Scientists at GEO600 have pushed the available technologies to the limits: laser stabilization, absorption-free optics, control engineering, vibration damping and data acquisition and processing got new impulses. A specialty of GEO600 is the amplification of laser light and signal called "dual recycling": By means of highly reflecting mirrors the laser light is constructively superposed with itself and thus enhanced ("power recycling"); on the other hand by means of an additional mirror the signal is superposed with itself ("signal recycling"). This technique allows a tuning of the detector to a certain frequency. The suspension of the mirror on glass fibers is anther one of the many groundbreaking developments of GEO600. GEO600 is also the first gravitational wave detector that uses squeezed laser light in order to improve sensitivity!
GEO600 sensitivity has now reached the limits set by the laws of nature. At frequencies around 1000 Hertz – exactly where signals from supernovae and the birth of neutron stars are expected – the quantum nature of light comes into play. Just like pellets fired from a shotgun, individual photons will hit the detector at an uneven rate. Because of quantum fluctuations resulting from the Uncertainty Principle, this ‘shot-noise’ will show up as a fluctuating background signal that could completely obscure the expected short gravitational wave signal from the event itself. GEO scientists have managed to tame this unwanted signal noise by manipulating the fluctuations so as to produce what is called “squeezed light”. GEO600 was upgraded with a source of squeezed light in mid-2010 and has since been testing it under operating conditions. Now, GEO600 uses two lasers: its standard laser of about 10 Watt power, and the new squeezed-light laser that just adds a few entangled photons per second but significantly improves the sensitivity of GEO600.
A highly stable laser
In close cooperation of AEI and LZH a new type of high-performance laser has been developed for use in the next generation of gravitational wave detectors. These new lasers have been installed in the Advanced LIGO project. A special feature is that it provides 200 Watt of power at a wavelength of 1064 nanometers. Its unsurpassed stability in both output power and frequency is what makes the high sensitivity of the new generation of gravitational wave detectors possible.
The central elements in all gravitational wave detectors are mirrors weighing up to 10 kilograms, which are used to direct the laser beams. These mirrors are suspended as pendulums, so that they are isolated from various disturbances. The mirror suspensions must meet several special requirements: they have to hold the heavy mirrors securely and must not cause disturbances of their own.
The Institute for Gravitational Research (IGR) of the University of Glasgow has developed suspensions meeting these requirements: thin threads made of quartz glass – fused silica fibres. Such fibres have far less internal losses than equivalent steel wires, for instance. They are bonded directly onto the mirrors and a second pendulum mass, which means there is no friction at the point of contact. This increases the overall sensitivity of GEO600 through reduced mechanical loss.
Many of the disturbing influences in a gravitational wave detector are seismic in origin. They are especially noticeable when making measurements in the low-frequency range below 100 Hertz. To reduce their influence, the Glasgow scientists have developed a multiple pendulum system which works to dampen these disturbances. They can reduce external vibration by nine orders of magnitude. In addition, the individual components of the pendulum are driven by electromagnetic or electrostatic actuators to reduce the residual noise. The result is that only an earthquake with the strength of 6 and above on the Richter scale occurring anywhere in the world can jolt the gravitational wave detector out of alignment. In all, 260 control loops are needed to move the mirror back into alignment, hold it there and dampen external vibrations on the system.
Tuned signal recycling
GEO600 is the only detector amplifying the signal of the laser beam. A special signal-recycling mirror at the signal exit reflects the interference beam back into the interferometer so that the part of the laser light containing the expected gravitational wave signal is amplified. This process is repeated until the signal is 10 times stronger. GEO600's signal recycling mirror especially can amplify the signal in a broad frequency band. Tuned signal recycling is one of the main reasons why GEO600 has a similar sensitivity to Virgo at higher frequencies, despite its shorter arm lengths. The advanced LIGO is already using this technique and the Virgo detector will also use this technology.