Study On Nonlinear Frequency Transformation Based On Lithium Niobate Waveguide

Second-order frequency transformation is widely used in nonlinear optics.It plays an important role in different fields such as spectroscopy,signal processing,ultra-short pulse frequency doubling and optical communication.In order to achieve efficient frequency conversion,such as second harmonic（SHG）generation,phase matching（PM）or quasi-phase matching（QPM）conditions need to be satisfied.In phase matching,the effective wavelength can be converted by the dispersion of waveguide.By using quasi-phase matching,the output of frequency doubling can be achieved in periodic pole-reversal lithium niobate materials.In this paper,based on the theory of nonlinear optics,a suitable ridge-shaped waveguide structure is designed,and the influence of the size and polarization mode on the effective refractive index of the waveguide is studied by changing the thickness,width and temperature of the upper layer of the waveguide.On this basis,the parameters of Li Nb O₃ ridge waveguide were optimized to obtain wavelength points which could realize phase matching.The simulation model is built to realize the second harmonic generation,and the relationship between the frequency doubling optical conversion efficiency and the length of the waveguide is analyzed.It is also shown that the phase matching wavelength of the Li Nb O₃ridged waveguide changes with the change of temperature,and the conversion efficiency of the second harmonic normalization increases with the increase of temperature.In order to overcome the limitation of waveguide refractive index to meet the phase matching wavelength,using periodic polarization structure,to meet the phase matching conditions,lithium niobate waveguide in the second harmonic generation are simulated and analyzed the thickness,width and matching type on the influence of phase match second harmonic conversion bandwidth,studies have shown that using periodic polarization structure of lithium niobate waveguide.The broadband second harmonic output can be achieved.Compared with the Type-0（e+e→e）and the Type-0（e+e→o）,quasi-phase matching has a larger frequency band width,and the bandwidth value reaches 260nm and 67nm,respectively.