Research On The Optimized Characteristics Of Terahertz Parametric Oscillators

Terahertz（THz） technology is now regarded as a popular subject which can be applied in material diagnostics, remote atmospheric sensing and monitoring, drug and food inspection, security checking and so on. Owning to the compactness, high output power and wide tenability, terahertz parametric oscillators have a bright prospect in the research of single frequency terahertz sources. Several aspects of research work are mentioned in this thesis, including the study of optimizing pump beam of terahertz parametric oscillators and exploring new noncolinear crystals for improving the output power of terahertz parametric oscillators, the study of injection-seeding in Stokes wave for narrowing the linewidth of output terahertz wave, and a detailed study of terahertz wavemeter based on scanning Fabry-Perot interferometer for accurate measurement of terahertz wavelength for the research of the tunability of terahertz parametric oscillators.The main shining points of this paper are as follows:1. We theoretically and experimentally studied the dependence of output characteristics of terahertz parametric oscillators on the transverse mode of the pump beam. The physical mechanism of the influence of multimode pumping on the output characteristics of terahertz parametric oscillators is explained with the coupling coefficients of the parametric process and the multimode effects in stimulated Raman scattering. The output power and the optical quality of output terahertz beam of terahertz parametric oscillator were studied experimentally under different pump modes. It can be concluded that the output power of the terahertz parametric oscillator is much higher under better pump modes, and beam quality of output terahertz wave is also better when the pump mode is better.2. Narrowing the linewidth of terahertz parametric oscillators by injection seeding is studied theoretically; the narrow-linewidth terahertz parametric oscillators with a reduced size is designed. An injection-seeded terahertz parametric generator is designed, and the requirements for successful injection locking is analyzed. 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. Besides, a designation for terahertz parametric oscillators with narrow-linewidth, high conversion efficiency and reduced size is proposed with the introducing of external cavity enhancement of the pump beam.3. The application of BaGa₄Se₇ crystal in terahertz wave and mid-infrared radiations is studied. The refractive indices and absorption coefficents of the crystal is measured with a standard THz-TDS system. The phase-matching conditions for terahertz difference frequency generation utilizing BaGa₄Se₇ crystal is calculated with the measured values. Besides, an optical parametric oscillator for mid-infrared radiation with an BaGa₄Se₇ crystal pumped by an Nd:YAG laser is experimentally studied, and mid-IR radiation with 3.9μm wavelength and 60μJ pulse energy is achieved.4. A terahertz wavemeter based on scanning FP interferometer is demonstrated for better researching the tenability of terahertz parametric oscillators. And an algorithm for uncertainty estimation of measured wavelength is demonstrated for the first time to our knowledge. The uncertainty of the measured wavelength can be estimated using the algorithm for both methods of wavelength calculation, which are direct determination from the interferogram and calculated from the fitted values of the measured curve. The algorithm can be used for both uncertainty estimation of the measured results, and the optimization of the wavelength designation.