By making some simple modifications to semiconductor lasers, a pair of University of Rochester researchers has devised a way to make them perform with the power and precision that laser surgeons routinely demand. These small, relatively inexpensive lasers might replace more powerful and costly gas lasers in treating a variety of medical conditions: removing warts, moles, unwanted hair, and tumors, for instance, performing gum surgery, or treating glaucoma.
The work, done by graduate student John Marciante and Professor of Optics Govind Agrawal, was presented last week at the annual meeting of the Optical Society of America in Rochester.
Most laser surgery today is done using more powerful and expensive gas lasers, since their semiconductor counterparts usually lack the power necessary for most surgery. The Rochester work cleans up the beam produced by a semiconductor laser, creating a sharply focused beam with more power and precision than the beams produced by most such lasers.
"This sounds like a very useful technology for surgery," says Raymond Lanzafame, director of Rochester General Hospital's Laser Center and a practitioner of laser surgery. "The boost in power provided by this technology could make a single semiconductor laser an option for many procedures where other types of lasers are currently used. If you've got a high-quality beam and a high power output, this offers a convenient alternative for use in the operating room."
The useful power output of semiconductor lasers is limited by the tendency of their beams to fragment into a number of parallel but weaker beams. This doesn't pose a problem for applications that don't require much power, such as laser printers or supermarket scanners, but it limits their use in surgery, where higher power and precision are critical. The new laser produces a unified beam no wider than a grain of sand, with the power efficiently packaged in the center. Rochester researchers believe the new design makes possible semiconductor lasers with 6 to 12 watts of power, two to four times as powerful as current devices. Currently the only way for semiconductor lasers to produce such power is for several to be used in tandem, but this results in poor beam quality.
"Traditionally, with higher power you lose the ability to focus a semiconductor laser on its target," Marciante says. "Rather than a single strong beam, you get three to five weaker beams, greatly diminishing the laser's power and performance."
To compensate for this breakdown of semiconductor laser beams -- known as "filamentation" -- Agrawal and Marciante propose inserting two extra layers of slightly modified semiconducting material on either side of the active layer of gallium arsenide, where the beam is formed.
"It's long been known that in filamentation, you get a 'positive' bending in the beams of high-powered semiconductor lasers, but nobody has been able to successfully counteract that with a 'negative' bending before," Marciante says. "The layers we're inserting induce negative bending, resulting in a net bend of zero for the laser beam and keeping it sharply focused."
While a prototype has yet to be built, Agrawal says that the new laser performed well in computer simulations. He adds that since the inserted layers are made of the same semiconducting materials used in the other layers of the laser -- with a small amount of aluminum added to give them slightly different properties -- the new laser shouldn't be significantly costlier or more difficult to produce than current semiconductor lasers.
Lanzafame, who was not involved in this research, says that semiconductor lasers offer several advantages over the gas lasers more widely used in surgery. They're more durable and more user-friendly, he says, since they have no gas tanks to replace or mirrors to care for. Semiconductor lasers are also more efficient than gas lasers: A gas laser can take 5,000 watts of power to run a one-watt laser, but a semiconductor laser requires only five watts to produce the same power. Since they can be mass-produced, semiconductor lasers usually cost only a few thousand dollars, compared to tens of thousands of dollars for a comparable gas laser.
Semiconductor lasers also have an advantage over gas lasers in portability: gas lasers are often immovable refrigerator-sized units. By contrast, a semiconductor laser is the size of a coarse grain of salt. Even when a power source is added, a high-power semiconductor laser apparatus is still no larger than a fist.
The technology may also be applied in the area of information storage; for instance, the new laser's more precise focus might permit the storage of more data on CD-ROMs. Semiconductor lasers are also used in telecommunications and in communications between satellites.
The work, funded by the University and the U.S. Air Force's Phillips Laboratory, which employs Marciante, was published in the July 29 issue of Applied Physics Letters. Agrawal and Marciante are now considering corporate partners to help build the new laser.