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news digest ♦ Lasers


multi-mode variants that can produce 1.4 W and have a wall-plug efficiency of more than 23 percent.


Raring believes that the excellent performance produced by these lasers stems from the use of non-polar and semi-polar planes.


Turning to these orientations improves the radiative recombination rate of these devices through increased overlap of the electrons and holes in the quantum wells. What’s more, it aids hole injection, thanks to a reduction in the effective mass of this carrier. However, Raring believes that the most exciting aspect of these novel planes is the far greater design freedom that they enable.


Osram improves green laser performance


Refinements to the epitaxial structure of green laser diodes have led to an increase in output power and longer lifetimes.


At the ninth International Conference on Nitride Semiconductors, Uwe Strauss from Osram Opto Semiconductors described the company’s efforts to improve the performance of its green laser diodes that are fabricated on the conventional plane of GaN.


Strauss explained that these lasers will target pico- projectors, where they can combine with red and blue equivalents to provide a lighting source with a small footprint that is free from a focusing element.


The most promising market for Soraa’s lasers is projection displays, which employ a combination of red, green and blue lasers to form colour images. In this market lasers are competing with lamps and LEDs, but the latter two deliver an inferior optical throughput, typically by a factor of three.


Most commercial green lasers on the market today employ some form of frequency doubling of an infra- red source. Replacing such devices with a single, green-emitting chip will lead to improvements in efficiency, compactness, ruggedness and speckle, according to Raring.


He estimates that the pico-projector market could consume 200 million units per year, and a similar number may be needed to supply desktop and high- end projectors.


According to him, the green source in a pico- projector producing a 100 lumen brightness must have: an emission wavelength of 515 nm; an output power of at least 50 mW; a wall plug efficiency of 5 percent or more; and, in both the lateral and vertical directions, a single mode output. If a shorter wavelength source is used – for example, a 505 nm laser – the power output requirements are higher.


In the lab, Osram has exceeded these requirements. It’s best lasers produce 70 mW at 522 nm with a wall plug efficiency of 5-6 percent, and have a spectral width of 1.8 nm. What’s more, reliability – defined as the time taken for the operating current to increase by 30 percent – is more than 1000 hours.


116 www.compoundsemiconductor.net August/September 2011


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