Comcast released a press release that announced it is the first ISP in the U.S. that is trialing hollowcore fiber. Hollowcore fiber takes advantage of the phenomenon where light can travel 50% faster through air than it can through fiberglass. As is described by the name, hollowcore fiber is a fiberglass strand with a small hollow tube in the center.
Comcast is interested in the technology because the faster light speed translates into as much as a 33% reduction in latency. Comcast would use the hollowcore fiber for applications that demand low latency.
The press release described a test where Comcast made a 40-kilometer connection between two locations in Philadelpia to be able to test the performance of the fiber in a real-world application. Comcast was able to successfully establish a bidirectional transmission using simultaneous traffic paths ranging from 10 to 400 gigabits per second to 400 Gbps on a single strand of hollowcore fiber.
Hollowcore fiber has been developed by Lumenisity. The concept of hollowcore fiber was born a decade ago in a DARPA lab working with Honeywell to improve fiber performance. Those tests showed that it was possible to create a single straight path of light in tubes that was perfect for military applications. The light could carry more bandwidth for greater distances without having to be regenerated. By not bouncing through glass, the signal maintained intensity for longer distances. DARPA found the fixed orientation of light inside the tubes to be of great value for communication with military-grade gyroscopes.
Until some recent breakthroughs, the hollow tube fiber was plagued by periodic high signal loss when the light signal lost it’s straight-path coherence. Lumenisity has been able to lower signal loss to 1 dB per kilometer, which is still higher than the 0.2 dB loss expected for traditional fiber. However, the lab trials indicate that better manufacturing processes should be able to significantly lower signal loss.
Lumenisity big breakthrough came when it developed the ability to combine multiple wavelengths of light while avoiding the phenomenon known as interwave mixing, where different light frequencies interfere with each other. By minimizing signal dispersion, Lumenisity eliminated the need for digital signal processors that are used in other fiber to compensate for chromatic dispersion. This means repeater sites that can be placed further apart and require simpler and cheaper electronics.
Lumenisity doesn’t see hollow core fiber being used as a replacement on most fiber routes. The real benefits come in situations that require low latency along with high bandwidth. For example, the hollow core fiber might be used to feed the trading desks on Wall Street. The fiber might improve performance for the fiber serving large data centers.






