Study On Spatial Intensity Distribution Of Terahertz Parameter Source And Stokes Parametric Oscillator Of KTA Crystal

In the past decades,the terahertz wave has been widely used in the fields of the counter-terrorism,food and drug inspections and medical imaging.Research on terahertz wave radiation sources has become more and more important.Terahertz parametric sources are one of major terahertz wave radiation sources and have many advantages such as wide tunable spectral range,high peak power,operation at room-temperature,easy operating,compact structure and so on,which attract considerable interests.The principle of the terahertz parametric source is Stimulated Polariton Scattering(SPS)When a strong laser beam is incident in a nonlinear crystal which has both infrared and Raman active transverse optical(TO)phonons,the laser will couple with the TO phonon.A Stokes photon and a polariton will be generated.The polariton could generate terahertz wave at small wave vector.Due to the large terahertz wave refective index and large dispersion,terahertz parametric sources adopt non-collinear phase matching condition.By changing the non-collinear phase matching angle,the Stokes lasers of different wavelengthes and terahertz waves of different frequencies can be obtained.So terahertz parametric sources could be used to provide tunable Stokes lasers and terahertz waves.The crystals used in the SPS must have TO modes which are both infrared and Raman active.At present,there are few kinds of crystals which can be employed in the SPS process.Terahertz wave parametric sources composed of lithium niobate(LiNbO3)crystals are common.Its output energy and frequency characteristics have been widely studied.In recent years,other new crystals used in terahertz parametric sources have begun to enter researchers' vision,such as KTiPO4(KTP)and KTiOAsO4(KTA).However,the researches on terahertz wave parametric sources composed of new nonlinear crystals are not complete,especially on the basic characteristics of the tunable Stokes lasers.The study of the infrared intensity and dielectric constant of TO phonon modes involved in the SPS process of KTA crystals is lackingIn this paper,the tuning characteristics of the Stokes laser based on KTA crystal Stokes parametric oscillator are studied experimentally,and the dielectric constant of the KTA crystal is calculated.The coupled wave equation is a common tool for quantitative analysis of the SPS characteristics.During the SPS process,the three waves are coupled to each other by nonlinear polarization to achieve three-wave mixing.The nonlinear polarization intensity is also affected by TO phonon modes.Therefore,studying the nonlinear polarization intensity of SPS is of great significance for solving the coupled wave equation and expounding the mechanism of interaction between light waves and TO phonon modes.Previous theoretical studies on the SPS coupled wave equations describe the gain characteristics based on the small signal approximation,ignoring the terahertz wave walk-off and the pump depletion.With the rapid development of high peak power laser technology,new crystal materials and terahertz detectors,the terahertz parameter source has made great progress and the conversion efficiency has been greatly improved.The existing small signal gain are no longer suitable to describing terahertz wave parametric sources with high conversion efficiency.Moreover,the existing theory does not consider the terahertz wave walk-off under non-collinear phase matching conditions,so the influence of non-collinear phase matching on the spatial intensity distribution of the terahertz parameter source has not been studied.In this paper,the spatial intensity distribution of the injection seed terahertz parametric generator(is-TPG)with high conversion efficiency is numerically calculated and experimentally explored.And the parameters of is-TPG are optimized by numerical simulation results.The detailed contents of this desertation are as follows.1.Research on KTA-based tunable Stokes lasers.Firstly,the output of tunable Stokes laser was realized by Stokes parametric oscillator based on KTA crystal.The wavelength tuning characteristics,energy tuning characteristics were experimentally explored.When the angle between the pump and Stokes beams changed from 1.875° to 6.500°,the tunable near-infrared Stokes laser changed from 1077.9 nm to 1088.4 nm.The maximum output energy was 33.9 mJ at 1078.6 nm.At the same time,four wavelength discontinuities of 1079.0 nm-1080.1 nm,1080.8 nm-1082.8 nm,1083.6 nm-1085.5 nm and 1085.8 nm-1086.8 nm appeared in Stokes tuning process.Then,the LiB3O5(LBO)crystal was used to perform intracavity frequency doubling of Stokes laser,and a tunable green laser output of 539.0 nm to 544.2 nm was obtained.The maximum output energy was 15.7 mJ and the corresponding wavelength was 543.8 nm.2.The dielectric constant formula of KTA crystal was fitted by using the experimentally measured spectral characteristics of the Stokes laser.The infrared intensity,high-frequency dielectric constant and polariton dispersion relationship of the TO phonon in KTA crystal were obtained for the first time.The dispersion relationship of KTA were used to explain the gap in the Stokes laser wavelength tuning process in detail.3.The spatial intensity distributions of is-TPG was studied.Based on the maxwell's equations and energy density matrix,the nonlinear polarization formula and coupled-wave equation model of SPS were derived.For the is-TPG with high conversion efficiency,the coupled wave equations were solved numerically by the fourth order runge-kutta method.It was the first time that the spatial intensity distributions of the three waves were obtained.The simulation calculations of rectangular pump pulse and Gaussian pump pulse were' carried out respectively.We studied the spatial intensity distributions under different pump energies,different Stokes seed powers and different interaction lengths.Also the characteristics of transverse spatial intensity distributions of the pump and Stokes beams,and the terahertz wave intensity distribution along the pump propagation direction were obtained.4.The spatial intensity distribution of the is-TPG based on the LiNbO3 crystal was studied experimentally.The spatial intensity distributions of the pump,Stokes,and terahertz waves were studied.The is-TPG system used in the experiment was composed of a Q-switched pulse laser at 1064.2 nm,a continuous wave seed laser at 1070.0 nm,and the LiNbO3 crystal.The spatial intensity distributions of the three waves were measured under different pump energies,different seed powers,and different interaction lengths.The characteristics of transverse spatial intensity distributions of the pump and Stokes waves,and the terahertz wave intensity distribution along the pump propagation direction were obtained.The spatial intensity distributions measured by the experiment verified the correctness and reliability of the numerical simulation method of the coupled-wave equations.5.The parameters of is-TPG were optimized by the numerical simulation method.Using the numerical calculation of the coupled wave equations,the numerical simulation of the spatial intensity distributions and output intensity distributions of the three waves in the is-TPG under different gain and loss coefficients were respectively simulated.Optimization rules of the transverse dimensions of the crystal under high loss and the terahertz wave output position at high gain were obtained.The innovations of this desertation are as follows1.The characteristics of the Stokes laser based on KTA crystal were studied experimentally in detail.By changing the angle between the pump and Stokes resonator axis of the Stokes parametric oscillator,the Stokes wavelength was changed.The wavelength range of the output Stokes wave was from 1077.9 nm to 1088.4 nm.The maximum output energy was 33.9 mJ at 1078.6 nm.2.The tunable frequency-doubled laser based on KTA Stokes laser was performed by LBO crystal for the first time.The maximum output energy was 15.7mJ at 543.8nm.3.The infrared constant Sj of TO phonon and the dispersion relation of the polariton in KTA crystal were obtained for the first time.Performing curve fitting to the frequency-dependent dielectric constant formula and the measured polariton refractive indexes at different frequencies,we obtained S;and high frequency dielectric constant.For the first time,the four gaps in the tunable Stokes wavelength range were explained.4.For the first time,the numerical solution of the coupled wave equations of SPS was obtained by considering the pump depletion.The fourth order runge-kutta method was used to solve the coupled wave equations of SPS.The spatial intensity distribution,output energy density distribution,and temporal intensity distribution in the is-TPG were obtained by iterative calculation of the time and space discrete elements.5.The spatial intensity distribution of the is-TPG based on LiNbO3 crystal was investigated experimentally for the first time.The spatial intensity distribution of terahertz wave along the x-axis was obtained by measuring the terahertz wave output intensity along the emitting surface of the trapezoidal LiNbO3 crystal.6.Using the coupled wave equations of SPS to numerically simulate the three-wave spatial intensity distributions under different losses and different gains.Optimization rules of the transverse dimensions of the crystal under high loss and the terahertz wave output position at high gain were obtained.