Research On The Mid-Infared Optical Parametric Oscillator Based On Stoichiometric Lithium Tantalate

Optical parametric oscillator (OPO) is one of the most important ways to get tunable lasers. The pump light could be converted into signal light and idler light by this way. Meanwhile,it can generate tunable coherent light. The broadband tunable laser generated by OPO have shown great value in data communications, optoelectronic measurements, laser range finder, laser radar, photoelectric confrontation and other fields, particularly the mid-infared laser with 3⁵μm-wavelength has great value in military. It is one of the most effective ways to get 3⁵μm tunable laser through optical parametric conversion by using quasi-phase-matched (QPM) techneque. In the past, congruent LiTaO3, congruent LiNbO₃ and KTiOPO₄ were used to make QPM devices. Compeared with these materials, stoichiometric LiTaO₃(SLT) has a serious of outstanding properties of nonlinear optics, especially the lower corecive field and higher anti-photorefractive ability, which make it one of the most attractive materials for making nonlinear optical devices. Periodic poled stoichiometric LiTaO₃ (PPSLT) based on QPM technique is widely used in parametric process. 3⁵μm tunable laser also could be generated through PPSLT. Based on this backgroud, this dissertation focus on the growth techneque of stoichiometric LiTaO3 and fabrication techneque of mid-infrared OPO based on SLT. In this thesis, the main contents are following:Stoichiometric lithium tantalate crystals were grown by flux method. To obtain SLT crystal with large size and uniform component, mixture ratio of raw material is studied and optimum value is gained. Processing method of subsequent crystal is also presented. By optimizing temperature distribution and selecting appropriate growth technical parameters, crystals with good optical homogeneity and no macroscopic defects have been grown. The composition of the grown crystal was assessed by measuring the UV absorption edge, Curie temperature and so on. The assessed Li content in the grown crystal is 49.8% by testing Curie temperature. The coercive field of the grown crystal is 2400V/mm. The distribution of coercive field in a wafer was tested and the result indicates that SLT is a good candidate for QPM devices.The domain reversion kinetics of SLT was studied and the factors that influence domain reversion were determined. Based on the microstructure of SLT, every stage of domain inversion in external field poling procedure at room temperature was discussed. According to the poling results, the functions between the parameters such as poling voltage and poling current were built. Theory of phase matching and quasi-phase matching is studied. Superlattice structure with wavelength of 3-5μm and its tuning mode are both designed with the consideration of SLT.Periodical polarization experiments were peformenced while the SLT crystal and poling equipment made by ourselves were uesed. In the experiments, metal electrodes, composite electrodes and liquid electrode were used. Based on the periodical poling results and domain reversion kinetics of SLT, the best electrode structure and poling curve were determined. PPSLT with 30μm periods was fabricated successfully.The interrelated basic theorics of the tunable mid-infrared OPO, such as the three wave interaction in the OPO or OPG,QPM theory,and the basic theory of the OPO have been analyzed and diseussed. How to design the OPO system based on PPSLT were introduced.A mid-infrared QPM OPO is demonstrated, in which a PPSLT crystal is pumped by a high-repetition all solid state Nd:YAG laser to realize the mid-infrared QPM OPO. The wavelength range of 3.34³.64μm is obtained through the temperature tuning, and when the average pump power is 7.5W, the average mid-infrared output power reaches 800mW.