asers drive modern communications. From the huge banks of servers and switches
powering Web search engines and online shopping to the undersea cables linking
continents together, the data and images we need and crave originate with the tiniest flashes of
infrared laser light.
According to experts, lasers have replaced LEDs as signal sources in all but the most specialized
applications of optical fibers. Semiconductor lasers have significantly faster response times than
LEDs, which is crucial to the development of ultrafast networks and computers. LEDs also
fail to meet the spectral-width and power requirements of today’s high-speed fiber systems,
although they work with large-core multi-mode fibers in some short-range applications.
Communications lasers operate in the near-infrared bands tailored to the transmis-
sion properties of optical fibers. ;e most important transmission window is the
C or “conventional” band from 1,530 to 1,565 nm, centered on 1,550 nm—the
wavelength at which single-mode fibers have the least signal loss. ;is is also
called the “erbium window” because it is the best suited for erbium-doped
fiber amplifiers, which are crucial to long-haul communications. Other
important bands include the L or “long” band from 1,565 to 1,625 nm,
for long-haul telecom; the O or “original” band at 1,310 nm and the S
or “short” band at 1,490 nm, for intermediate distances; and the un-
named region centered on 850 nm, for short-range data applications.
Over the past 15 years, engineers have developed the tunable
semiconductor laser, whose operating wavelength can be adjusted
within the relevant communications band, and the vertical-cavity
surface-emitting laser (VCSEL), whose top-emitting geometry
lends itself to low-cost manufacturing techniques. Without
these essential elements, it is unlikely that fiber-optic commu-
nications would be crisscrossing the globe, running the “last
mile” to homes and businesses and forming the backbone of
mega data centers.
VCSELs burst onto the communications scene about
15 years ago, once they became practical. ;ese short-
cavity lasers are easy to fabricate on a single semiconduc-
tor wafer, so they have become low-cost and popular
in many applications. ;e VCSEL in your computer
mouse probably cost its manufacturer 10 cents in U.S.
funds to make.
L
