Dual-wavelength demodulation technique for interrogating a shortest cavity in multi-cavity fiber-optic Fabry-Pérot sensors.

This paper demonstrates, for the first time, a novel demodulation technique that can be applied for interrogating a shortest cavity in multi-cavity Fabry–Perot (F–P) sensors. In this demodulation technique, using an amplified spontaneous emission (ASE) light source and two optical fiber broadband filters, the interference only occurs in a shortest F–P cavity that is shorter than the half of the coherence length. Using a signal calibration algorithm, two low-coherence interference optical signals with similar coherence lengths were calibrated to obtain two quadrature signals. Then, the change in the cavity length of the shortest F–P cavity was interrogated by the two quadrature signals and the arctangent algorithm. The experimental results show that the demodulation technique successfully extracted 1 kHz and 500 Hz vibration signals with 39.28 µm and 64.84 µm initial cavity lengths, respectively, in a multi-cavity F–P interferometer. The demodulation speed is up to 500 kHz, and the demodulation technique makes it possible for multi-cavity F–P sensors to measure dynamic and static parameters simultaneously. The results show that the demodulation technique has wide application potential in the dynamic measurement of multi-cavity F–P sensors.