Recirculating light in a fiber optic gyro can cause significant scale factor distortions and uncertainty when the signal processing makes use of the second harmonic of the bias modulation to sense rotation. Significant reduction of this problem can be realized by operating the gyro at particular modulation depths and/or at one-half the proper frequency.
James N. Blake,
"Second harmonic detection for rotation sensing in fiber optic gyros", Proc. SPIE 1795, Fiber Optic and Laser Sensors X, (5 March 1993); doi: 10.1117/12.141279; https://doi.org/10.1117/12.141279