I recently read an article about fiber broadband that described optical fiber as a hollow glass tube that transmits light down the hole in the center. I’ve seen that mistake made often since fiber optics began taking over communications many years ago. The light-carrying core of the optical fiber is pure glass, of course, but hollow fibers are in development.

There is a particular reason for this—latency, or the time it takes a signal to be transmitted to a distant point. It’s tied to the speed of light, which is much slower in glass than in air or a vacuum.

The speed of light and latency issues

A beam of light in a vacuum travels at approximately 300,000 kilometers per second (or 186,000 miles per second). That’s why it takes light about eight minutes to travel from the sun to the Earth. 

In glass, the speed of light slows to roughly 200,000 km per second (or 124,000 miles per second). We say glass has an index of refraction of about 1.5, the ratio of the speed of light in a vacuum to the speed in glass. That change in speed also causes light to bend, making it possible to focus light with a lens. More subtle changes in the glass composition allow optical fiber to trap light in the core and transmit it for long distances.

That speed change means that light travels slower in the fiber than light, or even radio waves, in the air. The difference is substantial over long distances, about 1.5 microseconds per kilometer.

Who cares about a microsecond or so? Well, it is important if you are working on systems with critical timing or engaging in activities such as high-speed stock trading. It’s stock trading that is really obsessed with latency; lower latency means faster trades and more profit. Since the majority of stock trades are done by computers, quite a few people are interested in reducing latency in communications systems. The traders are the ones pushing dedicated microwave links and hollow-core fibers to reduce latency.

Hollow-core fiber transmits light in an air core instead of a solid glass core. A simple hollow glass tube with an air core won’t work well because it has high losses caused by scattering off the core/cladding interface. Research has led to a cladding with a honeycomb-like microstructure, which confines light in the hollow core and reduces losses.

Attenuation characteristics

However, the attenuation of hollow-core fiber is still high. Commercially available hollow-core fiber has a typical attenuation of about 3–5 decibels/km, limiting its use to short links. Research and development has shown promising gains in the laboratory. A British company reported hollow-core fiber designs with losses at or below that of conventional single-mode fiber, but has not released a real product using that technology.

Hollow-core fiber also has unusual attenuation characteristics caused by the microstructure of the cladding. To get low loss, you have to carefully choose transmission wavelengths. The microstructure makes the fiber sensitive to bending and stress, which makes cabling hollow-core fiber difficult. Some companies have solved those problems and offer cabled hollow-core fiber. You also need a special fusion splicer for hollow-core fiber that can rotate the fiber to allow matching the microstructure to get low loss.

As you might guess, the complicated process of manufacturing, cabling and splicing it makes hollow-core fiber very expensive. Field trials recently demonstrated 800 Gb/s and 1.2 Tb/s transmission using coherent transceivers. If low-loss hollow-core fiber can be made in quantity, it could be a good choice for very long links like submarine cables connecting the continents.

Hollow-core fiber is not for every application, but if latency is important, it can be the right solution. High-speed stock traders already use it, often connecting data centers to microwave towers. They use microwave links for the same reason; the speed of microwave signals through the air is faster than light in a fiber. 

Hollow-core fiber also has applications in data centers, supercomputers and many other areas where latency is important.

jim hayes