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30 percent faster:
DARPA's new hollow-core optical fiber to enable high-power military sensors

Design is first to include three key properties needed for military applications.

The intensity of light that propagates through glass optical fiber is fundamentally limited by the glass itself. A novel fiber design using a hollow, air-filled core removes this limitation and dramatically improves performance by forcing light to travel through channels of air, instead of the glass around it. DARPA's unique spider-web-like, hollow-core fiber design is the first to demonstrate single spatial mode, low loss, and polarization control -- key properties needed for advanced military applications such as high-precision fiber optic gyroscopes for inertial navigation.

Although hollow-core fiber has been available from overseas suppliers for years, DARPA's ongoing Compact Ultra-Stable Gyro for Absolute Reference (COUGAR) program has brought design and production capacity inside the United States and developed it to a level that becomes the new state of the art.

A team of DARPA-funded researchers led by Honeywell International Inc. developed the technology. The hollow-core fiber is the first to include three critical performance-enabling properties:

• Single spatial mode: Light can take only a single path, enabling higher bandwidth over longer distances;
• Low loss: Light maintains intensity over longer distances;
• Polarization control: The orientation of the light waves is fixed in the fiber, which is necessary for applications such as sensing, interferometry, and secure communications.

Hollow-core fiber can also be bent and coiled while guiding light at speeds 30 percent faster than conventional fiber.

"Previous instantiations of hollow-core fiber have shown these high propagation speeds, but they weren't able to do so in combination with the properties that make it useful for military applications," says Josh Conway, DARPA program manager. "The real breakthrough with COUGAR fiber is that it can achieve a single spatial mode, maintain polarization, and provide low loss, all while keeping more than 99 percent of the optical beam in the air."

DARPA began COUGAR to enhance fiber-optic performance for military-grade gyroscopes and to develop a world-class, hollow-core fiber production capability in the United States.

"While we are still working on integrating this new technology into a gyroscope, the fiber itself is revolutionary," says Conway. "This type of technology may also lend itself to other types of high-power sensors and additional applications where intense optical beams are required. Hollow-core fiber is also naturally radiation hardened, so it may open up fiber applications to space systems."

Source: DARPA

Published September 2013

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