Supercontinuum white light laser, like the SuperK EXTREME from NKT Photonics, combines the broadband nature of a lamp with the power and robustness of a fiber laser. One can access any wavelength in the 400 to 2400 nm range enabling optimal dye excitation. Also fluorescence lifetimes are easily measured with the short pulses from the lasers. Today supercontinuum lasers are used for numerous bio-imaging applications including OCT, confocal microscopy, FLIM, FRET and optical molecular imaging.

Broadband sources in bio-photonics

White light sources have allowed users to illuminate, interrogate, and excite biological materials and chemicals for several hundred years. Traditionally, filament or gas discharge lamps have been used and they are today supplemented by LEDs and other white light sources. However, the output power and/or bandwidth of these sources remain inadequate for many applications.

Lasers provide excellent beam quality and power but they are inherently single wavelength devices. To address several wavelengths one must combine a number of lasers adding cost and complexity. Moreover, only certain discrete wavelengths are available.

The supercontinuum white light laser combines the broadband nature of the lamp with the power and robustness of a laser and has been used in bio-photonics since their introduction to the market by NKT Photonics in 2003. A supercontinuum laser typically provides a continuous output in the entire 400-2400nm range and by filtering one can address any part of the spectrum individually. The diffraction limited output is fiber delivered with a typical spectral density of several mW/nm – many orders of magnitude brighter than lamp-based sources and the total power can reach several Watt. Moreover, since supercontinuum lasers are monolithic fiber lasers, they require no alignment or service and NKTs sources have lifetimes exceeding thousands of hours.

Conclusion

Supercontinuum white light lasers provide numerous advantages as a light source for bio-imaging and many of the leading players in the field today are using SuperK supercontinuum lasers. Among the most important advantages are:

• Only one source needed to cover all wavelength regions

• No alignment of multiple sources leading to better stability and less maintenance

• Excitation can be tuned to the absorption peak of the dye maximizing the signal-to-noise ratio.

• Reduced cross excitation in samples with several dyes

• Robust maintenance free system

Bio-Imaging Applications

In confocal fluorescence imaging, the continuous spectrum allows the choice of any excitation wavelength and the diffraction limited light ensures great resolution. The excitation wavelength can be controlled using acousto-optic tunable filters (AOTFs), like the SuperK SELECT, that allows the user to choose any wavelength or combination of wavelengths in the visible and nIR spectrum. This ensures optimal excitation of the fluorescent dyes in terms of absorption and minimization of cross excitation between dyes, leading to higher contrast images.

In vivo imaging often requires the use of the near-infrared wavelengths between 650 and 900 nm where tissue absorption is lower than at visible wavelengths. Not many light sources are available at these wavelengths making supercontinuum lasers very attractive for e.g. in vivo molecular imaging.

Lifetime measurements (FLIM) are easily added since the supercontinuum lasers are naturally pulsed. The SuperK EXTREME comes with adjustable repetition rate so that excitation can be matched to the decay times of the sample and the integrated NIM trigger can be connected directly to FLIM equipment for timing.

Optical Coherence Tomography (OCT) utilizes the continuous broadband nature of the supercontinuum output. In combination with a Gaussian shaping filter, like the SuperK GAUSS, one can get a smooth broadband spectrum covering more than 200 nm.  The smooth spectrum reduces artifacts in the OCT images and the large bandwidth increases resolution. Moreover, since the source covers the entire 400 to 2400 spectrum, one can freely choose which wavelength range to use depending on the sample studied.