Oscillations, Waves, and Interactions - GWDG

286 Schreiber
Rotation Rate [rad/s]
10 -11
10 -12
10 -13
10 1
10 2
10 3
Time [s]
Figure 5. Computed quantum noise limit and measured sensor resolution for G. The sensor
almost reaches the shot noise limit.
with a width of the cavity resonance of
10 4
10 5
1 − R
∆νc = ∆FSR
π √ . (5)
R
Using ∆FSR = c/L = 18.75 MHz and R = 0.999892 one obtains ∆νc ≈ 645 Hz and
hence ∆νL ≈ 275 µHz. Since a He-Ne laser is a 4-level system one may expect to
set N1 ≈ 0 which assumes full inversion. For an industry type He-Ne laser with a
capillary diameter of 1-2 mm and a moderate excitation current this assumption is
certainly true. However, our lasers use wider capillaries with 4 mm (C-II), 5 mm (G)
and 6 mm (UG2) diameter. This slows down the wall-collision induced de-excitation
of the Neon atoms from the 1s state. As a result the laser gain reduces substantially
and, by the process of electron-Neon collision, pumping leads to a an increase of N1,
a reduction of the inversion [3, 16], and consequently to an increased linewidth. In
the absence of any variations of the scaling factor according to Eq. (3) one obtains a
constant lower limit for the gyro resolution, which then is only depending on fluctuations
of the laser beam power in the cavity. A typical value for the stability of the
beam power due to mode competition of 0.01% over a time period of about 1 hour
has been observed from intensity measurements in G.
In order to investigate the mid-term stability of large ring lasers, Fig. 6 shows an
Allan deviation plot of most of the lasers specified in Table 1. One can see that the
monolithic constructions are more stable than the heterolithic structures. This is
not unexpected. Compared to UG1, C-II experiences much more perturbations from
backscattering and comes with a much smaller scale factor. The performance of UG2
on the other side falls off despite the enhanced scale factor. This may be due to the
rectangular sensor layout or comes with the increased arm length of the instrument.
G0 as the only vertically mounted ring laser does not have the mechanical stability
to perform well. Actually this prototype ring laser construction was only intended
to test the feasibility of heterolithic ring laser designs. The GEOsensor is designed