A simple test to confirm the Piezoelectric Phase Modulator is electrically sound.
Being Piezoelectric, this Modulator is pressure sensitive. Simply applying pressure to the Piezoelectric Cylinder will cause a small voltage to be seen across its wires.
A portion of the 4km long Fibre Optic Cable is tightly wrapped multiple times around a Piezoelectric Cylinder. Applying a voltage to the Cylinder will cause it to swell slightly, stretching the cable and thus alter its length. The Gyro can use this to finely control the Phase and calibration of the unit.
This very elegant and simple system is not without its issues, as Piezoelectrics are affected by Air pressure, vibration and temperature.