An Ultrasensitive Fiber-End Tactile Sensor with Large Sensing Angle Based on Specklegram Analysis

Space-tight micromanipulation such as interventional diagnosis requires miniature, highly sensitive sensors to detect contact forces from all directions. This work reports a fiber-end tactile sensor (FETS) based on specklegram analysis, consisting of a multimode fiber (MMF) and a probe. The probe is fabricated using a less demanding dip-coating process, with a two-step process to develop the elastic layer and the reflective layer directly on the MMF end face. Data were collected over a range of loading angles from axial to 90° relative to the axial direction. A dual-output residual network model was trained. On the test set, FETS achieves contact force estimation with a mean absolute error (MAE) lower than 2% of the range and 100% accuracy in direction classification across the ten directions studied. Furthermore, the resolution is demonstrated to reach the micro-Newton level by specklegram correlation analysis, with a best resolution of $0.61~\mu $ N. Additionally, an anti-bending method suitable for fiber specklegram sensors (FSSs) is proposed, achieving force estimation with an MAE lower than 4% of the range under the MMF bending state changes. The FETS exhibits a full 180° sensing angle and the ability to recognize force directions, representing significant advances over previous micro-Newton fiber-optic force sensors. The FETS holds significant potential for micromanipulation applications, particularly for force sensing in interventional surgeries.