Lighting with laser diodes

Solid-state lighting continues to gain popularity as an efficient replacement for conventional incandescent and fluorescent illumination. The light sources typically combine an LED that emits in the blue or near-UV with one or more phosphors that are excited by the LED and emit at longer wavelengths. When an LED is driven at high current densities, however, its efficiency falls off steeply (see Physics Today, July 2013, page 12 ) and the spectral outputs of the LED and the nearby phosphors shift. But as Kristin Denault, Michael Cantore, and colleagues at the University of California, Santa Barbara (UCSB), report, laser diodes are developing into an attractive alternative to LEDs. The coherent laser output doesn’t droop or shift, and the phosphors can be placed far enough away and kept cool so that their spectral contributions also remain stable. In two of the UCSB devices, a commercial near-UV laser diode excited red, green, and blue phosphors. By varying the ratios of phosphors, the researchers produced one device that gave off yellow-tinted “warm” white light similar to incandescent light and another that gave off a bluer, “cooler” white (the square and circle, respectively, in the color plot; the inset shows the warmer output). Both produced excellent color rendering. Although their brightness was comparable to commercial white LEDs, the devices’ respective luminous efficacies—the perceived light output per watt—were 16 and 19 lumens/watt, similar to that of halogen bulbs. A third device, incorporating a blue laser diode coupled to a yellow phosphor, reached 76 lm/W, comparable to many LEDs, but poorer color rendition than the triphosphor devices. The team expects that advances in laser-diode technologies and phosphor properties will further increase the efficacy and competitiveness of laser-based lighting. (K. A. Denault et al., AIP Adv. 3, 072107, 2013, doi:10.1063/1.4813837 .)