08 Jul 2004
A military program to ramp up laser-diode performance is on track to deliver highly efficient devices in 2006. James Tyrrell reports on the progress of the SHEDS project.
From Opto & Laser Europe July/August 2004
In less than a year SHEDS has achieved its 18-month target of 65% electrical-to-optical (wallplug) efficiency. By September 2006, the project is looking to deliver 480 W 80% efficient diode stacks operating at 50 °C for the pumping of solid-state lasers.
"The motivation was an interest in improving the efficiency of diode-pumped solid-state lasers," SHEDS program leader Martin Stickley of the Defence Advanced Research Projects Agency (DARPA) told Opto & Laser Europe. "They are very inefficient - probably not more than 10-15% efficient - and the worst culprit in the inefficiency is the diodes."
DARPA's longer-term goal is to develop a portable 100 kW diode-pumped solid-state laser for defence applications. By driving down the waste heat generated by the diodes, the agency is looking to dramatically reduce the size of the refrigeration units needed to cool the laser. Currently these have to be transported on 18-wheel vehicles, which are impractical for military use.
Three US laser-diode manufacturers - JDS Uniphase, nLight Photonics and Alfalight - are participating in SHEDS. Although they have been tackling the goal independently, the firms are united in recognizing the project's commercial value. The knock-on effects of increased laser-diode efficiency will extend well beyond defence applications.
"All types of diode-pumped laser benefit from these improvements," a spokesperson from Alfalight told Opto & Laser Europe. "Kilowatt-class solid-state lasers are rapidly moving towards using diode pumping for improved efficiency and reduced maintenance." Biomedical, sensing and display markets would also profit from the research, given the success of diode-pumped devices such as fibre lasers and microlasers.
SHEDS tackles the 880-980 nm window: the range for pumping directly into the upper laser level of Nd and Yb:YAG lasers. Due to the narrow linewidth of some of the transitions being pumped, the linewidth of the radiation emitted over the whole bar has been specified as 1-2 nm. Additionally, each bar in the stack must have the same peak wavelength within a ±0.5 nm margin.
Efficiency boost Currently, all three participating laser-diode manufacturers are reporting wallplug efficiencies in the 65-70% range at wavelengths between 915 and 970 nm. "They have moved from 45 to 55 to about 65% by paying careful attention to grading the junctions where dissimilar materials come together," said Stickley. "Those voltage barriers lead to sources where energy is dissipated, and of course that's just what we want to eliminate."
"To date we've been working at 940 nm." nLight's Jason Farmer told Opto & Laser Europe. "We do the majority of our work with single emitters and then once we make progress of the order of, say, 3-5% improvement in efficiency, we'll make a few bars."
Stickley believes it will take more ingenuity to reach the final goal of 80%. All three companies are tackling the problem in different ways. For example, Alfalight is attempting to grow gallium arsenide (GaAs) in a different orientation to give it higher gain and bring greater efficiency by lowering the threshold. JDS Uniphase is investigating quantum-well structures and low-loss distributed feedback. And nLight is looking to inject carriers at the edges of the device. "Imagine etching big trenches on either side of the laser diode that you fill up with GaAs - the carriers go right down through the GaAs, but then come in sideways when they reach the bottom," explained Farmer. "You don't have to put the carriers across all those heterobarriers and see all those losses."
The SHEDS project also benefits from university input. nLight has teamed up with the University of Southern California (USC) to look at waveguides and photonic-crystal structures. "If you can confine all that spontaneous emission directly to the laser mode then you will lower the amount of current it takes to get to threshold," said Farmer.
Amnon Yariv's research group from Cal Tech is also working towards harnessing this wasted emission. Yariv is studying transverse Bragg reflectors, again to reflect spontaneous emission back into the gain region. The idea is that you could increase the area of the mode, as well as lowering the threshold.
The University of Central Florida (UCF) is providing expertise on two fronts. Leon Glebov and his team are developing volume Bragg gratings to provide spectral control of the laser diodes. UCF's second contribution, led by Michael Bass, has already made good progress in modelling high-average-power solid-state lasers, including designs for uniform energy deposition.
Completing the list of project partners is the National Institute for Standards and Technology (NIST). Their role is to measure the power, efficiency and spectral properties of the new diodes.
Despite funding only three semiconductor manufacturers, Stickley believes the project is influencing a larger audience. "I sense that other companies are also now bragging about their increased efficiency," he said. Stickley feels that SHEDS will also help non-military applications, particularly in materials processing. "I think this efficient diode laser technology will eventually overtake the kinds of gas lasers that are being used today."
Achieving the final goal of 80% laser-diode efficiency may promise huge benefits, but Stickley acknowledges that it is a challenge. "People have worked on models which say that it's within reach. But at the same time, getting to that level requires doing a number of things in parallel." The key issue is predicting how these different processes will interact in the final device - a difficult task. The final word goes to Stickley: "We're just going to have to go out there experimentally."
SHEDS partners and their specialities JDS Uniphase
Radically asymmetric waveguides; quantum-well designs combining best materials; longitudinal current differentiation; low-loss distributed feedback; interface bandgap engineering.
Epi growth of mirrors on large optical cavity laser facets to alleviate catastrophic optical mirror damage; design of transverse junction stripe to bypass heterojunction barriers and achieve superefficient injection; spontaneous emission coupling efficiency through introduction of photonic band gaps in non-lasing directions.
Thorough investigation of Al-free material system; quantum dots for reducing threshold; growth of quantum wells in <110> direction to achieve higher gain.
Cal Tech (Amnan Yariv)
Transverse Bragg resonance for increasing gain and power and lowering threshold.
UCF (Leon Glebov)
Holographic gratings in glass for wavelength definition and control of bars and stacks.
UCF (Michael Bass)
Modelling of high-average-power solid-state lasers, including determination of designs for uniform energy deposition.
High-power diode-laser bar metrology; independent measurement of efficiency and spectral width, stability and uniformity of radiation from diode bars.