The Study Of Terahertz Radiation Source Based On Nonlinear Optical Frequency Conversion

Terahertz wave can be extensively used for biomedicine, safety inspection, environmental monitoring and so on. All the applications can be realized based on the terahertz radiation source with superior performance. There are many methods for genera- ting terahertz wave among which there are lots of advantages for the method of nonlinear optical frequency conversion, for example simple optical structure, easy tuning, high output power and so on. Two methods were intruduced in the Master's thesis, which are Difference Frequency Generation (DFG) and Terahertz Parametric Oscillator(TPO).Three Phase-matching methods were intruduced, which are Phase-matching of nonlinear, birefringence collinear and reststrahlen band collinear. The angle tuning curves were obtained by numerical simulations. When the pump source of DFG is CO2 Laser, the corresponding optical design was obtained. Because two laser beams can be generated in the same cavity for the Ceramic Fiber Laser(CFL), Compact terahertz radiation source with high output power can be realized with the CFL chosen as the pump source of DFG.The expressions of parametric gain and lattice wave dispersion were obtained by analyzing the theory of lattice vibration in the process of TPO. The angle tuning curve of LiNbO₃ based on the relation of lattice wave dispersion and Phase-matching was obtained by numerical simulation. The change of the parametric gain with the terahertz wave fruquency and pump energy was calculated with the distribution of the lattice vibration modes.The angle tuning curves were experimently obtained for the crystals of LiNbO₃ and MgO:LiNbO₃, the tuning range of frequency were 1.22-2.43THz and 1.33-2.59THz. The change of terahertz output energy with the frequency and pump energy was measured. When the pump energy was 85.7mJ, the voltage corresponding maxium output energy obtained was 877.7mV at 1.33 THz for MgO:LiNbO₃. The absorption cofficients were measured as 41.3cm^-1 for LiNbO₃ at 1.53 THz and 31.5cm^-1 for MgO:LiNbO₃ at 1.63 THz.