Robust optical fiber bending sensor to measure frequency of vibration

•Robust acoustic sensor based on a modal Mach–Zehnder fiber interferometer.•The interferometer consists of two fiber tapers in series.•The interferometer can detect frequencies in the KHz order.

A simple technique for sensing the acoustic vibration of a cantilever beam, using a single-fiber Mach–Zehnder interferometer, is presented. The interferometer consists of two concatenated low-loss fused fiber tapers, with a waist diameter of 60 μm, separated by an un-tapered fiber section of 10 mm length. The interferometer transmitted signal is modulated when the device is bent under the presence of an external acoustic signal. The optical fiber device glued directly on a metallic cantilever beam is capable of measuring frequency of the resonant modes. The interrogation set-up is simple consisting of a single tunable diode laser and a photodetector. The measured frequencies of the resonating modes agree with the numerical results obtained by the Finite Element Method.