Acoustic frequency vibration sensor based on tapered SMS fiber structure

The propagation of light from a singlemode fiber (SMF) to a multimode fiber (MMF) arouses a series of eigenmodes, which interference together along the MMF where the positions of the local maximum intensity are the self-imaging points (SIPs). When the light in the MMF propagates to a SMF, the maximum coupling efficiency is obtain when the SMF and MMF connect at a SIP. However the connection generally deviates the SIPs for the accurate connection is difficult to be achieved, which decreases the accuracy of the sensor severely. Herein a novel vibration sensor based on the singlemode multimode singlemode (SMS) fiber is developed, in which the MMF is etched, which turns the SIP to the connection of the MMF and the output SMF accurately. The etched SMS structure dispenses with the requirement of highly accurate connection at a SIP and provides a succedaneous scheme that is sensitive to vibration. The etched sensor responses to the vibration more easily comparing to the un-etched one. Experimental results demonstrate that the sensitivity of the SMS sensor with the both ends etched is respectively over 5 and 40 times as that of the middle etched and un-etched one. When the MMF length decreases from 8 to 2 cm, the vibrating sensitivity of the sensor increases 13.5 times. Experimental results demonstrate that the etched structure with MMF length of 2 cm responses well to the acoustic wave from 20 to 300 Hz, where a pressure sensitivity of 1.49 mV/mPa and linear correlation index of 0.9980 can be attained at 120 Hz.