Broadband Quasi-Phase-Matching Harmonic Variation Utilizing Complementary Inverse Lattice Vectors

Light waves obtained based on second-order nonlinear wavelength conversion have many applications in all-optical communication networks,optical storage,high-definition displays,and medical and military imaging.In order to obtain wide-spectrum output,this paper carried out research from the theoretical derivation and numerical simulation based on the effective nonlinear coefficient model,transfer function method and electro-optical effect to realize the wide-spectrum output.The details of the study are as follows:1.The research background and significance of wavelength conversion technology are summarized,several phase matching methods and common quasi-phase-matching grating structures are compared and analyzed,and the current status of domestic and international research and current problems are specifically analyzed.The coupled wave equation of wavelength conversion is derived,and the coupled wave equation is solved by using the transfer function method to derive the multiplier expressions of the uniform periodic grating and chirped grating at the incidence of Gaussian beam.Based on the electro-optical effect of Lithium Niobate crystal,the change of refractive index of the crystal is deduced under the application of transverse and longitudinal electric fields,which provides a theoretical basis for the subsequent research of extending the second harmonic bandwidth in uniform periodic gratings.2.Based on the dispersion characteristics and electro-optical effects of Lithium Niobate crystal,a method to extend the second harmonic bandwidth by applying temperature and electric field gradients in a uniform periodic crystal is proposed.The dispersion equation of Lithium Niobate crystal is used to analyze the variation of second harmonic generation wavelength at different operating temperatures,operating wavelengths and actual temperatures.Simultaneously,based on the electro-optical effect,the changes of effective nonlinear refractive index caused by nonlinear phase shift around the phase-matching condition are inferred,and the effects of different temperatures,crystal lengths and electric field strength on the transmittance and second harmonic generation wavelength are compared and analyzed.With the temperature or electric field gradient imposed at both ends of the crystal,the second harmonic bandwidth gradually increases along with the growth of the temperature gap and electric field intensity difference between the two ends of the gradient,while the second harmonic bandwidth shows a linear relationship with the temperature difference between the two ends of the gradient and an exponential relationship with the electric field strength difference between the two ends of the gradient,and 2.4 nm and 1.6 nm are obtained at a temperature difference of 40?and an electric field strength difference of 4.4 V/?m before and after the gradient,respectively.3.A broadband quasi-phase-matching grating structure with complementary inverse grating is proposed to utilize the broadband inverse vector characteristic of chirped quasi-phase-matching grating.The broadband quasi-phase-matching grating structure is used to calculate the inverse vector bandwidth from the effective nonlinear coefficient model,and the effects of the crystal length and chirp rate on the inverse vector bandwidth and effective nonlinear coefficient are compared and analyzed based on different grating structures.Using the effective nonlinear coefficient model and the transfer function method,the broadband quasi-phase-matched grating structure with complementary inverse grating provides a large bandwidth of inverse grating from 0.3?m^-1 to 1.26?m^-1,and a broad spectrum output from 450 nm to 820 nm is obtained at the incidence of a tunable light source.