A Novel Structure With Ultra Short Multimode Fiber for Fiber Specklegram Sensor and Its Application in Multi-Bending Sensing. Proof and Concept

Fiber specklegram sensor (FSS) is widely used for structural monitoring due to its sensitivity to mechanical deformation and ease of manufacturing. The stability of the specklegram is crucial for the application of FSS. In our work, a novel fiber structure named S-ShortM with a millimeter scale short MMF connected with a section of single mode fiber (SMF) is proposed. Different from the conventional FSS, the SMF is used as the sensing fiber, while the ultra short MMF is used as a miniature beam expander to amplify the change experienced by the SMF. A comprehensive theoretical analysis and numerical simulation are conducted to explain the formation of annular specklegram of the S-ShortM, and how a slight change in the mode fractional power in the SMF induces obvious change in the specklegram of the S-ShortM. Multi-bending sensing by the S-ShortM is then demonstrated based on the analysis of the specklegrams from the S-ShortM under different bending state by a multivariate regression convolutional neural network (MRCNN). The generalization ability of the MRCNN for the unknown bending state is also tested and performs well. The result shows the proposed S-ShortM can not only be used to demodulate a single bending and bending position but also distinguish the angle and position of two or three bendings simultaneously. Moreover, the S-ShortM has experimentally presented a significantly improved stability in contrast to the typical SMF-MMF structure used in FSS, which thereby taking a further step for practical application of FSS.