Autore: ExtremeTech

DARPA's hollow-core photonic-bandgap optic fiber

DARPA has succeeded in creating hollow-core photonic-bandgap optical fiber, which allows light to travel along its length at around 99.7% the speed of light, or a 30% improvement over conventional (silica glass) optic fibers. In almost every fiber-optic network, light travels through plastic or glass fibers — in DARPA’s fiber, light travels through an air gap, allowing for networks that are faster, have more bandwidth, and traverse greater distances.

As you can see in the image above, in hollow-core fiber light travels through hollow tubes, rather than through glass or plastic. Light travels much faster through air than other materials — which might seem counterintuitive, but just look around you: If light didn’t travel quickly through air we’d have a hard time seeing. The problem is, transmitting a beam of light over a long distance (think tens or hundreds of miles) without the signal breaking down is incredibly hard. With glass and plastic fiber, the signal bounces along the length of the fiber until it gets to a repeater, where the signal is cleaned up and retransmitted. Take away the fiber, and the signal just hits the outside cladding of the cable and fizzles (see diagram below).

Submarine cable cross-section (from Wikipedia)

The cross-section of a submarine fiber optic cable. #6 and #7 are the cladding that would usually destroy a hollow-core signal.

The secret to hollow-core fiber is doing away with the cladding and replacing it with photonic crystals. The light shoots down the hollow core, and when it strikes the edge, the photonic crystals bounce the photons. By doing away with the plastic/glass, these hollow-core fibers have lower signal loss (allowing for longer distances between repeaters), and the increased speed of light (about 30% faster than plastic/glass) reduces latency. According to DARPA, the fact that each fiber is physically separated (single-spatial-mode) allows for higher bandwidth, and any polarization of the light is kept in tact (important for sensing, secure communications, and other interesting applications).

As for how DARPA’s hollow-core fiber was actually created, we have very little in the way of actual details — probably because this is a military project and DARPA isn’t ready to spill the beans. It’s probably very similar to the hollow-core fiber produced by the University of Southampton in March, though.

A ring-laser gyro

DARPA isn’t the first to create hollow-core fiber, but this is the first time that it has been produced in the US, and with specifications suitable for military use. While you might think that the obvious use for these fibers is in data centers and the internet backbone, the technology was actually developed as part of DARPA’s Compact Ultra-Stable Gyro for Absolute Reference (COUGAR) program. COUGAR aims to create an incredibly accurate gyro that can be used for navigation where GPS is being actively or passively denied (i.e. in a warzone or indoors). Without getting into the complexities of ring laser gyros, suffice it to say that the new hollow-core photonic-bandgap fiber should allow for the creation of a very accurate optical gyro. (Read: Think GPS is cool? Indoor positioning systems will blow your mind.)

Moving forward, the main takeaway is that the US now has the ability to construct militaryhollow-core optical fiber, which will first be used in military applications — but eventually, just like the internet, which originated from the ARPAnet, hollow-core fiber should find its way into commercial settings.

Now read: The secret world of submarine cables

[Image credit]