City of Light: The Story of Fiber Optics

RECIPES FOR GRAINS OF SALT 157
solve the problem. Having done their job of showing the feasibility of roomtemperature
lasers, Panish and Hayashi were happy to give device specialists
the responsibility for developing practical lasers. 47 The greatest strength of a
place like Bell Labs is not individual superstars, but its tremendous depth of
talent to apply the perspiration vital to practical inventions. Management
handed the job to Barney DeLoach, a hero for inventing a vital high-frequency
semiconductor device that helped restart the millimeter waveguide program.
At first, DeLoach’s group measured laser lifetimes in one-minute intervals.
‘‘It was a rare device that lasted that long,’’ recalls Robert Hartman. ‘‘It would
emit light constantly for a couple of seconds, then boom, it’s off. The better
ones would last 30 to 40 seconds, and die in a second or two.’’ 48 He spotted
two distinct failure modes: sudden burnout and slower dimming to darkness. 49
They soon pinned down the cause of sudden failures. The laser beam emerged
from a spot only about half a micrometer high and 20 micrometers wide on
the edge of the chip. The total power wasn’t high, but concentrating it on
such a tiny spot damaged the edge or ‘‘facet’’ where the beam emerged.
Applying special coatings and refining the laser structure reduced the damage.
Gradual degradation was a more stubborn problem until Hartman noticed
it resembled changes in mechanical springs and wondered if it might arise
from strain in the crystalline lattice. Studies with polarized light showed that
the more strain in the crystal, the faster the laser died. Then he set up a
microscope to watch laser emission through holes in its electrical contacts.
The laser stripe started as a bright zone, then dark lines grew across the bright
region, choking out laser emission. Growth of these dark lines was an uncanny
sight for a solid-state physicist trained to think of atoms as locked
immobile in a crystalline lattice. ‘‘Matter was moving around at room temperature,
as you watched. Nobody ever heard of diffusion rates like that in a
solid,’’ recalls DeLoach. 50
Detailed studies showed that the problem was flaws, usually in the gallium
arsenide substrate on which the whole laser structure was deposited. The
intense laser light triggered their formation and spread. Solving it required
years of meticulous development of better technology to grow laser crystals. 51
The British Post Office, STL, RCA, and Nippon Telegraph and Telephone had
their own programs. Life testing became an art, with racks of lasers, each
mounted with its own drive electronics and a heat sink to keep its temperature
constant. Sensors monitored drive current and output power, with feedback
circuits keeping one or the other constant. When output power dipped too
low or drive current rose too high, the testers declared the laser dead and
plugged another one into the socket.
Thousands and thousands of lasers lived brightly and briefly in labs around
the world. By the end of 1972, DeLoach had 25 to 30 people ‘‘going like
gangbusters’’ on reliability. Lifetimes crept up from minutes to hours and then
days. Then top management slammed on the brakes. DeLoach summarizes
their attitude: ‘‘We’ve already got air, we’ve already got copper. Who needs
a new medium?’’ 52