Performance of a 2400 m long suspended optical cavity

Using one arm of the Michelson interferometer and the power recycling mirror of the interferometric gravitational wave detector GEO 600, we created a cavity with an optical path length of 2400 m. The main purpose of this experiment was to gather first experience with the main optics. The residual motion of the mirrors is about 150 nm rms. Stabilising the length of this cavity to the pre-stabilised laser beam we achieved an error point noise of 100 µHz/ √ Hz at 100 Hz. We also demonstrated the good performance of all included subsystems by 10-hour-periods of continuous stable operation. Thus the full frequency stabilisation scheme for GEO 600 was successfully tested. Performance of a 2400 m long suspended cavity 2