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Since the January coincidence run the detector commissioning
continued. Several subsystems have been changed and improved:
– The length of the second modecleaner was changed in order
to make its free spectral range coincide with the correct modulation
frequency needed for interferometer locking.
– The beamsplitter suspension has been changed in order to
get rid of an undamped resonance which was caused by one of the
suspension wires touching the edge of a clearance hole.
– A high power photodetector has been installed.
– The local damping of a beam director had to be
repaired.
– The scattered light at the output bench was identified as the
limiting noise source and has been reduced.
– The fast automatic alignment system and the slow alignment
drift control system of the power-recycled Michelson interferometer
have been installed.
– The data acquisition system has been improved
(installation of whitening filters, replacement of timing card,
adjustment of signal levels).
Due to all these changes the sensitivity of GEO has been improved by more than one order of magnitude compared to the January sensitivity. The automatic alignment system extended the duty cycle by a conciderable factor. At the end of this commissioning period, the interferometer has been running over the weekend with a duty cycle of 99.7 %. It has been continuously in lock for 32 h 12m (August 5th).