GEO600 has reached a sensitivity of 2·10^(-22)/√Hz
The first direct detection of gravitational waves will mark the beginning of gravitational wave astronomy, opening a new window to the universe. The necessary high detection sensitivity calls for ultra-stable high-power lasers with outstanding beam quality. The AEI Laser Group (in close collaboration with the Laser Zentrum Hannover (LZH)) is in charge of the development of the pre-stabilised laser system (PSL) for the second generation gravitational wave detector Advanced LIGO and was chosen out of three competing groups based on its experience in the development, stabilisation and operation of GEO600.
Meanwhile the laser system for Advanced LIGO (consisting of two laser-diode pumped Nd:YAG lasers in a Master-Slave configuration with an output power of 200 W) has been demonstrated by the LZH and is currently being characterised and stabilised by the AEI Hannover. The optical breadboard, which was developed for fast and accurate characterisation of the laser, comprises high power photodiodes (specifically designed at the AEI for shot noise limited detection of optical power levels of more than 100 mW), a rigid Fabry-Perot optical resonator with automatic alignment system and several quadrant photodiodes. These tools serve the purpose of analysing the laser with respect to its beam profile, pointing fluctuations, power noise and polarisation. The experimental setup was carefully optimised to reach the most demanding requirement for the LIGO PSL, the power stability: The relative power fluctuations have to be below 2·10-9/√Hz, which corresponds to the shot noise of 100 mW of detected light power. These requirements were met in our group by the successful power stabilisation of a laser down to frequencies of 100 Hz (Seifert et al.). This represents the world-best results so far in this field.
Seifert F., Kwee P., Heurs M., Willke B. and Danzmann K. Laser power stabilization for second generation gravitational wave detectors, Opt. Lett. 31, No. 13 (2006) pp2000-2002