结构化波导设计增强GHz飞秒超短脉冲锁模克尔效应的理论与实验研究

Mode-locked femtosecond lasers with repetition-rates in excess of 1 GHz have many applications in areas such as industrial, medical, scientific research and military fields. However, within multi-GHz repetition-rates the relatively low intracavity pulse energy would enormously weaken Kerr-effect of the gain medium, which is detrimental to shortening the pulse duration of mode-locked lasers with a broad spectrum. Currently the pulse durations of mode-locked lasers with high repetition-rates are still on the order of picosecond, which are far away from the transform-limit of laser spectra. In this project, we will use the waveguide structure with special design to replace the bulk medium, providing a stronger Kerr-effect and self-phase-modulation in the gain medium. In order to balance the dispersion management intra-cavity, in this project we will employ x-standard shape cavity to separately control the repetition-rate and group delay dispersion instead of ultra-compact linear cavity. Our aim is to achieve gain bandwidth Fourier-transform-limited femtosecond laser pulse in multi-GHz repetition-rates by inserting in the group delay dispersion compensation elements, which has a smooth variation in a broad spectrum range. The development of this project is promising to realize femtosecond laser pulse within few optical-cycles in multi-GHz repetition-rates, providing coherent optical source in practical applications with self-owned intellectual property.

高重复率（＞GHz）的飞秒脉冲相干光源在频率梳测量,光通信,激光雷达，光学制样非线性显微技术等领域具有显著的应用优势。但过高的重复频率会大幅降低激光的单脉冲能量，从而相应地减弱由克尔效应导致的光谱展宽，极大限制了GHz飞秒激光的脉冲宽度，目前的高重频飞秒脉冲光源普遍仅达到亚皮秒量级，远未达到增益带宽极限。本项目提出精确设计结构化波导，替代传统的块状激光材料,增强激光介质的克尔非线性效应和光谱自相位调制，实现增益带宽极限的锁模激光发射;为解决紧凑线型腔导致的色散管理困难问题，项目拟采用折叠腔型分离控制重复频率和色散管理，引入宽波段范围变换平缓的色散补偿元件，最终产生近增益带宽限的GHz飞秒脉冲激光。项目研究成果，有望在GHz超短激光振荡器中实现若干光学周期的飞秒脉冲，从而为应用领域提供一种具有自主知识产权的高重频飞秒相干光源。

GHz量级的高重复率超快激光脉冲，可直接从紧凑型锁模激光系统获得，在天文学、基于频率梳的精密计量学、集成芯片光子学等各种科学应用中引起极大关注。本项目在完成波导主体材料常规锁模超短脉冲表征的基础上，结合波导尺寸和高重频谐振腔设计，理论计算材料的非线性光克尔效应强弱。本项目采用了飞秒激光直写的方法来制备波导样品，在钨酸镁(MgWO4)晶体表面制备了三种不同包层尺寸的通道波导。该方法基于将飞秒激光束聚焦在透明材料内的小体积内，导致受损区域的部分非晶化，从而导致永久折射率变化。本项目采用了折叠腔型来进行高重频（GHz）锁模实验，在优化连续激光输出性能之后，将全反镜替换成可饱和吸收镜（SESAM），调整折叠腔和增益介质以及SESAM之间的距离，改变波导层内和SESAM上的腔模大小，以达到启动SESAM辅助克尔透镜锁模的机制。目前在引入负色散量-3700fs2和SESAM的实验条件下，获得重复频率1.13GHz，最短脉冲宽度为357fs的超短脉冲输出。