City of Light: The Story of Fiber Optics
City of Light: The Story of Fiber Optics
City of Light: The Story of Fiber Optics
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RECIPES FOR GRAINS OF SALT 155<br />
as light which increased laser efficiency and reduced threshold current. In<br />
the Bell Labs lasers, this region was the plane <strong>of</strong> the junction across the entire<br />
chip, 0.37 millimeter long and 0.08 millimeter wide. 33 Alferov concentrated<br />
current flow further by covering parts <strong>of</strong> the laser chip with silica, an insulator,<br />
so electrons had to flow through a 0.03-millimeter stripe in the 0.20<br />
millimeter wide chip. 34 This extra confinement increased the likelihood the<br />
electrons would recombine with holes—improving laser operation. Later diode<br />
laser developers have followed the same philosophy, refining structures<br />
to better confine current. Performance also improves when the layers confine<br />
light by total internal reflection, trapping it in the active stripe like a cladding<br />
traps light in an optical fiber.<br />
Ironically, the narrow-stripe laser originated at Bell Labs in 1966. Jack<br />
Dyment hoped that limiting current flow to a narrow stripe would reduce the<br />
current needed to drive the laser. He didn’t make much progress toward that<br />
goal; the real problem was the homojunction structure <strong>of</strong> his laser. But the<br />
stripe geometry eased another problem <strong>of</strong> early semiconductor lasers—poor<br />
beam quality. While gas lasers generated steady pencil-thin beams, the uneven<br />
pulsed light from diode lasers spread out so fast that calling it a beam<br />
was barely justified. When he added the stripe, Dyment says, ‘‘We could start<br />
to see repeatable mode patterns coming out <strong>of</strong> these devices, the first time<br />
anyone could see anything repeatable happening in diode lasers.’’ 35 Mode<br />
control was an important advance, and stripe-geometry lasers helped focus<br />
light into single-mode fibers. 36<br />
Panish and Hayashi knew about stripe-geometry lasers, but they didn’t<br />
bother with the extra step <strong>of</strong> adding a stripe to their laser. Alferov’s group<br />
did, covering their wafer with a thin insulating layer <strong>of</strong> silicon dioxide, in<br />
which they etched a 30-micrometer stripe for current to flow through. That<br />
extra concentration <strong>of</strong> current flow did the job, and the Russians had the first<br />
semiconductor laser to emit steadily at room temperature. 37<br />
It probably didn’t last very long. <strong>The</strong> informal rules <strong>of</strong> the laser-making<br />
game specify only that a laser has to emit a steady beam long enough to<br />
measure its properties. It might last for only seconds or minutes. Hayashi was<br />
a stickler and insisted that his operate repeatedly; it emitted for a couple <strong>of</strong><br />
hours altogether. <strong>The</strong> Russians left the precise lifetime discretely unmentioned.<br />
<strong>The</strong> Russians also were vague on other details and announced their success<br />
only in a Russian-language journal. By the time word trickled through<br />
the Iron Curtain, Bell Labs had already claimed a first. <strong>The</strong> lack <strong>of</strong> details and<br />
the penchant <strong>of</strong> Soviet <strong>of</strong>ficials for claiming their scientists had invented almost<br />
everything left many Americans skeptical. You have to go back and<br />
compare submission dates to discover that Alferov submitted his report on<br />
May 6, a month before Hayashi submitted his. Neutral observers consider the<br />
Russian claim credible. 38<br />
Both groups realized they had something to learn from each other. In<br />
October, Hayashi and Panish visited Alferov in Leningrad. In November, Alferov<br />
came to America 39 to spend six months at the University <strong>of</strong> Illinois with