City of Light: The Story of Fiber Optics

158 CITY OF LIGHT
Downs and Ups at Bell Labs
The early 1970s brought tough times to AT&T. Long-distance traffic slumped
with the economy, and the first signs of competition appeared. It was time to
cut back on unpromising research, so one grim day in January 1973, management
told DeLoach to shut down diode laser development. They shifted
his whole group to work on silicon integrated electronic circuits. The next
morning DeLoach returned, pleading to stay with the laser project if anything
was to be kept alive. 53 He got his wish, although he was one of three people
left. 54
The cutbacks were not irrational. Other gallium arsenide devices had
earned a reputation for poor reliability, and diode lasers were doing nothing
to change that. 55 Ironically, progress was close at hand. By mid-1973, Bell
Labs had operated a double-heterojunction laser continuously for more than
a thousand hours at 30�C, the temperature of a warm room. Its output power
dropped a mere 10 percent over that 42-day period, although threshold current
increased and mode structure changed. No fundamental limits were in
sight. 56
Longer lifetimes posed new problems in testing. At the British Post Office,
says Alan Steventon, ‘‘We got to the stage where the predicted life of lasers
was increasing at the same rate as time was passing.’’ 57 The longer the lasers
lasted, the longer experiments took to yield results. One thousand hours is 42
days; 10,000 hours is over a year. System developers wanted lasers to last
up to a million hours (100 years), but nobody could wait that long. Test
specialists turned up the heat, figuring that higher temperatures would accelerate
aging.
As the research engineers debated the merits of accelerated aging, a little
New Jersey company decided the time was ripe to start selling doubleheterojunction
lasers. Commercial solid-state and gas lasers had reached the
market within a couple of years after the first laboratory successes. In June
1975—five years after Hayashi taped his note to Panish’s door—Laser Diode
Laboratories sent out its first new product announcements. The RCA spin-off
was too small to wait for long-term life tests; it had to cash in on its investment
early. The press release conceded that life tests had only reached 1000
hours, but boldly predicted that the lasers would last 10,000 hours. Nobody
else was brash enough to offer room-temperature diodes, but Laser Diode bet
that system developers would pay $250 to $350 for diode lasers emitting
steady powers of 5 to 10 milliwatts. 58
System designers wanted to verify much longer lifetimes, so laser developers
turned up the heat. They put fresh new lasers into ovens at 50 to 90�C
(122 to 194�F) and watched how long they lasted. Then they multiplied that
number by a suitable factor to extrapolate room-temperature life. It was a
simple way to test laser reliability without waiting forever for the results.
Researchers debated the proper scaling factors for a few years, but it was
never a crucial issue.