Fiber-based Large Dynamic Range Vibration Sensing with Dual-Wavelength Phase Unwrapping

A large dynamic range vibration sensor is demonstrated with a fiber-based Mach-Zehnder interferometer, in combination with the $3\times 3$ fiber coupler based phase demodulation and dual-wavelength linear regression phase unwrapping. Due to the relief of the $\pi$ -phase constraint in the digital phase demodulation, the measurement range of the vibration sensor is significantly enlarged, with a phase noise level the same as that of the single-wavelength technique. A sampling rate much higher than the Nyquist rate, which is used to limit the phase change within a sampling period, is no longer necessary, and the trade-off between the dynamic range and the measurement bandwidth is eliminated. The proposed vibration sensing system is able to measure signal frequencies ranging from 10 Hz to 7.0 kHz, with almost flat frequency response, good linearity, and a dynamic range independent of frequency.