Optics & Photonics News

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.”

The so-called “million hour paper,” demonstrated to us and to the world’s laser community that it was possible to construct semiconductor 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
characteristics

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 laser’s mirrors.

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

Early applications

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.

AuZnAu p+ -InGa AsP ( 1-2 μm)

InGa AsP ( 1-3 μm) Au TiPt SiO2

n-InP
SI InP :Fe (OM VPE)
n-InP
n-InP substrate
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).