[Image above] Clemson University professor John Ballato and former graduate student Stephanie Morris (now a research scientist with Corning International) studying optical properties of a new type of optical fiber. Credit: Clemson University
Light is a $7.5 trillion industry driven largely by demand for information transmission and storage, with an estimated contribution of $19 billion from the U.S.-based photonics industry in 2014. Light plays such a central role in how we live that the United Nations declared 2015 the International Year of Light and Light-based Technologies.
An infographic by Statistica shows that the U.N. International Year of Light coincided with a significant point in communication history. In 2015, for the first time, the number of cell-phone-only American households exceeded the number of homes with landlines. The trend of trading landlines for wireless phones shows no sign of stopping.
However, growing market demand for wireless service and faster transmission of more data is bumping up against the limits of the material properties of optical fibers.
“Present fiber-based communication and high energy laser systems are limited in the level of optical power that can be propagated,” says Clemson professor John Ballato in the introduction of a new paper. Limitations in power scaling—pushing more light/data through a fiber—arise from optical phenomena such as stimulated Brillouin scattering, stimulated Raman scattering, transverse mode instabilities, nonlinear refractive index, and other phenomena related to wave mixing.
This collection of “nonlinearities” creates problems for optical engineers. Thus far, optical engineers have gotten around these materials limitations by manipulating the signals or by distributing light intensity across a larger cross-section to keep intensity below the threshold that stimulates nonlinearities.
However, these tactics do not address the root cause of limitations of silica-based optical fibers. Ballato and his team are taking a different approach—searching for new materials and processes to fabricate fibers with intrinsically low optical nonlinearities. “A materials approach is arguably the more direct and efficient route since the interaction of the light with the material is where the nonlinearities fundamentally originate,” he says in a new paper.
Ballato and collaborator Peter Dragic of the University of Illinois have been working on this problem for quite some time.
“DoD was having issues with their fiber lasers and didn’t like how every ‘solution’ involved ever more complicated fiber designs and without any consideration of materials. So, we won a grant to attack one of the nonlinearities, then another, and then began to fully appreciate the power, beauty, and simplicity of a unified approach to dealing with everything,” Ballato writes in an email.
The paper is the first of four new papers in an open-access “trilogy” by Ballato and collaborators on their work developing a unified approach to mitigating optical nonlinearities in optical fibers. The four-article series communicates a trilogy of ideas.
The first paper evaluates the nonlinearities problem by describing thermodynamics of optical scattering. The second and third papers (part 2 of the trilogy) dive into glass science and additivity models that determine properties and nonlinearity. The fourth paper offers a path forward with examples and a materials roadmap.
In the course of the trilogy, Ballato et al. conclude that silica-based glasses still are the best materials for efficient optical transmission. However, the compositional adjustments needed are not compatible with traditional CVD preform processing. Instead, the team proposes a “molten core” fabrication process, where a molten core glass surrounded by a clad glass are pulled directly into a fiber. Because the core is molten and quenches so quickly during fiber fabrication, compositions can result that are not feasible using conventional methods.
“The papers are, perhaps not surprisingly, somewhat provocative. For our industry friends, where transitions to new systems/methods is not feasible (or desired), we plan another paper that is more tailored to your processes. That said, the trends discussed, even taken in small increments, could still have beneficial impacts,” Ballato writes in an email.
The sheer size of the optical fiber industry means change will be challenging and costly. But which change is more likely—that the market will cease to demand more data transmission, or that industry will change to meet market demand?
The articles are all open-access in the International Journal of Applied Glass Science.
- “A Unified Materials Approach to Mitigating Optical Nonlinearities in Optical Fiber. I. Thermodynamics of Optical Scattering” IJAGS; DOI: 10.1111/ijag.12327.
- “A Unified Materials Approach to Mitigating Optical Nonlinearities in Optical Fiber. II. A. Material Additivity Models and Basic Glass Properties,” IJAGS; DOI: 10.1111/ijag.12328.
- “A Unified Materials Approach to Mitigating Optical Nonlinearities in Optical Fiber. II. B. The Optical Fiber, Material Additivity and the Nonlinear Coefficients,” IJAGS; DOI: 10.1111/ijag.12329.
- “A Unified Materials Approach to Mitigating Optical Nonlinearities in Optical Fiber. III. Canonical Examples and Materials Roadmap,” IJAGS; DOI: 10.1111/ijag.12336.
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