Phase modulation error reduction

In a fiber optic rotation sensor, a portion of the coiled optical fiber is a part of and affected by a bias optical phase modulator which causes a varying phase relationship between the light waves at a fundamental frequency. However, the varying phase relationship has at least one harmonic which is due to mechanical vibration of the optical fiber that is a part of the modulator. This harmonic contributes to an erroneous indication of rotation. The amplitude of such harmonic is reduced by lowering the Q of the resonating fiber. This is effected by applying a damping material to the optical fiber and a portion of the modulator. A photodetector detects the counter-propagating waves and sends a corresponding electrical signal on to a signal component selection device that provides an indication of sensed rotation, including error due to the mechanically resonating optical fiber of the modulator.