Brighter and More Efficient Solid-State Lighting using Laser Diodes

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How do you combat ‘energy droop’ in LED lights? Image by emlyn

Laser Diodes for Remote Phosphor Design

In a recent paper, Kristin A. Denault and a research team at the Solid-State Lighting and Energy Center and Materials Department, UC Santa Barbara tested the lighting output of a remote phosphor design using laser diodes. The group tested color output using a blue laser to produce a cool white light with a CCT of 4,400 K and a near UV laser to produce warm white light with a CCT of 2,700 K.

The UCSB team also used their data to calculate the “maximum efficacy” for blue laser diodes, a measure of how well a light source can potentially produce visible light. This calculation allows the group to gauge the possible capabilities of future laser based white light devices. They found a maximum efficacy of 78 lm/W for the blue laser diode which is comparable to a luminous efficacy of LEDs currently on the market. Improvements in wall plug efficiency to 75% could increase this number to 200 lm/W, bringing it in line with the Department of Energy’s goal for solid-state lighting and making laser diodes a viable high-power solid-state lighting candidate for the future.

Solid-State Lighting: The Future

Laser diodes and LEDs both suffer limitations when it comes to high power lighting.

LEDs suffer from “efficiency droop”, a problem first reported in 1999 where light output decreases if the current is increased beyond a critical point.

Laser diodes problems stem from their low wall plug efficiency, currently around 30% but an increase to 75% would make laser diodes an attractive high power lighting option.

Technological advances are overcoming the problems of both devices.

A recent paper by Rensselaer Polytechnic Institute researchers has identified the mechanism behind droop – a phenomenon known as “electron leakage” due to the slight differences in the movement of charge carriers through a semiconducting material results in energy loses.

The group believes their findings could lead to the development of new semiconductor structures that will lead to more efficient devices.

A fundamental factor limiting wall-plug efficiency of laser diodes is the material properties of the semiconductor itself. Each material contributes a certain electrical resistance, optical loss and thermal resistance – researchers are making improvements all the time. Though laser diodes are currently less efficient than LEDs, they have the potential to go beyond where LEDs are currently.

When asked about the future of laser diode lighting and its comparison to LEDs, Denault, co-author of the study on laser-diodes for solid-state lighting, told Decoded Science, I think that as laser diode technology improves and people begin to be aware of the benefits of using laser diodes as an excitation source in solid-state white lighting, more applications and advantages will become apparent.

Resources

Denault, K. A., Cantore, M., Nakamura, S., DenBaars, S. P., & Seshadri, R. Efficient and stable laser-driven white lighting. (2013). AIP Advances. Accessed August 18, 2013.

Meyaard, D. S., Lin, G.-B., Cho, J., Fred Schubert, E., Shim, H., Han, S.-H., Sun Kim, Y. Identifying the cause of the efficiency droop in GaInN light-emitting diodes by correlating the onset of high injection with the onset of the efficiency droop. (2013). Applied Physics Letters. Accessed August 18, 2013.

Xu, Y., Chen, L., Li, Y., Song, G., Wang, Y., Zhuang, W., & Long, Z. Phosphor-conversion white light using InGaN ultraviolet laser diode.  (2008). Applied Physics Letters. Accessed August 18, 2013.

Future Lighting Solutions. Remote Phosphor. (2013). Accessed August 18, 2013.

Lighting Research Center, What is an LED? (2003). Accessed August 18, 2013.

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