= laser ring gyros.
There will be a better explanation on Wikipedia, but the basic idea is that laser light is split and routed around a triangle of very stable glass. The light is recombined so that the light from the clockwise and anticlockwise route around the triangle is exactly in phase. A detector measures this phase and if the triangle is turning, then the laser light will take longer to travel one way around the triangle and shorter round the other and so will not be in phase when it is recombined. This phase error is used to give a movement signal.
Laser ring gyros are much more stable and accurate than the old mechanical types, although I believe that there is a dither motion applied to assist the phase measurement, so they are not completely solid state.
PS the '@' instead of 'a' in laser is not my doing, but the forum software. Something to do with laser pointers.