The hidden network that makes the internet possible - Sajan Saini
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That’s how a virtual world of interconnected computers was imagined in the classic SF novel. Today, the internet has realized a version of this alternate reality, as it marches towards ever-more immersive user interfaces. What’s the unique contribution of fiber optics and integrated photonics to this new cloud computing era?
Learn more on state-of-art fiber optic data transfer demonstrated by the Japanese-Danish research team, and a short review comparing the power efficiency of copper coaxial cables with fiber. Then, check out Nokia’s history timeline on the data capacity of copper telephone lines.
This historical review by Jeff Hecht details the roots of 18th century light-messaging, the invention of fiber, and Charles Kao’s radical insight to purpose it for global communications. The Snell’s Law description for total internal reflection of light in glass fibers is governed by a refractive index difference between an inner doped fiber core and an outer cladding, and this online primer explains how different core diameters correspond to distinct types of signal propagation. This video shows how fiber is drawn-a process you can mimic with a model draw tower. Today, practiced vendors such as Corning routinely draw state-of-the-art strands for thousands of kilometers.
Undersea fiber networks then carry the data across and between continents, provided light signals get a power boost every hundred or more kilometers by optical amplifiers. This Vox video reveals the grand engineering challenge to lay such submarine cable.
The unmatched ability of optical fiber to carry great amounts of data around the globe with little power loss has led to a modern cloud computing economy (see Cisco’s cloud forecast report) that relies on massive quarter-mile long hyperscale data centers, and their power demands are leading to a global energy crisis. The servers in these centers have efficiency and heat load limits; as more servers alleviate with parallel computing, power is lost in their cabling. While some data centers exploit arctic climates to cool down, silicon-based integrated photonics offer a key solution to electrical power loss.
Swapping server coaxial cables for optical fiber is a first step reduce power consumption, and as data centers grow larger, industry leaders like Google and Facebook are pioneering designs for faster photonics relays between servers, with ambitious collaborations involving Microsoft and Nokia.
Silicon photonic wires, or waveguides, guide light like fiber, but silicon waveguides have a large refractive index difference between core and cladding, and this confines light to small core areas. That said, this simulation from AIM Photonics Academy shows how some light always overflows out of waveguide (or fiber) cores. Adjusting a core’s cross-section confines more or less light, and at a core-cladding boundary with atomic scale roughness, some overflowing light scatters out. This scattering loss is larger in high confinement waveguides, than in fiber.
This AIM Academy simulation also shows how, unlike optical fiber, high confinement waveguides move light through tighter turns with less loss. Learn of the trade-off between scattering and turning loss in this MIT online course and review article by Prof. L.C. Kimerling: silicon waveguides are ideal to build micron scale devices in advanced integrated photonic chips (see this Nature article), while fiber optics are suited for large scale applications. The two technologies are deeply complementary and compatible.
As the drive for increased wireless access (see Cisco’s mobile forecast report) leads to 5G and eventually 6G innovation, signals will shift from microwave to higher data rate millimeter and terahertz waves. This article by Shams & Seeds describes terahertz short-range limits and how integrated photonics with fiber may mediate its long-distance communication needs. While the integrated photonics chips will employ silicon devices, generating terahertz waves is yet another major photonics challenge that may rely on compound semiconductor resonant diode and high mobility transistor devices, or specialty optical fiber.
To learn more on integrated photonics and how the technology is transforming low power cloud computing, hyperfast wireless, smart sensing, and augmented imaging, visit the AIM Photonics Institute and its education program at MIT, AIM Photonics Academy. Then, step back to learn about similar advanced Manufacturing institutes like AIM that are transforming robotics, smart fabrics, flexible electronics, 3D printing, bio-fabrication, and other high-tech fields.
Sajan Saini is a former materials scientist and science writer. He directs the educational curriculum for AIM Photonics Academy at MIT. He has written for Coda Quarterly, MIT Ask an Engineer, Harper's Magazine, and TED-Ed. Learn about Sajan here.
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