Simulation Research For Quasi-phase Matched Second Harmonic Generation Of Broadband Light Source

Because of the rapid development of broadband laser light source, especially super continuous spectrum laser technology and the extensive application in fields such as FOC, sense, biomedicine and spectrum analysis, the study on second harmonic conversion of broadband laser has fundamental theoretical and practical significance. However, owing to harmonic converse of high efficiency and broadband need to simultaneously satisfy phase matching and group velocity matching, which is impossible for traditional frequency doubling manner, it greatly restraints the development of harmonic generation of broadband laser towards high efficiency and broadband directions. Compared with traditional frequency doubling of birefringence phase matching, the advantage of frequency doubling technology of Quasi-phase matching is that it can broaden the application range of nonlinear crystal and provide new thought for high efficient harmonic generation of broadband laser.Currently, the study on Quasi-phase matching mainly centers upon how to improve generation efficiency of narrow band laser and lower temperature sensitivity, and the discussion on Quasi-phase matching of broadband laser is mainly directing at ultrashort pulse light source, while the frequency doubling process for normal broadband laser has not been paid concern by researchers. Directing at Quasi-phase matching frequency doubling of broadband laser, the paper has developed theoretical analysis and simulation study, explored and analyzed the feasibility and actualization condition of second harmonic generation with high efficiency and broadband, provided theoretical basis and data support for further study for high efficient harmonic generation of broadband laser.The paper first reviewed the proposal and development course of Quasi-phase matching technology and a series of study achievements obtained in recent twenty years, broadly introduced the type, feature and application range of broadband laser light source, stated the scientific and practical value of broadband laser frequency doubling and concluded the advantage of Quasi-phase matching technology’s application in broadband laser frequency doubling on this basis.Starting from Maxwell’s equations, the paper drew forth the coupling wave equation with three-wave interaction and detailed discussed the actualization method of production process of second harmonic wave and phase matching. It also further analyzed the fundamental principle of frequency doubling technology of Quasi-phase matching and discussed on the factor that influences frequency doubling efficiency of Quai-phase matching in theory by employing small signal approximation.Making further study on ordinary frequency doubling of monochrome continuous laser and frequency doubling of Quasi-phase matching through numerical value simulation means. The calculation results demonstrated that in ordinary frequency doubling crystal, when the phase mismatches, the frequency doubling light’s operating distance increases while periodicity vibrates, and the periodicity of smaller mismatching factor has apparent difference with conclusion of small signal approximation. While for PPKTP crystal of poling period, the vibration periodicity of frequency doubling light field intensity is far larger than ordinary frequency doubling of Quasi-phase matching, and there is fluctuation within a narrow rance in local.It has made correction aiming at the coupling wave equation of Gaussian light beam towards description of frequency doubling process and preliminarily analyzed the difference of frequency doubling process of Gaussian light beam and plane wave in theory. Through the frequency simulation of PPKTP crystal of poling period, it found that Gaussian beam divergence will lead to decrease of frequency doubling efficiency. Besides, under condition of phase mismatching, the divergence of Gaussian beam will generate damping action to frequency doubling light generation. For data of frequency doubling efficiency reported by GD. Milier in 1997, the improved calculation result of Gaussian light beam is 54.1%, which is closer to the experimentally reported frequency doubling efficiency of 42%.It has conducted analysis of broadband and poling period tolerance of Quasi-phase matching frequency doubling in theory and studied on the influence of each parameter of PPKTP, PPLN and 7mol% mixing with PPMgLN crystal on the frequency doubling efficiency and frequency doubling broadband through numerical value simulation, with comparing of reported experimentally data. The analysis results indicated that the frequency doubling broadband presents diminishing reciprocal relationship along with crystal length approximation, and crystal with smaller length will have better broadband performance; the selection of poling period and dispersion property of material seriously influences frequency doubling broadband. Material with smaller mismatching factor derivative and periodicity will significantly improve performance of frequency doubling; matching order has almost no influence on frequency doubling broadband, only influencing the generation efficiency of frequency doubling.