前向反馈式飞秒光梳放大与相干合成

High power Optical Frequency Combs has been widely used in many areas, such as precision spectroscopy and optical frequency metrology. As promising laser sources for optical frequency comb, mode-locked fiber lasers have attracted much attention for their compactness and the capability of stable, long-term operation. Besides, developments in large-mode-area ytterbium-doped double-clad photonic crystal fibers have greatly contributed to pulse amplification up to hundreds of watts and therefore stimulated the advance of high-power fiber-based frequency combs. In order to stabilize CEP of the high-power lasers, either a phase servo loop or a difference-frequency-generation scheme can be used. However, each approach has its own distinct drawbacks: the former suffered from its limited feedback bandwidth and negative side effects on laser oscillators, while the latter had a zero offset frequency seriously influenced by increased 1/f noise and other low frequency noises. Recently, a self-referenced feed-forward scheme based on an acousto-optic frequency shifter was demonstrated to be able to overcome those shortcomings. In this project, high power optical frequency combs will be realized by feed-forward scheme and Coherent Combining technology. Consequently, the CEP frequency will be locked on the laser repetition rate with a relative linewidth of mHz level. The integrated phase noise of the CEP frequency amounted to hundreds of mrad, corresponding to a timing jitter of hundreds of attoseconds. Such a precise CEP control of a fiber laser paves the way for further development of compact fiber-based high-power frequency combs.The merging of a self-referenced feed-forward scheme with coherent combining of beams has led to precision control of high power fiber lasers. High power optical frequency combs have a major impact on nonlinear spectroscopy.

高功率飞秒光学频率梳在精密光谱学、光学频率测量等领域有着广泛的应用。光纤激光以其紧凑型、稳定性等特性，是实现光梳的理想激光源之一。大模场光纤放大技术，促进了高功率光梳的发展。为了使激光载波包络相位稳定，相位伺服锁相环和差频产生的方法被广泛采用，然而，两者存在着对振荡源有负面影响、锁定精度受限的不足。在本项目中，我们提出基于前向反馈式飞秒光梳放大与相干合束，拟利用声光频移器，采用自参考前向反馈技术，锁定后光纤光梳线宽将可控在毫赫兹量级，相位噪声和时间抖动分别可控在百毫弧度和百阿秒量级。这种精确控制光纤光梳为进一步相干合束实现高功率光梳奠定了基础。光梳相干放大合束，突破单一放大系统的功率放大瓶颈。拟通过前向反馈式操控分束后的激光的载波包络相位漂移，实现零频噪声检测与补偿，实现光梳相干合束。光纤高功率光梳放大系统的研制不仅可以为全波段光梳提供高能脉冲光源，同时它还是实现光梳精密光谱的基础。

高功率飞秒光学频率梳在精密光谱学、光学频率测量等领域有着广泛的应用。光梳相干放大合束是突破单一放大系统的功率放大瓶颈的有效方法。为了实现两路时频域精密控制的超短脉冲相干合成，我们采用钛宝石激光器的红外光谱成分作为种子光；搭建两路并行的光纤放大器，采用级联式结构，合理设计每级放大器的增益，分别采用单模光纤、光子晶体光纤，实现平均功率从毫瓦到百瓦的超短脉冲放大。通过测量放大后超短脉冲的时频域特性，证明放大过程中的自发辐射噪声、自相位调制噪声均得到有效抑制。采用基于声光移频器的前馈式载波包络相位补偿方式，分别对两路放大器引入的载波包络相位噪声进行有效控制，通过对拍频信号进行滤波、放大然后驱动声光移频器，自适应地补偿了放大前后脉冲的载波包络相位飘移。对脉冲载波包络相位飘移测量的结果表明：加入反馈后，飘移范围得到有效控制。实验上，采用偏振式结构实现两路超短脉冲的相干合成，合成前两路超短脉冲的脉冲宽度分别是254 fs和296 fs，合成后脉冲宽度为277 fs。通过测量合成前后的光谱，在光谱上观察到了明显的干涉条纹，说明放大过程中的相位噪声得到有效抑制，脉冲的相干性得到很好的保持。同时，为了实现稳定的偏振式相干合成，本项目采用最小值算法，根据合成后功率的变化得到两路放大器的相位差信息，进而决定相位控制器的补偿方向和补偿量大小，结合单片机，实现数字化的锁定，获得大于90%的合成效率。本项目采用前馈式载波包络相位补偿方式和数字化相位补偿技术，实现了脉冲宽度小于300 fs的超短脉冲偏振式相干合成，为载波包络稳定的多路超短脉冲相干合成和光谱组束提供了技术参考。