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Husain ImamKoheras, Blokken 84, DK-3460 Birkerød, Denmark

e-mail: hui@koheras.com

T he adoption of fibre-laser technology by the commercial sector has seen rapid growth over the past five years.

The lower price of fibre-laser systems, the reduction of maintenance costs, good beam quality and high wall-plug efficiency make this technology attractive for the replacement of conventional lasers in many industrial and medical applications.

Although fibre lasers are already a popular choice for materials processing, fibre-laser technology is also enabling the creation of light sources with unique and attractive characteristics for other applications. Good examples are supercontinuum lasers and ultranarrow-linewidth fibre lasers, which are beginning to attract interest in the fields of metrology, biomedicine and sensing (see Box 1).

Supercontinuum light can be best described as ‘broad as a lamp, bright as a laser’. Incandescent and fluorescent lamps, such as those made from tungsten, halogens or xenon, provide a very broad spectrum, typically 400 nm to 1,700 nm, but the intensity is limited to the quality of the filament or the efficiency of the gas excitation. Furthermore, as the light is not

spatially coherent, coupling the light into the fibre is a challenging affair, resulting in a low-power, low-brightness source with mediocre beam quality. Lasers on the other hand have high spatial coherence and very high brightness, which enables optimum coupling to a fibre and outstanding single-mode beam quality. However, lasers are usually monochromatic, and thus if more than one wavelength is required extra lasers at specific wavelengths are required to cover a broad spectrum.

A supercontinuum source bridges this gap, providing an ultrabroadband white-light spectrum but with single-mode beam characteristics and excellent pointing stability and the brightness of a laser. Figure 1 shows the broad emission spectrum that can be obtained from a SuperK Extreme source. If such light enters a dispersive medium (such as a three-dimensional photonic crystal), the resulting diffraction pattern, shown in Fig. 2, yields a rainbow spectrum illustrating the white nature of the supercontinuum.

Supercontinuum light is generated by promoting highly nonlinear optical processes in materials by pumping them with pulsed light at high energies. These nonlinear processes in the material break up the pulsed nature of the light

and give rise to broadband emission of light — supercontinuum light. This phenomenon was previously confined to the study of research institutions that used large expensive pulsed Ti:sapphire lasers together with photonic-crystal fibre to generate the supercontinuum white light. However, the increasing maturity of mode-locked fibre-laser technology and photonic-crystal fibres has paved the way for the development of all-fibre supercontinuum sources, such as the SuperK product range from Koheras. Such an all-fibre approach provides the end-user with a more cost-effective, reliable and low-maintenance solution when compared with conventional technology.

An excellent example of the unique advantages that supercontinuum technology can provide is in the field of confocal microscopy in the life sciences. In confocal microscopy, image formation of structures outside the microscope’s focus is suppressed by using an arrangement of pinholes that act as a point light source and a point detector. This approach enables very-high-resolution analysis of specimens, which is particularly important for fluorescence imaging, where dyes or labels are attached to specific cells. Furthermore, the images obtained can either be two-dimensional (the samples

Fibre-laser technology is enabling the creation of new types of compact light sources with unique ultrabroad or ultranarrow spectral characteristics. These lasers are now finding applications in diverse fields ranging from biotechnology to test and measurement apparatus.

Metrology

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Figure 1 A supercontinuum source. a, the supercontinuum white-light source Koheras SuperK extreme. b, Broadband output spectrum from the SuperK extreme compared with other sources.

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being scanned in the horizontal plane) or also three-dimensional (by additionally moving the focal plane).

The development of fluorescent imaging techniques based on probes, dyes and staining is constrained by the fact that today’s microscopes are limited to excitation light sources, which are fixed in wavelength and based on gas or solid-state lasers or laser diodes. This gives rise to two major problems. First, the wavelength of the laser usually does not match the excitation wavelength of the dye, giving rise to low excitation efficiency and hence, low fluorescence intensity. To improve the intensity, the laser power can be increased, but there is always the risk of damage to the cells in the sample. Second, a suite of lasers operating at different wavelengths are required to cover the excitation spectrum of the wide variety of dyes and probes that are available at present. This results in an expensive piece of analysis hardware. A supercontinuum source offers an attractive answer to both of these problems.

When using a supercontinuum system for confocal microscopy, an

acousto–optic tunable filter (AOTF) provides a selection of between one and eight simultaneous wavelengths from

within the supercontinuum spectrum to be directed onto the sample. Because a single supercontinuum light source is able to illuminate many dyes in the sample at one time at the exact wavelengths of the excitation, an efficient fluorescent response is achieved and the image is bright. This is extremely important in the detection of cancer cells. Biopsies that have been stained with dyes generally require higher excitation powers to fluoresce efficiently and be detected. It is therefore vitally important that the excitation wavelength is as close to resonance as possible, to maximize the strength of the fluorescence.

Figure 3 shows the benefits of using a supercontinuum source in confocal microscopy. Here human cancer cells (HeLa immortal cancer cells) were labelled with dyes Alexa 488, TRITC (red) and Mitotracker 590. Each of these dyes attach to specific parts of the cancer cells, namely the microtubules, F-Actin and mitochondria. The image was taken using a Leica TCS SP5 confocal microscope equipped with a Koheras SuperK Extreme supercontinuum source. The excitation

Figure 2 the laue diffraction pattern created when a three-dimensional woodpile photonic crystal is illuminated with a supercontinuum source. Courtesy of the Institute of Applied optics and the Nanotechnology Institute (Helmholtz) at the University of Karlsruhe, DFg-Center for Functional Nanostructures (CFN) and Nanoscribe.

Distributed feedback (DFB) fibre lasers are now becoming the ideal source for advanced coherent sensing and measurement applications. Characteristics such as ultranarrow (0.5–3 kHz) linewidths, long coherence lengths, low levels of frequency and intensity noise, exceptional stability and mode-hop-free operation have made fibre lasers an attractive tool. Other benefits include robustness, their lightweight form and immunity to electromagnetic interference.

The long coherence length and low noise characteristics of DFB fibre lasers are important in applications where interferometric sensing plays a major role in the measurement process. As an example, it has been shown that a Koheras DFB fibre laser locked to a reference frequency can have a coherence length of 8,000 km. Applications, such as wind measurement by LIDAR (light detection and ranging), exploration for oil or gas, and perimeter security, benefit from such a high coherence length and low noise for their measurements. Wind Doppler LIDAR measures wind speed by detecting the Doppler shift created by reflected laser light from moving particles in the wind. In oil and gas exploration optical hydrophones are trawled behind

a ship over seabed areas suspected of containing lucrative fuels. Coherent light from a DFB fibre laser is transmitted to the optical hydrophones and a depth charge designed to induce seismic shock waves is detonated. The optical signal received by the ship includes information about the seismic shock waves reflected from the seabed. The signature of the reflected shock waves (and hence the resulting interferometric signal from the hydrophones’ optical reflection) determines the type of material under the seabed and assists in the search for oil or gas. Similarly, in perimeter sensing, optical fibre laid underground reflects laser light from a DFB fibre laser owing to stresses and vibrations that the fibre is subjected to by footsteps or traffic on the surface. In all of these cases, the DFB fibre laser interrogates by detection of beat signals between distributed reflections from the measurement target and the fixed DFB fibre-laser reference signal. The high-coherence and low-noise performance of fibre lasers from Koheras have enabled coherent sensing to be a robust, sensitive and enormously viable technique. After years of development, DFB fibre lasers are now beginning to be commercially adopted as a standard

for coherent sensing applications. As an example, Fig. B1 shows the ZephIR laser anemometer wind-speed Doppler system from the UK company Natural Power, which uses a Koheras DFB fibre laser, to determine where to deploy wind-turbine farms.

Box 1 From the ultrabroad to the ultranarrow

Figure B1 the ZephIr laser anemometer wind-speed Doppler system from Natural Power is driven by a Koheras DFB fibre laser and is used to assess prospective sites for the deployment of wind turbine farms. Courtesy of Natural Power.

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maxima of the dyes were obtained by scanning the wavelengths contained in the supercontinuum white light over the range from 470 nm to 620 nm with a step size of 5 nm using the AOTF. Three excitation maxima were found at 500 nm, 550 nm and 585 nm. The composite image was acquired through line multiplexing scans with light at 500 nm, 550 nm and 585 nm illuminating the sample

simultaneously. The resulting image enables the clinician or researcher to see the morphology of the cells by virtue of colour.

Flow cytometry is another field where, like confocal microscopy, a plethora of light sources is required to excite dyes and fluorophores, which flow in solution. In this field, lasers are typically used to illuminate the solutes at wavelengths of 355 nm, 405 nm, 488 nm and 635 nm.

As in confocal microscopy, dyes are attached to certain parts of the cancer cell structures and then excited with specific wavelengths of light to cause fluorescence. Flow cytometry measures the amount of species in a particular sample, for instance, the number of T-cells in bone marrow. However, another class of fluorescent dyes (extracted from marine life) is arousing interest. These dyes contain genetic information and when modified and inserted into specific cells, the host cell’s DNA begins to express (or reproduce) these specific fluorescent species. The advantage of this technique is that samples can be analysed for specific behaviour of cells that have been treated — T-cell behaviour in a bone-marrow transplant, for instance.

The issue here is that the wavelengths that excite these dyes are primarily in the orange part of the spectrum and there are not many standard fixed-wavelength lasers in this region. Custom-built lasers are an option but can be expensive. Thus, supercontinuum light provides a convenient solution in such applications. Experiments at the National Cancer Institute (NCI) in the National Institute of Health, Bethesda, USA, with the SuperK Extreme supercontinuum source confirm that such sources are viable for these applications.

It can be seen from the unique examples given that supercontinuum sources have a range of applications as broad as their spectra. The use of fibre lasers to create a supercontinuum has made this technology available to industries that previously did not have access to such a broad and bright light source. Innovative applications of fibre-laser supercontinuum sources are still emerging every year so the future looks bright for this fascinating technology.

Figure 3 Supercontinuum sources eliminate the need to use multiple light sources for confocal microscopy. A supercontinuum source has been used to determine the optimum wavelength with which to illuminate human cancer cells. In this case three excitation maxima were found and the composite image was acquired by illuminating the sample simultaneously at all three optimized excitation wavelengths. Courtesy of leica Microsystems.

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