Research On Nonlinear Frequency Conversion Technology Of Solid-state Laser Based On Quasi-phase Matching

Because of the good monochromaticity,directionality,coherence and high brightness,laser has been widely applied in our daily life,industrial manufacturing,and scientific research.As the development of laser technology,the wavelength of laser covers from deep ultra-violet to infrared at several microns.In high-density optical data storage,color displays,biological and medical diagnostics,blue laser has advantages like its short wavelength and high single photon energy.And ultra-violet laser can be obtained through the frequency up-conversion of blue laser.It is one of the hot spot in the field of laser technology.There are many ways to produce blue laser source.Because of the high output power,high beam quality and narrow spectral linewidth,diode-pumped solid-state laser(DPL)can produce blue laser source with high efficiency,high output power and high beam quality by nonlinear frequency conversion.It benefits from the development of nonlinear optics and nonlinear crystal.Frequency up-conversion will be generated in nonlinear crystal by utilizing the second order nonlinearity.And blue even ultra-violet laser can be obtained after nonlinear frequency up-conversion of solid-state laser.The phase-matching condition must be fulfilled during frequency up-conversion in nonlinear crystal.Quasi-phase matching technology can make up for the phase mis-matching by utilizing the periodical modulation of nonlinear crystal's polarizability.The phase mis-matching will be compensated by correct poling period.This technology can utilize the major nonlinearity of crystal,avoid the walk-off effect,and produce frequency conversion with high efficiency and theoretically,the whole transparency range of the crystal can be utilized and the wavelength can be tuned.In this paper,the research focuses on red and blue solid-state laser based on quasi-phase matching technology.And it is as follows:1.According to the coupled wave equation in periodically poled crystal,the conversion efficiency and tolerance of second-harmonic-generation(SHG)and sum-frequency-generation(SFG)have been studied,especially their correlations with periodically poled crystal and fundamental laser.Crystal length is the key factor after the confirmation of poling period and working temperature when there are loss and phase mis-matching.Under the condition of phase matching,a high efficient SHG or SFG process can be achieved at lower power density.2.The study on high efficient SHG of solid-state laser.In order to meet the phase matching condition,the parameters of crystal need to match those of the fundamental laser.The factors may cause phase mis-matching in SHG have been studied and it turns out that the deviation of poling period is the key to phase mis-matching.The phase mis-matching caused by average deviation of poling period can be compensated by temperature regulation in monocycle crystal.It mainly depends on the standard deviation of poling period and usually small.According to theoretical simulation and the analyzation of phase mis-matching,the crystal length,working temperature and the poling period of PPLN have been determined.The design of PPLN has been optimized according to theory,and a high efficient red-laser source has been produced.The conversion efficiency is 78.25%where the power density of fundamental laser is less than 2MW/cm2.The experimental results correspond with the theory,and it proves that the high efficient SHG at lower power density is practicable.3.Study on cascaded third-harmonic-generation(THG)of solid-state laser produced by PPLN.The cascaded THG process consists of SHG and SFG successively.To realize high efficient THG,the photon ratio between fundamental and second-harmonic laser is 1:1.It requires an optimum conversion efficiency of SHG.According to the theoretical simulation of cascaded THG,the poling period,working temperature and crystal length for SHG and SFG have been confirmed.The cascaded process of SHG and SFG has been matched by temperature regulation,and a 440-nm blue laser has been produced by cascaded THG in PPLN.It proves that the cascaded THG of solid-state laser in monolithic PPLN crystal at the same temperature is practicable.There are two different poling periods in PPLN for THG.The phase mis-matching can be decreased by the improvement of period accuracy and then it is possible to achieve THG process with higher efficiency.The red laser by SHG and blue laser by cascaded THG in monolithic PPLN crystal have been obtained successfully.It is promising to produce high efficient laser with high output power by SHG or cascaded THG.In the trend of high efficiency,high output power,tunability and miniaturization,it will have wide prospect.