Fiber optics of the future: Multifunctionality through multimaterials
Three of the general methodologies for multimaterial fiber preform fabrication. (a) rod-in-tube, (b) extrusion, and (c) stack-and-draw methods. Credit: Tao et al.; IJAGS.
Over the last 50 years, optical fibers have moved from novelty to ubiquity. Although we pay little attention to the composition of these fibers, really, why should we? The fact is that nearly all of the optical fiber in use today is of the “plain vanilla” variety composed primarily of silica glass. These workhorse fibers now, of course, are the backbone of global telecommunications that deliver high-speed data and entertainment across continents and into homes and offices. Basic optical fibers also have made possible remarkable advances in surgery, structural-integrity systems and manufacturing, including advancements in fiber-based lasers.
While ordinary silicate optical fibers will be counted on to play a big role in the foreseeable future, a number of scientists and engineers believe that totally new types of uses for optical fibers could be in reach if the “right” kinds of fibers were available. What are some of the suggested new uses? Some of the examples mentioned include fibers that react with an electrical signal when exposed to external light, temperature changes or ultrasonic signals, fibers that monitor their own performance and even fibers that may play a role in various types of “cloaking” à la metamaterials. Some even have suggested that revolutionary types of fabrics that incorporate electronic and optoelectronic fibers are easily foreseen.
Clearly, researchers have something in mind beyond ordinary optical fibers, and one of the ideas emerging in recent years is the concept of multimaterial fibers, i.e., using the introduction of new materials into the fiber composition to yield new structures, functionalities and applications. (See, for example, our story on work by John Ballato’s group at Clemson University on high-alumina fibers.)
But imagining new fibers and actually producing them is easier said than done. The method used for manufacturing ordinary optical fibers—”pulling” a continuous fiber from a single-material macroscopic preform —is not robust enough to to do the trick. In fact, traditional drawing processes were not really up to the task of making a new class of optical fibers, photonic band gap fibers, despite the fact that making the PBGs does not involve adding new materials to the silica.
Thin film PBG fiber fabrication and characterization. (I) The chalcogenide glass is thermally evaporated onto both sides of the polymer film. (II) This multilayer film is then rolled onto a teflon-lined mandrel, and additional polymer-cladding layers are rolled for mechanical support. (III) The entire structure is thermally consolidated under vacuum until the materials fuse together into one solid preform. (IV) The preform is then thermally drawn into hundreds of meters of fiber by applying uniaxial tension. The ratio of the preform down-feed speed (v1) to fiber draw speed (v2) dictates the final layer thicknesses. Credit: Tao et al.; IJAGS.
In response, a fascinating range of new multimaterial fiber fabrication methods are emerging that are making more exotic forms of fibers a reality. Indeed, fabrication techniques must often be customized to the materials in use and the functionalities desired. Along these lines, the International Journal of Applied Glass Science has just published a tour de force overview of these techniques in a paper authored by Guangming Tao, Ayman F. Abouraddy and Alexander M. Stolyarov (Tao and Abouraddy are from CREOL, the College of Optics & Photonics, University of Central Florida, and Stolyarov is from the Research Laboratory of Electronics at MIT).
The trio’s paper (available in the Early View section of the journal) first discusses some of the thinking that goes into fiber drawing and the general constraints on the construction of multimaterial preforms dictated by the various materials. They elaborate on four techniques used to create the preforms: the rod-in-tube approach, extrusion, stack-and draw approach and thin-film rolling.
The authors then review the emerging palette of exotic photonic and optoelectric multimaterial fibers, including hollow-core PBGs, radially emitting fiber lasers, flouride and chalcogenide glass fibers, semiconductor photodetecting fibers, and piezoelectric fibers.
I really can’t do the paper much justice here. Tao, Abouraddy and Stolyarov are obviously excited about this new field and they cover a lot of ground, and even they admit they that some intriguing work was omitted. But, please know that what I have summarized above only scratches the surface, and I guarantee that readers will find that the authors’ excitement for where this work is going is extremely contagious. If you have any interest in this topic, find time click over to the IJAGS website (ACerS members—it is free, but remember to log in via the ACerS homepage) and read “Multimaterial Fibers.”