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Astronomical Spectrograph Calibration at the Exo-Earth Detection Limit

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Astronomical Spectrograph Calibration at the Exo-Earth Detection Limit Telescopes and Instrumentation Astronomical Spectrograph Calibration at the Exo-Earth Detection Limit Gaspare Lo Curto1 length (such as in Å), as a fractional For the measurement of frequencies, Luca Pasquini 1 wavelength, e.g. 10–7, or by scaling by the LFCs represent the ultimate level of preci- Antonio Manescau1 speed of light to express it in m/s. The sion, as they are locked to the energy Ronald Holzwarth 2, 3 limitation on Th-Ar wavelength precision level of a well-known atomic transition via Tilo Steinmetz 3 is due not only to the measurement pro- an atomic clock. They are the most pre- Tobias Wilken 3 cess (Palmer Engleman, 1983), but also cise time-keeping devices, and the unit of Rafael Probst 2 to the production method (contamination) measurement of time in the International Thomas Udem 2 and aging of the lamps. Even when aver- System (SI), the second, is defined by the Theodor W. Hänsch 2 aging over a wide spectrum with say, caesium transition via a caesium atomic Jonay González Hernández 4, 5 10 000 lines, measurements are limited clock. LFCs have many applications, from Massimiliano Esposito 4, 5 to an overall precision of 10–9 at best metrology and precise time-keeping to Rafael Rebolo4, 5 (the achievable precision is furthermore laboratory atomic and molecular spec- Bruno Canto Martins 6 degraded by line blending, and non- troscopy. Now the period is beginning Jose Renan de Medeiros 6 uniform density of the Th lines across the when LFCs will “look at the sky”. spectrum). 1 ESO This precision in the measurement of the The demonstrator programme 2 Max-Planck-Institut für Quantenoptik, positions of spectral lines is not sufficient Garching, Germany for various compelling science cases: In 2006 ESO approached the group of 3 Menlo Systems GmbH, Martinsried, – the measurement of the variation of the Prof. Theodor Hänsch at the Max Planck Germany fundamental constants, which requires Institute for Quantum Optics (MPQ), to 4 Instituto de Astrofisica de Canarias, a precision at least as good as the study the possibility of using an LFC for La Laguna, Spain precision with which the constants are the calibration of high-resolution astro- 5 Departamento de Astrofísica, determined; for the best known (the nomical spectrographs. Due to its unsur- Universidad de La Laguna, Tenerife, proton to electron mass ratio and the passed stability, HARPS at the 3.6-metre Spain fine structure constant) this is telescope in La Silla provided the best 6 Universidade Federal do Rio Grande de ~ 3–4 × 10–10 (Beringer et al., 2012); candidate to validate the performance of Norte, Natal, Brazil – the amplitude of the recoil motion this technique (Mayor et al., 2003). A fruit- imprinted by the Earth on the Sun ful collaboration was initiated between is ~ 9 cm/s. The radial velocity detec- ESO, MPQ and Menlo GmbH, a spin-off Following the development of the laser tion of an extrasolar planet with company from MPQ which markets LFCs, frequency comb which led to the 2005 the mass of the Earth in a 1 AU orbit with the goal of demonstrating the feasi- Nobel Prize in Physics, we began in­­ around its solar-type star therefore bility of operating an LFC to calibrate an vestigating the possibility of using this requires a measurement precision of astronomical spectrograph, thus opening novel technology for precise and accu- about 3 cm/s, or 10–10; up a new horizon for the current and next rate spectrograph calibration. A pro- – the direct measurement of the expan- generation high-precision, high-resolution gramme was begun, aimed at demon- sion rate of the Universe, which can spectrographs. strating the capabilities of laser fre- constrain the cosmological parameters quency combs (LFC) when coupled to that define the metric of the theory The demonstrator programme started in an astronomical spectrograph. In the of gravity, requires a measurement pre- 2007 (Araujo-Hauck et al., 2007) and last three years we have tested an LFC cision at the level of cm/s (10–10) for concluded in 2011. In this period a proto- connected to HARPS at the 3.6-metre over ten years (Liske et al., 2008). type LFC dedicated to astronomy (or telescope in La Silla, the most precise astro-comb) was developed and refined. spectrograph available. Here we show The new class of giant telescopes with The astro-comb was tested in the labora- the very promising results obtained so greatly increased light-collecting power tory; in the infrared (IR) regime on a tele- far, and outline future activities, including will naturally ease the photon noise limi- scope for the first time (Steinmetz, 2008); the provision of an LFC system for rou- tation to high precision spectroscopy by to validate technical solutions; and in four tine operation with HARPS, to be offered a factor of five to ten. A new calibration campaigns in the visible, with HARPS to the community in the near future. source is therefore needed, which ena- at the 3.6-metre telescope in La Silla, to bles these science cases and capitalises test the global performance (Lo Curto on the great opportunity that giant tele- et al., 2010; Wilken et al., 2010; Wilken et The most widely used wavelength refer- scopes open up for high precision spec- al., 2012). ence in astronomical spectroscopy, the troscopy. The ideal calibration source thorium–argon (Th-Ar) lamp, can achieve would be at least ten times more precise An LFC consists of thousands of equally a precision on the determination of indi- than Th-Ar lamps allow and would have spaced frequencies over a bandwidth vidual line positions of several tens of many unblended lines, with approximately of several THz. It is based on the proper- metres per second (m/s). The uncertainty the same intensity and equally spaced ties of femtosecond (fs) mode-locked in the wavelength of a spectral line can across the spectrum. The obvious choice lasers. The frequency difference between either be expressed as an absolute wave- is the laser frequency comb. two neighbouring lines corresponds to 2 The Messenger 149 – September 2012 Figure 1. The spectrum of the LFC dispersed by a Although an LFC based on fibre lasers are detrimental to nonlinear conversion low-resolution grating and projected on a wall. The is essentially an off-the-shelf product, its processes. The approach to overcome individual lines are not resolved and only the contin- uum is visible: its spectral structure is due to the adaptation for use in an instrument like this dilemma employs two key compo- spectral broadening stage and can be flattened out HARPS requires major developments. nents. First, a comb system based on by a spatial light modulator (SLM). The basic turn-key LFC systems available an Yb-fibre laser enables the use of Yb- on the market today deliver a comb of fibre high-power amplifiers to reduce the repetition frequency (frep) of the lines centred at 1025–1050 nm, with a re­­ the problem of low pulse energies. Sec- pulsed laser and is therefore constant petition frequency of 250 MHz and gen- ond, when using specially designed across the comb spectrum. The entire erally cover only few nanometres in wave- photonic crystal fibres (PCFs), relatively LFC spectrum can be described by the length. Resolving spectral lines 250 MHz low pulse energies are sufficient to obtain simple equation f = foff + n∙frep where frep apart in the visible range requires a spec- spectral broadening. is the repetition frequency, i.e. the dis- tral resolution of more than six million, tance in frequency between two adjacent which is not practical for the typical use Another challenge comes at the FPC fil- lines, and foff is the “offset frequency” that of an astronomical spectrograph. An LFC tering stage and arises from the fact that can be interpreted as a “zero point”; n with such a line spacing would appear the nonlinear processes (interactions is a large integer which projects the radio as a continuum source to instruments between photons) in the PCF can amplify frequencies foff and frep into the visible such as HARPS or UVES (see Figure 1). spectral lines that were intended to be domain. Since all frequencies used in the suppressed. We saw this effect during system are radio frequencies, they can New developments are needed to oper- our first test run in La Silla. The solution be stabilised to an atomic clock using ate an LFC on an astronomical spectro- was to employ more FPC cavities, essen- well-established electronic phase-locking graph in the visible with a spectral resolu- tially one of them replicating the sup- techniques. In this way, each optical tion of ~ 105: pression of the unwanted modes, and frequency obtains the accuracy and long- 1) increase the repetition frequency to with a higher finesse. Finally in our system term stability of the atomic clock. ~ 18 GHz; we used three FPCs in series to guar- 2) double the frequency of the spectrum antee a sufficient suppression of the An LFC acts like a gear, transferring the to have it centred in the visible at intermediate lines. A series of four FPCs precision of atomic clocks from the micro- ~ 520 nm; was also tested, but no improvements wave regime to the optical. LFCs are 3) broaden the spectrum to increase were noticed from the fourth cavity. The the ideal calibrator for astronomical spec- wavelength coverage. line spacing is increased to 18 GHz, trographs if they can cover the spectral These are the three steps where most of which is well resolved by HARPS (the in­­ bandwidth of the spectrograph with a the efforts of the programme have been strument has an intrinsic resolution of sufficiently flat spectrum and if their line focussed.
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