as well as scientists who have not been
involved in difficult high-tech development
programs, do not appreciate what it takes to
succeed with these types of endeavors.
While still in London, I reflected on
our reliability achievements as well as
the hard parts of the laser development
program still ahead. I recall taking some
encouragement from the words of the great
statesman Winston Churchill, referring to
much more serious issues than ours: “Now
this is not the end. It is not even the beginning of the end. But it is, perhaps, the end
of the beginning.”
“million hour paper,”
demonstrated to us
and to the world’s
that it was possible
laser devices with
very long lifetimes.
used by Eric Ippen and coworkers to produce
optical pulses as short as 1. 3 picoseconds—
which were believed to be the shortest pulses
produced at that time (1980) with a semiconductor laser.
Improving laser device
As we became better able to fabricate and age lasers, we also
refined the testing of device characteristics and our ability to
analytically model these devices. These developments greatly
aided our early identification of lasers with deficiencies and
also pointed the way to eliminating those problems.
Deficiencies came in several forms, including “kinks,”
“spikes” and “pulsations.” Kinks manifested themselves as nonlinear light-current characteristics and spikes as high-intensity,
short-duration, leading edge light output when the laser was
excited with current with a very fast rise time. It turned out
that kink and spike nonlinearities were due to subtle distortions of the lasing mode in the device.
When the lateral position of the lasing mode was not well
enough constrained by the structure, it could move slightly as
the excitation current changed the thermal or gain profile. We
developed a testing technique using derivative measurements
of light-current-voltage characteristics, which was helpful in
identifying such lasers. Because the pumped volume is better
controlled by their more complex design, these problems are
significantly reduced in properly fabricated, index-guided,
buried heterostructure lasers.
We used the term “pulsations” to describe light output
that exhibited high-frequency oscillations, often beginning
after considerable aging. This characteristic was undesirable not only because it could potentially cause transmission
errors in high-data-rate applications but—more important for
us—because they interfered with procedures for qualifying
and certifying high-reliability devices. We came to understand that this problem in otherwise good lasers (e.g., those
with no DLDs and with well-behaved lowest order spatial
modes) was associated with poorly pumped regions near the
These pulsations were essentially removed when we covered
the mirrors we fabricated in a precision cleaving process with
half-wave coatings of Al2O3. Some of these non-uniformly
pumped lasers, with their built-in saturable absorbers, were later
The first applications of these lasers in the
Bell System were in system experiments
that were not intended to carry commercial
traffic. These used 50-µm core multi-mode
fiber and data rates such as 45 and 90 Mb/s.
After that, they were tried in short-distance
trials carrying live traffic, including a suc-
cessful May 1977 installation in which
fibers were used to connect three telephone
central offices in downtown Chicago. The
small physical size and large capacity of the fiber system helped
to relieve crowding in the underground (and sometimes under-
water) ducts that connected the offices.
p+ -InGa AsP ( 1-2 μm)
InGa AsP ( 1-3 μm)
SI InP :Fe (OM VPE)
An InP/InGaAsP channel substrate buried heterostructure
laser (CSBH) for operation at 1. 3 µm.
D.P. Wilt et al. Electron Lett. 22, 869 (1986).
[ A 1.7Gb/s transmitter ]
Courtesy of R. W. Dixon
A 1.7Gb/s transmitter, which contains a 1. 3 μm multi-wavelength
CSBH laser, of the type shown above.
46 | OPN Optics & Photonics News