Titanium-doped sapphire (Ti:sapphire) ultrafast lasers have come a long way since the devices were introduced in the early 1990s, and their makers are hoping they’ll continue their march out of the lab and into industry. Ultrafast lasers operate with pulses ranging from tens of femtoseconds to a few picoseconds. The short-pulse durations allow the lasers to deliver energy precisely without causing heat to spill over beyond their target. This permits measurement of still-living tissue, for example, or treatment of materials without damaging the surrounding area. Sandia National Laboratory (Albuquerque, NM), for instance, uses ultrafast lasers to cut explosives without setting them off.
One of the big markets for ultrafast lasers is biological science, for example in a variation of confocal microscopy called multiphoton excitation (MPE), says Marco Arrigoni, general manager of the scientific business unit at Coherent (Santa Clara, CA). A pair of photons strikes a tissue sample that has been treated with a fluorescing dye, which fluoresces only in the focal spot. The advantage of Ti:sapphire is that it is tunable from 700 to 1000 nm, covering the excitation wavelengths of most standard visible dyes.
In the old days, researchers had to be equal parts laser physicist and biologist to perform experiments using temperamental ultrafast lasers. With the advent of diode pumping, systems became more compact and turnkey, but even recently, Arrigoni says, lasers that covered a range greater than 100 nm required manual tuning and were two-box systems, with the pump laser separated from the main resonator. Coherent recently introduced the Chameleon, a computer-controlled single box tunable from 720 to 930 nm.
“Doctors and biologists don’t want to deal with knobs and tuning the laser,” says Arnd Krueger, director of marketing for the industrial and scientific laser unit at Spectra-Physics (Mountain View, CA), which makes its own hands-off, tunable femtosecond laser, the Mai Tai. “In the past, these ultrafast lasers have been huge systems that occupied an entire table, and you needed a laser jock to operate them,” Krueger says. “Everything is really easy now.”
While ease of operation is making ultrafast systems attractive to users, new applications are also beginning to open up. For instance, as printed circuit board designers try to pack more circuits into a tighter space—inside cell phones, for example—femtosecond lasers provide a way to drill holes in the boards without damaging the area around the hole. The systems can perform drilling operations in the manufacture of ink-jet cartridges or fuel injectors for automobiles. Such applications generally don’t require that the lasers be tunable, but they need to deliver higher pulse energy than MPE requires. Whereas energy for MPE is usually measured in nanojoules per pulse, an application like writing waveguides in glass would require tens of nanojoules. Drilling holes in stainless steel fuel injectors requires millijoules. Researchers are studying material systems that might deliver more energy and are trying to extend available wavelengths into the UV spectral region.
“The reality is that every one of these customers has an application-specific requirement,” says Jeremy Weston, president of Positive Light (Los Gatos, CA), which manufactures the amplifiers used to increase the power of ultrafast lasers. Adding amplifiers and oscillators, of course, increases the complexity of the systems as well as the cost. Some complex systems can run from $225,000 to $300,000, Arrigoni notes.
“It’s not very easy to build generic lasers for industrial customers,” Weston says. “It’s not really possible to just build a laser for micromachining.”
Industry journalist David Belforte says a potential customer for these lasers needs to be able to justify the cost. “The customer has got to have a real need that makes economic sense, and those things are difficult to find,” he says. That doesn’t mean they won’t be found, though—it may take time. “Look how long it took excimers to get accepted.”
(By Neil Savage, OEmagazine, September, 2002)