All systems go for high-brightness pico-projectors as Osram Opto Semiconductors launches green laser diodes, reports Compound Semiconductor.

The race to deliver green lasers for pico-projectors embedded in mobile phones grew more intense last month as Germany-based Osram Opto Semiconductors launched direct diode lasers, with a view to ramping to volume production by the end of this year.

A first InGaN laser diode emits at 515 to 530nm, pretty much in the centre of range needed for pico-projectors, has a continuous 50mW output and 5 to 6% efficiency, while a second emits at 510 to 520nm with 30mW output.

The company remains a little vague over lifetime, although Osram's director of laser development, Uwe Strauss, asserts the laser diodes will last for several thousand hours, depending on application and lasing conditions.

And crucially, the diodes come in a 3.8mm diameter package, much smaller than the bulky frequency doubled infra-red lasers already used in pico-projection, and perfect for using in the pico-projectors that will be crammed into mobile phones. What's more, Osram claims the devices cost less to make, have a high modulation rate and reduced speckle.

“We don't need any additional optics parts such as frequency doubling crystals, which makes these lasers compact and reduces the mid-term low cost potential,” says Strauss. “These lasers also have a high temperature stability compared to frequency-doubled lasers.”

Osram's latest move can only spell good news for an industry that still falls back on lower brightness LEDs for pico-projectors, only truly effective in darkened rooms. However, the company is hardly alone in its quest to deliver a commercial green laser diode for pico-projection.

Japan-based Nichia now mass produces a 515nm laser with a maximum 100mW output while Sony and Sumitomo Electric Industries have jointly pioneered a range of green lasers that deliver output powers as high as 169mW, efficiencies up to 8% and emitting up to 526.6nm. At the same time Soraa, a California-based venture from blue laser diode inventor Shuji Nakamura, has unveiled a 520 to 525nm laser diode delivering 75mW output.

But despite the potential number of green laser diode options for the pico-projector market, the road to successful manufacturing has been rocky. Difficulties in depositing InGaN quantum well layers onto GaN substrates have been rife, and manufacturers have ultimately chosen one of two crystal cuts, depending on whether they wanted to grapple with strain-inducing lattice mismatches or piezo-electric fields.

Sony and Sumitomo, and Soraa have focused on the semi-polar plane, choosing to master the epitaxy of high-indium content quantum well layers on GaN in a bid to avoid the efficiency-quashing fields that characterise the alternative cut.

However, Osram, and indeed Nichia, have persevered with the polar plane, also known as the c-plane, and as Strauss points out, he and colleagues have actually made the most of its piezo-electric fields.

“The semi-polar plane doesn't have the piezoelectric field, [so you don't struggle to] get the right overlap between electrons and holes, giving efficient light generation,” he says. “But the c-plane has a big advantage; the internal field reduces the bandgap, so you don't have to put in as much indium to the light-emitting layers, which reduces the [lattice mismatch] strain and defects.”

So which route will LED manufacturers opt for? As Strauss puts it, clearly each works. “Products on each plane have results that are very good, so both ways are possible,” he says. “The question is, which one gets the better product with the lower cost ...the product cost is a lot less when using c-plane [substrates] compared to semi-polar substrates.”

But still the future is far from clear cut. Strauss asserts that epitaxy growth on his competitor's semi-polar GaN substrates is difficult, but concedes that solving the handicap of piezoelectric fields on polar planes is equally challenging. “For me it's not clear which product is best in the last three or four years,” he admits.

But could issues over colour sway manufacturers? To date, the longest, and arguably greenest, wavelengths have been reported on semi-polar substrates, not Osram's choice of plane.

Strauss agrees, but adds: “We've said, 'ok, [the longer wavelength] is nice but what wavelength do we really need?' The high volume projection market [only] needs 520nm, maybe 525nm, and our customers are very happy with this.”

And given its happy customers, the company now plans to focus on improving the output power of its green laser diodes.

“On a research and development level we have already demonstrated power outputs in excess of 100mW,” says Strauss. “When we look at the high volume market of laser projectors in mobile phones, this is the future.”



Image caption: Osram's latest diode signals a green light for high-brightness pico-projection.