Through a glass brightly: Making the first low-loss optical fiber
Dr. Donald Keck was a member of the team at Corning that developed the first low-loss optical fiber in 1970, an advancement that made fiber practical for communications applications. Here, Dr. Keck tells the story of the work behind this achievement.
Editor’s Note:Dr. Donald Keck was a member of the team at Corning that developed the first low-loss optical fiber in 1970, an advancement that made fiber practical for communications applications. Here, Dr. Keck tells the story of the work behind this achievement.
The power and pervasiveness of optical fiber means that today, we are a world that runs on connections and communications moving literally at the speed of light.
Forty years ago, however, Dr. Bob Maurer, Dr. Peter Schultz, and I were brought together to provide a communications solution, with no idea of the revolution we would create. On June 17, 1966, members of the British Government asked Corning to help create pure glass fiber optics. Their request was for a singlemode fiber (100 micron diameter with a 0.75 micron core) with a total attenuation of less than 20 dB/km. The very best bulk optical glasses of the day had attenuations of around 1000 dB/km. This meant we had to see an improvement in transparency of 1098 to reach the 20-dB/km figure. The goal seemed impossible -- no one had any idea if we could reach it.
Corning handed the challenge to Dr. Bob Maurer, a Corning physicist known for his work on measuring light scattering in glasses. Bob had published two studies, in 1956 and 1960 respectively, indicating that flame hydrolysis fused silica had the lowest Rayleigh scattering of all glasses measured. These studies were built upon the innovations of two giants within Corning’s history, Dr. Frank Hyde and Dr. Martin Nordberg.
In 1930, Dr. Hyde invented the flame hydrolysis process, in which vapors of silicon tetrachloride, when passed through a flame, would hydrolyze to form a fine powder of fused silica glass that he called soot. Nine years later, Dr. Nordberg added titanium tetrachloride to Dr. Hyde’s process and formed a very low-expansion doped fused silica glass used today in many applications, including astronomical telescopes.
Dr. Bob Maurer, Dr. Peter Shultz, and Dr. Donald Keck (left to right) tackled the problem of creating a low-loss optical fiber.
Our time of trial and error
Bob began his work the summer of 1967 by making a rod-in-tube fiber using Corning’s fused silica as the cladding and a slightly higher refractive index titania-doped fused silica as the core. While losses were still very high, he was encouraged enough to request that two additional scientists join the team, Dr. Peter Schultz and, in 1968, myself.
So began our time of trial and error. Based on Bob’s earlier results, we focused our efforts exclusively on fused silica fibers made by flame hydrolysis. Our approach was somewhat contrarian because we were essentially adding an “impurity” to the pure fused silica to raise the refractive index to create the fiber core. We had no idea if this “doping” technique would work, and indeed our initial results were no better than Bob’s.
We tried various methods of cleaning and polishing the rods of ULE and tubes of fused silica, but still the losses remained too high. We tried depositing the fused silica cladding onto a core rod inside Peter’s small boule furnace to no avail.
One source of the high loss was the formation of reduced titanium color centers during the high-temperature fiber drawing step, so we learned to anneal these away by heat-treating the fibers at 800 to 1000°C. But this in turn weakened the fibers due to surface crystallization. The second, equally large loss came from light-scattering centers at the core-clad interface, created by dirt particles during the rod-in-tube process. We kept on failing, but we also kept on learning.
Then we hit upon an idea that proved to be the key. Rather than inserting a core rod, we directly deposited a thin layer of core glass inside our carefully flame-polished cladding tube. This gave us intimate contact between core and clad materials.
Crude but effective, our equipment consisted of a lathe headstock made from a large ball-bearing that held the rotating cladding tube in front of a flame hydrolysis burner. The burner produced a soot stream containing titania-doped silica that was coaxed into our tube, believe it or not, by our lab vacuum cleaner. This coated tube was then placed in our fiber draw furnace where the soot sintered into a clear glass layer. The hole collapsed to form a solid rod containing the doped core, and the entire structure was drawn down into fiber.
I will never forget measuring that first low-loss fiber. After heat-treating a piece of our latest fiber, I positioned it in my attenuation measurement apparatus. I had a viewing telescope so that I could observe and position the focused He-Ne laser beam on the fiber-end. When the laser beam hit the fiber core, I was suddenly blinded by a bright returning laser beam. It took me a moment to realize the laser was being retro-reflected off the far end of the fiber and coming back through my system. With considerable anticipation I measured the fiber loss and, to my delight and surprise, it was 17dB/km. I registered my delight in my now well-known lab-book entry … “Whoopee!”
We had done it, but we were far from done. In fact, it wasn’t until 1972 that we had the first truly practical low-loss fiber, when Pete learned to replace titanium with germanium and avoid the heat treatment, and we invented the outside vapor deposition process. Because of the investment involved in producing it, Corning made no profit until 16 years after our first demonstration. Talk about patient money! But we all know how the story ends -- or, I should say, continues to be re-written.