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Apl. Prof. Dr. Benno Willke
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Dr.  Benjamin  Knispel
Dr. Benjamin Knispel
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The world’s most stable high-power laser now tested at California Institute of Technology

Laser will form the heart of the world’s largest Gravitational Wave Detectors

October 12, 2007

The world’s most stable high performance laser, developed by the Albert Einstein Institute (AEI) Hanover and the LaserZentrum Hanover (LZH)/Germany, is currently completing its test phase at the LIGO Laboratories of the California Institute of Technology (Caltech) group in Pasadena/USA. “We are proud that the exceptionally bright and stable laser light will form the heart of the largest gravitational wave detectors in the world by the end of the year. It will make a decisive contribution to the detector’s sensitivity. Researchers will then be able to listen further out into our Universe than ever before”, says Prof. Karsten Danzmann, Director at the AEI Hanover. The new laser technology combines extremely high stability and reliability with a high output power.

„The dedication of our colleagues from Germany and their commitment to the utmost precision has enabled this development. Thanks to the new lasers from Hanover, we can improve the sensitivity of our detectors by a factor of 2,” adds Dr. Jay Marx, LIGO Director.

During the last months before it was shipped the laser was put through its paces in the AEI/LZH laboratories. With 35 W output power it is 3 times brighter than previous types. It will allow measuring the smallest length changes imaginable with unprecedented precision. If it performs as expected in the current tests, two more lasers will be sent to the US from Hanover and will be installed in the LIGO gravitational wave detectors in Livingston/Louisiana and Hanford/Washington. They will significantly improve the sensitivity of the detectors, increasing the number of potential sources by almost a factor of ten, and thus greatly increase the probability of directly detecting the first gravitational waves – ushering in the age of gravitational wave astronomy.

The scientists from Hanover will continue their work in the US during each stage of development. Their American colleagues already familiarized themselves with the new technology in Hanover over the last few months.

The American LIGO project decided to use the „new light“ from Hanover in late summer of 2006. The technology will be a vital element in future generations of gravitational wave detectors to be installed from 2011 onwards. The full responsibility for this core system of stable lasers thus rests with the AEI, strengthening the leading role of the AEI/LZH group in laser development worldwide.

The German-British GEO project enjoys an excellent reputation worldwide for innovative and reliable technology. Besides running the gravitational wave observatory GEO600, it is regarded as an important think tank for gravitational wave research. The development of the new laser system is a fine example: after the laser system developed for GEO600 had proved its reliability and stability in years of operation, the new laser with an output 3x more powerful was developed based on this experience. The French-Italian VIRGO-Project is already using a laser system from Hanover today and will implement the new laser technology into the next stage of development: Virgo+. Recently the Japanese LCGT project (Large Cryogenic Gravitational Wave Telescope) also indicated interest in implementing this laser in their future project.

Background

The future of gravitational wave astronomy begins today
The direct detection of the gravitational waves predicted by Albert Einstein – tiny distortions of space-time – is one of the most important and fundamental open questions of modern science. Their direct observation will open the era of gravitational wave astronomy and will thus allow totally new insights into our universe inaccessible to any other technology – including clues as to its very beginning.

Observation of gravitational waves would have far-reaching consequences, aside from verifying the General Theory of Relativity: It would become possible to cast an eye on the “childhood” of our universe for the first time. Up to now observation of the sky is limited to the electromagnetic spectrum (e.g., radio and X-ray telescopes and astronomy in visible light). The information thus available to us can reach us from the past only from a time at least 380,000 years after the Big Bang. Epochs dating back further have thus far remained hidden, as the universe became transparent for electromagnetic radiation only at that time. The various theories on the early universe have therefore remained unverified experimentally. The direct measurement of gravitational waves may allow “listening” back as far as the very first trillionth of a second following the Big Bang: This would give us totally new information about our universe: with gravitational wave astronomy, totally new areas of science will become accessible.

Measuring gravitational waves – the challenge
Lasers of the highest possible stability are necessary for the direct detection of gravitational waves. Only they are capable of detecting the extremely small changes of length in space that indicate the passing of a gravitational wave.

The gravitational wave detectors of the first generation, e.g. the German-British detector GEO600 at Ruthe near Hanover, are able to record length changes of the order of 10-19 m – a length of one 10,000th of the proton diameter even today. With this sensitivity, neutron stars as far out as 40 million light years can be observed. The portion of the universe thus accessible encloses several dozen galaxies.

But the scientists aim at even higher precision, in order to measure even smaller length changes and thus look even farther out into the universe. The direct detection of gravitational waves will then be easier as more potential sources of gravitational waves come in reach the further you can look out. The catch is: for that purpose researchers need more light. Highly stable in this context means that the laser is not allowed to vary in its intensity (brightness) or frequency (colour), for instance. The requirements on stability refer to all laser parameters: laser power, laser frequency (wavelength or colour of light), spatial beam profile (deviation from a circular power distribution), and of course reliability and ease of maintenance.

A close collaboration of scientists from the AEI Hannover with their colleagues at the LaserZentrum Hanover has now achieved the breakthrough: for the first time, lasers were developed that are bright and quiet enough to fulfil the requirements of the second generation of German-British and American gravitational wave detectors (GEO-HighFrequency and Enhanced LIGO) and the French-Italian Virgo+ project.

The lasers developed in Hanover are also destined for the third generation of gravitational wave detectors such as Advanced LIGO. They will allow the observation of 10,000 times more gravitational wave sources than today – for example “hearing” the coalescence of neutron stars a billion light years out in the universe.

Cooperation and funding
The development of lasers for gravitational wave detectors is financed by the Max Planck Society and by the State of Lower Saxony from funds of the Volkswagen Foundation. In this framework a research and development contract over the sum of € 2.4 million has been signed in July 2006 between the Hannover branch of the Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI) and the Laser Zentrum Hanover (LZH).

In this agreement, the LZH is charged with the construction of a reliable laser with a power of 200 W, which will then be stabilised by the scientists of AEI and installed in the US gravitational wave detectors of the next generation. By virtue of the latest research results, the AEI is now able to reduce the power fluctuations down to one billionth of the power output and thus reach the quantum limit of the detectable power.

The GEO project is funded jointly by the Max Planck Society in Germany and the Science & Technology Facilities Council in the UK.

The AEI Hannover
The AEI Hannover is a joint research centre of the Max-Planck-Gesellschaft and the Leibniz Universität Hannover for doing experimental gravitational wave research. That includes fundamental as well as applied research in the fields of laser physics, interferometry, vibration isolation, and classical and quantum optics. In combination with the theoretical section of the Max Planck Institute for Gravitational Physics at Golm near Potsdam, Germany has a centre covering all aspects of gravitational physics – unique in the world.
In cooperation with British research facilities, the AEI Hannover operates the gravitational wave detector GEO600 at Ruthe near Hannover. The scientists of the institute are leading members of the LISA (Laser Interferometer Space Antenna) project, the gravitational wave detector in space. This joint project of NASA and ESA is scheduled for launch in 2018 to measure gravitational waves in space and to “listen” into space farther out than ever before.

LIGO
The Laser Interferometer Gravitational-Wave Observatory (LIGO) consists of two widely separated installations within the United States, operating in unison as a single observatory. There are two interferometers in Hanford, Washington, having 2 and 4 km arm lengths, respectively, and one 4 km arm length interferometer in Livingston, Louisiana.

LIGO is funded by the National Science Foundation (NSF) and was designed and constructed by a team of scientists from the California Institute of Technology, the Massachusetts Institute of Technology, with contributions from the University of Florida and industrial contractors. Construction of the facilities was completed in 1999. After years of construction and commissioning, the LIGO interferometers started their first long science run in fall 2005. The LIGO detectors will be enhanced in 2007/2008.

Virgo
The Virgo detector with its 3 km arm lengths was completed in June 2003 and has been followed by a commissioning phase to activate and adjust the control systems. It started its first science run on May 18th 2007. A Virgo upgrade is planned for 2008.

Virgo is funded jointly by the Italian Istituto Nazionale di Fisica Nucleare (INFN) and the French Centre National de la Recherche Scientifique (CNRS) through the EGO Consortium.

 
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