Research On Cascaded Difference Frequency Generation Of High Efficiency Terahertz Wave

In recent years,high power and tunable terahertz(THz)radiation sources have important scientific research value and broad application prospects in the fields of imaging,spectral analysis,security detection,communication and military security,and have become the key areas of research in the world.High power,high efficiency,compact structure,room temperature operation of terahertz radiation source is the key to achieve these applications.Terahertz radiation source based on optical difference frequency effect and optical parameter effect has the advantages of high power,wide tuning and room temperature operation,which is widely concerned by various research groups.However,at present,the low optical conversion efficiency of the terahertz radiation source generated by the above optical effects seriously limits the application of THz waves in the above fields.In view of this,this paper proposes a new method to generate high power and high efficiency THz wave by optimizing cascaded differential frequency combined with optical parametric oscillator,which greatly improves the conversion efficiency of THz wave.The main research contents of this paper are as follows:1.The cascaded optical coupling wave equation is derived through the nonlinear optical three-wave coupling equation,and the multi-wave coupling model is constructed.The polarization wave vector of the aperiodic polarization crystal is accurately set,and the phase mismatch of the cascaded anti-Stokes difference frequency is increased while the phase mismatch of the cascaded Stokes difference frequency is reduced.The pump photon is guided to transfer to the cascaded Stokes frequency domain,and the transfer of the pump photon to the anti-Stokes frequency domain is suppressed at the same time,which effectively improves the energy conversion efficiency of terahertz wave.2.A new method for generating high-power and high-efficiency THz waves by optimizing cascaded differential frequency combined with optical parametric oscillator is designed.The optical parametric oscillator is composed of aperiodically polarized lithium niobate(APPLN)crystal and butterfly optical resonator.By setting the required polarization period along the length of APPLN crystal,the phase mismatch of each order difference frequency can be reduced in the cascade Stokes difference frequency process,and the phase mismatch of each order difference frequency can be increased in the cascade anti-Stokes difference frequency process.By setting the reflectivity of the optical resonator mirror,the pump energy is transferred to the cascade Stokes frequency domain and limited to the anti-Stokes frequency domain.By combining the optimized cascaded difference frequency with the optical resonator,the cascaded Stokes difference frequency can be effectively enhanced and the cascaded anti-Stokes difference process can be effectively suppressed.The theoretical calculation shows that at low temperature of 100 K,when the pump intensity is 10MW/cm~2,the energy conversion efficiency from light wave to THz wave exceeds 40%.This scheme can be applied to generate pulse,quasi-continuous and continuous THz waves.