Ultra-Broadband Laser Frequency Combs for Precision Spectrograph Calibration

天文学界面临的一个关键问题是如何为新的超高分辨率光谱仪提供永久的在线校准。超高分辨率光谱仪在回答今天的“重大问题”方面发挥着核心作用，比如发现太阳系外的类地行星，以及“基本”常数的变化。大约从2007年开始，光子学团体就一直在与天文学家合作，提供一种解决方案，以一种超稳定的激光校准源的形式，产生由数千个规则间隔的光学频率组成的“梳子”。诺贝尔奖得主霍尔和汉斯开创的技术展示了这样一个梳状结构是如何稳定的，允许组成梳状线的频率被冻结，精度接近1000,000,000,000,000,000分之一（实际上，这个水平比许多天文学背景所需要的要精确得多）。HIRES和ESPRESSO分别是在E-ELT和VLT上提出的高分辨率光谱仪，其基础科学案例包括寻找类地系外行星，原始核合成和基本常数的可能变化。这两种仪器都需要卓越的径向速度精度和稳定性（HIRES高达2 cm/s），这只能通过以宽带激光频率梳的形式嵌入永久在线校准来实现。目前还没有一种激光频率梳技术能够完全提供必要的波长覆盖范围和模式间隔。此外，目前用于获得必要波长覆盖和模式间隔的技术引入了一些伪影，这些伪影会在光谱仪上部署时破坏校准结果。因此，需要研究探索替代激光频率梳概念的可行性，以满足ESPRESSO和HIRES项目的需求。基于HWU独特的激光频率梳专业知识，并与HIRES和ESPRESSO仪器的利益相关者合作，该项目将评估基于光学参数振荡器的宽带激光频率梳架构的几个新概念，并解决稳定性、均匀性、精度和梳线间距的基本校准源标准。我们的主要工业合作伙伴激光量子有限公司参与了这个项目，将为该项目提供高重复率的钛：蓝宝石泵浦激光器，以及重要的技术诀窍。进一步的开发路线是通过赫瑞瓦特新成立的Chromacity有限公司提供的，该公司成立的目的是将赫瑞瓦特的飞秒OPO技术商业化。项目的结果将采取技术评估的形式，总结候选梳子架构的适用性，以及最有前途的系统的演示。

The astronomy community faces a critical problem in how to provide perpetual online calibration of new ultra-high-resolution spectrographs, which play a central role in answering today's "big questions" such as the discovery of extra-solar Earth-like planets, and the variation of "fundamental" constants. Since around 2007, the photonics community has been working with astronomers to provide a solution, in the form of an ultra-stable laser calibration source producing a "comb" of thousands of regularly spaced optical frequencies. Techniques pioneered by Nobel laureates Hall and Haensch showed how such a comb could be stabilised, allowing the constituent comb lines to be frozen in frequency to precisions approaching one part in 1,000,000,000,000,000,000 (actually a level rather more accurate than is needed in many astronomy contexts).HIRES and ESPRESSO are proposed high-resolution spectrographs at the E-ELT and VLT, respectively, whose underpinning science cases include the search for Earth-like exo-planets, primordial nucleosynthesis and the possible variation of fundamental constants. Both instruments demand exceptional radial velocity accuracy and stability, (up to 2 cm/s for HIRES), which can only be realized by embedding perpetual online calibration in the form of a broadband laser frequency comb.No laser frequency comb technology fully offering the necessary wavelength coverage and mode spacing has yet been demonstrated. Furthermore, the current techniques used to obtain the necessary wavelength coverage and mode spacings introduce artifacts which corrupt the calibration results when deployed on a spectrograph. Consequently research is needed to explore the feasibility of alternative laser frequency comb concepts which could meet the needs of the ESPRESSO and HIRES projects. Building on unique laser frequency comb expertise at HWU, and working with stakeholders in the HIRES and ESPRESSO instruments, this project will evaluate several new concepts for broadband laser frequency comb architectures based around optical parametric oscillators, and addressing the essential calibration-source criteria for stability, uniformity, accuracy and comb-line spacing.Engagement in the project by our principal industrial partner, Laser Quantum Ltd., will support the project with Ti:sapphire pump lasers of high repetition rate, and with vital technical know-how. A further exploitation route is provided via the new Heriot-Watt spin out company Chromacity Ltd., formed to commercialise Heriot-Watt's femtosecond OPO technology.Outcomes from the project will take the form of a technical assessment summarizing the suitability of the candidate comb architectures, and a demonstrator of the most promising system.