基于光子晶体光纤飞秒激光双波长参量振荡差频的超宽带可调谐超快太赫兹源

In this project, an high-power ultra-widely tunable ultrafast terahertz（THz）wave source based on the difference frequency generation from a femtosecond dual-wavelength optical parametric oscillator pumped with a femtosecond photonic crystal fiber laser is proposed, to meet the application needs of real-time online rapid detection in an range of widely bandwidth. The project includes three parts: a high repetition rate femtosecond photonic crystal fiber laser, a widely tunable femtosecond dual-wavelength optical parametric oscillator (OPO) and an ultrafast THz generator by the OPO difference frequency. The main studies on the subject are as follow: the performing conditions of dual-wavelength operation in the femtosecond OPO, including the nonlinear crystal structure, the stability and adjustment for the resonant cavity, the characteristic of dual-wavelength OPO pulses and the phase matched condition. We are going to explore the possibility to keep the identical space-temporal characteristics between the dual-wavelength OPO pulses, and to research the high-power THz generation and tuning by the difference frequency with the dual-wavelength OPO pulses, including the structural design and selection for the nonlinear materials, the phase matched condition and the corresponding techniques for ultra-widely tunable. The interactional mechanism and the control techniques between the THz waves generated by optical rectification and by difference frequency in a nonlinear material, and how to actualize the phase matching with a widely bandwidth as well as the related physical issues in the femtosecond OPO and the difference frequency generation, will be investigated.

本项目提出研究基于光子晶体光纤飞秒激光双波长OPO差频高功率超宽带可调谐超快THz源，以适用于宽谱范围的实时、快速检测的应用需求。本项目包括三部分：高重复频率光子晶体光纤飞秒激光泵浦源；飞秒级宽带可调谐双波长OPO和差频超快THz波发生器。主要研究在光子晶体光纤飞秒OPO系统中实现可调谐双波长运转的条件，包括非线性晶体的结构、谐振腔的稳定和调节、双波长参量脉冲的特性和相位匹配条件；探索在可调谐范围内保持双波长参量脉冲时空特性相同的可能性；研究利用双波长OPO脉冲的差频过程实现高功率超快THz的产生和调谐，包括：可利用的非线性材料的选择和结构设计，实现宽带可调谐的相位匹配条件和相应技术；研究和探索双波长参量脉冲在采用光学整流技术的非线性材料中由光学整流产生THz波和由差频产生THz波的相互作用规律及其调控；研究双波长飞秒OPO和差频产生THz过程中如何实现宽带相位匹配的技术及其相关物理问题。

本项目包括三部分：高功率光子晶体光纤飞秒激光泵浦源；飞秒级宽带可调谐双波长OPO和差频超快太赫兹波发生器。项目取得的主要进展和成果如下：（1）在理论和实验上，系统研究了几种光子晶体光纤飞秒振荡级和放大级结构，及其相互耦合的动力学，最终实现了整个泵浦源系统的优化和性能提升。（2）提出并研制成功了双路独立双腔的宽带可调谐OPO/OPG/OPA的设计方案，该方案与原有单腔双波长运转方式相比，具有更宽的调谐范围，且调谐方便；特别是采用种子注入的参量频率变换系统，大大降低了整个系统的噪声水平和明显提高了整个系统的运转稳定性。该研究成果实现了本项目提出的高功率宽调谐双路差频源的需求。（3）为了提升太赫兹转换效率，我们在理论和实验上系统研究了不同太赫兹产生晶体（GaP、LiNO3、DAST）和泵浦方式（单向、双向、共线、非共线、脉冲面倾斜）下光学整流法产生太赫兹波的效率与泵浦激光特性的相互关系，并且通过不断优化泵浦脉冲参数提升了光学整流法法的太赫兹产生效率。利用单路OPO/OPA泵浦DAST晶体，分别实现了光学整流法的太赫兹产生和可调谐；在双路OPO/OPA同步共线泵浦DAST晶体条件下，已经实现差频产生太赫兹信号。