Laser Frequency Combs for Astronomical Observations
T. Steinmetz1,2, T. Wilken1, C. Araujo-Hauck3, R. Holzwarth1,2, T. W. Hänsch1, L. Pasquini3, A. Manescau3, S. D'Odorico3,
M. T. Murphy4, T. Kentischer5, W. Schmidt5, 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, 5th September 2008

The paper
Published version: Science
Preprint version (full resolution figures): PDF (5.7M)

Media releases:
Swinburne media release: HTML, PDF
ESO media release: HTML, PDF
Max-Planck Institut media release: PDF

Other media:
New Scientist: Laser 'comb' used to disentangle Sun's light
ABC (Australia) News: Laser precision puts universe within reach

Some background
Time and frequency:
Time, and its inverse, frequency, are the two quantities physicists can measure best. For example, today's best atomic clocks would lose or gain only one second after about a million years. A so-called "optical laser 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 way to transfer this incredible precision to the many sharp spectral features of laser light. A frequency comb is a special laser which emits extremely short pulses of light only femtoseconds long (i.e. thousandths of a millionth millionth of a second) in millions of extremely well-defined colours. The frequencies (or wavelengths) defining all these colours are controlled by an atomic clock and so they are known to super-high accuracy. They are also spaced equally with respect to each other, like the markings on a ruler.

Cosmic speeds:
The colours emitted or absorbed by heavenly bodies - e.g. stars, galaxies, or the gas clouds in between different stars and galaxies - can tell us a lot about astrophysical phenomena. In particular, they tell us about the speeds the stars, galaxies and other astrophysical objects are moving at. How? Via the Doppler effect, the same effect which makes a passing ambulance sound high pitched then, as it passes, low pitched. It's the bunching up or stretching out of waves: in the ambulance's case, it's sound waves; in astronomy, it's light waves as they emerge from (or travel through) objects and gas clouds and then travel through the Universe.

For example, a planet will impart a small wobble to the motion of star around which it orbits. The change in speed of the star will cause a very small change in the frequencies (or colours) we see coming from it. By taking the light of the star, spreading it out with a prism into its components colours, and looking for periodically shifting features in that colour spectrum, astronomers can infer the existence of planets around stars outside our own Solar System. But to detect a planet like the Earth orbiting a star like our Sun, we would need to measure changes in speed of only about 10 centimeters per second over a period of about a year.

Another important example is the expansion of the Universe itself. As light travels to us through the cosmos, it gets stretched - it's wavelength increases - as the Universe expands. We see this effect as an overall "redshift" of cosmological objects: their light is stretched out, becoming redder as it travels to us. Redshifts themselves are important quantities in astrophysics because they indicate how far away objects are from us. Furthermore, in an ever expanding Universe, watching how redshifts of objects change with time is also very important. That would tell us a lot about what is causing the expansion in the first place. However, this experiment has never been done because the colours of objects (their redshifts) change by such tiny amounts over, say, a human life-time. The European Southern Observatory aims to do this in future, using its planned 42-meter diameter European Extremely Large Telescope to observe extremely distant quasars. Quasar spectra are riddled with absorption lines arising in intergalactic gas clouds throughout the Universe. Therefore, quasars are the best targets to use when trying to watch the Universe expand in real time.

Frequency combs meet astronomy:
This is where frequency combs can help astronomers. The "teeth" of the comb - the many narrow spectral features emitted by the laser - provide a sort of `frequency ruler' with which to measure the colours in the light from astronomical objects. In this paper we made the first demonstration of how to do this in practice. While observing the Sun with an astronomical spectrograph (like a prism, to disperse the colours), we simultaneously launched the light from a frequency comb into the system. The recorded shows the familiar patterns of features in the light spectrum of our Sun. However, the teeth of the comb are overlaid, providing a very accurate frequency scale.

The future:
Even though we've now demonstrated how to use frequency combs to do ultra-precise astronomy, there are many challenges ahead. We want to broaden the comb light to cover more colours, even the entire optical spectrum, from ultra-violet through to infrared light. At the moment, we only have the system working in infrared light. Also, we'd like to make sure we get the same amount of light at all colours; at the moment, there's too much light at some colours, too little at others, for the system to be of general use. But we think these things should be relatively easy to overcome. We aim to have a new way of doing astronomical spectroscopy in the very near future.

Publicity images
JPG or GIF image (click for full resolution)Other formatsDescription/captionCredit information
None Scheme of our experiment: Using a telescope, the light from the Sun was coupled to an optical fibre that guides it to a spectrometer (prism) in order to resolve its spectral lines. The spectral lines from the Sun (Fraunhofer lines) appear as dark bands because they represent the wavelengths of light that has been absorbed by the Sun's photosphere as it emerges from deeper within the it. Superimposed are many short, bright (white) spectral lines of the laser frequency comb. ESO
None The spectrum of colours in arriving at the Earth from a quasar. The Universe the light passes through is made up of galaxies, which we see in this picture, but these galaxies are interconnected by streams and clouds of hydrogen gas which causes a "forest" of absorption lines to appear in the quasar spectrum. Michael Murphy
None Same as above but now, instead of galaxies, a simulated universe's intergalactic structure is shown. The filamentary structure is traced by hydrogen gas which causes a "forest" of absorption lines to appear in the quasar spectrum. It is these absorption lines which, over several decades, will shift very very slightly. We need to carefully measure our quasar spectra with a frequency comb if we are to pick up this tiny effect. Michael Murphy

None The frequency comb, which is the light from pulsed laser, consists of many colours which are 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
None A planet (green) orbiting 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 Tilo Steinmetz (left) and Constanza Araujo-Hauck (right) aligning the frequency comb at the VTT solar telescope at Tenerife. Constanza Araujo-Hauck

Publicity movies
JPG or GIF image (click for full resolution)Other formatsDescription/captionCredit information

Versions without inset graph:
Quasar light's long journey to Earth. This movie follows the light from a quasar - a supermassive black hole, gobbling up the center of a distant galaxy - as it travels through the Universe to Earth. As light emerges from a distant. It encounters an intervening galaxy and some of the quasar light is absorbed in the interstellar medium of this galaxy. The inset graph shows the spectrum of colours from the quasar. As the light travels, the overall spectrum shifts to the red side because of the expansion of the Universe. The absorption from the intervening galaxy manifests itself as sharp lines in the spectrum, indicating the colours of light absorbed by the galaxy. Centre for Astrophysics & Supercomputing,
Swinburne University of Technology
None A planet (green) orbiting 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
See also: Similar animation at HST website
The expansion of the universe means all galaxies seem to move away from us (and from each other). This causes their spectral lines to shift towards the red end of the spectrum. On the other hand, if the universe contracted, the spectral lines of the galaxies would appear shifted towards the blue. Through a tiny change of the magnitude of the shift, as is now measurable with the frequency comb, one can decide whether the universe's expansion is actually accelerating, as several indirect observations currently suggest. Thomas Udem

Colour figures from the paper
JPG or GIF image (click for full resolution)Other formatsDescription/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 by Michael Murphy