Lessons on Laser Diodes

This story was originally published in Architectural Lighting.

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 Versus LEDs
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. Manufacturers create white LEDs by directing light from blue chips onto phosphors—chemical compounds that emit yellow light when illuminated by blue light. The emission from this yellow phosphor and the blue LED combine to produce light that appears white to the human eye.

Laser diodes have two mirrors on the opposite ends of the semiconductor chip, one of which is partially transparent, like a two-way mirror. At low power levels, a laser diode works essentially like an inefficient LED. However, once the electrical power reaches a threshold density of around 4 kilowatts per square centimeter, the semiconductor emits enough light for a portion of the wavelengths reflecting between the mirrors to stimulate the semiconductor to emit more light, surpassing an LED’s output. Furthermore, the light reflecting between the mirrors emerges through the semi-transparent mirror to create a narrow blue beam that can be directed onto a phosphor to generate yellow light.

Blue LEDs have a high luminous efficacy, converting up to 70 percent of the electrical power passing through them into light at a power density of 3 watts per square centimeter. That’s considerably more efficient than blue laser diodes, whose power conversion peaks at around 30 percent when electrical power density tops 10 kilowatts per square centimeter, according to “Comparison Between Blue Lasers and Light-Emitting Diodes for Future Solid-State Lighting,” a 2013 paper published in Laser & Photonics Review by Jonathan Wierer Jr., an associate professor of electrical and computer engineering at Lehigh University. However, LEDs can only achieve that high efficacy at low-current levels, which would require large areas of expensive semiconductors.

Pumping more current through LEDs can make them painfully bright—an outcome easily illustrated by removing the diffuser from an overhead LED fixture. While increasing the current reduces LEDs’ efficacy sharply, a phenomenon known as “droop,” the efficacy of laser diodes appears unaffected. So at electrical power densities of about 5 kilowatts per square centimeter, LEDs become less efficient than diode lasers, and that performance difference increases with the power level.

The output of a laser beam is only 1 to 2 degrees as compared to the 90-plus-degree light emission cone of LEDs. And, notably, the wavelengths of laser light fall within one nanometer, as compared to a few tens of nanometers for LED light. These differences make lasers valuable in some applications where LEDs fall short.

Within a diode, the laser can be focused onto a tiny spot on a phosphor to produce a narrow, intense beam with a luminance as much as 20 times greater than that of an LED. “We can generate 500 lumens from a focal spot of only a few hundred micrometers,” says Paul Rudy, co-founder and senior vice president of business development at the Fremont, Calif., office of SoraaLaser, which produces blue laser diodes. (SoraaLaser, a spinoff of lighting manufacturer Soraa, is one of the few companies beginning to explore laser lighting applications for an architectural lighting audience.) “With lasers and 1-inch optics, we get a spotlight beam of around 1 degree,” he adds. “That’s revolutionary. You can think of kilometer flashlights and long-range headlights.”

Automobile manufacturer BMW, which has deployed laser lights in some of its models, reported in 2015 that a blue laser emitted from a 30-micrometer by 4-micrometer surface emitted as much optical power as LEDs covering an 800-micrometer square. To reach the maximum high-beam range allowed in the European Union, BMW designed a headlight that combined a wide angle, LED-illuminated phosphor unit with a narrow angle, long-range, laser-illuminated phosphor that could deliver an illuminance of 1 lux at 600 meters (1,968 feet). After being modified to meet U.S. headlight standards, a version of the headlights is now available domestically.

Design Considerations for Laser Fixtures
Fixtures utilizing laser sources will inherently have different design considerations from those for LED fixtures, says Faiz Rahman, an optoelectronics expert and visiting Stocker professor at Ohio University’s Russ College School of Electrical Engineering and Computer Science. A laser diode and a phosphor must be separated by enough space for the laser beam to focus and to keep the phosphor from overheating; conversely, phosphors can be adjacent to or coated directly on LEDs. Software, Rahman says, can help designers model the optics in laser luminaires.

SoraaLaser’s current laser lighting products utilize blue lasers emitting near 450 nanometers, the standard wavelength output for white LEDs. Thus, they can use the same yellow phosphors used on LEDs to create white light. However, the blue laser light must be scattered or diffused by materials, such as frosted glass, to blend properly with the phosphor emission.

Laser lighting also can leverage the mature technology of 405-nanometer violet lasers, developed for Blu-Ray optical discs, Rahman says. Producing white light requires adding phosphors to convert the violet light into blue light at 450 to 460 nanometers to complement the yellow phosphors. This conversion costs energy, Wierer says, but violet laser diodes’ increased efficacy over blue lasers could make up the difference.

The Quest for Monolithic White Light Lasers
Phosphor-based white LEDs dominate the solid-state lighting market because of their simplicity. Combining light from red, green, and blue LEDs to produce white light is another option, with the added ability to modulate color, as exemplified by several LED lamps on the market that have color-changing functionality.

In principle, RGB lasers can also be combined to produce white light, but the technology is still in research and development. One problem is the need to control or diffuse laser light for safety reasons and to prevent artifacts from laser illumination (see “Safety and Speckle in Lasers,” right). Another is the challenge of finding suitable RGB laser sources.

Philips, for example, uses separate LEDs as RGB sources in its Hue lamp, with a higher proportion of green diodes because they are less efficient and emit less optical power than red or blue LEDs. The performance difference becomes larger for semiconductor lasers, with blue being the most powerful color, red less powerful, and green the weakest and shortest-lived. (Green laser pointers can be dangerously bright, but this light comes from crystalline lasers, not semiconductors.) To complicate matters, semiconductor lasers emitting each of the three wavelengths cannot be integrated on the same chip—desirable for mass production and quality control—because they are made from different semiconductor compounds that are deposited in different ways.

“It has been very difficult to have a monolithic piece of material that can make all the colors,” says Cun-Zheng Ning, a professor at Arizona State University’s School of Electrical, Computer and Energy Engineering. His group succeeded in integrating different colored laser diodes by eschewing the standard compounds—gallium, indium, nitrogen, and arsenic—used in semiconductor laser diodes in favor of a family of semiconductors composed of cadmium, zinc, sulfur, and selenium. By depositing different mixtures of those elements in thin layers, his team made a monolithic device that combines disparate diodes emitting blue, green, light red, and deep red light to produce white light. However, the technology is still solidly experimental.

At Aston University’s Institute of Photonic Technologies, in Birmingham, England, professor Edik Rafailov’s group has taken a different approach to producing white or color-tunable light from lasers. “We decided to use a broadly tunable infrared laser and convert it to a broadly tunable visible laser,” says Rafailov. Infrared light can be shifted into the visible spectrum by combining two infrared beams in a thin, microstructured material—potassium titanyl phosphate—with a high nonlinear effect that adds their frequencies together. Mixing the laser outputs together produces red, green, and blue wavelengths, the group reported at the 2016 Conference on Lasers and Electro-Optics.

Architectural Applications for Laser Sources
The high intensity of lasers works well for spotlights and other lighting applications requiring narrow beams. Lasers with tiny optics can also illuminate precise areas with a large-angle, ultrashort throw, Rudy says. “You can project a 100- to 200-inch image even though you’re only a couple of inches from the wall.” Laser excitation of phosphors can produce very high contrast between bright and dark areas, with light gradients more than 10 times sharper than with LED-based sources. For example, a laser light source can uniformly illuminate a five-story building exterior from a single fixture near the ground floor. SoraaLaser’s first-generation outdoor laser lighting systems have a nominal color temperature of 5700K and color-rendering indexes of 70 to 80.

Laser light can also be concentrated and directed into optical fibers or waveguides for transport, a difficult task with large-area LED sources. SoraaLaser is developing a fiber system to transport blue laser light to phosphors located in a remote luminaire so lighting designers and architects can specify fixtures for locations in which heat or electricity is undesirable.

The next big thing for SoraaLaser is spatially dynamic lighting, which Rudy calls “the convergence of display and lighting.” Light from a static source is passed through a light-processing chip to create changing patterns, such as the shape of a spotlight, a company name, or dynamic imagery.

SoraaLaser expects the first wave of commercial, static lighting products to be available by 2019, with dynamic lighting to follow. Outstanding refinements include improvements to color rendering, energy efficiency, and performance for specific applications such as steerable spotlights, which will enable lasers to go where LEDs have yet not prevailed. •

Resources
“Comparison Between Blue Lasers and Light-Emitting Diodes for Future Solid-State Lighting,” by Jonathan Wierer Jr., et al., Laser & Photonics Reviews, 2013. Available at: bit.ly/2hwBsBp.

“Lasers Light the Road Ahead,” by Abdelmalek Hanafi and Helmut Erdl, Compound Semiconductor, 2015. Available at: bit.ly/2mo0J2s.

“A Monolithic White Laser,” by Fan Fan, et al., Nature Nanotechnology, 2015. Available at: go.nature.com/2AGx1J2.

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