Ultra-Broadband Laser Frequency Combs for Precision Spectrograph Calibration

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.

The Photonics community now plays a pivotal role in providing astronomers with the tools they need to make further advances at the leading edge of the subject. Questions such as, "Are the fundamental constants time-varying?" can only be addressed by using a new class of optical sources known as "laser frequency combs", which uniquely offer the traceable precision needed to calibrate the high-resolution spectrographs employed to detect tiny changes in the wavelength of starlight. Existing frequency-comb sources have already been tested on astronomical spectrographs, obtaining impressive results but also revealing some of the limitations presented by the current comb technology. This project aims to develop a new class of comb sources which will bypass the main limitations of state-of-the-art combs, extending their wavelength coverage and providing a purer* frequency scale for use in spectrograph calibration. In this way, we expect to contribute significantly to the international effort which will, in time, deliver exciting and important results of the kind that captivate the public imagination and motivate a new generation of scientists and engineers.

On a shorter timescale, the technology to be developed within the project will have several immediate impacts for UK industry. Firstly, our collaboration with Laser Quantum Ltd. will provide them with a test-bed for their new pre-commercial 10-GHz femtosecond Ti:sapphire laser, delivering "white-paper" results that can be used as they open up new markets for this disruptive technology. The Heriot-Watt spin-out company Chromacity Ltd. offers a route to commercialising the OPO sources to be developed in the project, with the potential for job creation and wealth creation in the UK.

Industry also stands to benefit from the technology itself, since new applications are opened up by high-repetition-rate tunable lasers. These include in metrology where, for example, the National Physical Laboratory could benefit from access to such high repetition rate sources, either in optical frequency metrology or in microwave metrology. In the defence sector, there is a growing recognition that ultra-stable "flywheeling" oscillators have a role to play in protecting our national infrastructure from the consequences of a GPS outage. High repetition rate modelocked oscillators are amongst the most intrinsically stable oscillators ever invented, and the tunable 10-GHz source developed in the project presents possibilities to distribute this timing signal in fibre or free-space. Another sector, life-science imaging can also expect to benefit from a simple "near-continuous-wave" broadband source, as a means of exciting wavelength dependent fluorophores used in confocal and time-resolved imaging.

In the project we have included plans (outlined in more detail in the Pathways to Impact document) to engage with non-academic stakeholders, industry and the general public. These plans include the use of industry-academic workshops, dissemination at trade events, public lectures and outreach events.

*originating from the elimination of parasitic frequency sidebands