Laser Gyro
Some time ago I found a nice description in the Boeing Maintenance Manual of the B737. I'll try to recall here:
A laser is emitting monochromatic light (that is light with a single wavelength).
This light is sent through the gyro. One part of it through 2 mirrors in a right hand triangle pattern and another part through the same 2 mirors in a left hand triangle pattern.
At the end there is a receiver, the result of this two light patterns is a bright-dark-bright-dark-bright-dark-bright-... pattern.
If the gyro now is turned right or left, this pattern also moves right or left as the time and as a result of the time the distance travelled through the device changes.
This movement of the resulting pattern can be measured and the computer can calculate the rate of turn with some easy maths.
Hope it helps...
FL