Passive Quadrature Phase Demodulation of a Dual-Cavity Fabry-Perot Ultrasound Sensor

In this work, we propose a passive quadrature phase demodulation technique based on a modulated grating Y-branch (MG-Y) laser, and demonstrate its applicability in demodulating a dual-cavity Fabry-Perot (F-P) ultrasound sensor. We analyze the cavity length difference condition that satisfies the quadrature phase relationship. Due to the wide tuning range of the MG-Y laser, the requirement for the cavity length difference of the dual-cavity sensor is reduced, so that the fabrication difficulty of the sensor is greatly simplified. The key parameters of the phase calculation can he measured directly by laser frequency scanning. Experimental results show that the proposed demodulation system can successfully detect ultrasound signals in the range of 20 similar to 100 kHz and random partial discharge (PD) signals generated by electronic pulse igniters. This scheme extends the dynamic range and eliminates any consideration of' whether the operating wavelength deviates from the quadrature point (Q-point), which offers the prospect of PD detection and localization against environmental fluctuations.