Oscillations, Waves, and Interactions - GWDG

A B
Large ring laser gyroscopes 281
D C
F
603 m
Figure 1. Sketch of the experiment of Michelson and Gale. The interferometer had a
length of 603 m and a width of 334 m.
itself. Therefore they integrated a second much smaller interferometer corresponding
to the contour A,B, C and D into their device, which created a negligible fringe
shift. A comparison between the two different fringe pattern provided the required
evidence.
3 Active Sagnac interferometers
In the days of coherent light sources it is possible to increase the sensor resolution
of a Sagnac interferometer substantially. In particular the transition from a phase
measurement to a frequency measurement promises a great improvement for the
sensor sensitivity. By placing a laser gain medium inside, a closed light-path set up
by three or four mirrors converts the apparatus into a laser with a ring cavity. Along
with the lasing condition that the perimeter P corresponds to an interger number i
of waves around the cavity, P = i · λ, one obtains the ring laser equation
δf = 4A
n · Ω , (2)
λP
with δf the beat frequency, λ the wavelength of the optical beam, n the normal
vector on the area, and Ω the rate at which the entire apparatus rotates with respect
to the universe. This beat frequency is usually called the Sagnac frequency. In the
classical approach one can depict the co-rotating optical resonator as slightly larger
compared to the anti-rotating resonator. Therefore both optical beams are shifted
by the same small amount but in different directions away from the optical frequency
which both beams would have when the apparatus were at rest.
Ring lasers have several advantages. First of all there are no moving parts in the
design of this sensor. Secondly, they are sensitive to a very large range of rotation
rates, covering more than six orders of magnitude with a linear system response.
E