SEMICONDUCTOR SOURCES: Laser plus phosphor emits white light without droop

Combining a blue-emitting indium gallium nitride (InGaN) LED with a yellow YAG phosphor has produced one of most successful commercial photonic devices ever—the white-light LED. But there is a problem: InGaN LEDs suffer from efficiency droop, in which their efficiency at high currents, and therefore high optical outputs, is lower than that for low currents and optical outputs. However, other InGaN light sources—blue- and near-UV-emitting laser diodes—do not suffer from this problem. With this in mind, researchers at the University of California, Santa Barbara (UCSB), led by material scientists Kristin Denault and Michael Cantore, have combined laser diodes with inorganic phosphors to create efficient, stable sources of white light.¹

Other white-light lasers

A bit of background: Other laser setups have also been, and are being, considered for use as white-light sources. For example, four lasers of different colors were combined to make a lighting-oriented white-light source by researchers at the University of New Mexico, Sandia National Laboratories (both of Albuquerque, NM), and the National Institute for Standards and Technology (NIST; Gaithersburg, MD). The pulsewidth-modulated laser source was preferred by test subjects as a white-light source over both warm and cool white LEDs.

BMW has been working at least since 2011 on combining blue-emitting laser-diode light with a phosphor to create efficient white light for car headlights; however, no BMW cars yet have laser-based headlights.

Supercontinuum lasers, which produce white light from the nonlinear interaction of ultrafast laser pulses with optical fiber, are used in research and the medical arena for optical coherence tomography (OCT) and other applications, and can replace broadband light sources, but are too expensive for use as general indoor and outdoor lighting.

Similarly, in a setup built by Taiwanese researchers, a near-UV laser was used to pump a fiber with a sapphire core, producing white light to be used for OCT, as well as for fluorescence microscopy and flow cytometry. This approach is also presumably too expensive for use as general lighting.

Very high-quality white light

To obtain white light, the group at UCSB created three different setups, all using commercially available laser diodes. The first two were built around a near-UV laser diode emitting at 402 nm with a full-width at half-maximum (FWHM) of 2.6 nm and a wall-plug efficiency (WPE) of 20%; the only difference between the setups was the red, green, and blue (RGB) phosphors chosen (see figure). The laser was operated at a current of 450 mW, which corresponds to its peak efficiency.

For the RGB1 and RGB2 setups, the color-correlated temperature (CCT), color rendering (R_a), luminous flux (lm), and luminous efficacy (Im/W) were: 3600 and 2700 K, 91 and 95 (dimensionless), 47 and 53 lm, and 16 and 19 lm/W, respectively. An R_a of 100 corresponds exactly to the appearance of blackbody radiation, which is optimum; any R_a over 90 is high-quality white light. In particular, the R_a of 95 for the RGB2 setup is very high quality, and far exceeds standard commercial white LEDs in color-rendering quality.

The researchers believe that improvements in near-UV laser-diode efficiency can boost the luminous flux and efficacy and flux of their white light sources to levels higher than those of commercial white-light LEDs. An additional advantage of the near-UV version is that the laser light itself can be completely filtered out, eliminating any potential laser-safety concerns.

Potentially high luminous efficacy

For the third setup, the researchers combined a blue laser emitting at 442 nm at a FWHM of 2.7 nm and a WPE of 30% with a yellow-emitting phosphor made of three high-quality materials: yttrium oxide (Y₂O₃), aluminum oxide (Al₂O₃), and cerium oxide (CeO₂). (This form, combining a blue emitter and yellow phosphor, is much more similar to that of commercial LEDs than the near-UV and RGB form.) Here, the CCT was 4400 K, the R_a was a rather low 57, the luminous flux was 252 lm, and the luminous efficacy was 76 lm/W.

The researchers note that if the WPE of a blue laser diode could be improved to 75%, this type of white-light emitter would reach a luminous efficacy of almost 200 lm/W. This is notable because although a few conventional LEDs have reached this figure, they were running at very low currents and thus low optical outputs to prevent efficiency droop; in contrast, the UCSB blue-laser version can be run at high currents and optical outputs.

With other phosphor combinations, the researchers also achieved a variety of other color temperatures with high color rendition, broadening the range of applications for these new lights, notes Kristin Denault. Luminous efficacies for all these versions will rise with improvements in blue and UV laser diodes, inorganic phosphors, and packaging optics.

REFERENCE
1. K. A. Denault et al., AIP Adv. 3, 072107 (2013); http://dx.doi.org/10.1063/1.4813837.