There's no way you can measure time difference round the 2 laser light paths. The laser light path in each direction is about 15 cm. Light travels at 300 million metres per second. Any timing device is doomed to failure.. .. .No, you've missed the point. Why do they use lasers? Because laser is COHERENT light, ie, single frequency light - completely unlike the light in a sunbeam or from a light-bulb, which contains the whole spectrum from the infra-red through to the ultra-violet.. .. .If it's single-frequency, it's also single wave-length - that's the whole point. When 2 pure sine waves of light (which is what you've got with laser light) arrive together at a destination, they make a static interference pattern. When they're slightly out of phase - which they are if the path length is different - then the interference pattern moves across the photo-electric cell with respect to time. It is that movement which is detected - as a change in heading, pitch, or roll.. .. .Simple, isn't it? - if it's explained properly.