 5.1?%that of periodic structures.As for Type-0interaction of QPM DFG,the acceptance bandwidth for idler are 1045.9 nm and 96.6 nm around3.043?m,enhanced 3.70 and 2.73 times,correspond to the fixed signal of 1.550?m or pump wavelength of 1.064?m. |
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