S-tapered flexible optical fiber sensor for wide-range pressure sensing and directional slip recognition

Flexible optical tactile sensors hold great promise for wearable electronics and robotic perception; however, existing devices typically face a trade-off among wide sensing range, simple readout, and dynamic slip recognition. Here, we propose a single-element flexible optical fiber sensor based on an asymmetric 3D S-tapered polymer optical fiber embedded in polydimethylsiloxane (PDMS). Leveraging geometry-dependent curvature modulation, normal pressure is transduced into optical intensity variations, while slip direction is intrinsically encoded into distinguishable temporal waveforms. A deformation-optical loss model is established via finite element simulations and optical analyses to elucidate the underlying sensing mechanism. The sensor delivers a pressure sensitivity of 1.55 MPa^-1 across a broad linear range of 0-299 kPa, exhibiting an ultrafast response time (<1 ms) and robust cyclic stability over 3,000 loading/unloading cycles. Furthermore, the device demonstrates reliable tensile strain sensing and unambiguous slip-direction discrimination. Its practical utility is validated through applications in radial pulse monitoring, finger-joint motion tracking, and tactile slip perception. This work presents a streamlined paradigm for integrating wide-range pressure sensing and directional slip recognition within a single flexible optical fiber, paving the way for advanced, low-complexity tactile interfaces.