Stretchable Polymer Optical Fiber with An Unusual Relationship between Optical Loss and Elongation

Stretchable optical fibers are essential for applications that adapt to objects' contours and motion. The relationship between the optical loss and the elongation of stretchable optical fiber plays a vital role for sensing applications such as strain sensors, motion sensors, etc. It is common to consider that the optical loss and the elongation of stretchable optical fiber are roughly in line with the Beer-Lambert law. Here, we report an unusual relationship between the optical loss and the elongation of a stretchable optical fiber. The optical fiber is fabricated from a thermosetting elastomer, and along with the fiber elongation, the optical loss decreases until the elongation reaches 70% in the visible to the near-infrared range, contradicting with the Beer-Lambert law. The optical loss remains stable in the range of 0.170±0.007 dB/cm for wavelength at 800 nm when the elongation is between 70%-90%. Even when the elongation is further increased to 200% and the optical loss increases from 0.17 dB/cm to 0.44 dB/cm (at 800 nm), the loss is still smaller than that in the initial relaxed state (1.81 dB/cm at 800 nm). We attribute the reason behind the unusual relationship to microscopic scattering and macroscopic deformation. The observed unusual relationship between the optical loss and the elongation of the stretchable optical fiber shows great potential for applications in smart textiles, soft robotics, and health monitoring.