Soft Fiber Optic Sensors for Precision Measurement of Shear Stress and Pressure

This paper reports a soft fiber optic sensor based on polymer fiber Bragg gratings (PFBGs) for simultaneous measurement of shear and normal stresses. The sensor comprises two PFBGs embedded in soft polydimethylsiloxanes (PDMS) matrix, and one of them is horizontally placed while the other is tilted. The transduction principle is that applied normal and/or shear stresses lead to deformation of the matrix and induce strains on gratings. Finite element simulation reveals non-uniform strain distribution along two fibers for direct imbedding that is verified by theoretical analysis, and therefore four gaskets are used to stretch polymer optical fibers from two ends and to produce strains on gratings. The fabricated sensor is tested by concurrently applying normal and shear forces, and Bragg wavelength shifts of two gratings were found to be linearly dependent on stresses in two directions. The measured pressure sensitivity is 0.8 pm/Pa in the range of 2.4 kPa and the shear stress sensitivity is 1.3 pm/Pa for a full range of 0.6 kPa. Such soft sensors are especially suitable for high precision measurement of contact stresses involving human skins and tissues because they are similar in Young's modulus to the used PDMS.