qwertyuiop
I thought the IRUs used 3 "laser gyros" and then clever maths to sense the yaw.
They do. A LASER gyro is a RATE sensing device. The interference pattern created by the two laser beams (ok one laser beam split into 2) changes as the RATE of rotation about the axis changes. The clever maths converts this rate of change of the interference fringe into rotational rate and by intergration rotational distance (i.e angle)