Remembering the Million Hour Laser

In the late 1960s, Bell Labs had a problem. The nation’s demand for long-distance telecommunications services was steadily increasing, but the technologies then in use— coaxial cable and point-to-point microwave transmission through the air—could not keep up with the pace. The major reductions in optical fiber waveguide losses reported in the early 1970s were therefore of great interest. The lowest-loss regions of these fibers were in the 0.8 to 0.9 μm range, which could in principle be accessed by devices built using the GaAs-GaAlAs material system. Thought was given to the possible use of GaAs light-emitting diodes (LEDs), but it was immediately obvious that semiconductor lasers would be much better sources—if they could be developed reliably in commercial quantities. One could easily imagine an efficient GaAs laser that could couple a milliwatt of optical power into a fiber with a core diameter of about 50 μm. Thus was defined the first generation of fiber-optic telecommunication systems. In the late 1960s and early 1970s, the author was a young supervisor working on the development of LEDs for Bell System applications. In that process, he learned quite a bit about the physics, technology, and transfer-to-volume manufacture of III-V semiconductors. One result of this programwas the successful implementation of green-emitting GaP LEDs for nighttime dial illumination in the handset of the Dreyfuss-designed Trimline phone. Something like 100 million of these sets were subsequently produced. In 1973, the author transferred to a small exploratory development group working on semiconductor lasers. The group had benefited from an excellent research effort that happened just down the hall. Most notable was the demonstration in 1970 of a continuously operating room-temperature GaAs-AlGaAs heterostructure semiconductor laser [1] (see Fig. 1). However, these broad-area lasers had high operating currents (around 400 mA) and very short lives (they were sometimes referred to as flashbulbs), but they showed the way forward! The group’s choice of a laser structure for initial development consisted of four planar epitaxial layers grown sequentially by liquid-phase epitaxy (LPE) on a GaAs substrate. We inhibited lateral carrier flow by using proton bombardment to define a “stripe-geometry” wherein only a narrow stripe, 10 × 250 μm, was electrically pumped (see Fig. 2). These “stripe-geometry lasers” became the workhorses of the early Bell Labs semiconductor laser development. They allowed the sorting out of many reliability and device performance issues. In a typical week, half a dozen or so wafers were processed into some thousands of lasers. Fast turnaround made it possible to quickly and systematically iterate device, processing, and material innovations. Many of the early stripe-geometry lasers had very erratic properties. Some would lase for a time but would then suddenly become inoperable. Others would die slowly. Still others would not work from the outset. Typical continuous-wave operating lifetimes at room temperature were on the order of minutes to days. Many devices also had other undesirable characteristics, for example, nonlinear light output versus current. It was clear that the group had a very difficult development project on its hands! Some thoughtful observers, including one key Bell Laboratories vice president, opined that success was unattainble. Important clues to improvements came in early 1973 from an experiment in which “windows” were fabricated on the substrate side of stripe-geometry lasers in such a way that spontaneous emission (and scattered stimulated emission if present) from the stripe region of the laser could be observed with an infrared optical microscope. Dark-line defects (DLDs), which 1975–1990