🔬 Fibre-Optic Gyroscope: Sagnac Phase, Bias Modulation, Shot-Noise Limit and Earth-Rate Gyrocompassing (2D)

A fibre-optic gyroscope on a rate table: two beams circle a coil in opposite directions, the Sagnac phase measures rotation, and bias modulation, photon shot noise and the rotation of the Earth set what it can sense.