Editing Laser diode (section)

===Vertical-external-cavity surface-emitting-laser===
{{main|Vertical-external-cavity surface-emitting-laser}}

Vertical-external-cavity surface-emitting lasers, or [[VECSEL]]s, are similar to VCSELs. In VCSELs, the mirrors are typically grown [[Epitaxy|epitaxially]] as part of the diode structure, or grown separately and bonded directly to the semiconductor containing the active region. VECSELs are distinguished by a construction in which 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&nbsp;cm.

One of the most interesting features of any VECSEL is the small thickness of the semiconductor gain region in the direction of propagation, less than 100&nbsp;nm. In contrast, a conventional in-plane semiconductor laser entails light propagation over distances of from 250&nbsp;μm upward to 2&nbsp;mm or longer. The significance of the short propagation distance is that it causes the effect of ''antiguiding'' nonlinearities in the diode laser gain region to be minimized. The result is a large-cross-section single-mode optical beam that is not attainable from in-plane ("edge-emitting") diode lasers.

Several workers demonstrated optically pumped VECSELs, and they continue to be developed for many applications, including high-power sources for use in industrial machining (cutting, punching, etc.) because of their unusually high power and efficiency when pumped by multi-mode diode laser bars. However, because of their lack of ''p''–''n'' junctions, optically pumped VECSELs are not considered ''diode lasers'', and are classified as semiconductor lasers.{{citation needed|date=December 2012}}

Electrically pumped VECSELs have also been demonstrated. Applications for electrically pumped VECSELs include projection displays, served by [[frequency doubling]] of near-IR VECSEL emitters to produce blue and green light.