Laser vs. LED: New research studies are exploring the effectiveness of laser diodes in producing white light that could be used in ambient lighting. Laser-based white light is created by University of California, Santa Barbara (UCSB) researchers. Blue or UV laser diodes are directed at a phosphor target to produce white light. (Photo: Kristin Denault)
While you’ve undoubtedly heard about the rise of Light Emitting Diodes (LEDs) and the potential rise of Organic Light Emitting Diodes (OLEDs), you may not be familiar with the latest form of solid-state lighting that is further out on the horizon: Laser Diodes (LDs). A number of recent research studies have demonstrated the ability of laser diodes to produce efficient white light that could potentially satisfy general illumination requirements.
Laser diodes have both similarities and differences to LEDs. Both are produced from similar “sandwiches” of semiconductor materials, including Gallium nitride (GaN), to produce blue light. The most familiar uses of laser diodes are in laser pointers and Blu-ray disc players, however, neither of these examples involves white light.
There are three demonstrated ways to create white light with laser diodes.
- Blue Or UV Lasers Plus Phosphor
Researchers Kirstin Denault and Michael Cantore – Ph.D. students specializing in Phosphor Material Science at the University of California, Santa Barbara – have recently created white light by directing a blue or ultra-violet (UV) laser at a phosphor target. In one experiment, the team excited red, green, and blue phosphors with a near-UV laser. In another, they excited a yellow phosphor with a blue laser. Both experiments produced white light, but with differing trade-offs of efficacy (lpW) and color rendering (CRI). The blue laser with yellow phosphor had much higher efficiency, but much lower CRI. The near-UV laser with RGB phosphors had high CRI and much lower efficacy. The trade-off between efficacy and CRI also exists when creating white light with LEDs.
UCSB researchers, Michael Cantore and Kirstin Denault in their lab. (Photo: Kristin Denault)
When asked to compare the blue laser versus UV laser approach, Denault has concerns about the safety of a blue laser + phosphor system. White light using a blue laser depends on a portion of blue light passing through the phosphor, with the resulting blue and yellow components combining to create white light. The blue laser light that passes through the phosphor is what concerns Denault. Other experts interviewed for this article didn’t share that safety concern. Julian Carey, Senior Director/ Marketing at Intematix – a leading provider of phosphors to the LED lighting industry – believes blue lasers + phosphor lighting could be safe, and pointed to existing Casio laser projectors, as an example.
Denault is planning additional research into the mixing of various wavelengths to produce white light, as well as the limits of phosphors to withstand higher-power lasers (especially the heat).
- Combining Red, Green, And Blue Lasers (Without Phosphors)
Another possible method for creating white light from lasers involves combining three or more colored lasers to achieve white light. This is analogous to the early RGB white LEDs. Four lasers of different colors were mixed to make white light by researchers from the National Institute for Standards and Technology (NIST), Sandia National Laboratory, and the University of New Mexico. Test subjects preferred their white light source over LEDs (both warm and cool CCT LEDs).
According to several experts interviewed for this article, an RGB laser approach to white light is impractical today due to the inefficiency of currently produced green lasers. Denault also believes safety issues could be a potential concern with RGB lasers, as the laser light doesn’t get converted by phosphors.
Recall the trade off in CRI versus efficacy mentioned above? Andrew Chalmers and Snjezana Soltic, researchers at New Zealand’s Manukau Institute of Technology, published a 2013 study about the optimization of efficacy and CRI using combinations of four to six narrow-band lasers of different colors.
The study showed how four, five, and six narrow-band laser combinations could optimize CRI + efficacy for mixed laser systems in the future. The team acknowledges that their research was purely theoretical and didn’t take into account present-day laser diode efficacy for different colors, nor costs. Chalmers and Soltic pointed out that “for a laser mixture to be a useful white light source, the laser outputs must be thoroughly mixed to avoid effects such as speckle and color shadows. The mixture will need to be diffused to assist the mixing process and remove speckle. At this point, it was considered important to establish the feasibility of optimizing laser mixtures, but it is recognized that these aspects will have to be taken into account in the design and implementation of a practical white-light source.”
- Supercontinuum Lasers
A third method for generating white light from lasers involves “pumping” ultra-fast, monochromatic laser pulses through an optical fiber. Complex interactions within the fiber transform a single wavelength of laser light into a broad continuous spectrum, producing white light. A team of Taiwanese researchers recently used a near-UV laser to pump a fiber with a sapphire core to produce white light. The resulting broad spectrum is ideal for several very specialized medical applications, including optical coherence tomography (OCT), fluorescence microscopy, and flow cytometry. Supercontinuum approaches to producing white light, however, are currently far too expensive for use in general illumination.
The Problem of Droop
Will white-light laser diode systems one day surpass LEDs in general illumination? Some experts say “yes,” and others “no.” To explore the question involves digging into very specific advantages and disadvantages of laser diodes compared to LEDs. The largest advantage of laser diodes is that their peak efficiency occurs at operating currents up to 2,000 times greater than that of LEDs. Meanwhile, LEDs suffer from a rapid decline in efficiency as operating currents (and output) are increased. This undesirable LED phenomena is known as “droop.”
Carey, at Intematix, shared that as a result of LED droop, LEDs emit a couple hundred lumens per chip, while laser diodes can emit thousands of lumens per chip.
Another complex trade-off emerges. The higher output of laser diodes means that higher chip costs can be acceptable at very high outputs. Yet, the practical limits of heat-sinking work against laser diodes at very high outputs. The lower-cost for higher-output laser diodes must be balanced against the thermal management challenges of high-output lasers.
What About Efficacy?
LEDs have power-conversion efficiencies (PCE) today of 70 percent, while laser diodes (LDs) are currently at 30. Paul Rudy, Advisor & VP/Business Development at Soraa® – a manufacturer of full-spectrum GaN on GaN LED lamps – explained that in the short-term, laser diode systems are significantly less efficient compared with LED systems. However, there are very rapid efficacy improvements in LDs, year over year. Rudy estimates LD efficacies have doubled over the last five years — and are not slowing down. After years of development, Soraa is now going to market with its first laser diode production, competing with OSI and Nichia.
Another large advantage of laser diode systems are their ability to emit very tight beams, placing more of their light output on an intended target. This can result in lower output requirements. Rudy points out that laser efficiency at putting light on target could allow laser systems to compete with LEDs long before their power-conversion efficiencies catch LEDs.
How About Price?
Intermatix’s Carey ballparks LED pricing at roughly $1 per chip with 100+ lumens per watt, while LDs are roughly $10 per chip with 60+ lumens per watt. Rudy, at Soraa, stated that LDs have significantly higher costs than LEDs today, in part due to much smaller production volume. He estimates a factor of 5 to 10 times smaller global annual volume for LDs than LEDs. The comparative cost question is complex and involves:
- Lack of droop in LDs
- LDs’ more expensive substrates
- Relative volumes of production
- Rate of annual efficacy increases for LDs vs. LEDs
Rudy says that it is very possible that if LD progress continues at current rates, LDs systems could become competitive with LEDs for general illumination.
What Types of Applications?
Laser diodes are already being used in a variety of applications today, such as the aforementioned laser pointers and Blu-ray disc players.
BMW has been developing laser-based automotive headlights for its i8 “hybrid-supercar” for several years. (This model will be released in Spring 2014.) BMW claims its laser headlights will require only half the power of LED headlights, in large part because of its focused beam.
Other applications for laser illumination include Powerpoint projectors, movie & IMAX projectors, miniaturized “pico” projectors, TVs, computer monitors, and head-mounted displays like Google Glass.
General illumination applications for LDs are possible. The first illumination applications that could adopt laser diodes would be those that play to lasers’ strengths:
- Lack of droop favors high power, high output applications
- Very remote phosphors become possible (even feet removed from the laser diode)
- Very tight beam angles, permitting long throw
- Low-cost fiber optics become possible, leading some to predict centralized light banks distributing light throughout a building using fiber optics
- Much smaller emission area, up to 10,000 times smaller for an LD than an LED: 10-20 microns-squared for LDs versus 1mm-squared for LEDs
Possible applications could include high output, commercial, outdoor flood and area lights, stadium lighting, as well as stage and theater lighting.
Denault, at UCSB, is reluctant to predict success for general illumination from lasers. Carey, at Intematix, thinks laser illumination has potential, but that it is years behind LEDs. Rudy, at Soraa, believes laser illumination could happen in the next 5 to 10 years, if progress continues at its current pace.
About the author
David Shiller is President of Lighting Solution Development, a consulting firm and OEM rep agency serving energy-efficient luminaire manufacturers. Email: firstname.lastname@example.org .