Chris Chinnock
Recent breakthroughs in the development of blue-laser diodes and blue LEDs portend higher-capacity CD-ROM discs, brighter projection displays, and replacements for today's short-lived light bulbs. Although you probably won't find blue lasers in
commercial products
for another three or four years, researchers say they are confident that blue lasers will replace less efficient red lasers by the end of the decade.
Blue lasers will deliver increased CD-ROM storage capacity because of the shorter wavelength of the blue light compared to that of the red lasers used in today's CD-ROM drives (see the table
"Blue Laser Capacity Improvement"
). Shorter wavelength means the "pits" formed in the CD-ROM can be smaller, permitting higher pit densities and mo
re data storage. Currently, the blue lasers in existence aren't light and compact enough for consumer electronics. Researchers are now developing semiconductor laser diodes and LEDs that offer a better cost/performance/size solution.
LEDs are less complex than lasers. They have a wider distribution of colors in their output spectrum, emit into a broader output cone, and emit dimmer light than lasers. Unfortunately, you can't simply increase the current on an LED and expect it to lase. Lasers require a more sophisticated device architecture than LEDs to generate their concentrated light.
But historically, high-quality LEDs have been precursors for laser-diode development. "It's kind of like learning to walk before you run," says a spokesman for Durham, NC-based Cree Research. "You need to understand how to build an LED before you build a laser diode." That is why researchers see advancements in LED commercialization as a harbinger of future laser availability.
For several years, Cree has a
ctually been selling rather dim blue LEDs made from silicone carbide (SiC). But recently the company announced the prototype development of a super-bright blue LED made from a combination of materials: a gallium nitride (GaN) layer grown on top of an SiC wafer. These LEDs are over 20 times brighter than Cree's previous SiC LEDs. Neal Hunter, president of Cree, says that by the end of 1995, the company should be producing up to 5 million of the new LEDs per month. "These blue LEDs emit about 0.5 milliwatts of power at 435 nanometers and have passed our accelerated life test, so they are quite robust," Hunter says. "Half a mW is sufficient power to enable 90 percent of the applications envisioned for blue LEDs. A little more power--perhaps 1 mW to 1.5 mW--is needed for outdoor signs, and improvements beyond that make applications brighter and more power efficient."
Cree joins Japan-based Nichia Chemical Industries as a blue LED supplier. Nichia, which also has an office in Lancaster, PA, startled the ind
ustry about two years ago by demonstrating a blue LED based upon the GaN-sapphire combination. Nichia said its LEDs have a lifetime of over 10,000 hours (about 42 days continuous operation), stable enough for commercial applications.
Nichia's breakthrough shifted much development work to GaN, a material that others had previously tried but rejected as too inefficient. Paul Maruska, considered by many to be the "father of GaN," demonstrated light emission from the material while working at RCA in 1968. "Nichia uses the same basic process that we developed at RCA, but they improved the device efficiency by adding a heat-treatment step," Maruska says. "RCA gave up too soon. With a little more work, we could have done this 20 years ago."
Maruska is now working with a new start-up company called NZ Applied Technologies (Woburn, MA) to develop its own blue LEDs. Blue LEDs, when used in conjunction with red and green LEDs, could be used in displays ranging from small message signs to large outdoor disp
lays. A more intriguing use of blue LEDs is to illuminate rooms: A blue LED combined with a green-yellow LED in a single package emits white light that is similar to normal light. Such devices could replace short-lived incandescent bulbs with stable, inexpensive LED pairs.
As for blue lasers, many organizations are researching these devices, including Sony, Philips, 3M, Panasonic, APA Optics, and numerous universities. Officials at Cree believe they can develop blue lasers from the GaN-SiC materials the company is commercializing. Cree has partnered with Philips Laboratories (Briarcliff Manor, NY) in a contract from the Advanced Research Projects Agency to deliver a 3- to 5-mW, room-temperature blue-laser diode in two years.
A critical goal in blue-laser diode development is to reduce the number of defects in fabrication materials. Cree's Hunter believes his company is close to getting the defect densities low enough for laser development.
LASER WAVELENGTH NUMBER OF MUSIC ALBUMS ON CD-ROM
(due to (due to all
wavelength factors including
changes compression and
only) improved tracking)
860 nm (near infrared) 1 7
635 nm (red) 2 14
430 nm (blue) 4 28
350 nm (ultraviolet) 9 63
Blue Laser Applications
-- Higher density optical storage (audio/video/data)
-- Spectrography and sensing
-- Materials processing
-- Optical communications
-- Brighter, more efficient projection displays
-- Higher resolution printing
Blue LED Applications
-- Direct view displays (laptop backlighting)
-- Laptop scr
een backlighting
-- Room illumination
-- Indicator lamps
Blue Laser, Bright Future
