LEDs exciting phosphors can produce a bright white light, but for highest intensity a laser diode is better.
An LED and a semiconductor laser (or laser LED) work in essentially the same way, that is, light is emitted as electrons and holes combine, with emission wavelength dependent upon the materials used. But while LEDs emit light in a narrow spectral range, semiconductor lasers emit light at essentially a single wavelength. Semiconductor lasers emitting wavelengths ranging from infra-red to ultraviolet are widely applied in areas like fiber optic communication, barcode readers, disc readers, and laser printing, but use of these products in general illumination applications has proven impractical up to now.
Like traditional lasers, a semiconductor laser requires a resonant cavity to facilitate amplification, and this is accomplished by means of two parallel planes, separated by a few hundred µm, that act as mirrors to bounce emitted photons back into the cavity. At low power levels, the semiconductor laser functions essentially like a conventional LED. When sufficient power is applied (approx. 4 kW/cm2), the photons bouncing between the two “mirrors” begin to stimulate the semiconductor material to emit more photons. When stimulated photon generation exceeds internal losses, the device begins to “lase,” that is, emit coherent light of a single wavelength.
There are similarities as well between conventional LEDs and semiconductor lasers: both are powered via a driver that converts AC to DC current and both experience a drop in light output at increasing temperatures. However, unlike conventional LEDs, semiconductor lasers do appear to be immune to the phenomenon known as “droop,” for which increasing drive current results in lower efficacy (output lumens/input watts). In terms of application in illumination products, conventional blue LEDs have a higher efficacy than that of semiconductor lasers, but only at lower input currents, so the real estate that would be required to generate the same level of light from a conventional blue LED is not practical.
Although laser diodes have been available since the 1960s, only recently have efficacies been high enough for consideration in illumination applications, notably in high-end headlamps for automobiles. BMW, which offers laser headlamps, claims that they are 10 times brighter than LED headlamps and 30 percent more efficient. The headlamps produce white light beams by bouncing blue semiconductor laser light around inside the headlamp housing using precisely placed mirrors, then focusing the beam through a phosphor-filled lens, resulting in a high-intensity white light.
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.
While the number of companies actively pursuing semiconductor lasers for general illumination is unclear (more information may be forthcoming at the Light Fair International conference), at least one company has an offering. In 2016, 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 fist semiconductor laser-based light source to achieve UL 8750 safety certification.
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.
On a lighter note (pun intended), as an example of how semiconductor lasers are being incorporated into all sorts of products, check out Baja Designs’ photos and videos of their lighting accessories, including semiconductor laser-based lighting for off-road vehicles. Baja Designs claims that their OnX6 Hybrid Laser/LED and XL Laser High Speed Spot are capable of throwing light 350% farther than conventional LED off-road lighting products, making it easier to navigate during night-time off-road races.
—Yoelit Hiebert has worked in the field of LED lighting for the past 10 years and has experience in both the manufacturing and end-user sides of the industry.