City of Light: The Story of Fiber Optics

184 CITY OF LIGHT
On March 27 Horiguchi measured a low-water fiber at longer wavelengths
and found it had loss of 0.47 decibel per kilometer at 1.2 micrometers, with
all but 0.1 decibel due to scattering. That was their best fiber so far, and it
was no fluke. Total attenuation was below 1 decibel from 0.95 to 1.37 micrometers.
They looked very very closely at loss over a range of wavelengths
and concluded that their clearest fiber contained only 80 parts per billion of
water, one-sixth as much as the previous record low. 45 They had hit the
jackpot, a new window for fiber optic communications.
Crucially, their new window offered both low loss and low material dispersion.
Not only could signals go farther at the longer wavelength, but also
they could carry much more information because shorter pulses could be
packed more closely together. Opening the new window was encouraging
news, and even while Bell Labs was designing its Chicago system, researchers
on the cutting edge began considering the new window at 1.3 micrometers.
A New Laser Family
The most obvious problem in moving to a new wavelength was the need for
a new laser source. That seemed a formidable obstacle because after a dozen
years of development, gallium arsenide lasers still didn’t meet telephone industry
reliability requirements. However, as Horiguchi and Osanai opened the
second window in fibers, a Chinese-born scientist at the MIT Lincoln Laboratory
invented a new family of long-wavelength lasers.
J. Jim Hsieh was in grade school when his father, an air force officer, fled
the communist revolution and took his family to Taiwan. After a year in the
Taiwanese Navy, the tall young Hsieh came to America, where he earned a
doctorate studying the then-exotic semiconductor gallium nitride. 46 He started
working on gallium arsenide when he arrived at Lincoln Lab in 1971, but
his interest soon wandered. The Air Force wanted 1.06 micrometer diode
lasers for space communications, and Hsieh thought such wavelengths might
be useful in fiber optics. After standard materials showed little promise, about
1973 he turned to an unconventional approach, making semiconductor lasers
from a mixture of four elements, gallium, indium, arsenic, and phosphorus.
The conventional wisdom held that as a bad idea. Two-element semiconductors
such as gallium arsenide must be mixed in the right proportions to
make working devices. Adding a third element to the blend caused more
problems because they had to be grown on substrates of simpler compounds
with almost identical atomic spacing. That worked for gallium aluminum
arsenide on gallium arsenide, but not for other semiconductor mixtures. Most
specialists thought there was no hope of balancing four elements in what
they called a ‘‘quaternary’’ compound.
Hsieh saw an opportunity instead of a problem. A two-element compound
has a fixed lattice spacing and gap between valence and conduction bands—
which determines the laser wavelength. Adding a third element generally