FIBER LASERS: Supercontinuum lasers begin to shine in biomedicine

Optical supercontinuum lasers produce light across a broad spectrum in high-power, ultrafast pulses. Originally developed by researchers seeking devices with high power and high speed, they soon found application beyond the lab in metrology, military affairs, and homeland security. Now they have begun to show promise in another field: biomedicine. Recent studies have revealed the lasers’ potential use in flow cytometry, confocal microscopy, and optical coherence tomography, among other areas.

Vladimir Kozlov, vice president of sales and marketing in the U.S. branch of Fianium (Southampton, England), explains the lasers’ value in life-science laboratories. “What is really attractive is continuous spectral coverage of the whole visible range,” he says. “Look at cytometry and fluorescence microscopy. People use different excitation sources to excite different molecules. They often need three or even five to gain the full range. We offer continuous tunability over the wavelengths. We also cover some windows that have no discrete laser sources, such as the orange region around 590 nanometers wavelength.”

Supercontinuum lasers have another useful quality. “Their output consists of ultrashort, ultrafast pulses,” Kozlov says. “Some applications, like the fluorescence decay times of certain compounds, are based on that.”

To generate light for their supercontinuum lasers, researchers originally used liquids and nonlinear crystals. More recently, companies such as Fianium have relied on nonlinear crystal fiber. “What we’re building is not fundamentally new,” Kozlov says. “But our whole system consists of fiber-based components. That changes the reliability and stability of generation.” Three recent reports reveal the scope of biomedical research facilitated by supercontinuum fiber lasers.

A group from the National Cancer Institute (NCI), in conjunction with researchers from Fianium, reported in Nature last year the first application of supercontinuum lasers to flow cytometry (Nature Methods 4, 678; Sept. 1, 2007). This use benefits specifically from the lasers’ tunability over multiple wavelengths. “Even the most modern cytometers typically provide for not more than four laser wavelengths,” explains William Telford, an NCI researcher. “Supercontinuum white-light lasers provide wavelengths that are difficult to produce using traditional technologies, allowing virtually any fluorophore to be analyzed by flow cytometry.” Telford’s team used Fianium’s SC450 white-light source.

Scientists at Harvard Medical School headed by Lev Perelman used the same type of laser in the first application of optical supercontinuum technology to confocal light absorption and scattering spectroscopic microscopy. The technique combines the principles of light-scattering spectroscopy (LSS), which relates the spectroscopic properties of light elastically scattered by small particles to their sizes, refractive indices, and shapes, and confocal microscopy (see Inside Instrumentation, p. 22). Because it is multispectral, LSS can measure internal cell structures much smaller than the diffraction limit without causing damage to the cell or otherwise influencing the cell’s function. Fianium, meanwhile, has recently developed a system to be adapted to fluorescence lifetime imaging microscopy (see figure).


Tunable supercontinuum sources can be use in a hyperspectral FLIM microscope to acquire full excitation-emission-lifetime-resolved images at every pixel in a sample (Owen et al., Opt. Lett. 32, 3408; 2007), The monochrome figure on the left is the intensity image (integrated over excitation and emission spectra and decay profile), the middle figures are excitation-emission matrices (EEM) corresponding to the indicated points in the image and the decay profiles on the right are examples taken from the indicated points on the EEM images.
Click here to enlarge image

A research group headed by Felix Spöler at Germany’s RWTH Aachen University has used a supercontinuum fiber laser to produce ultrahigh-resolution optical coherence tomography (UHR-OCT). The laser substitutes for the complex and costly femtosecond lasers typically relied on to obtain UHR-OCT. Spöler’s team filtered two wavelength bands centered on 840 and 1230 nm. The use of two wavelengths reduces the amount of speckle in images.

Other biomedical applications beckon as the brightness of supercontinuum lasers increases and sources at wavelengths below 400 nm become available. “Most of our customers are using just the visible part of the spectrum,” Kozlov says. “This has a lot of molecules that need to be excited. But only 20% of the excitable continuum is in the visible range. My expectation is that we’ll see more developments take advantage of the new infrared part of the spectrum.”

–PG

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