Laser diodes versus LEDs

Nov 11, 2013 by K. Maedefessel-Herrmann
Laser diodes versus LEDs

Solid-state lighting based on light-emitting diodes (LEDs) is the most efficient source of high color quality white light. Nevertheless, they show significant performance limitations such as the "efficiency droop". Blue laser diodes operated in stimulated emission offer a potential alternative.

Solid-state lighting (SSL) has recently become competitive with conventional light sources and is now the most efficient source of high color quality white light ever created. At the heart of SSL is the light-emitting diode (LED). The current standard architecture for SSL is the phosphor-converted (PCLED) in which high-brightness InGaN blue LEDs are combined with one or more wavelength-downconverting phosphors to produce composite white light of virtually any color temperature and color rendering quality. Despite this success, blue LEDs still have significant performance limitations, especially a nonthermal drop in efficiency with increasing input power density called "efficiency droop" which limits operation to relatively low input , contrary to the desire to produce more photons per unit area of the LED chip and to thereby make SSL more affordable.

An alternative could be a blue laser diode (LD). LDs can in principle have high efficiencies at much higher input power densities than LEDs. Above the lasing threshold, parasitic nonradiative recombination processes, including those likely responsible for efficiency droop in LEDs, are clamped at their rates at lasing threshold. Indeed, at high input power densities state-of-the-art, high-power, blue, edge-emitting LDs already have reasonably high (30%) power-conversion efficiencies, with the promise someday of even higher efficiencies. A team from Sandia National Laboratories, Albuquerque (NM, USA) and Corning Incorporated, Corning (NY, USA) compared LEDs and LDs and discuss their economics for practical SSL.

The scientists refer to the tremendous progress made in both device types, with current state-of-the-art power-conversion efficiencies (PCEs) of 70% for LEDs and 30% for LDs. The input power densities, at which these PCEs peak, are vastly different at about 10 W/cm2 for LEDs and 25 kW/cm2 for LDs. As the areal chip cost necessary for economical lighting scales as input power density, areal chip cost can be much higher for LDs than for LEDs. The authors conclude that it appears to be much more challenging to achieve areal chip costs low enough for LEDs than for LDs to be operated at the input power densities at which their PCEs peak.

Yet, as heat-sink-limited single-chip white-light output scales inversely as input power density, heat-sink-limited single-chip white-light output can be much higher for LEDs than for LDs. A white-light output high enough for practical illumination applications should be more challenging to achieve for LDs than for LEDs.

The researchers conclude, that for both, LEDs and LDs, the solution will be to shift the input power density at which their PCEs peak. Whereas LEDs need to shift to higher input power density to offset higher areal chip cost, LDs need to shift to lower density to enable higher white-light output. In other words, both LEDs and LDs will be made more practical and economical if they can move into and fill the "valley of droop."

Explore further: Bright, laser-based lighting devices

More information: Jonathan J. Wierer Jr., Jeffrey Y. Tsao, Dmitry S. Sizov, Comparison between blue lasers and light-emitting diodes for future solid-state lighting, Laser Photonics Review., 7:6, 963-993 (2013); dx.doi.org/10.1002/lpor.201300048

add to favorites email to friend print save as pdf

Related Stories

Bright, laser-based lighting devices

Sep 27, 2013

As a modern culture, we crave artificial white lights—the brighter the better, and ideally using less energy than ever before. To meet the ever-escalating demand for more lighting in more places and to ...

New material for warm-white LEDs invented

Jan 18, 2013

Light emitting diodes, more commonly called LEDs, are known for their energy efficiency and durability, but the bluish, cold light of current white LEDs has precluded their widespread use for indoor lighting.

LED's efficiency exceeds 100%

Mar 05, 2012

(PhysOrg.com) -- For the first time, researchers have demonstrated that an LED can emit more optical power than the electrical power it consumes. Although scientifically intriguing, the results won’t ...

Recommended for you

'Comb on a chip' powers new atomic clock design

Jul 22, 2014

Researchers from the National Institute of Standards and Technology (NIST) and California Institute of Technology (Caltech) have demonstrated a new design for an atomic clock that is based on a chip-scale ...

Creating optical cables out of thin air

Jul 22, 2014

Imagine being able to instantaneously run an optical cable or fiber to any point on earth, or even into space. That's what Howard Milchberg, professor of physics and electrical and computer engineering at ...

New material puts a twist in light

Jul 18, 2014

Scientists at The Australian National University (ANU) have uncovered the secret to twisting light at will. It is the latest step in the development of photonics, the faster, more compact and less carbon-hungry ...

User comments : 0