astrO cO m b s

The 3.6-metre telescope at ESO‘s
La Silla observatory.
Image credit: Iztok Boncina/ESO

measurinG cOsmic velOcities
Spectrographs connected to telescopes areused to identify planets around stars outside
our solar system. Astronomical spectrographs
are typically calibrated against ThoriumArgon lamps or Iodine absorption cells. A
more precise way of calibration should make itpossible to explore deep space more accurate
than ever before, making identification ofEarth-sized planets or direct detection ofcosmic acceleration possible.
Traditional calibration techniques are subject
to uncertainties that unavoidably degrade thewavelength resolution: lines are not evenly
distributed in the spectral range of interest,
have a wide range of intensities and sometimes appear blended. The frequency comb
technology may offer a solution.
a ruler made OF cOmb mOdes
When laser pulses pass through a spectrometer, the regular train of modes is overlapped with the light collected by the spectrograph and hence can be used as a perfectruler to calibrate the apparatus with unprecedented resolution.

6The main challenges are to generate a frequency comb with a sufficiently large mode
spacing that can be resolved by the astronomical spectrograph and to have sufficient comb light in the full spectral operating
range.
Fabry-Pérot cavities can serve as mode filters
to increase the fundamental mode spacing.
Using more than one filter cavity has theadvantage of achieving a higher suppressionof the unwanted modes. Power losses due tothe rejected modes of the filter stages need
to be replenished to ensure sufficient lightintensity for non-linear processes such as frequency doubling and spectral broadening necessary to match the frequency comb spectrum
to the optical bandwidth of the spectrograph.

A solar telescope collects light that is superimposed with the frequency comb light. Together
they are fed to a spectrometer. Since the original mode separation of the frequency comb
(250 MHz) is too close to be resolved by thespectrometer, the light is first filtered using a FabryPérot filter cavity to 15 GHz.

tailOrinG tHe ytterbium
Fiber cOmbs
The FC1000-250 Optical Frequency Synthesizer is an ideal choice for such a system.
Based on an Ytterbium-doped fiber laser, it
allows for amplification to high power levels.
The automated ASTRO Extension Package canbe configured to meet the individual requirements posed by high performance astronomical instruments in need of high-precision calibration.
One OF astrOnOmers’ dreams
FulFilled
Menlo Systems teamed up with scientists from
the Max Planck Institute of Quantum Optics inGermany and the European Southern Observatory to develop calibration instruments

Tilo Steinmetz from Menlo Systems and Constanza
Araujo-Hauck set-up the comb mode filter cavity at
the VTT optical laboratory, in Tenerife.
Image credit: ESO

specially designed for high resolution spectrographs. After successful tests in the laboratory, in 2008 the team has successfully
tested a prototype device using the laser comb
at the Vacuum Tower Telescope (VTT) solar
telescope in Tenerife, measuring the spectrumof the Sun in infrared light. For more information on this proof of principle campaign see
Steinmetz et al. in Science Vol. 321: 13351337, 2008.
Current work focuses on the calibration ofthe HARPS planet-finder instrument on ESO‘s
3.6-metre telescope at La Silla in Chile.
Recently published results by Wilk et al. (inMonthly Notices of the Royal Astronomical
Society Vol. 405: L16–L20, 2010) announce
an era of a promising technique to achieve
the accuracy needed to study the big astronomical questions.

The solar spectrum measured at the VTT on Tenerife. A section is magnified on top; the dark lines
are caused by absorption of gaseous elements inthe photosphere of the Sun and by absorption in
Earth’s atmosphere. The spectral lines of the frequency comb appear as bright streaks that areused as precise calibration lines for the entire solar
spectrum.

The laser frequency comb calibration spectrum isdetermined by fitting individual comb modes withGaussians, which serve as a good approximationof the spectrometer point spread function. A section of the extracted comb spectrum (red dots) isshown together with a sum of Gaussians fitted tothe data (green line). The inset shows one full echelle order (out of 72) with the spectral envelopeof the frequency comb. The range of the zoomed
region is highlighted.

oRDeRInG InFoRMAtIon
menlO systems’ laser selectOrFc1000-250
Optical Frequency Synthesizer

pulse energy in nJJ

1,000orange-MP

astrO

1021orange A

orange A 515

C-Fiber A

C-Fiber A 780

M-Fiber A

C-Fiber 780/T-Light 780
M-Fiber A 780

orange/orange-Combo-Light
1030

520

Extension Package for Mode Spacingin the GHz range

780
1020

orange one

C-Fiber/M-Fiber/M-Comb
OMO
1560

1050

C-Fiber HP/T-Light

wavelength in nm

1120

7