基于级联光学参量转换技术的波长快速可调谐单频中红外激光光源研究

Rapid-wavelength-tunable single frequency mid-infrared laser sources are potential candidates for important applications including atmospheric sounding, environmental monitoring and medical diagnostics. Periodically poled and aperiodically poled magnesium-doped lithium niobate (PPMgLN/APMgLN) crystals are usually adopted as the nonlinear materials to generation such laser radiation via optical parametric oscillation (OPO) or difference frequency generation (DFG) processes. However, the wavelength tuning bandwidth for a PPMgLN crystal is limited while the effective nonlinear coefficient for the chirp APMgLN crystal is comparatively small. As a consequence, neither of the crystals can provide high efficiency and wide tuning bandwidth simultaneously in a practical OPO or DFG system. In order to break this restriction, the cascade optical parametric conversion technique is introduced by converting the redundant signal wave from the OPO process to the mid-infrared idler output via intra-cavity DFG process, by which the pump-to-idler conversion efficiency can be enhanced without narrowing the wavelength tuning range. Hence, in this project, we will design and then fabricate the compound chirp APMgLN with apodization for such cascaded optical parametric conversion. The optimized ratio of the effective nonlinear coefficient for OPO and DFG processes will be achieved in the design procedure. An acousto-optic tunable filter based wavelength tunable single frequency pulsed fiber laser will be developed as the pump source of the cascaded OPO. A single frequency seed laser at the wavelength of the oscillating signal wave will be directed into the cavity to reduce the build-up time and the laser linewidth of the parametric waves. Finally, the rapid-wavelength-tunable single frequency mid-infrared laser system will be developed based on the cascaded optical parametric conversion technique.

波长快速可调谐单频中红外激光在大气探测、环境监测、疾病诊断等领域具有重要应用前景。使用周期性或非周期性畴极化反转掺镁铌酸锂（PPMgLN/APMgLN）晶体作为非线性材料，通过光学参量振荡（OPO）或光学差频产生（DFG）等非线性光频转换方式是研制此类光源的常用技术手段。但由于PPMgLN晶体的波长调谐带宽有限而啁啾APMgLN晶体的有效非线性系数较低，因此，这两种晶体无法兼顾转换效率和调谐带宽。为了解决这一矛盾，本项目引入级联光参量转换技术，将OPO产生的多余信号光通过腔内DFG，放大中红外闲散光，在维持调谐带宽的同时提高泵浦到闲散光的转换效率。通过设计并制备复合啁啾切趾的APMgLN晶体，优化OPO和DFG过程的有效非线性系数配比；研制基于声光可调滤波器的波长可调谐单频脉冲激光器作为泵浦；利用单频信号光注入，缩短参量光的建立时间压缩线宽；最终实现波长快速可调谐的高效单频中红外激光输出。

中红外波段覆盖了诸多气体分子吸收光谱的“指纹区”，利用这些吸收特性，使用波长快速可调谐的单频中红外激光作为主动探测光源，通过差分吸收的探测方式可以快速获知待测气体的成分和浓度信息；因此，这种中红外波段的可调谐单频激光光源在大气探测、环境监测、疾病诊断等领域具有广泛的应用前景。使用周期性或非周期性畴极化反转掺镁铌酸锂（PPMgLN/APMgLN）晶体作为非线性材料，通过光学参量振荡或光学差频产生等非线性光频转换方式是研制此类波长可调谐的单频中红外光源的常用技术手段。但由于PPMgLN晶体的波长调谐带宽有限而啁啾APMgLN晶体的有效非线性系数较低，因此，这两种晶体无法兼顾转换效率和调谐带宽。为了解决这一矛盾，我们研制出一种新型的放大辅助型光学差频系统，该系统使用单块APMgLN晶体将差频产生的近红外信号光通过光参量放大过程继续放大中红外波段闲散光，从而在兼顾调谐带宽的同时极大提升中红外激光的产生效率。为了更好地优化系统性能，研究工作首先从理论建模与数值仿真入手，建立可调谐单频中红外激光系统中的泵浦源、信号光、非线性晶体以及谐振腔等核心组部件的物理模型，获得泵浦和信号激光器工作的速率方程以及非线性光频转换过程的耦合波方程的数值模拟方法。根据数值计算的结果，项目组提出了使用方波的脉冲泵浦光与连续波信号光共同注入前后分段级联式铌酸锂光学超晶格并驱动放大辅助型光学差频系统的最佳工作模式。方波的泵浦脉冲使得高效的非线性转换贯穿整个脉冲，连续的信号光彻底解决了脉冲间的同步难题，分段式的铌酸锂光学超晶格的设计也大大降低了晶体的研制难度。使用这种方案，在仅仅0.4W的连续单频信号光注入条件下，该系统的泵浦到闲散光转换效率比相同泵浦峰值功率情况下使用高斯脉冲的转换效率提升超过50%，相比于传统单级光学差频产生的效率提升更是超过100%，即使在如此高的转换效率下，研制系统的泵浦和信号激光的波长调谐范围还能大于10nm。较高的转换效率、合适的调谐范围、无跳模的运作以及更方便的晶体设计也使得本项目所研制的基于级联光参量转换技术的波长快速可调单频中红外激光非常适合用于远距离大气定量化遥感,有望大幅度提高目前此类系统的探测速度和精度。