Balanced polarization maintaining fiber Sagnac interferometer vibration sensor.

To achieve a nearly zero-delay operating point in a polarization-maintaining (PM) fiber Sagnac interferometer, two identical PM fibers were incorporated so that their two main axes were orthogonally coupled to each other. A simple fiber vibration sensor system was constructed with a light emitting diode and a balanced PM fiber Sagnac interferometer, in which one of the PM fibers was used as a sensing cable and the other as a reference cable. The vibration sensor was confirmed to be temperature-compensated and generated a phase shift per unit length and unit strain of the sensor of 4.7 milliradian/(m·με) when mechanical vibrations with 1 kHz sinusoidal and triangular waves were stably observed under an input power of 10 μW.