Study On Optical Difference-frequency Generation Of Terahertz Radiation

THz-wave has great scientific research values and wide applications in imaging, material detection, environmental monitoring, communication, astronomy, life sciences, and national defense security and so on. Now many methods have been employed to obtain the THz radiation including electronics and photonics methods, among them the difference-frequency generation (DFG) of terahertz wave based on the nonlinear optical frequency maxing has attracted great interest for its numerous advantages of operating at room temperature such as widely tunable range, high peak power, low cost and compactness.The main contents and structure are as follows:1. According to the coupled-wave equation, the energy transfer among the three waves has been studied in the process of optical parametric oscillation and the frequency tuning characteristics have been analyzed. Then KTP OPO is investigated theoretically and experimentally. First, the low-repetition-rate, high-pulse-energy eye-safe KTP OPO is researched. In the experiment, eye-safe lasers are achieved from noncritically phase-matched KTP OPO pumped by electro-optically Q-switched pulsed Nd: YAG lasers in an external ring cavity configuration. The maximum pulse energy of KTP OPO at 1572nm is 37mJ, corresponding to the optical-to-optical conversion efficiency of 22.06%. Second, using a 532nm green laser with repetition frequency of 10Hz and pulse width of 10ns as a pumped source, a dual-wavelength KTP OPO/OPG operating at the degenerated point around 1064nm is realized. The conclusion is that KTP OPO is one of the most potential pump sources for the THz-wave DFG.2. The energy transfer among the three waves has been studied in the process of the terahertz-wave DFG and the absorption effect on the output power of terahertz wave has been analyzed. The phase matching angle, tuning curves and the effective nonlinear coefficient are calculated. Quasi-phase matching (QPM) nonlinear optical frequency conversion has the advantage of especially high conversion efficiency and has received growing attention in many fields. And the basic principle of QPM is analyzed.3. Theoretical analysis is performed on the principles and process of cascaded DFG for THz wave in GaAs crystal and in quasi-phase-matched (QPM) GaAs. We obtain the optimum pump condition and crystal length through solving the coupling wave equations of cascaded DFG, and analyze the influence of absorption, wave-number mismatch and pump intensity on the cascade DFG process. Based on our calculations in which the pump intensity of 50MW/mm2 and the pump wavelength about 2.1μm are taken for example, the generated THz intensity at 1THz reaches 2.84MW/mm2. The output intensity is increased by 9 times. Last, theoretical analysis is performed on the principles and process of cascaded DFG for THz wave in other zinc-blende structure crystal.