基于单体PPMgLN晶体的双波长参量共振产生6微米波段激光的差频转换技术研究

Radiation with a wavelength of around 6μm is useful for many applications in atmospheric spectroscopy, molecular spectroscopy, the thermal dissociation of cholesterol, and DNA probing. Using the magnesium-doped periodically poled domain lithium niobate (PPMgLN) as a nonlinear crystal material, 6μm laser radiation can be obtained by direct optical parametric oscillation (OPO) and difference frequency generation (DFG). Because of the relatively high absorption in the crystal, direct parametric conversion is difficult and DFG conversion efficiency is extremally low. By utilizing the parametric conversion and difference frequency conversion properties of the crystal, a monolithic PPMgLN crystal with cascaded domain structure can be designed and fabricated to achieve infrared dual-wavelength generation and the difference frequency conversion simultaneously. In this project, we will design and fabricate a specific PPMgLN crystal with cascaded domain structure for both infrared dual-wavelength generation and difference frequency conversion to 6μm. The dual signal and idler output from the dual-wavelength OPO system can be converted to 6μm simultaneously by the chirped or aperiodically domain structure, and high conversion efficiency and high power output of 6μm waveband can be achieved.

6微米波段附近的中红外激光是大气光谱学、分子光谱学、胆固醇热分解以及DNA分子探针等领域的重要光源。利用周期性畴极化反转掺镁铌酸锂（PPMgLN）作为非线性晶体材料，可通过直接参量转换和级联差频转换两种方法获得该波段激光输出。由于晶体在6微米波段相对较高的吸收，直接的参量转换难度大，常规的差频转换效率低。综合PPMgLN晶体的参量和差频转换特性，可以设计和制备复合结构的单体PPMgLN晶体器件，以同时实现中红外双波长振荡并以其实现高效的差频转换。本项目将在单块PPMgLN晶体上，制备适用的畴极化结构，以分别实现双波长中红外振荡和差频转换的功能。差频转换的畴周期采用复合或啁啾结构设计，使前端双波长信号光和闲散光均可转换，最大程度利用泵浦功率，实现6微米波段更高转换效率的激光输出。

本项目的基本思想是通过研制基于周期性畴极化反转的复合周期结构的掺镁铌酸锂晶体材料，获得双波长中波红外激光输出，并将其应用于差频转换从而在6微米波段获得中波红外激光输出。.项目深入研究了单周期和复合周期结构的PPMgLN晶体的制备技术，制作成功了高质量的非线性晶体材料，并将其应用于本项目的各项研究内容中。项目研究了增益调制型脉冲光纤激光器泵浦的光参量振荡器系统以及准同步泵浦的光参量振荡器系统。同时，利用增益调制型脉冲激光器作为泵浦源获得了双波长中波红外激光器，验证了温度调制双波长输出的可行性。项目研究了基于ASE光源和自种子系统的光参量放大器和差频系统，就差频系统的输出特性等做了细致研究。.在以上研究的基础上，项目利用双波长中波激光器作为泵浦源，复合周期结构的PPMgLN晶体作为非线性晶体材料，研究了非内腔式光参量振荡器系统，实现了6微米波段的激光输出，输出激光功率达到了50mW。项目进一步研究了单体式复合级联周期结构的PPMgLN晶体材料，其前半段作为双波长泵浦激光的增益介质，后半段作为6微米波段差频转换的增益介质。利用内腔式光参量振荡器系统，我们在单体PPMgLN晶体材料上，同样成功获得了6微米波段的激光输出。