啁啾脉冲放大(CPA)技术:从衍射光学角度的回顾与展望

啁啾脉冲放大(CPA)技术使飞秒脉冲的峰值功率从原来的千瓦级飞速发展到现在的拍瓦级,这一突出贡献使其获得了2018年诺贝尔物理学奖。过去几十年里已有很多论文从激光物理的角度,向公众解释了运用CPA技术将激光脉冲放大的原理,即使用一对高密度光栅可以将激光脉冲放大到最强。

啁啾脉冲放大技术原理示意图

从衍射光学的角度,光栅的压缩和展宽是CPA技术的核心。但是,很少有论文指出除了等密度光栅之外,其他的光栅结构也可以用于压缩或展宽激光脉冲。

中国科学院上海光学精密机械研究所周常河研究员在Chinese Optics Letters 2020年第18卷第11期发表的综述(C. Zhou, Chirped pulse amplification: review and prospective from diffractive optics [Invited])系统介绍了多种控制飞秒脉冲的光栅技术。除了CPA采用的非常著名的等密度光栅技术之外,还有其它光栅结构,如达曼光栅、双倍密度光栅、深刻蚀石英光栅等,都可以压缩或展宽激光脉冲,为CPA技术提供了一个完整的图谱。

双倍密度光栅、深刻蚀光栅用于压缩飞秒激光脉冲及大尺寸的棱镜对压缩器

达曼光栅:是一种可以将入射单色光高效生成均匀光强点阵的位相光栅,在飞秒脉冲加工中可以将激光脉冲在不同的衍射级次分成多个等强度的光斑,每个级次的衍射需要相应倍增密度的光栅做补偿。由于达曼光栅产生的光斑阵列光强均匀性不受入射光波影响,近年来广泛应用于光纤星状耦合、多激光干涉、光互连、并行读取或作为逻辑阵列器件的光源等。

双倍密度光栅:意味着两块光栅,其中第二块光栅的密度是第一块光栅密度的两倍,经过第一块光栅衍射的激光脉冲可以被第二块光栅衍射而原路返回,由此构成飞秒激光脉冲压缩器。其优点是小巧紧凑,可能替代庞大棱镜对装置。

深刻蚀光栅:也是表面浮雕光栅,是具有亚波长和深刻蚀双重特征的微纳光子学元件。比如深刻蚀石英光栅组成的飞秒脉冲压缩装置体积小、结构紧凑、造价低。由于石英光栅的激光破坏阈值很高,可同样用于高能光纤激光器中。

达曼光栅、双倍密度光栅、深刻蚀光栅已经被用于处理飞秒激光脉冲并应用于上海光源工程等大型科研项目。此外还有其他光栅结构用于产生激光的多脉冲,以满足各种应用需求。

制造高质量的光栅,尤其是大尺寸米级光栅,并将光栅栅距精度控制在皮米量级,是一个挑战极限的工作,不仅可以促进激光的更多独特应用,还使人类得以进入皮米光学的世界。

周常河研究员于2019年发明了皮米光梳,基于皮米光梳的物理思想,也许我们最终会打开皮米光学的大门。

Chirped Pulse Amplification: review and prospective from diffractive optics

Chirped pulse amplification (CPA)technique has been awarded Nobel prize in Physics in 2018.There are rich literatures to explain this technique to public, most from the view of laser physics, that CPA is a powerful tool, using a pair of high-density gratings, to amplify laser pulses to the most powerful laser pulses in the past decades.

From the view of diffractive optics, compression and stretching using gratings are the essence of CPA technique. However, there are few reports to explain that other grating structures can also be used to compress and stretch the femtosecond laser pulses. Prof. Changhe Zhou from Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, systematically elucidates the full picture of diverse grating technics that enable the manipulating of femtosecond laser pulses in Chinese Optics Letters, Volume 18, Issue 11, 2020 (C. Zhou, Chirped pulse amplification: review and prospective from diffractive optics [Invited]). Apart from the well-known CPA technique using equal-density gratings, the femtosecond laser pulses can also be compressed by using other grating structures, such as Dammann grating, doubled-density gratings, and deep-etched gratings, and most of these techniques have been invented by Prof. Zhou's research group over the past ten years.

Dammann grating can split the femtosecond laser pulses into multiple equal-intensity spots at different diffraction orders, when the diffraction at each order is compensated by using a corresponding increased-density grating. Doubled-density gratings means there are two gratings where the density of the second grating is doubled of the first grating. The laser pulses at diffractive orders of the first grating can be returned back by placing the second grating whose density is doubled of the first grating, therefore, a compressor of femtosecond laser pulses can be constructed. Deep-etched gratings are the surface-relief gratings, such as the deep-etched fused silica gratings, which can compose a small-size compressor of laser pulses compared with the traditional large-size compressor of prism pair, which is very attractive for building a small-size compressor of femtosecond laser pulses. Besides, other structures of gratings have been invented to generate the multiple pulses of laser pulses for practical applications.

Making high-density gratings with unique property for applications of laser pulses is a challenging work, particularly controlling the period of large meter-sized grating in picometer accuracy. A new world of pico-optics is open for new researches right now, and the research group of Prof. Zhou has invented picometer comb taking advantages of the new concept. It might eventually enable the researchers to explore picometer optics in the future

Doubled density gratings and Deep-etched gratings for compression of femtosecond laser pulses in comparison with the commercial large-sized prism-pair compressor.