Another high-power laser for the wave chasers

April 04, 2012

The third and, for the time being, last laser system for the US LIGO gravitational wave detectors has started off on its journey from Hannover to Hanford (Washington). The powerful laser for the “Advanced LIGO” phase was developed by the Laser Zentrums Hannover (LZH e.V.) together with the Albert Einstein Institute Hannover (AEI) and the company neoLASE.

If all goes according to plan, the over 350 kg laser head plus several hundred kilograms of cables, electronic equipment and lenses should soon reach its destination in the USA. After the successful installation of two identical systems over the past few years, the third 200 W high-power laser from Hannover will soon be ready for integration into the US gravitational wave detectors.

Starting in 2014, the first direct measurements of tiny spacetime changes are expected at the LIGO locations in Hanford and Livingston. These gravitational waves were predicted over 90 years ago by Albert Einstein. In 1974, Russell A. Hulse and Joseph H. Taylor, were able to indirectly prove gravitational waves. They were awarded the Nobel Prize for their efforts in 1993. Now, the first direct proof of gravitational waves is within reach, as the required highly precise measuring technology is available. The lasers from Hannover are at the heart of these technologies.

“The lasers for Advanced LIGO are a good example for the central role of our German-British gravitational wave detector GEO600 in the international network of the gravitational wave observatories: GEO600 is an experimental technology ‘forge’. The technologies developed in the GEO project enable extremely precise length measurements, which are required for the direct observation of gravitational waves,” explains Dr. Benno Willke, project head of Advanced LIGO laser development at the Albert Einstein Institute, Max Planck Institute for Gravitational Physics and Institute for Gravitational Physics, Leibniz-­Universität Hannover.

In order to be able to meet the extremely precise measurement requirements for gravitational waves, laser oscillators of the highest beam quality and durability are a must. Scientists from the Laser Zentrum Hannover (LZH) and the Albert ­Einstein Institute Hannover (AEI) have, together with the company neoLASE, developed, over a ten-year period, several prototypes, each with improved power and efficiency. The current laser system for the “Advanced LIGO” phase is, with an output wattage of around 200 W at a wavelength of 1064 nm, 5-times more powerful than the laser of the preceding “Enhanced LIGO” phase.

Whereas the laser system used in the “Enhanced LIGO” phase was a pure amplifier system, the current laser system for “Advanced LIGO” is an amplifier system coupled with a high-power laser oscillator. The overall system then combines the outstanding properties of the subcomponents involved: the single-frequency amplifier system determines the stability of the frequency, the high-power oscillator the beam quality, and the output wattage is the result of the sum of both subsystems.

“One of the biggest challenges for us scientists and engineers was to develop the system, from an initial laboratory prototype on which the fundamental specifications were demonstrated, to the extent that it will be able to be operated reliably with consistent power and frequency round-the-clock and for many years,” says Dr. Peter Weßels, describing the particular demands of the past few years. He heads the Single Frequency Laser group (Laser development department) at LZH, which played a key role in the development of the LIGO laser.

The lasers are responsible for the intrinsic measurement in a Michelson interferometer of significant magnitude. This interferometer is installed in a vacuum in the 4 km-long arms of the observatory, which are set up perpendicular to each other. Should a gravitational wave traverse the observatory, the relative length of the arms of the interferometer would change. Whereas one arm would be lengthened, the other arm would be shortened, which then brings about a phase shift of the partial waves of the laser light. The resulting interference alters the intensity of the measured light at the end of the interferometer. The set up allows for a relative difference in both arm lengths of 10−22.

After integration of the recently delivered laser into the gravitational wave detector in May, the detector still has to be equipped by companies and institutes in the US and other countries with further components that are in line with the new light source. Then, in two years time at the earliest, the first “science runs” with the new laser will be possible, meaning real measurements with the kilometre-long interferometers. The work also continues today for the LZH and AEI researchers after this provisional last delivery: they are already working on the development of lasers for “gravitational wave detectors of the 3rd generation”.

GEO600: The German-British observatory is located near Hannover and is operated by researchers from the AEI, as well as from the British Universities of Glasgow, Cardiff and Birmingham. The GEO project is financed by the Max Planck Society, the German Federal State of Lower Saxony, the Volkswagen Foundation and the British Science and Technologies Facilities Council (STFC). GEO works closely together with the QUEST (Centre for Quantum Engineering and Space-­Time Research) excellence cluster in Hannover.

LZH: The Laser Zentrum Hannover e.V. is an independent, university-affiliated non-profit institution that focuses on applied laser research.  A scientific staff of more than 120 researchers from the areas of physics, chemistry and engineering work together across all fields to solve laser-based problems. The laser development department alone has around 30 scientific staff members who work on solid-state lasers, fibre lasers and their respective applications. LZH is one of the biggest research facilities in Europe devoted to laser technology.

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