In the very early days of fiber-optic communications, developers desperately seeking low-loss materials turned to liquids. They filled thin silica tubes with tetrachloroethylene, a dry cleaning fluid that is extremely clear and has a refractive index higher than fused silica. The index difference was adequate to guide light, and developers eventually reduced loss to several decibels per kilometer, very good for the time, and better than current plastics.
Liquid-core fibers were far from a practical communications technology. Filling the tiny capillary tubes took a very long time, but the real problem was thermal expansion. The liquid expanded at a different rate than the tube that held it, so the liquid-core fiber acted like a thermometer, with liquid rising and falling with temperature. If you weren’t careful, the liquid could squirt out the ends.

Now larger diameter liquid-core light guides are finding a new life transmitting visible light short distances for illumination. Single liquid-core light guides 2 to 10 mm thick are an alternative to standard illuminating bundles. Using suitable fluids, they have lower attenuation than standard bundle fibers, particularly at green and blue wavelengths. The liquid is housed in a plastic tube rather than glass, so the liquid waveguide is more flexible than a large solid fiber. Because lengths are modest——at most 20m and typically only a few meters—thermal expansion poses little problem.