Development of New Synthetic-Heterodyne in Sinusoidal Phase Modulation for Optical Fiber Sensors

摘要

In this study a new algorithm for modulating a PZT stack using a sine-wave signal instead of a saw-tooth signal was developed in order to resolve the flyback problem in a PZT driver. An improved synthetic heterodyne demodulator circuit was designed and arranged to interrogate the output signal from an interferometric sensor with a sine-wave modulation (phase-generated carrier). A depth of modulation or a gain amplifier to the circuit design was adjusted such that the standard heterodyne signal could by formed. The result is a conventional PM modulated carrier that can be demodulated using standard techniques, followed by a lock-in amplifier or a phase meter, to recover the phase shift in the sensing information. Implementing it into a path-matching differential interferometer (PMDI) for the signal modulation validates this new method. The new synthetic heterodyne is applied to a differential optical fiber refractometer for measuring the refractive index change. A lock-in amplifier is chosen as the demodulator to extract the phase shift. Therefore, the resolution of this sensing system is 7.52 * 10~(-7) refractive index unit (RIU) in 120 μm length of the sensing cavity.