Real-time in situ phase sensitivity calibration of interferometric fiber-optic ultrasonic sensors

Output from a fiber-optic interferometric ultrasonic sensor can be affected by the operating point changes and/or the optical power fluctuations. We report a method for real-time and in situ calibration of the responses to ultrasound-induced phase delays for a low-finesse Fabry-Perot (FP) ultrasonic sensor demodulated using a phase-generated carrier. The carrier is produced from an electro-optic phase modulator, and its zero- and fundamental-carrier frequency components yield two quadrature signals. The same phase modulator is employed to generate controllable laser frequency modulations and calibration phase delay modulations. The response to the known calibration phase delays is used to gauge the sensor output in real time and in situ. The experiment has verified the effectiveness of the calibration method against the signal variations from both operating point changes and optical power variations.