Inter-Modal Interferometry Using Two-Mode Region of Single-Mode Fibers for Vibration Sensing

Interest is growing in the use of telecom fiber networks for sensing purposes. Notably, vibration sensing has the potential not only for network health monitoring but also for generating new revenue sources. Interferometry is an appealing option for vibration sensing because of its simple structure and low cost. This method can be implemented using two inactive single-mode fibers (SMFs). However, two inactive SMFs may not always be available. In addition, interferometry using two SMFs typically requires access to both ends of the SMFs, which can sometimes be impractical. Therefore, a method for achieving single-end vibration sensing using a single SMF while maintaining simplicity and low-cost is necessary. In this paper, we propose a novel vibration sensing method that is based on inter-modal interference using the two-mode region of conventional SMFs. The proposed method forms an interferometer with one SMF using the fundamental and second-order modes as two separate paths. By analyzing the interference between the two modes, one can detect the vibration applied to the SMF. We also extend the method to enable single-end vibration sensing by adding a reflector at the far-end of the SMF. Proof-of-concept experiments demonstrated the capability of single-end vibration sensing.