Experts say the improved energy efficiency brought by LEDs is only a prelude to what will be possible with laser-generating diodes.
Leland Teschler | Executive Editor
An interesting deal popped up on eBay recently for a replacement headlight assembly. The headlight goes on a BMW i8 coupe. You can get one – sent from Latvia – for a mere $7,355, plus shipping. That’s for one right headlight assembly. If you want the left one too, it will be another $7,355.
One reason for the high price tags: The headlights use laser-diode light sources rather than LEDs. Though laser diodes have been around since the 1960s, they have only recently become efficient enough to be considered candidates for general illumination. Besides being energy efficient, they also put out a lot of lumens. In the BMW headlights, the laser diodes make possible a lighting range of almost 700 m. There is an LED version of the same headlight (about $3,000 each on eBay) that only beams out to about 300 m. A typical incandescent headlight only reaches out about 100 m.
The promise of laser diode illumination has prompted some experts to predict big things for the technology. One is Dr. Shuji Nakamura, inventor of the blue LED for which he shared the Nobel Prize in physics. Nakamura claims laser diodes are the future of lighting. He has backed up this vision with action, founding a company called SoraaLaser (now called SLD Laser) to make laser diodes optimized for illumination. The laser diodes they create use gallium nitride crystals grown on a semi-polar crystal plane, not normally done in conventional GaN laser diodes.
(Ordinary crystal planes are electrically neutral. Polar planes are those that aren’t electrically neutral, so there is a charge between neighboring crystal planes. The charge is big in GaN and similar type III-N materials. It can degrade the efficiency of light-emitting devices. But growing the material along certain crystal planes reduces the polarization charge. Materials grown along these special crystal planes are called semi-polar.)
SLD Laser says semipolar GaN devices have a gain 3-5X higher than ordinary laser diodes because the semipolar orientation eliminates internal electrostatic fields and provides maximum overlap between energy levels. The semi-polar material also makes it easier to fabricate diodes operating at different wavelengths. The company says it has produced laser diodes exhibiting a continuous-wave output of more than 4 W multimode, and blue laser diodes putting out over 1 W. It also says it has demonstrated green (525 nm) continuous-wave laser diodes putting out 200 mW.
But as the price of the BMW headlights indicate, it may be awhile before laser diodes can become widely used sources of illumination.
Inside a laser diode
All solid-state lasers need a gain or amplifying medium and a resonant cavity. In a diode laser, electrons and holes are injected across a p-n junction when there’s a forward bias, as in an LED. But the injected current is a lot higher than in an LED.
The semiconductor material in the diode is typically GaN, used because it has a bandgap that allows efficient light emission. Laser diodes described so far generally use a p-type magnesium-doped GaN film grown with GaN buffer layers on sapphire substrates.
To make laser diodes practical, fabricators had to give the GaN a high level of p-type doping. The resulting semiconductor material has what’s called a double-degeneracy. Here, the doping level is so high that the material in some ways acts more like a metal than as a semiconductor. Photons having the right energy level can then stimulate the laser action.
Generally, in a semiconductor diode laser, a resonant cavity is created by cleaving two parallel faces about 250 to 400 µm apart. The cleavage planes act as mirrors, allowing the laser to build up energy. The diode will lase when the system gain exceeds the total losses.
Laser diode fabricators use a double heterostructure (DH) as a way to reduce the thickness of the active region and thereby reduce the threshold current density for lasing action. The active region acts as a dielectric waveguide because the active layer material has a larger refractive index than the cladding layers on either side. The reflectivity of the facets can be controlled by using coatings on the exposed semiconductor surface. If the refractive index of the active layer exceeds that of the cladding layers, light reflects at the interfaces, as in optical fiber. The larger the difference in refractive indices, the better the optical confinement and the sharper the lasing modes.
To reduce the total current flow, the current injection area is usually confined to a narrow stripe contact region on top of the diode. Then injected carriers stay in the region under the stripe contact. The stripe geometry has another advantage in that if the stripe is narrow enough, single-mode lasing can take place.
The resulting action generates blue laser light. But blue light isn’t optimum for illumination, so the laser light must be converted to something warmer. In the BMW headlights, laser-created light bounces off a series of mirrors in the headlamp assembly and eventually gets focused into a lens. The lens contains a yellow phosphorus that reacts to the laser light. Laser light interacts with the yellow phosphorous to create a bright, highly intense white light. The light then goes through a diffuser to reduce its intensity and gets reflected out through the front of the headlight.
The white light projected from the headlights is ten times brighter than LED-created light. BMW says that this technique is also 30% more efficient than LED alternatives. The laser light optics occupy a smaller volume than an equivalent LED version, so designers can reduce the size of the housing and optimize the headlights for aerodynamics.
It is also interesting to note that the drive electronics for laser diodes resembles that for LEDs because both devices work best when powered from a current source. And as with LEDs, light output from a laser diode drops with rising temperature, so heatsinks will be a part of most laser lighting circuits.
You might think that the high cost of BMW’s laser headlights doesn’t bode well for the practicality of laser diodes in everyday uses. But the headlight’s high cost doesn’t arise from the laser diodes alone. For example, the headlights can spotlight animals and pedestrians up to 300 ft away and carry an infrared camera that illuminates these obstacles. The headlights also connect with the car’s GPS which helps predict upcoming turns in the road so drivers can better see them. A built-in camera detects oncoming vehicles to automatically dim the beam. And the laser diodes can be controlled to literally project information like MPH onto the road in the driver’s field of view.
However, technological hurdles do remain. One is in the area of efficiency. Energy efficiency is typically measured as wall-plug efficiency (WPE), the fraction of the electrical input power emitted as light output. Some GaN-based blue LEDs can hit 80% WPE, but GaN-based laser diodes are generally below 40%. The mechanism behind the poor efficiency isn’t well understood.
Recent research indicates that a phenomenon called Auger recombination might be the source of the problem. Here, the excess energy given off by an electron recombining with a hole gets absorbed by a second electron instead of being emitted as a photon. The newly excited electron then gives up its additional energy in a series of collisions with the GaN crystal lattice. Indications are that Auger recombination worsens with the high currents that are involved in laser diodes used for illumination. There is also some self-heating in the semiconductor material that lowers the optical gain, boosting the amount current carriers needed to compensate for optical losses.
A WPE lower than that of LEDs won’t keep laser diodes out of illumination applications. SDL Laser points out that the key metric in directional lighting applications is the lumens per target area per watt of power consumption. SDL says laser-diode illumination excels at this metric for distances of 5 m or more and improves dramatically beyond 10 m compared to LEDs. In terms of raw output, laser diodes can hit 40-50 lm/W levels today. Expectations are that laser diode outputs can rise to 100 lm/W and beyond.
As an example of what’s possible today, consider one of SDL’s products: a laser-diode-based white-light source packaged as a 7×7-mm surface-mount device (SMD). It produces up to 500 lumens of output from a 300-μm emitting area in collimated beam angles as low as 1-2°.
The SDL device contains the same optical elements as the BMW laser headlight, but all within the SMD package. Inside is a high-power blue indium gallium nitride (InGaN) semipolar laser diode that excites a tiny (1 × 1 mm) remote phosphor target (<300 μm diameter) that converts the laser light to directional white light. There is also a beam dump, essentially a passive absorbing element, that blocks any blue light that could reflect from the single-crystal phosphor and leave the package.
Additionally, there are collimating optics that narrow the output beam to about one-tenth that of an LED of the same size and lumen output. SDL says it gets 1,000 candela/mm² spread over 120° from a single 7-mm² SMD. The same approach can produce luminaires with one-tenth the diameter but 100X lumen output of LEDs, SDL says.
SDL has a second form factor for its laser diodes wherein the laser and phosphor element are in two different packages with a length of fiber optics carrying the laser light to the phosphor. The firm uses this configuration when it’s advantageous to keep the laser separate from the phosphor – the phosphor can heat up significantly when the laser passes through it.
SDL says the fact that laser diodes approximate a point source simplifies the process of shaping their output. For example, laser beams lend themselves particularly well to diffractive type optical elements for beam sizing and shaping. A light shaping diffuser element can transform a 1° spotlight to a rectangle of 1° ×10° with an efficiency exceeding 92%. And liquid-crystal lenses added downstream can electronically control and dynamically change the beam angle and its shape.
Currently, laser diodes aimed at illumination can emit up to 500 lumens. As illustrated by BMW’s headlight design, laser diodes work well when combined into a single beam. Their overall lumen output is expected to rise with new designs, and future work in phosphors and laser diode fabrication is expected to result in devices covering a wider light spectrum complete with warmer, higher light levels for indoor applications. DW