基于非线性光谱调制的3微米矢量超短脉冲产生及其机理研究

The 3 μm ultrafast lasers with polarization singularity property have potential applications in high-resolution microscopy, electron acceleration, and high-energy attosecond laser. However，it is a big challenge for the conventional mode-locked solid laser to produce stable optical-cycle-duration pulse trains due to the intrinsic limited gain bandwidth of laser materials. A novel intracavity method of generating few-cycle pulses with vector beam from all-solid-state laser is proposed based on the nonlinearity spectrum broading effect induced by the super-diffraction limit of radially polarization beam. The novel Er-doped sesquioxide ceramics pumped by the 976 nm fiber laser system and the sub-wavelength deep-focusing structure of the radially polarization beam will be constructed to control the intensity of nonlinearity. Based on the spectrum broading induced by the self-phase modulation, it will yield ~3 μm few-optical-cycle pulses with polarization singularity property directly output from the laser oscillator. The corresponding spectrum bandwidth will not be clamped by the limited fluorescence bandwidth of laser material again. The proposed mechanism on nonlinearity spectrum broading has broken the theoretical limitation of pulse-shortening due to the gain bandwidth limitation from conventional mode-locked lasers. It will make it possible that optical-cycle pulses will be directly generated from the laser oscillator with any narrow bandwidth laser materials. It will also largely increase the development of ultrafast-solid-lasers.

具有“偏振奇点”特性的3微米矢量超快激光在超分辨率成像、电子加速和高能阿秒脉冲产生等前沿领域具有重要的潜在应用价值，然而现有锁模方法受增益介质的带宽限制很难直接产生近光学周期量级脉宽的超短脉冲。课题基于矢量光束的超衍射极限特性，提出了一种非线性光谱展宽机制直接产生近光学周期量级的全固态矢量光束超短脉冲新方案。拟采用976nm单模光纤激光系统高亮度泵浦掺铒倍半氧化物陶瓷产生径向偏振矢量光束，通过其独特的亚波长聚焦特性实现非线性光谱展宽，将可获得3微米近光学周期量级的超短脉冲，其光谱带宽不再受增益介质带宽限制。课题所提出的非线性光谱展宽机制突破了传统锁模增益介质带宽的原理性限制，使所有具有窄光谱的增益介质直接产生周期量级脉冲成为可能，将对整个固体超快激光技术发展产生巨大的推动作用。

具有“奇点”特性中红外超快激光在超分辨率成像、电子加速和高能阿秒脉冲产生等前沿领域具有重要科研应用价值。.本课题面向“奇点光束”，研究了周期量级超短脉冲的产生和控制机理，搭建了以3微米，2微米和1.6微米为代表的中红外超短脉冲激光系统，获得了 “偏振奇点”和“相位奇点”光束：. 1) 理论上研究了Er3+、Tm3+离子掺杂的中红外周期量级超短脉冲激光产生和控制机理，基于金兹伯格-朗道方程建立了超短脉冲传输模型，模拟了被动克尔介质(Cr3+：ZnSe)作为非线性元件实现超短脉冲光谱调控和脉冲窄化行为以及高阶孤子的动力学过程。研究了折叠腔型(X-腔型)固体锁模激光系统中高阶横模产生机理，首次建立了腔镜三维空间折叠引起的高阶横模调控模型，数值模拟了古伊相位调控下的高阶横模产生和演化行为。. 2) 实验上搭建了覆盖1.6-2.8um波段的中红外超快激光系统，获得了具有矢量和涡旋特性的超短脉冲激光。基于SESAM，首次在1.6微米掺Er3+固体激光振荡器中获得连续锁模脉冲激光，脉宽4ps，平均功率达1W；基于a-cut的Tm:CYA，在2微米波段首次实现了具有“相位奇点”特性的超快涡旋（LG01阶）激光腔内直接产生，平均功率320mW，脉宽为3ps，其手性可控。基于c-cut的Tm:CYA实现了“偏振奇点”特性的径向和角向矢量光束输出。基于掺Er3+:Y2O3陶瓷，实现了3微米激光输出，输出波长在2.72微米和2.82微米之间可切换。通过三焦点系统，采用Cr3+:ZnSe和SESAM调控元件，获得了稳定部分锁模脉冲激光输出，最短脉宽小于30ps，这是3微米波段固体激光振荡器获得最短脉宽。. 3）在超快激光器的控制方面，还开展了基于AI算法的智能化超快激光研究。基于遗传算法搭建了2微米智能化控制超快激光系统，脉宽325fs，实现了环境扰动下锁模脉冲的自恢复和自调节，这是首次在中红外波段超快激光系统中实现了AI控制。. 本课题研究成果突破了传统锁模增益介质纵模和空间横模限制，使具有窄光谱的增益介质和高阶横模超短脉冲直接产生成为可能，开拓了超短脉冲激光特性研究新维度，将有力推动对中红外激光技术进一步发展。