Semiconductor diode that produces visible or infrared light when subjected to an electric current, as a result of electroluminescence. Visible-light LEDs are used in many electronic devices as indicator lamps (e.g., an on/off indicator) and, when arranged in a matrix, to spell out letters or numbers on alphanumeric displays. Infrared LEDs are used in optoelectronics (e.g., in auto-focus cameras and television remote controls) and as light sources in some long-range fibre-optic communications systems. LEDs are formed by the so-called III-V compound semiconductors related to gallium arsenide. They consume little power and are long-lasting and inexpensive.

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A vertical-external-cavity surface-emitting-laser (VECSEL) is a small semiconductor laser similar to a vertical-cavity surface-emitting laser (VCSEL). VECSELs are used primarily as near infrared devices in laser cooling and spectroscopy, but have also been explored for applications such as telecommunications.

Comparisons with VCSELs

Unlike a VCSEL, in which two high-reflecting mirrors are incorporated into the laser structure to form the optical cavity, in a VECSEL one of the two mirrors is external to the diode structure. As a result, the cavity includes a free-space region. A typical distance from the diode to the external mirror would be 1 cm. Several workers demonstrated optically pumped VECSELs, and they continue to be developed for many applications including very high power diode laser sources for use in industrial machining (cutting, punching, etc.) because of their unusually high power (see below) and efficiency when pumped by multi-mode diode laser bars. These lasers are in the process of challenging conventional high power lasers such as solid state (e.g., Nd:YAG) and carbon dioxide lasers for machining operations.

However, electrically pumped VECSELs (another matter entirely), were the brainchild of Aram Mooradian, a well-known and highly credentialed worker in the laser field. (Mooradian worked for many years at MIT Lincoln Laboratory in Lexington, Massachusetts. He is known for fundamental contributions to diode laser linewidth studies.) Mooradian formed a company, Novalux, Inc., for this express purpose (www.novalux.com) which was the first to demonstrate VECSELs (which they call "NECSELs"). Applications for electrically pumped VECSELs include frequency doubling of near-IR VECSEL emitters to attain compact powerful sources of single-mode blue and green light for projection display purposes.

Semiconductor gain

One of the most interesting features of any VECSEL is the thin-ness of the semiconductor gain region in the direction of propagation, less than 100 nm. In contrast, a conventional in-plane semiconductor laser entails light propagation over distances of from 250 µm upward to 2 mm or longer. The significance of the short propagation distance is that it causes the effect of "antiguiding" nonlinearities (the same phenomenon is coincidentally quantified by the linewidth enhancement factor relating to Mooradian's above-mentioned earlier work) in the diode laser gain region to be minimized. The result is a large-cross-section single-mode optical beam which is not attainable from in-plane (a.k.a. "edge-emitting") diode lasers.

In a VECSEL, the external mirror permits a significantly greater area of the diode to participate in generating light in a single mode, resulting in much higher power than otherwise attainable. Monolithic VCSELs emit powers in the low milliwatt range. By contrast, at the 2004 Optical Society of America "Conference on Lasers and Electro-Optics," held in San Francisco, California, one company (Coherent, Inc.) announced 45 watt continuous wave single-mode emission from an optically pumped VECSEL. Numerous other companies and organizations world-wide have adopted the optically pumped architecture for its simplicity.