Military Project Aims to Improve Diode Laser Efficiency

Breck Hitz, Senior Technical Editor

The Defense Advanced Research Projects Agency’s Super High Efficiency Diode Sources (SHEDS) program is a military undertaking with huge implications for the commercial laser marketplace. SHEDS’ goal is to increase the efficiency of semiconductor pump lasers to 80 percent from the current state of the art of 50 percent, and to maximize their coupling to high-power solid-state lasers. If this goal is achieved — or even if it is partially achieved — it will make many of today’s lasers obsolete and will usher in a new generation of smaller, more efficient and more powerful commercial lasers.

Perhaps a more realistic, and more impressive, way to state the goal is to consider the waste heat generated in a diode laser. In boosting diode efficiency to 80 percent, SHEDS will reduce the waste heat by a factor of 2.5, from 50 percent to 20 percent. Because thermal management is an overriding consideration in designing any diode laser, this means that as much as 2.5 times more laser power could be generated from an outwardly unchanged package.

Even at 50 percent efficiency, diode lasers are more efficient than any other laser, and for this reason, they already are very strong contenders in the commercial marketplace. Their drawback is their inherently poor beam quality, but over the past two decades, a number of successful techniques to overcome this have surfaced.

The first was the diode-pumped solid-state laser, which appeared 20 years ago and which today is a mature technology. More recently, fiber lasers have emerged from the laboratory and are appearing in commercial products. In just the past half-dozen years, a third entry, the thin-disk laser, has graduated from Adolf Giesen’s laboratory at the University of Stuttgart in Germany into the commercial market. And yet a fourth entry, the alkali vapor laser, is poised to make the transition from the workbench of William F. Krupke at Lawrence Livermore National Laboratory in California.

One quite reasonably could argue that these and similar technologies are the current and future heart of the commercial market, and that all will be dramatically affected by the efficiency improvements envisioned. Under the guidance of C. Martin Stickley, the Defense Advanced Research Projects Agency is funding SHEDS at $8 million this first year, and the program is expected to run through the fall of 2006. It comprises nine distinct projects, seven of which are described in the following essays. Essays describing the two newest projects, addressing vertical-cavity lasers, will appear next month.

New approaches

In the first, Leonid B. Glebov describes his efforts at the University of Central Florida’s Center for Research and Education in Optics and Lasers in Orlando to develop holographic gratings that will lock the laser’s frequency and bandwidth rock solid, despite variations in temperature and other environmental changes. Next, Jason Farmer of nLight in Vancouver, Wash., reports on his project to identify and minimize each of the loss mechanisms in a diode laser. Following that, Amnon Yariv and John Choi of California Institute of Technology in Pasadena give an account of the transverse Bragg resonator, a new approach to laser resonators that shows promise for efficiently generating significantly higher power from a small package.

In the fourth essay, Matthew Peters of JDS Uniphase Corp. in San Jose, Calif., describes a radically asymmetric wavelength stabilized laser that can reduce the intracavity loss from free-carrier absorption. Then, Michael Bass of the Center for Research and Education in Optics and Lasers explains his mathematical model of the interaction between a diode-pumped laser and a high-power solid-state laser, and the sometimes surprising predictions the model makes.

The sixth, contributed by Manoj Kanskar of Alfalight Inc. in Madison, Wis., addresses that company’s careful design improvements and their impact on enhancing laser efficiency. And last, but not least, Christopher L. Cromer of the National Institute of Standards and Technology in Boulder, Colo., describes his unique design for a laser radiometer that can accurately capture and measure the rapidly diverging power from a diode bar, because without accurate measurement techniques, it would be impossible to calibrate efficiency.