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Fiber lasers are closing the technology gap

BioPhotonics
Mar 2011
Marie Freebody, Contributing Editor, marie.freebody@photonics.com

Ultrafast fiber lasers, found in countless research laboratories around the globe, are popular tools for physicists and biologists. Favored for being compact and reliable, fiber lasers also come at a fraction of the cost of comparable solid-state sources, which means that they take up a smaller portion of research grants.

What’s more, maintenance-free operation of these lasers leaves scientists with more time to focus on their core research rather than on having to be part-time laser physicists, constantly tweaking and adjusting, as is required by the solid-state counterparts.

In fact, there are many positives when it comes to fiber lasers: They are robust, efficient and easy to power scale, and they have lower cooling requirements. Surely, then, the biomedical industry can’t be far behind in accepting this small but mighty laser into its collection?

Many laser manufacturers and researchers seem to think so. We are already seeing some ophthalmic surgeons adopting fiber lasers for lasik surgery. The vision correction procedure uses a femtosecond laser to cut a flap in the cornea prior to ultraviolet laser ablation to adjust the refractive power of the eye.

Femtosecond fiber lasers are fast becoming the technology of choice for this procedure because they offer a route to bringing down the capital equipment cost and hence reduce the cost per procedure. Furthermore, the compact size of the fiber-based system enables footprint reduction of the surgical tool, which will become increasingly important as ophthalmic surgery further penetrates global markets.

Fiber laser adoption could extend into other areas, such as multiphoton microscopy, surgical procedures, and imaging techniques that offer higher speed and resolution, as well as lower-cost instrumentation, making them more available to clinics and research facilities.

In Southampton, UK, Fianium is working on an all-fiber approach to creating discrete-wavelength laser sources at any wavelength within the visible and ultraviolet ranges. Sources within this range typically are used for biomedical imaging. To create such sources, nonlinear frequency conversion must be exploited. “We are able to achieve these with 100-mW power levels which will challenge conventional diode laser technology,” said John Clowes, director of business development at Fianium.


Turnkey supercontinuum or white-light lasers from Fianium have been enabled by highly reliable ultrafast fiber laser technology. These state-of-the-art lasers provide every wavelength in the visible and near-infrared region of the spectrum, from below 400 nm to beyond 2 μm. Courtesy of Fianium.


Fianium is a fiber laser company that manufactures ultrafast, high-power laser systems spanning the 240- to 2500-nm region. But one of its specialties is its supercontinuum fiber laser sources, which have applications in fluorescence microscopy, imaging and spectroscopy. A supercontinuum laser is an ultrabroadband source of light that maintains the spatial beam properties of a laser.

“In the past, laser-based imaging and spectroscopy applications have had to make do with the limited available discrete wavelengths offered by diode and argon-ion lasers to excite fluorescent molecules,” Clowes said. “The supercontinuum offers every wavelength from the blue to short-wavelength infrared. For fluorescence excitation, all that is needed is a supercontinuum and a set of filters to choose the optimum wavelength; it is therefore an ideal source for imaging.”

Fianium has seen rapid growth in supercontinuum installations over the past six years, with scientific researchers forming the majority of early adopters of the technology; however, Clowes points out that with increasing maturity of the technology, including higher powers, and brighter and whiter supercontinuum sources, as well as significant cost reduction in volumes, we are beginning to see many industrial applications develop.

“In terms of the fiber supercontinuum systems, we have scaled the power to beyond 10 watts and are delivering systems with more than 1.5 watts of power in the visible region of the spectrum – a fivefold increase in the six years since we developed the technology,” Clowes said. “In the laboratory, we have pushed this level to several watts and more than 15 milliwatts of power for every nanometer of the visible spectrum. We are now focused on the challenges of transferring this into commercial products with the reliability demanded by our customers.”

Obstacles to overcome

Fiber lasers are not without drawbacks, however, particularly when it comes to generating higher energy and shorter pulses. Because of the small confinement area of light in the fiber, high-intensity pulses can produce several nonlinear and thermal effects that can affect performance.

A group at Cornell University in Ithaca, N.Y., working under professor Frank Wise, is hoping to develop methods that will produce high-intensity pulses despite these nonlinearities.

“A short pulse of even modest energy can experience significant nonlinear effects on propagation through a long length of fiber with a small core,” Wise said. “These distort, or even destroy, a pulse.”

One of the most significant developments in the technology – and one that has helped the community realize the potential of fiber lasers – is the fabrication of fibers with larger cores. This serves to reduce nonlinear effects and has led to high-power fiber lasers.

In the past couple of years, the Wise group has reported new ways of developing femtosecond-pulse fiber lasers that exhibit much higher energies than in the past. These methods appear in papers published in Optics Letters in 2009 and 2010.

The performance (power and pulse duration) of these lasers now reaches, and even exceeds, the performance of femtosecond Ti:sapphire lasers that set the standard in the field. Since the pulses generated by the fiber lasers can tolerate large nonlinear effects, they can be stable at higher energies. A drawback, however, is that fiber lasers are not broadly wavelength-tunable.

Wise and colleagues recently reported the incorporation of a new high-power femtosecond fiber laser into a multiphoton microscope built in professor Chris Schaffer’s laboratory, also at Cornell. Together, they performed a craniotomy on an anesthetized mouse to obtain optical access to the brain. “Schaffer used a transgenic mouse that expresses yellow fluorescent protein in a subset of pyramidal neurons in the mouse cortex,” Wise said. “To visualize vasculature [the circulatory system], he intravenously injected Texas Red dextran.”

Two images of neurons were taken using a Fianium supercontinuum fiber laser within a confocal and stimulated emission depletion (STED) microscope. STED is a superresolution imaging technique developed to overcome the diffraction limit. The images clearly show the improved resolution of STED, right, over conventional confocal microscopy, left. Images courtesy of the Department of NanoBiophotonics, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany.


And the result: two-photon-excited fluorescence images of unparalleled quality obtained at depths of up to 900 μm – almost the full thickness of the mouse cortex.

It is the long wavelength (1060 nm) of the fiber laser that aids transmission of light through the tissue of the mouse and that Wise believes is a good indication that a simple and cheap fiber laser will one day be able to replace Ti:sapphire in multiphoton microscopy.

While two-photon-excited fluorescence microscopy requires the structure of interest to be labeled, other imaging techniques can operate without that restriction. Coherent anti-stokes Raman scattering (CARS) imaging is one such method, and fiber lasers could offer advantages here, too.

CARS imaging does not require the injection of fluorescent labels; instead, the image is generated from the cell tissue itself. Wise has worked in collaboration with professor Sunney Xie’s group at Harvard University in Cambridge, Mass., to build a fiber laser for CARS imaging. When applied with endoscopic versions of CARS that are being developed today, the laser could bring the powerful imaging technique into the mainstream. “People are working to make CARS a real-time tool for pathology (of brain tissue, for example); for the technique to reach real clinical applications, the compact, (eventually) inexpensive fiber lasers will have advantages,” Wise said.

It is no surprise that fiber lasers, with their ability to operate at not only high but also low energies, are lending themselves well to biomedical applications. Procedures on sensitive tissue such as the eye or skin require low pulse energies, typically less than 20 μJ, and ultrafast fiber technology sits comfortably in this region.

As laser scientists continue to close the gap between fiber and solid-state sources, we should see more imaging technologies and medical procedures become cheaper and more accessible, thanks to fiber lasers.


GLOSSARY
fluorescence microscopy
Observation of samples using excitation produced fluorescence. A sample is placed within the excitation laser and the plane of observation is scanned. Emitted photons from the sample are filtered by a long pass dichroic optic and are detected and recorded for digital image reproduction. 
solid-state laser
A laser using a transparent substance (crystalline or glass) as the active medium, doped to provide the energy states necessary for lasing. The pumping mechanism is the radiation from a powerful light source, such as a flash lamp. The ruby and Nd:YAG lasers are solid-state lasers.
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