Formation of broad continuous spectra through propagation of short high power pulses through nonlinear media, also known as supercontinuum generation, was first observed in 1970 and has since then been studied extensively in many different materials. The term supercontinuum does not cover a specific phenomenon but rather a plethora of nonlinear effect leading to considerable spectral broadening of optical pulses and thereby potentially octave-spanning output. The involved nonlinear effects depend on the dispersion in the material and count effects like self-phase modulation (SPM), Raman scattering, phase matching and solitons.

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 fi bre 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 a specific wavelengths are required to cover a broad spectrum. A supercontinuum source bridges this gap, providing an ultrabroadband white-light spectrum but with singlemode beam characteristics and excellent pointing stability and the brightness of a laser. The figure below shows the broad emission spectrum that can be achieved.

Although supercontinuum generation can be observed in a drop of water given enough pumping power, photonic crystal fibers are ideal media for supercontinuum generation as the dispersion can be designed to facilitate continuum generation in a specific region. In this way it is possible to convert light to both higher and lower wavelength, just like super wide spectra covering more than an octave is achievable at previously unthinkable low power levels.

Our nonlinear fibers are attractive for studying nonlinear effects, as they have a high nonlinear coefficient, and are available with a long range of unique zero dispersion wavelengths. The fibers are designed to be single-mode at the operation wavelength. This approach has several advantages compared to multimode nonlinear fibers with large air-holes:

• The fibers are easier to splice to solid standard fiber due to the lower air-filling fraction
• Alignment and focusing with free-space coupling is less critical as light focused on the cladding region will not be coupled, unlike in high-air-filling faction fibers, where light can be guided in the silica "islands" between the large holes
• Many applications requires strict single-mode operation.

More over, some of our most popular supercontinuum fibers are available in polarization maintaining version to reduce the required pumping power, lowering the threshold for wide spectrum supercontinuum generation even further.

More information can be found in our application note on supercontinuum generation in photonic crystal fibers.

Commercial supercontinuum sources

At NKTPhotonics, the technical challenge was to ensure an industrially reliable supercontinuum source capable of being employed in demanding commercial and research applications rather than just being a lab tool. The development of high lifetime fiber mode-locked lasers, efficient, but highly reliable fiber amplifiers and the production of highly nonlinear fibers with outstanding reliability characteristics have enabled the SuperK sources to be deployed in applications from confocal micrscopy to critical semiconductor inspection equipment. The combination of robust fiber laser and photonic crystal fibre technology from NKT Photonics together with the modular concept in the construction of the SuperK platform results in a product which is not only the most reliable supercontinuum source on the market, but also the most servicable product in the field.