VCSEL research produces green laser diodes, paving the way for tiny projectors, other apps

May 4, 2009--New pioneering work promises to serve the growing commercial interest in using laser technology for tiny projectors to build into mobile devices such as laptop computers and cell phones. "Such devices would have extremely high market potential," says Mircea Guina of Tampere University's (Tampere, Finland) Optoelectronics Research Centre. "But so far the development has been technologically hindered by the 'green gap'--the lack of green laser diodes." Now that gap has closed, thanks to novel nanomaterials.

Guina was project manager of the EU-funded Nano-Photonics Materials and Technologies for Multicolor High-Power Sources (NATAL) project, established to develop new laser technologies that would not only plug the green gap but also make possible a host of applications that require high-brightness miniature lasers tuned to specific wavelengths.

Laser diodes are compact, mass producible and relatively cheap, but otherwise have been saddled by limitations that make them unsuitable for compact projectors and other applications. "Using traditional laser diode technology it is still difficult to produce high-brightness radiation at the wavelengths preferred for laser projection applications," Guina says. "In particular, the emission from typical direct-emitting red laser diodes is limited to about 640-650 nanometers while the eye is most sensitive to 620-640 nm. They are also affected by changes in temperature and require high working voltages. Even worse, there is still no suitable commercial solution for the green color."

The results of the NATAL project relate closely to developments in novel semiconductor gain materials and the demonstration of new lasers. "The key technology is the optically pumped VECSEL, which resembles the geometry of a solid-state laser while retaining the wavelength versatility offered by semiconductor gain media," says Guina. A vertical external-cavity surface emitting laser (VECSEL or VCSEL), a kind of semiconductor laser that produces a high-quality light beam perpendicular to the chip's surface, can work effectively at higher power.

Research in NATAL has focused on producing red, green and blue wavelengths by developing new nanomaterials to provide gain in a VECSEL--including 'quantum dot' structures that have not been used in a VECSEL before--and using 'frequency doubling.' Among the highlights from the project are high-power VECSELs operating directly in red light and frequency doubled infrared VECSELs that can emit in the sought-after green gap, as well as in the amber-orange-red part of the spectrum. The red lasers can themselves be frequency doubled to emit ultraviolet light.

"One of the partners, the Institute of Photonics at the University of Strathclyde, has for the first time demonstrated direct-emission red VECSELs pumped with commercially available blue diode lasers," Guina adds. "Another significant outcome of the project was a full 3D VECSEL simulation software that takes into account the laser geometry as well as optical and thermal properties of the laser."

Industrial uses for compact, mass-produced lasers are likely to be numerous. Two NATAL partners, OSRAM Opto Semiconductors and EpiCrystals, are in the midst of developing the green laser for projection technologies.

The third industrial partner, TOPTICA Photonics, is working on scientific applications. "Using all semiconductor VECSEL technology we have new opportunities for customised and wavelength-tailored solutions in the near infrared," says Wilhelm Kaenders, president of TOPTICA Photonics and NATAL project dissemination manager. "Combining this with our established frequency conversion modules we can finally plug the spectral niche between green and red. We can now supply spectroscopists with diode-based single frequency, fixed frequency and tunable lasers from 375 to 3000 nanometres."

Many other applications of the new lasers are possible, such as materials processing, UV lithography and in medicine. The amber-orange-red lasers, for example, could be used for photodynamic therapy and to make artificial guide stars for telescopes by exciting sodium atoms high in the atmosphere, supporting long-term goals of the European astronomical community.

There are also a number of scientific and medical applications where compact, reasonably high-power visible and UV lasers would be preferred over the existing high-cost, high-maintenance gas and ion lasers.

NATAL, which was funded through the EU's Sixth Framework program for research, ended in August 2008 but the work is being carried on in several other projects. Some of these aim to develop VECSELs emitting at longer infrared wavelengths which could be used in distance sensing, environmental monitoring of gases and tissue-welding in surgery. Lasers emitting ultra-short optical pulses are also in development.
"NATAL has helped to generate a significant amount of knowledge and new technologies in this field," Guina says. "Much of what we have done with VECSELs is now regarded as state of the art in the world."

The work on quantum-dot VECSELs is being continued in the FastDot project funded under the EU's Seventh Framework program. Other spin-out projects are expected.

For further information see the NATAL project web page.

Posted by Barbara G. Goode, [email protected], for Laser Focus World.