A Dark-Horse Green Laser Shines

New green-laser chips could power HD pico projectors

3 min read

If you want to show pictures and presentations anywhere, you'll increasingly have the option of buying a mobile phone with a pico projector. And if picture quality tops your wish list, you'll want a model that creates images by mixing the output from red, blue, and green lasers.

The green variant will initially be a cumbersome contraption that combines an infrared laser and a frequency doubler, but researchers at the German electronics company Osram Opto Semiconductors and the Japanese optoelectronics giant Nichia are independently claiming to have met the power and color requirements with a single laser chip. This potential successor to the infrared laser and frequency doubler combo has much to recommend it: It's one-tenth the current size, costs less to make, has a high modulation rate, and produces less speckle, too.

"Picture resolution is given by the modulation capability of the laser," says Osram's Uwe Strauss. "With a diode green laser you have the chance to go up to HD."

Osram won the race to produce the first green chip suitable for a pico projector. In late 2009 it produced a 516-nanometer laser with a continuous 50-milliwatt output, and thanks to further refinement of the device, it reported a slightly greener 524-nm variant this June in the Japanese journal Applied Physics Express. According to Osram, the more recent device is almost right in the center of the 515- to 535-nm range needed for a green laser in a pico projector.

But Nichia may have trumped that effort. It has just announced that it will start shipping 50-mW green lasers to customers this August.

Nichia's lasers have a lot going for them. They have an efficiency of 5 percent, a value more than twice that of Osram's and high enough to vanquish the infrared laser and frequency doubler. Typical lifetimes of 10 000 hours may also be long enough to win sales. However, these devices are let down by their emission wavelength, which is only 510 nm.

Among manufacturers it is highly controversial whether 510 nm is truly green or merely blue-green. Nichia insists that 510 nm is green enough for a pico projector, but Osram claims that 515 nm is the minimum.

Osram's success is something of a shock, because it follows a path that was thought to be a technological dead end. When the Japanese tech giant Sumitomo Electric Industries unveiled the world's first truly green laser a year ago, it seemed that a radically different kind of substrate held the key to success in this spectral range [see "Lasers Get the Green Light," IEEE Spectrum, March 2010].

Sumitomo's triumph came from growing the laser on an unusual cut of the gallium nitride crystal. Growing a laser on what's known as a semipolar plane created a device that could escape the full brunt of the incredibly strong internal electric fields that naturally occur in these nitrides. These fields are a menace because they impair efficient light generation.

Osram's triumph is surprising because it uses the conventional polar planes that seemed destined for making just violet and blue lasers. The German company has stretched emission into the green by exploiting the sole benefit of internal electric fields—they push output to longer wavelengths. However, you can't capitalize on this unless you carefully engineer the device. Fail to do this and you'd have to crank up the voltage to get lasing. That's no good, because it means you'd have to inject so many electrons and holes into the device that they'd oppose and cancel the internal electric fields you wanted in the first place.

It's hard to tell whether all this effort will reap commercial rewards for Osram. The company has to battle not only Nichia and Sumitomo but also Kaai, in Goleta, Calif., a spin-off cofounded by the inventor of the nitride laser, Shuji Nakamura, that is developing semipolar and nonpolar lasers. Its best results include 525-nm devices delivering 30 mW.

What is clear, however, is that the competition is good news for everyone hankering after high-quality pico projectors.

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