Laser diodes: high power in tiny arrays

By Jeanne Erdmann

This article was originally published online by IEC on January 16, 2007. It is re-published with permission.

By any measure, lasers are big business. The 2007 January issue of "Optoelectronics Report" cites worldwide commercial laser revenues at USD 5,6 billion dollars, which further divides into 44% for nondiode lasers and 56% for diode lasers.

In the 1972 the IEC created Technical Committee 76 to prepare standards for lasers. In some of its first work, published as IEC 60825, it created a system of classification for these devices according to their degree of hazard. Today, this four-class system is used by all of the world’s manufacturers of non-military lasers.

Gas versus semiconductor
The laser world splits these devices into two camps: nondiode and diode. In large part, this distinction boils down to differences in size, degrees of efficiency and ways in which they work. Each has its own strengths and limitations. Nondiode lasers use gas to convert electromagnetic radiation into a beam of coherent light at a single frequency. Diode lasers use semiconductor technology to produce a beam of light and thus can be as small as a grain of sand. The smallest non-diode laser is about the size of a cigar without the power supply.

Diode lasers are semiconductors (think of the tiny arrays on light emitting diodes that are revolutionizing the lighting industry). They are more efficient, and less expensive than non-diode lasers. The disadvantage of the diode laser is beam quality (there’s a high divergence, which can be overcome by the use of fiber optic cable), coherence length (a broader range of wavelengths makes interferometry more difficult), and limited energy storage compared with a YAG (yttrium-aluminum-garnet) laser, explains Robert Herrick, who is senior reliability engineer for Cisco Systems in San Jose, California.

The communications industry, for instance, uses diode lasers because they can switch on and off billions of times a second. This type of fiber optic communication is used in the back rooms of most large companies, universities, ISPs, server farms, and so on, to send large amounts of data around efficiently, notes Herrick

Power in numbers
In the early 1990s, cutbacks in defense spending by the U.S. government helped fuel the diode laser industry. Around this time, former employees of McDonnell Douglas Corporation in St. Louis, Missouri, licensed company technology and formed two start-ups, which have both grown in scope and size: Cutting Edge Optronics (CEO) is based in St. Charles, Missouri and is now a division of Northrop Grumman in Los Angeles. Nuvonyx Inc. is based in St. Louis and now a part of ICx Technology in Washington, DC.

These companies represent the diversity found in the diode laser field. CEO focuses on the original equipment manufacturing market. The company also supplies components to customers in many fields, for example medicine, graphic arts, and materials processing, such as laser-marking products. "Everything that we ship to our customers has a high-powered laser diode in it somewhere," says David B. Jones, director of sales and marketing for Northrop Grumman.

Nuvonyx developed their first diode laser to cauterize ruptured blood vessels and stop any bleeding. Today, the company uses laser diodes in many industrial and military applications.

Laser diodes can be used in such a variety of ways because of their semiconductor properties. Diodes are constructed of a thin strip of gallium arsenite, which sits across one end of a gold-plated copper bar. This bar can either be used directly, such as in a laser pointer, or used as a light source to pump another crystal as in a YAG laser. They’re water cooled to remove the heat. Efficiency runs at 60%, which is the "most efficient on the planet," says John Haake, vice president technology at Nuvonyx, Inc.

A small bar about the size of a postage stamp produces 100 watts of optical power, enough to weld plastic. When more power is needed, bars are stacked one on top of the other. The number of bars depends on the application. Nuvonyx makes huge lasers for the military up to 7 kW of optical power. Systems for laser cladding – coating metal particles onto another piece of metal – require 4 to 8 kW.

Stacking tiny power wafers makes this enterprise sound much easier than it actually turns out to be. Issues of increased current, increased heat, thermal and optical feedback, and a large power density spread all need to be dealt with in tiny, confined spaces. This makes for challenges that keep very smart people terribly busy. The hard work pays off. Not only can diode lasers bring increased speed and better control to many sectors of the manufacturing industry but these lasers can be used in some surprising industrial applications.

Lasing out of the box
Nuvonyx uses diode lasers in remanufacturing – adding metal back to a worn part. This method extends the useful life of train engines, military tanks, and aircraft engines, machinery that otherwise would be too expensive to replace. The company was the first to demonstrate that welding was possible through laser diodes. Haake says he’s always thought that diode lasers could be used in knit-point heating, a process in which a hot gas jet puts down thin layers of two materials. This type of layer produces strong and light composites, such as those already used in automobiles, racing cars and fishing poles. Even ships and submarines are using this material. Aircraft are heading this way as well.

It takes a lot of ribbon and tape to make the wings and fuselage of a commercial aircraft. Right now, the big limitation is speed. Lasers are orders of magnitude faster than a hot gas jet, which blows hot air into the knit point like a giant hair dryer, to make the surface tacky. "You have to create tack with heat and that’s what lasers can do. This is just one of many, many, many applications for diode lasers," comments Haake.

Down the road … disposable cars?
Diode lasers of the future will begin with more brightness; have more power, and more portability. At least in Europe, diode laser hair removal kits will be available for home use. We may see the day when diode laser technology would be used to recycle automobile engines by adding metal to worn parts in the same way that Nuvonyx remanufactures heavy equipment. These lasers could certainly be used to drive robots that strip the paint from material slated for recycling.

"Light is a wonderful thing," says Haake.

Source: International Electrotechnical Commission (IEC).