I thought that at rest, a stable interference pattern was formed by the recombined laser light. The pattern changes when it is accelerated and the shifting interference pattern is measured.
Yep, I dumbed it down a bit. The laser travels in opposite directions around the triangular path so when the gyro is rotated the light wave in one direction gets a increase in frequency shift where as the opposite path gets a reduction in frequency. The two light waves are combined on a photo detecter and cause a fringe interference pattern. The number of fringes is proportional to the frequency difference and the fringes are detected as pulses on the photocell so appear at the output in digital form ready for processing.