City of Light: The Story of Fiber Optics

THREE GENERATIONS IN FIVE YEARS 185
changes both wavelength and lattice spacing, but leaves only one degree of
freedom to adjust both parameters—like one knob both tuning a radio frequency
and changing its volume. Mix four elements together in the right
proportions and you get two degrees of freedom, so you can adjust both
wavelength and lattice spacing. Hsieh realized that mixing gallium, indium,
arsenic, and phosphorus would let him both select the laser wavelength and
match the lattice spacing of the thin quaternary layer to that of a substrate
of indium phosphide. 47
Like Panish and Hayashi, Hsieh had to expend considerable perspiration
to take advantage of his inspiration. Few others had worked on indium phosphide,
so he had to painstakingly test mixtures and growth techniques. He
and a technician set up four furnaces and grew samples, collecting one data
point per day per furnace, compiling the data he needed to make recipes for
his lasers. 48
With data in hand, Hsieh grew simple double-heterojunction lasers with
a blend of indium gallium arsenide phosphide (InGaAsP) mixed to emit at 1.1
micrometers. The first ones emitted only pulses at room temperature, but as
Horiguchi and Osanai measured their record low loss, Hsieh saw a steady
1.1-micrometer beam from a InGaAsP laser at room temperature. 49 When
Hsieh tried making longer-wavelength lasers to match the new window, he
found they were easier to produce. By the end of 1976, he had made roomtemperature
InGaAsP lasers emitting steadily at 1.21 and 1.25 micrometers
and generating pulses at 1.28 micrometers. 50
Other developers expected lifetime problems like those that continued to
plague gallium arsenide. But Hsieh was delighted to find that his quaternary
lasers ‘‘looked great’’ in early life tests. From the very start, they were much
more durable than gallium arsenide, although initially no one knew why.
Long wavelengths began looking good.
A Big Push in Japan
Osanai and Horiguchi were on a roll in the summer of 1976. Hydrogenoxygen
bonds absorbed very strongly at 1.39 micrometers, even in dry fibers.
But removing boron from the core glass opened a third window beyond 1.5
micrometers. The window opened further when they used cores of fused silica
doped with germanium. Their best fiber had loss of only 0.46 decibel per
kilometer at 1.51 micrometers. 51 They also dried the glass further, reducing
water to 30 parts per billion, although their clearest fiber at the time contained
150 parts per billion.
Opening the new window changed the ground rules for fiber optics. Systems
operating at 1.3 micrometers had a whole different set of operating
characteristics than 850-nanometer systems. Japanese engineers were quick
to see that lower loss and pulse spreading could be the basis of a second
generation of fiber technology. A team at Nippon Telegraph and Telephone
calculated that 1.3-micrometer LED transmitters could send several tens of