Optical fibers that use a liquid core to guide light

Significance 

Polymer optical fiber (POF) materials are attractive for sensing applications due to their low cost, improved flexibility and unique mechanical properties. They can be fabricated through different methods, including melt processing that allows the doping of the polymer waveguides with stable photoluminescent dyes to expand their applications. However, most of the existing POFs are fabricated from commonly available transparent polymers such as polycarbonate polymers that limits their mechanical properties. For instance, such polymers can only withstand minor elongations when subjected to strains making them unsuitable for some sensing applications. Additionally, the extensional fracture requires extra optical interfaces that further result in light scattering issues. Therefore, developing new polymers for fabricating high-performance POFs is a much-needed industrial application.

Polydimethylsiloxane (PDMS) polymer has been recently identified as a potential candidate for developing higher flexible POFs. However, effective utilization of such polymers faces obstacles associated with their low refractive index, small numerical aperture, which reduces their ability to capture luminescent light. These challenges make upscaling of the PDMS polymers difficult and expensive. Light guidance in liquids was first demonstrated over a century ago and has been extensively researched. In particular, optical fibers with liquid cores have exhibited stimulated scattering characteristics desirable for photonic applications. In fact, this approach has been used in different sensing applications like sensing refractive indices and temperatures. The main disadvantage of this approach is that it utilizes hollow tubes filled with different discontinue set-ups that may reduce the scalability of the process and limits its mechanical properties.

To address these limitations, Dr. Konrad Jakubowski, Wiebke Kerkemeyer, Dr. Edith Perret, Dr. Manfred Heuberger, and led by Dr. Rudolf Hufenus from the Swiss Federal Laboratories for Materials Science and Technology, Empa, developed a new approach for light guidance based on photoluminescent liquid-core polymer optical fibers (LiCo-POFs) fabricated by a continuous liquid-core fiber core-extrusion process. The authors further investigated the waveguide properties and mechanical performance of the drawn fibers. Their research work is currently published in the journal, Materials and Design.

In their approach, a continuous core-extrusion of a low refractive-index semi-crystalline fluoropolymer sheath was carried out to fabricate the LiCo-POFs. The experimental work was performed in two parts. In the first part, X-ray analysis was conducted to establish the correlation between the mechanical, optical, and structural properties of the fibers taking into account the effects of crystal orientation induced by drawing the fluoropolymer fibers. In the second part, the glycerol core was doped with a fluorescent dye to enhance the light conversion and guidance in the fiber. The properties of the resulting photoluminescent LiCo-POFs were discussed, and their practical applicability as strain sensor was validated.

Results showed that using transparent liquid as a substitute to the solid polymer resulted in LiCo-POFs with remarkable tensile and waveguide properties even at higher elongations combined with simultaneous guidance and light conversion than soft PDMS fibers and conventional solid-core POFs. The molecular orientation of the sheath exhibited a linear relationship with the tensile strength and a nonlinear relationship with the light attenuation. The elongated micro-voids induced by excessive drawing on the sheath materials resulted in light scattering that affected both light conversion and attenuation. Furthermore, the optical strain sensors allowed the detection of displacements in both elastic and inelastic regimes.

The study is the first ever to report a novel approach for producing luminescent POFs with liquid cores based on a continuous co-extrusion process. The sheath morphology, the dye concentrations and the core cross-sectional areas were identified as critical design parameters for enhancing the mechanical and optical performance of LiCo-POFs. Owing to the improved tensile, mechanical and waveguide properties, the luminescent LiCo-POFs emerged as potential candidates for optical strain sensing applications. In a statement to Advances in Engineering, Dr. Rudolf Hufenus who is the lead author and the R&D Manager for Polymer & Processing at Empa explained that their simple design approach, high flexibility, and robustness of the sensing principle present an opportunity to develop low-cost and high-performing strain sensors for various sensing applications.

Reference

Jakubowski, K., Kerkemeyer, W., Perret, E., Heuberger, M., & Hufenus, R. (2020). Liquid-core polymer optical fibers for luminescent waveguide applicationsMaterials & Design, 196, 109131.

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