Oscillations, Waves, and Interactions - GWDG

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Large ring laser gyroscopes 291
d 1 = 39.703 m
d 2 = 21.015 m
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Figure 9. Basic layout of the UG2 ring laser. The laser beams are steered around the
cavity by mirrors with precise mounting on the corner monuments.
UG2 has a rectangular layout spanning an area of 39.703 m by 21.015 m. The
basic design is shown in Fig. 9. Because of the long beam trajectories of 39.703 m and
21.015 m of the laser, small mirror tilts in the range of a few seconds of arc are causing
already a noticable beam displacement on the next mirror. Since this corresponds
to a change in the geometric scale factor, the beamwalk was monitored and the
instantaneous area and ring laser orientation (relative to the ring laser hardware
structure) was computed. By placing a CCD camera behind the mirrors at the
locations 1–3 indicated in Fig. 9 and recording the light leakage of the laser beam
through the mirrors, beamwalks on the order of a micrometer in displacement could
be monitored. Measurements were conducted by averaging over 4000 individual
images taken one after the other, with the maximum supported exposure time of the
cameras of 8 ms. Figure 10 shows a sample measurement sequence of the movement
of the beam spot center postion with time. In this particular dataset the excursion of
the horizontal position of the laser beam is two times larger compared to the vertical
movement. On other occasions both movements were of the same proportion.
Since the determination of scale factor variations are required with a relative precision
of 10 −9 one can expect a substantial improvement from the geometrical scale
factor correction. After the alignment of the UG2 ring laser to optimize it for minimum
losses, and after the laser was refilled with a clean supply of Helium and Neon,
a measurement sequence of approximately two weeks was started on Dec. 30, 2005
and lasted until Jan. 15, 2006. The Sagnac frequency was recorded with an integration
time of 30 minutes. Figure 11 shows the measurement sequence of raw data
as recorded on the logging system. From the experimental setup, one would expect
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