The third and last laser system for the American gravitational wave detector LIGO has been sent from Hannover to Hanford (Washington). This high power laser for the phase “Advanced LIGO” was developed by the Laser Zentrum Hannover e.V. (LZH) together with the Albert-Einstein-Institute Hannover (AEI) and the company neoLASE.
If all goes according to plans, a 350 kg laser head and several hundred kilograms of wiring, electronics and optics will soon reach their goal in the USA. This 200 W high power laser system from Hannover follows two identical systems which were successfully installed last year. The new system will be integrated in the American gravitational wave detector.
The first direct measurements of these miniscule ripples in space-time are expected to take place at the LIGO sites in Hanford and Livingston in 2014. Gravitational waves were first proposed by Albert Einstein over 90 years ago. In 1974, Russell A. Hulse and Joseph H. Taylor were able to indirectly prove the existence of gravitational waves, and they received the Nobel Prize for their work in 1993. Now, the first direct proof of gravitational waves is close at hand, since the high precision measurement technology is now available. At the heart of this technology are the lasers from Hannover.
„The lasers for advanced LIGO are a good example for the central role our German-British gravitational wave detector GEO600 plays in the international network of the gravitational wave observatories: The technologies developed in the GEO project make extremely precise length measurements possible, which are necessary for direct observation of gravitational waves”, says Dr. Benno Wilke, leader of the Advanced LIGO laser development project at the Albert-Einstein-Institute, Max-Planck-Institute for Gravitational Physics
and Institute for Gravitational Physics
at the Leibniz Universität Hannover.
In order to meet the extremely high demands placed on measuring gravitational waves, laser oscillators with the highest possible beam quality and stress resistance are needed. Scientists at the Laser Zentrum Hannover (LZH) and the Albert-Einstein-Institute Hannover (AEI), together with the firm neoLASE, have worked together during the past ten years to build several prototypes, each with a higher performance than the one before. The current laser system for the “Advanced LIGO” phase has an output power of 200 W at a wavelength of 1064 nm, and is 5 times more efficient than the laser of the last phase, the “Enhanced LIGO”.
Whereas the laser system used in the “Enhanced LIGO” phase is a pure amplifier system, the current “Advanced LIGO” laser system couples a high power laser oscillator to this amplifier system. The complete system combines the good properties of the subcomponents used. The single-frequency amplifier system defines the frequency stability, and the high power oscillator the beam quality. The output power is a result of combining the sums of both systems.
“One of the greatest challenges for the scientists and engineers was to take the system used in one of the first lab prototypes, which demonstrated the basic specifications, and to develop it into a system with constant output and frequency, that runs reliably day in, day out for several years,” says Dr. Peter Wessels, when asked about the special requirements on the system in the last few years. Wessels is head of the group working on the development of the LIGO laser, the Single Frequency Lasers Group (Laser Development Department) at the LZH.
The lasers are needed to carry out the actual measurements in a gigantic Michelson interferometer. This interferometer is situated in a vacuum, in the observatory’s 4 km long arms , which are perpendicular to each other. When a gravitational wave passes through the observatory, the relative length of the interferometer arms changes. One arm is lengthened and the other shortened, which in turn causes a phase shift in the laser light waves. This interference changes the intensity of the light measured at the interferometer exit. The whole setup can measure a relative difference in the arm lengths of only 10−22.
After the laser is integrated into the gravitational wave detector in May, companies and institutes from the USA and from other places in the world need to upgrade the new light source with other suitable components. In two years at the earliest, the first “science runs” with the new laser can take place, and real measurements with the kilometer long interferometer can then be made. But work for the scientists at the LZH and the AEI is not finished! They have already begun to develop lasers for “third generation gravitational wave detectors”.
GEO600: The German-British observatory is situated near Hannover and is run by scientists from the AEI and the British universities of Glasgow, Cardiff and Birmingham. The GEO project is financed by the Max Planck Society, the state of Lower Saxony, the Volkswagen Foundation, and the British Science and Technologies Facilities Council (STFC). GEO works closely together with the excellency cluster QUEST (Centre for Quantum Engineering and Space-Time Research) in Hannover. Further information can be found at: http://www.geo600.org.
LZH: The Laser Zentrum Hannover e.V. is an independent, university-related institute which concentrates on application-oriented laser research. Over 120 scientists from the areas of physics, chemistry and engineering work on interdisciplinary solutions to laser-based problems. In the Laser Development Department alone, nearly 30 scientists are involved in research centering around solid-state lasers, fiber lasers, and their applications. The LZH is one of the largest research institutes for laser technology in Europe