Medicine and the life sciences add fiber

Hank Hogan, Contributing Editor, hank.hogan@photonics.com

Compact, reliable, powerful and efficient, fiber lasers are being put to use in medicine and the life sciences, where one device can replace a half dozen or so other lasers or can enable higher-resolution imaging, thereby offering new capabilities. The key to expanding such applications, though, may lie in cutting costs and improving laser performance.

More than skin deep

For !%Solta Medical Inc.%! of Hayward, Calif., fiber lasers meet a fundamental need, in part because they offer a fundamental – as in single-mode – solution. Solta is in the medical aesthetics market, supplying skin resurfacing and dermatology systems to physicians. These devices require lasers that pack a punch.

Fiber lasers power skin resurfacing, in part because they provide the needed power at short-wave infrared wavelengths. Courtesy of Solta Medical.

“We require between 10 and 30 W of power at specific therapeutic wavelengths,” said George Frangineas, the company’s director of laser technology.

He noted that, in addition to supplying enough power at wavelengths between 1400 and 2000 nm, fiber lasers offer a number of other advantages. For example, the single-mode nature of the beam and its quality allow a more compact optical design and the focusing of the laser into a spot. The air-cooled nature of the devices, along with their reliability, size and efficiency, makes them suitable for a medical office.

Solta’s systems create an array of small, localized and well-controlled microthermal treatment zones, both on the skin’s surface and at some depth. The company offers fiber-laser-based systems at 1410, 1550 and 1927 nm, each intended for a different use. The 1550-nm beam, for example, penetrates 1.4 mm and is used to treat wrinkles, photodamage and acne scars. The 1927-nm device reaches only the top 200 µm and is used for surface treatments.

The lasers in Solta’s systems are pulsed under computer control, delivering a dose of 5 to 70 mJ per fractional microthermal treatment zone. A typical face and neck treatment will consist of a half a million spots. The laser’s attributes are key to making this therapy cost-effective.

“Because of the high power and the speed at which we can modulate the laser, physicians can deliver a treatment in about 20 minutes,” Frangineas said.

He noted that one of the company’s latest products combines erbium 1550-nm and thulium 1927-nm fiber lasers into one system, allowing physicians to switch easily between the two. Wavelength tunability, greater dynamic range, smaller size and higher efficiency in fiber lasers were some of the items on his wish list, Frangineas said.

Expanding at both ends

Solta buys all of its fiber lasers from !%7101%%IPG Photonics Corp.%! of Oxford, Mass. Bill Shiner, the latter company’s vice president of industrial products, noted that high-power tunable fiber lasers aren’t available, but it is possible to buy products from the high-performance fiber laser maker over an ever-increasing range. The company has products down into the green, at 532 nm, and has already frequency-doubled that into the ultraviolet, although no UV products have been released yet.

At the other end, IPG’s existing lasers top out at about 2 µm, a wavelength that has strong water absorption and is being investigated for use in prostate cancer treatment procedures, Shiner said.

An acquisition earlier this year gave the company the proprietary transition metal-doped ZnS- and ZnSe-based crystal laser material technology needed to move in steps up to as much as 5 µm. In addition to the increased range, Shiner noted, having a wider array of wavelengths could allow the creation of new lasers at medically important points in between.

“With fiber, you can get the sum and difference frequencies; you can frequency double. You can amplify,” he explained.

However, he noted that IPG would produce only lasers for which there is a business case. In the medical arena, this often means getting regulatory approval. That can be relatively simple, if the new laser is replacing one already approved. For new uses, such as the treatment of prostate cancer, the process is lengthier and more complicated.

The company’s products also show up in analytical applications. For some of these, both the wavelength and the beam characteristics are important. A case in point is optical tweezers, which are used to capture and manipulate objects. Shiner noted that this requires focusing the light to a very small spot, which can be done with a fiber laser to a diffraction-limited point.

Superresolution and more

Fiber lasers also allow researchers to do what was once thought impossible: image below the classical diffraction limit of about one-half the wavelength. One way to do this is through stimulated emission depletion, or STED, microscopy. In this technique, a laser causes adjacent objects to fluoresce one at a time by sequentially stopping their emission of light. This allows their location to be precisely determined, with values as low as 0.15 nm reported. It also enables resolution below the diffraction limit.

In an implementation first done by the nanobiophotonics group at the Max Planck Institute for Biophysical Chemistry in Göttingen, Germany, nanosecond-pulsed fiber lasers provided a convenient multiple-wavelength source through cascaded Raman shifting. Having this very broad and relatively dense spectral output avoids elaborate pulse preparation and offers multicolor imaging, the group reported.

A fiber laser enables better than diffraction-limited imaging of vimentin filaments in a mammalian cell labeled with the dye Oregon green. On the left is an image acquired using stimulated emission depletion (STED) microscopy; on the right is an image acquired using confocal microscopy. Courtesy of Gael Moneron and Stefan W. Hell, Max Planck Institute for Biophysical Chemistry.

The researchers have used the technique to image biological specimens. They’ve paid particular attention to protein distributions both in live and fixed cells, noted group head Stefan W. Hell.

“An important application was the first measurement of major differences between the diffusion of different types of lipids – sphingo- versus phospholipids – in the membrane of a living cell,” he said.

For another success, he pointed to a Jan. 18, 2010, Optics Express paper that described a system based on continuous-wave 592-nm fiber lasers from MPB Communications of Montreal. The system offered resolution of as little as 35 nm in the focal plane and captured a 6 x 12-µm image in 190 ms – fast enough to make a movie.

For future STED applications, Hell would like to see more commercially available continuous-wave fiber lasers offering more than 1.5 W. On the pulsed side, the desire is for pulse energy of more than 10 nJ and a repetition rate of 20 to 100 MHz, something not available commercially today. The wavelength range needs to be 580 to 780 nm.

Researcher Patrick Georges of the Institut d’Optique in Palaiseau, France, heads a group that developed a femtosecond fiber laser operating at 1.6 μm that is specifically for corneal surgery. They wanted a better tool for those times when the entire cornea must be removed and replaced. The system was described in a 2009 Optics Letters paper. Georges also was part of a team that used a fiber-laser generated supercontinuum source for total internal reflection fluorescence lifetime microscopy, with that work appearing in Applied Optics last year.

An erbium-doped fiber amplifier pumps a 1.6-μm femtosecond fiber laser intended for corneal surgery applications. Courtesy of Institut d’Optique.

“Our interest was the compactness,” he said of the decision to use a fiber laser. “All the fibers are spliced, so long-term stability is obtained, and no misalignment can occur.”

More spectral brightness would benefit such applications, he noted. An extension of the technology further toward the UV would also be an advantage.

Commercial instrumentation products based on these and other research efforts are beginning to appear, noted Colin Seaton, global vice president of sales and marketing for fiber laser maker Fianium of Southampton, UK.

A spectrally bright supercontinuum fiber laser, such as this variable repetition rate, picosecond-pulsed device made by Fianium, renders bioimaging and fluorescence techniques possible at many different wavelengths. Courtesy of Fianium.

For example, the company’s products have been used in the Max Planck STED implementation. Seaton said they have been used by the US National Cancer Institute in prototype cell flow cytometers, replacing multiple sources with a single fiber laser.

Chicken and egg, or cost and volume

While these and other emerging markets are of interest, Seaton said the driver for commercialization is volume, and that, in turn, depends on cost. An ultrafast, high-power supercontinuum fiber laser currently runs about $60,000, with substantial variation resulting from accessible power, accessories and other factors. That cost is too great for a high-volume instrumentation market, which would probably need the laser to be less than $20,000.

Seaton noted that Fianium makes its own laser modules. Thus, the company’s stance is that it will be possible to hit that target, provided potential component cost reductions happen.

“Because the technology is based mainly on telecom-like pumps, fibers and fiber-coupled components, cost scaling can be relatively dramatic,” Seaton said.

He reported that Fianium is working on potential commercial high-volume applications. The challenge is that the necessary volume won’t happen without the cost reduction, which won’t happen without the volume. It’s a problem that confronts many attempts to commercialize research, he said.

Fiber lasers are rugged, as shown here with an ultrafast fiber laser head on a vibration test rig. Courtesy of IMRA America.

Of course, even if it meets the most demanding cost targets, a fiber laser still would not be tunable for high-pulse energy without additional optical devices attached. Thus, it may not be ideal for initial investigations, noted Gyu Cho, executive vice president for fiber laser manufacturer !%6621%%IMRA America%! of Ann Arbor, Mich.

Once parameters are set, though, the compactness, reliability, efficiency, beam characteristics, high repetition rate and relatively high power of fiber lasers can all be reasons that make it worthwhile to switch to them. Cho noted that IMRA’s lasers, for example, are being deployed for refractive eye surgery worldwide, in part because of these advantages.

Of such technology changes, he said, “People realize there are a number of applications which can be done better by fiber lasers.”