Performance of a 1200 m long suspended Fabry–Perot cavity

Using one arm of the Michelson interferometer and the power recycling mirror of the interferometric gravitational wave detector GEO 600, we created a Fabry–Perot cavity with a length of 1200 m. The main purpose of this experiment was to gather first experience with the main optics, its suspensions and the corresponding control systems. The residual displacement of a main mirror is about 150 nm rms. By stabilizing the length of the 1200 m long cavity to the pre-stabilized laser beam, we achieved an error point frequency noise of 100 μHz Hz −1/2 at 100 Hz Fourier frequency. In addition we demonstrated the reliable performance of all included subsystems by several 10-hour-periods of continuous stable operation. Thus the full frequency stabilization scheme for GEO 600 was successfully tested.