Research On Inhibition Of High-power SHG Based On PPMgLN Crystal

With the development of laser technology, people studied the light not onlylimited linear but also began to research the nonlinear field after finding the laser.the development of frequency doubling technique makes laser to be compactand efficient high power.Lithium niobate crystal can be arbitrary wavelength, the phase matchingnonlinear coefficient, and can be processed into the waveguide and otheradvantages which has become the most attractive one of the selection in themanufacturing laser compact. But it has been reported that when the laser outputlight power which exceeds the value, it will be inhibition second harmonicgeneration. The phenomenon not only make the output power of the laser is notup to the expected value, but also the frequency doubling will be reduced. Thishas seriously affected the development of compact and high efficiency laser.Therefore we need to figure out why the phenomenon which has become oururgent the problem.Firstly, we describe the basic principle of second harmonic generation andgive the three waves coupled equation. We also quantitatively analyzed theprocess of second harmonic generation from the two kinds of technology toachieve phase matching and quasi phase matching.Secondly, we analyzed the inhibition of second harmonic generation at highpower which was caused by the thermal and photorefractive effect in detail:(1) This study investigates limitations of high power single-pass399nmlight generations in5%mol PPMgLN waveguides arising from optical absorptions and the resulting thermal loading from the understanding of LN. Acoupled thermo-optical model was employed to simulate399nm light generationprocess. It was found that after optimizing the waveguide length and theoperating temperature, the output power of0.393W with almost no thermalinhibition can be achieved. It is expected that a compact and efficient highpower399nm lasers can be realized by second harmonic generation in PPMgLNwaveguides.(2) This paper introduces that the inhibition of high power SHG in aperiodically poled MgO doped LiNbO3(PPMgLN) waveguide investigating theSHG process based on the coupled mode equations in combination with thephotorefraction and the temperature nonuniformities. The simulation resultsshow that significant refractive index nonuniformities are induced by thephotorefractive effect along the irradiated zone while thoseinduced by thethermal effect are very minor. Therefore, the photorefractive effect instead of thethermal effect is the main factor that inhibits the SHG conversion efficiency. Inaddition, comparison of PPMgLN waveguides with different transversedimensions shows that the waveguides with larger transverse dimension isadvantageous in high power SHG since the photorefractive effect is weaker.Finally, we summarized the inhibition phenomenon of second harmonicgeneration which caused by the thermal and photorefractive effect and discussedthe next work.