Liquid-core polymer optical fibers for luminescent waveguide applications

Abstract

Luminescent polymer optical fibers provide a flexible platform for sensor applications. Photoluminescent liquid-core polymer optical fibers (LiCo-POFs), produced by a co-extrusion of a low refractive index semi-crystalline fluoropolymer sheath with a glycerol core, are presented. Substituting a POF's solid polymer core with a transparent liquid yields luminescent waveguides with extra mechanical flexibility, enabling unique applications like strain sensing. Light conversion and guidance in LiCo-POFs were both achieved simultaneously by doping of the glycerol core with a fluorescent dye. X-ray analysis showed a strong impact of drawing on fiber sheath morphology, and in consequence on both mechanical and optical properties: as the molecular orientation of the sheath increases, tensile strength improves and light attenuation decreases. Excessive drawing leads to micro-voids in the sheath material, causing light scattering, which negatively affects both attenuation and light-conversion. It has been found that both the sheath structure and the sheath and core cross-sectional areas are important design parameters regarding the mechanical and optical performance of LiCo-POFs. A novel principle of an optical strain sensor based on self-absorption peak-shifts is demonstrated; it allows detection of sub-millimeter displacements, both in a reversible elastic as well as in an irreversible inelastic sensor range.

Keywords

• Optical fibers
• Molecular orientation
• Light conversion
• Strain sensor
• Semi-crystalline polymer