A LASER provides coherent light, that is the waves are all in sync. This allows you to observe/count their movment across a detector. Light is to all intents and purposes massless so it is not affected by the movements of the aircraft. Your detector can therefore count the number of wavefronts passing it and turn this into a physical displacement. The waves are obviously incredibly small so you need a very very fast counter. After that however it's just maths.
You'll get this off Google but to help try 'Ring Laser'