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We have performed long term tests using the following configuration: 1 W
laser, two sequential mode cleaners with automatic alignment control, and a
1200 m cavity between the power-recycling mirror and the east arm. (This
arm has the folded configuration consisting of a far and a near mirror.)
Highlights: The two mode cleaners stayed in lock for up to 38 h. The
longest locking time achieved for the 1200 m cavity was 8 h limited
by alignment drift; this test has been done without an alignment-drift control.
After these tests the mode-cleaner vacuum system was opened in order to clean the mirrors of the first mode cleaner with CO2 snow. During the installation process this mode cleaner was exposed to air for a long time and even locked in air. This increased the scattering losses of the MC mirrors. The cleaning increased the visibility from 87% to 94%, and the throughput went up from 40% to 80%.
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The injection-locked 14 W laser system has been installed at the site. It was decided to attenuate the power down to 1 W and to run the detector with low power until the installation of all mirrors will be finished. The injection locking showed stable performance, but occasionally the lock acquisition of the mode cleaners caused the slave laser to loose lock. This back action of the mode cleaner onto the laser needs more investigation even if an automatic re-lock system is available.
The installation of the north-arm suspensions (beam splitter, near and far mirrors) including the local damping is almost finished. As soon as the mirrors arrive we will install them and then operate the Michelson interferometer. In this configuration the two far mirrors will be suspended from fused-silica fibers while the near mirrors and the beam splitter still have steel-wire suspensions. Mirrors and beam splitter have been polished by Waveprecision (former General Optics) and coated by REO. After the coating, prisms for the fused-silica suspensions have been bonded to the mirrors, in collaboration with the Stanford University. Once these bonds are cured, the mirrors will be ready for installation.
