Research On Terahertz Radiation And Midinfrared Laser Based On Nonlinear Optical Frequency Conversion

Terahertz wave is between the infrared and millimeter wave,which has many unexplored characteristics.In addition,the region of 3-5?m is an important atmospheric window band.The laser in the band has a strong transmission in the atmosphere and becomes the minimum attenuation window in the atmospheric transmission,which also covers the absorption peaks of many atomics and has a strong penetration in the fog and smoke.Therefore,the electromagnetic wave technology of the two bands are now regarded as the popular subjects which can be applied in nondestructive testing,safety inspection,spectral analysis,biomedicine,terahertz communication,atmospheric monitoring,laser ranging and remote sensing imaging.Practical radiation sources in these bands are the basic elements for applications.The radiation sources based on optical frequency conversion technology has many advantages,such as good beam quality,high peak power,compact,wide tuning range,all-solid-state and room-temperature-operation.In this dissertation,the 3-5?m mid-infrared BaGa₄Se₇ OPO,as well as THz radiation sources based on terahertz parametric oscillator?TPO?in MgO:LiNbO₃ crystal,are theoretically and experimentally studied.Some new phenomena observed in the experiments are also further explored.The main contents of this paper can be summarized as follows:1.We have studied the fast tunning technology of terahertz-wave parametric oscillator. The pump source for the terahertz parametric oscillator with a reduced size is built, and terahertz wave generated with high efficiency is acquired with this pump source.The maximum THz-wave output energy of^uJ/pulse is obtained.In the experiment,the rapid tuning is achieved by rotating the injection angle of pump beam into MgO:LiNbO₃ crystal using a galvanometer.The wide tunable range is from 0.5 THz to 2.8 THz.2.We have studied the Characteristics of BaGa₄Se₇ crystal in the mid-infrared and terahertz regions.The phase matching condition of BaGa₄Se₇ crystal in the mid- infrared region,and the threshold of the mid infrared parametric oscillator based on BaGa₄Se₇ crystal are analyzed theoretically.This work is the basics of the following experiments.We have measured the refractive indices and absorption coefficients of BaGa₄Se₇ crystal using a standard THz-TDS system.We also calculate the phase-matching conditions for terahertz difference frequency generation utilizing BaGa₄Se₇ crystal pumped by 1064nm dual-wavelength.3.We have carried out the theoretical and experimental research of BaGa₄Se₇-OPO pumped by LD end-pumped and electro-optically Q-switched lasers.The phase matching condition and the threshold condition of the parametric oscillation are analyzed theoretically.In the experiment,a single pump passing singly-resonant OPO?SSRO?based on the BGSe crystal pumped by a 1.064?m Q-witched Nd:YAG laser is presented.The oscillator can be continuously tuned in the range from 3.12 to 5.16?m.It can provid 2.56mJ pulse energy at 4.11?m,corresponding to an optical-to-optical conversion efficiency of 4.16%.The peak power of signal wave amount to about 256 kW.