Laser Frequency Combs for Astronomical Observations

T. Steinmetz1,2, T. Wilken
1, C. Araujo-Hauck3, R. Holzwarth1,2, T. W. Hänsch1, L. Pasquini3, A. Manescau3, S. D'Odorico3,
M. T. Murphy
4, T. Kentischer5, W. Schmidt5 , and Th. Udem1
1Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Strasse 1, D-85748 Garching, Germany
2Menlo Systems GmbH, Am Klopferspitz 19, D-82152 Martinsried, Germany
3European Southern Observatory, Karl-Schwarzschild-Strasse 3, D-85748 Garching, Germany
4Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Mail H39, PO Box 218, Victoria 3122, Australia
5Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, D-79104 Freiburg, Germany

Published in Science 321, 1335 (2008)

The paper

Preprint version: PDF


Media releases:
Max-Planck Society: PDF
(in German)
ESO Media release: PDF
Swinburne Media release: HTML

Some background

Of any physical quantities time and frequency can be measured with the utmost precision. The best of today’s caesium atomic clocks would loose only one second after about a million years. A so-called optical frequency comb, for which the 2005 Nobel Prize in Physics was awarded to Theodor Hänsch (one of the authors of this paper) and John Hall, provides a simple means to transfer this accuracy to the spectral lines of a laser. For this purpose, a special laser which emits extremely short flashes is used. Such a laser emits up to a million spectral lines, i.e. colours, at once instead of the usual single colour laser. If the laser lines are superimposed on the star's spectral lines (Fraunhofer lines) the latter can be readily measured with the accuracy of an atomic clock. Due to the movement of the star these lines are shifted in frequency with respect to the corresponding lines observed in the laboratory. This is the so-called Doppler effect which might be more familiar in acoustics when the pitch of the siren of an ambulance changes as the vehicle moves towards or away from the observer. Using this effect the velocity of a star relative to an observer on Earth can be measured. With the frequency comb such a measurement can now be done much more accurately.

Cosmic velocities are very important quantities, for example to find extrasolar planets, i.e. planets outside our solar system, because they are generally very small as compared to stars and don't emit light themselves. They are not visible outside our solar system, not even when one uses the world's largest telescope and points it at the closest star beyond our Sun
(Alpha Centauri 4.3 light years away). In fact, until a few years ago it was not known whether there are other stars with planets besides our Sun. It seemed possible, albeit very unlikely that our Sun is the only one with a habitable planet among the approximately 200 billion stars that make up our galaxy alone. Meanwhile there are different ways to detect extrasolar planets. A very successful method is to look for a tiny periodic recoil motion of the star caused by the orbiting planet. Very high accuracy is required for this method. Our Sun is wobbling by only about 10 cm per second due to the orbiting Earth, which is superimposed on the movement of the Sun going around the galaxy with  at a speed of 220 kilometres per second. That's why with current velocity measurements only giant planets (like Jupiter) in a close orbit around low weight stars can be detected. With the accuracy of the frequency comb it will now be possible to detect Earth-like planets going around Sun-like stars within a distance that places this planet within the habitable zone.

Another important application of the precise measurements of cosmic velocities concerns the universe as a whole. Since the first velocity measurements by Edwin Hubble in the 1920's it is known that the universe is expanding. This is also explained by the theory of general relativity and understood to have been triggered by the huge explosion called Big Bang. Recent measurements of the microwave Wilkinson Anisotropy Probe (WMAP) suggest that this expansion is actually speeding up whereas one would expect it to slow down because the only known force between galaxies is gravity which is clearly attractive. To reconcile this effect with the equations of general relativity theoreticians invented a mysterious undetected energy
called "dark energy". The problem with this energy, which does not resemble any of the known forms, is that it does not shw up in any other processes observed in nature. A natural law that can only explain one process usually tells us that there is something wrong with the concept. The other way out of this dilemma is to assume that there is something wrong with the theory of general relativity, that is needed to interpret the WMAP data. The optical frequency comb offers a solution. Thanks to its very high accuracy changes of cosmological velocities can now be detected during observation periods that are much shorter than the age of the universe. Within 20 years of observation it should become obvious whether the universe is indeed speeding up or if there is something wrong with general relativity.

Frequently asked questions

Q: You demonstrated resolution of  9meters/second  - What would be needed to reach your target of 1cm/sec?

A: We need more comb lines than we have used so far. We hope to be able to increase their number from 58 to 100,000. In addition we need to use a suitable spectrometer. The VVT was never designed for that purpose and showed a drift of the calibration that corresponds to a spurious velocity change of up to 8m/sec within one minute, whereas specially designed spectrometers such as HAPRPS, are stable within 1cm/sec for at least a months. Clearly there are many orders of magnitude margin and we hope to gain the missing three in this way.

Q: You observed the solar spectrum - what would be needed to observe stellar spectra? How would you observe faint galaxies at high red shifts?

A: We would have to go to a larger telescope. For the real faint objects the Very Large Telescope (VLT) or the planned European Extremely Large Telescope (E-ELT) of the European Southern Observatory would be ideal.

Q: How important are the motions of hot gases in stars? Would the Doppler shifts blur the spectral lines, or do you have ways to overcome that limit?

A: They are very important if one wants to know absolute line positions. This could be interesting if one is to determine the gravitational redshift at the surface of the Sun which would be a very good test of general relativity (GR). Such a test  would go beyond the Newtonian terms of GR which is not possible to conduct on Earth since its gravity is too weak (or our best clocks aren't accurate enough). We don't mention this possibility in our paper because of problems with convectional blurring, Zeeman shifts etc. that are present at the Sun's surface. However, if we are interested in acceleration rather than velocity we would look only for a time dependence of the line positions and these systematics should cancel out as long as they are constant in time.

Q:  What signal to noise issues do you face with astronomical objects?

A: Of course the faintest objects, at the largest possible distance are also among the most interesting. To pick up enough photons from them one needs to have a telescope with a large diameter (the VLT has 8m and the E-ELT is planned to have 42m). The signal to noise ratio is also connected with spectral resolution. If we use a spectrometer with higher resolution, the limit light that the telescope can collect is distributed among more pixels (detectors) so that dark count rates will restrict the lowest signal to noise that can be detected. That's why resolution has to be traded in for signal to noise and we have to increase the mode spacing of our comb. This is actually the main challenge on the instrumental side.

Q: Would you observe from the ground or in space?

A: No plans to go to space. The combs do not yet work in the lab the way we want them (large mode spacing AND broad bandwidth). I guess it will be lot's of engineering to get them space qualified.

Q: How does your group's approach differ from that taken by the group from Harvard?

A: The Harvard group has so far only demonstrated that they can filter a comb in the lab to increase the repetition rate. We have gone one step further by using an actual telescope that we calibrate. However,so far at both places we will have to wait for interesting physics to come out. 

Publicity images
JPG (click for full resolution) Other formats Description/caption Credit information

none Scheme of the experiment: Using a telescope the light from the Sun or another star is coupled to an optical fibre that guides it to a spectrometer (prism) in order to resolve the spectral lines. The spectral lines from the Sun (Fraunhofer lines) appear in dark because they emerge from absorbing atoms and ions inside the Sun's photosphere. Superimposed are the short bright spectral lines of the pulsed laser that makes up the frequency comb. This also reveals the origin of the name for that regular structure, the frequency comb. ESO

none The frequency comb, which is the light from the pulsed laser, consists of many colours which is only revealed when observed with a high-resolution spectrometer such as typically used in astronomical telescopes. The spectral lines of the comb can be stabilized to the frequency given in the graph using an atomic clock. Theodor Hänsch

avi movie The orbiting of a planet (green) around a star imposes a wobbling movement of that star which is greatly exaggerated in this sketch. This motion is synchronized with the orbit of the planet and causes a periodic variation of the spectral lines or colour of the star. This color change is greatly exaggerated. In reality one needs the precision of an atomic clock to see it when dealing with a small planet like Earth. Thomas Udem
none The orbiting of a planet (green) around a star imposes a return movement of that star which is largely exaggerated in this sketch. The result in a quiver motion is synchronized with orbit of the planet that causes a periodic variation of the spectral lines or colour of the star. Also this colour change is greatly exaggerated. In reality one needs the precision of an atomic clock to see it when dealing with a small planet like Earth. Thomas Udem

avi movie Through the expansion of the universe all galaxies seem to move away from us. Because of that their spectral lines are all shifted towards red as Edwin Hubble figured out already in the 1920's. On the other hand, if the universe contracted the spectral lines of the galaxies would appear shifted towards blue as in the lower graph. Through a minute change of the magnitude of the shift, as is now measurable with the frequency comb, one can decide whether the universe is indeed speeding, asthe cosmic microwave background data suggests assuming the valitity of general relativity.
Thomas Udem
none Tilo Steinmetz (left) and Constanza Araujo-Hauck (right) aligning the frequency comb at the VTT solar telescope at Tenerife. Constanza Araujo-Hauck

Colour figures from the paper
JPG (click for full resolution) Other formats Description/caption

None Figure 1:The top left shows the solar telescope (VTT) on Tenerife which has been used for this work. The light from the Sun is superimposed on the frequency comb (shown below) with the help of a beam splitter. Together they are fed to a spectrometer (right). Since the original frequency comb has spectral lines that are too close to be resolved by the spectrometer, it is first filtered using a Fabry-Perot filter cavity.

None Figure 2: A section of the measured spectrum, magnified on top. The dark lines are caused by absorption of gaseous elements in the photosphere of the Sun and by absorption in Earth's atmosphere. The spectral lines of the frequency comb appear as bright streaks that are used as precise calibration lines for the entire solar spectrum. The frequency comb is connected to a rubidium atomic clock (Rb-clock) for that purpose.

Last updated: 5th September 2008 0:18 by Thomas Udem