The first incandescent light bulbs saw the light of day at the end of the 19th century. In the years that followed, compact fluorescent lamps were developed in the search for more energy efficient lighting and at the start of this century, LED lighting began making serious inroads. In the late-1990s, Shuji Nakamura, working at Nichia, developed the blue LED, which subsequently led to the white LED (a combination of blue die and yellow phosphor). Although it has been about 60 years since the first visible LED made an appearance in a laboratory at GE, it has only taken less than ten years for the descendant of those first LEDs to move from market introduction to industry standard, but what will come after LED.
There is a great push in society today to offer light sources that are brighter, clearer, and more energy efficient. For many years now, people have picked LEDs over other light sources. However, new research analyzes how laser-diode pumped phosphor light sources may provide previously unseen benefits. They offer higher luminance and scalable efficiency suitable for a number of applications, including projection displays. For decades, vibrant laser lights have dazzled concertgoers, sports fans, and others. But behind the spectacle were technological limitations. A laser beam could illuminate only one spot at a time and never in white. Further, illuminated patterns created using lasers were rife with the ever-shifting and somewhat eerie phenomenon of speckle. However, recent advancements in solid-state lighting have informed the use of lasers in a wider range of lighting applications, from the precise short-throw illumination of building façades to long-range automobile headlights.
Laser diodes are close technological cousins to light-emitting diodes, or LEDs. The diodes, or chips, both comprise two-terminal semiconductor devices that convert the flow of electrical energy into light of a specific wavelength, or color, which is dependent on the semiconductor blend used. Laser diodes have two mirrors on the opposite ends of the semiconductor chip, one of which is partially transparent, like a two-way mirror. A laser diode has a number of important benefits. For example, laser diodes are considerably smaller than their LED counterparts. The fact that a laser diode can produce up to 1,000 times as much light for only 2/3 of the energy means that it is even possible to light a complete house using only one laser diode.
The promise of laser diode illumination has prompted some experts to predict big things for the technology. One is Dr. Shuji Nakamura, inventor of the blue LED for which he shared the Nobel Prize in physics. Nakamura claims laser diodes are the future of lighting. He has backed up this vision with action, founding a company called SoraaLaser, then SLD Laser and now KYOCERA SLD Laser, to make laser diodes optimized for illumination. SoraaLaser was co-founded in 2013 by a group of leading global pioneers in solid state lighting, including Dr Shuji Nakamura (2014 Nobel Laureate in Physics), Dr Steve Denbaars, Dr James Raring, and Dr Paul Rudy. The independent spin-off from Soraa Inc holds an intellectual property portfolio with over 500 patents. In 2020 SLD Laser was acquired by KYOCERA Corporation and has commenced operations as a Kyocera group company under the name KYOCERA SLD Laser, Inc.
White-light lasers in particular continue to challenge LEDs. While LEDs are highly efficient and last longer, they produce divergence angles that are difficult to control. Engineers at KYOCERA SLD Laser in Goleta, California have worked on a white-light laser source that is up to 100x more luminescent than LED luminaire devices. This particular laser surface-mounted device uses semi polar gallium nitride (GaN) laser diodes and phosphors and can produce up to 500 lumens of output from a 300 um emitting area. SLD Laser has an ambitious roadmap of a wide spectrum of applications around the future of light.
While the number of companies actively pursuing semiconductor lasers for general illumination is unclear, at least one company has an offering. In 2016, KYOCERA SLD Laser, at that time SLD Laser, introduced its LaserLight surface mount device (SMD) that uses blue semiconductor lasers, phosphors, and high-luminance packaging to deliver about 500 lumens of white light from a 7×7 mm package with no inherent safety risks to the human eye. Precision optics enable beam angles of no more than 2 degrees. The LaserLight SMD package also has the distinction of being the world’s first semiconductor laser-based light source to achieve UL 8750 safety certification.
Is there a future for semiconductor lasers in general illumination? The theoretical efficacy limit for a phosphor-converted white light LED is about 350 lumens/watt with commercially available lighting products approaching 200 lumens/watt. Semiconductor lasers can deliver efficacies of 100 times or more that of conventional LEDs, enabling significantly higher light output with smaller die sizes. While it’s easy to see the appeal for applications for which physical size is a limitation (like headlamps), the issue for general illumination is the extremely narrow emission cone, on the order of 1-2 degrees.
Most probably, we’ll begin seeing laser semiconductors incorporated first into niche architectural lighting products for which a narrow, high intensity beam is advantageous. For example, lighting for museums, galleries, retail spaces, and other settings could potentially be located in one small area of the space instead of being spread throughout. Not only would this contribute to the aesthetics of the space, but control and maintenance functions could also be simplified. Because of the narrow beam spread, the use of fiber optics or waveguides to channel and deliver the emitted light may need to be incorporated to create viable products for general illumination using semiconductor lasers.
Laser lights could solve the problem of how to bridge the gap between traditional light sockets and more radical configurations of new lighting technologies. That’s because with just a few point sources of laser light installed in a building, their illumination can be redirected throughout a structure via plastic fiber optic cables that could be run along ceilings and around corners, just as the cable company runs its wires into buildings and through rooms without having to tear holes in walls or interface with the electrical system of a building. It’s potentially easier, in other words, to pipe light from one place to another in a building than to re-configure its electrical wiring. Once you can put a light source into glass or plastic fiber, it frees you from having to put a light fixture every 20 feet, rather than route the electricity to the bulb you can route the light to the sources. LEDs let you do that too, but lasers would take it a couple steps further. It would even be possible to channel light through “free space,” without any fiber optic cables at all. That is, a central laser light source could shoot across the ceiling or down a hallway, into some kind of glass or plastic waveguide, and from there it would illuminate an entire room. It’s a weird concept, but when you eliminate the light bulb, you end up with ideas that Thomas Edison never even dreamed of. A single light source can light up an entire house so this will give designers greater flexibility and provide us with brightly lit up households at high cost efficiency.
Currently, commercial applications are limited by overall cost considerations and regulatory issues. The cost of laser lighting continues to be significantly higher than that for blue LEDs, though the higher power densities may allow for reduced cost of blue laser lighting as less die area is required for a given amount of light output. With laser lighting, a radical change can be brought into being. As was mentioned above, with only a handful of laser diodes and some fibre optic cable, you can illuminate an entire building. Rather than transporting electricity to a lamp, you transport light to the place where it is needed. Not only is that easier to install, but it is also more energy efficient. Making that transition will probably take several years. Currently, LED lighting has only achieved a small fraction of its potential. We will be able to continue to use those little diodes for a long while to come. But one thing is certain. Our children or grandchildren will be using light in a totally different way.
This article was originally featured in the June issue of designing lighting (dl)