While I was being educated on fiber optics at Bell Labs in the late 1970s, optical fiber development was done mostly by Corning Glass. When fiber was first proposed as a communications medium, it had one major problem: attenuation. The best fibers in the beginning had high loss, greater than 20 dB/km—a problem caused by the purity of the best glass available to manufacture fiber. 

The breakthrough for low-loss fiber came from three researchers at Corning: Donald Keck, Peter Schultz and Robert Maurer. They developed a way of making high-purity, low-loss fibers called modified chemical vapor deposition. Instead of starting with pure glass, they deposited the glass as a frit or soot on the inside of a glass tube by chemical reaction at very high temperatures. The tube, called a fiber preform, was heated to solidify it, and then the fiber was pulled from the preform in a large tower.

The Corning fiber manufacturing method created fiber with much lower loss, adequate for the first field trials of fiber optic communications in 1976 and 1977 by AT&T in the tunnels under Chicago. Around the same time, GTE did field trials in Long Beach, Calif., and the British Post Office in Martlesham Heath in the United Kingdom. All field trials were successful and fiber optics became the medium of choice for future communications systems.

The field trials were all done with multimode fiber, which had a larger core diameter, so it was easier to manufacture, splice and terminate. But multimode fiber transmits many different rays of light called modes. Each mode travels at different speeds in the fiber, limiting its bandwidth. 

The rise of single-mode

When I was visiting Bell Labs not long after these field trials, the researchers told me that multimode fiber was not going to be used for much longer. Everybody was working on a new type, single-mode fiber, that had a core diameter so small that only one mode of light would be transmitted, providing virtually infinite bandwidth.

Manufacturing single-mode fiber was not easy. The smaller core diameter meant that manufacturing tolerances had to be smaller, too. And single-mode fiber was intended for use with a new laser at a longer wavelength of 1,310 nm, where the attenuation of the fiber was even lower, allowing longer fiber links without repeaters. Developing those lasers was still a work in process and I was involved; more on lasers in a future column.

While single-mode fiber was being developed, telecom companies began operating some fiber optic long-distance networks using multimode fiber and 850-nm lasers. AT&T built networks on the East and West coasts, although the networks were designed to also carry 1,310-nm signals when the lasers became available. 

Everything changed in late 1983. Single-­mode fiber and 1,310-nm lasers became viable products, and everybody began switching to single-mode. That fall, I remember a presentation at an industry marketing meeting by the engineering manager of MCI, the company that became an AT&T competitor with long-distance microwave links and satellites. He announced that MCI was switching to fiber optics and planned to build a nationwide telecom network based on fiber optics. The stampede had begun.

Fiber then and fiber now

The amazing fact is the single-mode fiber MCI used to build networks in 1984 is practically the same fiber we use today. It’s hard to imagine that in a fast-moving technology like fiber optics, anything could last virtually unchanged for more than 40 years! 

But indeed, the majority of optical fiber used today is practically the same as it was then. The one thing that has changed is the price. Experience manufacturing billions of kilometers of fiber has perfected the processes, and the price of fiber has fallen so much that cables with very high numbers of fibers are common today.

With single-mode fiber technology ready to go, telecom companies began installing fiber to replace all the aging long-distance copper wires and most of the microwave radio links and satellites. Some radio links and satellites are still in use today, mostly in mountainous terrain or very rural areas.

Some variations of single-mode fiber have been developed for very long distances like intercontinental submarine cables. More recently, new types of fibers such as multicore fibers (four or more single-mode cores in one fiber) and hollow-core fiber are being used for specialized applications, but single-mode continues to dominate the market.

Multimode fiber never died. It became widely used for premises networks with inexpensive LED sources. Its distance and bandwidth capability was much greater than unshielded twisted-pair Cat 5 copper cables, so it was the choice for longer links in buildings and campuses. 

stock.adobe.com / Péter Mács