Oscillations, Waves, and Interactions - GWDG

Large ring laser gyroscopes 283
Figure 2 shows an example of such a construction. C-II is a second generation
Helium-Neon ring laser [7]. The body of the gyro is made from a slab of Zerodur,
18 cm thick. All four corners are bevelled and polished so that discs of ULE with
optically contacted super mirrors can be wrung onto the ring laser body in order
to generate a pre-aligned closed light path. The beam path itself is drilled into the
neutral plane of the Zerodur slab, parallel to the sides, so that an area of 1 m 2 is
circumscribed by the laser beams. On one side, half way between the mirrors, there
is a cut-out in the ring body. A small adjustable capillary with a diameter of 4 mm is
placed in this gap. Two electrical loops around it act as an rf-antenna. They are used
to excite a gas discharge for laser excitation. A similar cut-out on the opposite site,
closed up with a 12 mm wide pyrex tube, is also integrated into the design for future
experimental purposes. There are also two diagonal holes drilled through the entire
ring laser structure. A UHV-valve located in the center above the Zerodur block
seals the cavity off from the environment. This valve can be connected to a turbo
pump in order to evacuate the ring cavity. Subsequently to the pumping, a mixture
of Helium and Neon can be applied to fill the entire inner part of the construction
with a few mbar of total gas pressure. The design of these ring lasers was chosen
such, that the quality factor of the cavity, Q, was made as large as possible. There
are no Brewster windows or mode selecting devices contained in the entire setup [8].
Together with the very low loss from the mirrors (≈ 20 ppm per mirror) a Q on the
order of 10 12 and higher is achieved for all the rings. The cavity quality factor Q,
defined as Q = ωτ, was established from decay-time measurements. High values for
Q result in a narrow linewidth of the laser and, equally important, in a much reduced
systematic offset of the Sagnac frequency from its true value due to the lock-in effect.
Essentially all large ring lasers built by the German-New Zealand collaboration
follow the same design principle. They are He-Ne gas lasers with rf excitation, optimized
for low loss operation. Since all viable active Sagnac interferometers have
to operate on a single longitudinal mode per sense of propagation, all the devices
are operated near the laser threshold. A free spectral range (FSR = c/P ) between
2.4 MHz and 75 MHz would otherwise allow many different longitudinal modes to
oscillate. The cross-section of the capillary used for laser excitation together with
the radius of curvature of the mirrors determines at which transversal mode laser
oscillation comes on. Preferably all rings operate on TEM-0,0.
Figure 3 shows the G ring laser during an upgrade of the vacuum tubing in 2006.
The dimensions of G are 4 m by 4 m and up to today this is the most stable and most
sensitive sensor. While the smaller rings C-II and G are of monolithic construction,
UG-1 (367.5 m 2 ) and UG-2 (833 m 2 ) are much too large for that. Therefore they have
a heterolithic design. The corners of these ring lasers, the laser gain section and the
connecting vacuum tubes are resting on small pedestals around the perimeter of the
Cashmere Cavern in the northern slope of the Banks Peninsula south of Christchurch
in New Zealand. Figure 4 gives an impression about the construction of these very
large rings.
Since ring lasers are also very suitable sensors for the monitoring of rotations
induced by earthquakes, a simplified version of an active Sagnac interferometer was
constructed in order to provide a relatively cheap but still sensitive device to the seismological
community. As there is no real need for long-term stability, this GEOsensor