Portland, Ore. - The extrusion techniques currently used to manufacture thin optical fibers require an aperture of the same diameter as the created cable, but physicists at the University of Chicago believe they have found a better way. With careful process controls that entrain a viscous liquid, a fiber of any nanoscale size can be manufactured without the need to extrude it through a like-sized aperture, the researchers claim. Both hollow and compound optical fibers can be made in a single step, instead of the two steps necessary today.
"Entrainment enables a nonviscous liquid to flow through an aperture into a viscous liquid and pull along some of the viscous material to make a long, thin fiber that is smaller than the aperture," said Wendy Zhang, assistant professor of physics at the University of Chicago. "We believe that with careful control of the processing, you can use viscous entrainment to create [either] submicron hollow waveguides or compound fibers with a core that would have a different index of refraction." By adjusting the speed, pressure and viscosity of such a flow, its diameter can be shrunk without limit.
Zhang demonstrates entrainment with an air bubble in a viscous liquid that rises through a small central aperture (see photo). The air bubble begins as a smooth parabolic shape but distends as it rises. Where the bubble should break off, the viscous material instead trails out behind the still-ascending bubble in a controllable, consistent, hollow stream.
"The thing that you might not think to do-which is key to the process-is to constantly adjust the pressure in the fluid at the back of the aperture in such as way that you maintain the same shape at the interface between the fluid and the air in front of the aperture," said Zhang. The theory is that equilibrium can be achieved by balancing the pressure in the fluids, to enable precise control of both the inner and outer diameters of fiber-even if the inner core is only air.
If air is used as the nonviscous fluid, then a hollow waveguide with precise inside and outside diameters results. If two polymers of different viscosities are used, then a compound optical fiber results.
A research group at Spain's University of Seville is interested in making fiber optics with viscous entrainment. The group is prototyping compound and hollow fibers that do not require a second step to remove the core.