Page 27 - Optical Switching And Networking Handbook

12 Chapter 1

was fine for looking inside the body but not for long-haul communi-
cations. Telecommunications operated over much longer distances
and required loss of no more than 10 or 20 decibels per kilometer.
At the Corning Glass Works (now Corning, Inc.), Robert Maurer,
Donald Keck, and Peter Schultz started with fused silica, a material
that can be made extremely pure but has a high melting point and a
low refractive index. They made cylindrical performs by depositing
purified materials from the vapor phase, adding carefully controlled
levels of dopants to make the refractive index of the core slightly
higher than that of the cladding without raising attenuation dra-
matically. In September 1970, they announced that they had made
single-mode fibers with attenuation at the 633-nanometer helium-
neon line below 20 decibels per kilometer.
The Corning breakthrough was among the most dramatic of many
developments that opened the door to fiberoptic communications. In
the same year, Bell Labs and a team at the Ioffe Physical Institute in
Leningrad made the first semiconductor diode lasers capable of
emitting a continuous wave at room temperature. Improvements
over time allowed for dramatically less fiber loss, aided both by
improved fabrication methods and by the shift to longer wavelengths
where fibers have inherently lower attenuation.
Early single-mode fibers had cores several micrometers in diame-
ter, and in the early 1970s, this bothered developers.They doubted it
would be possible to achieve the micrometer-scale tolerances needed
to couple light efficiently into the tiny cores from light sources or in
splices or connectors. Not satisfied with the low bandwidth of step-
index multimode fiber, they concentrated on multimode fibers with a
refractive index gradient between core and cladding and core diam-
eters of 50 or 62.5 microns.The first generation of telephone field tri-
als in 1977 used such fibers to transmit light at 850 nanometers.
These first-generation systems could transmit light several kilo-
meters without repeaters but were limited by loss of about 2 decibels
per kilometer in the fiber. A second generation soon appeared using
new lasers, which emitted at 1.3 microns. Fiber attenuation was as
low as 0.5 decibel per kilometer, and pulse dispersion was somewhat
lower than at 850 nanometers. Development of hardware for the first
transatlantic fiber cable showed that single-mode systems were fea-
sible, so when deregulation opened the long-distance phone market