The Laser Zentrum Hannover e.V. (LZH) and other project partners have
delivered a new laser system to Hanford, Washington, USA, which will be
integrated in the American gravitational wave detector. The first
direct measurements of gravitational waves are expected to take place
at the LIGO sites in Hanford and Livingston (Louisiana) in 2014.
 |
| LIGO
besteht aus mehreren Interferometern mit einer Armlänge von
jeweils vier Kilometern, die an den Standorten Hanford/Washington (hier
im Bild) und Livingston/Louisiana in den USA platziert sind. |
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”.
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