The objective of a "gyro" is to provide a static directional reference in space. Therefore all it has to do is either
- maintain its orientation in space so you can measure your offset -> classic mechanical gyro
- measure every tiny change in its orientation in space and add it all up -> laser gyro
Now, how does the laser gyro measure rotation? Imagine a monochromatic light beam, that travels around in a circle.
Without movement, a detector will see nothing but the original wavelength of the laser beam.
As soon as you move, the wavelength at the detector will change, depending on rotation speed and direction in relation to the laser loop.
All you need, really, are three laser loops arranged in 3D and some expensive hardware to do the maths - pronto!