Supercontinuum Generation in Photonic Crystal Fibers
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our application note on supercontinuum generation 
Formation of broad continuous spectra through propagation of short femto- or picosecond-range high power pulses through nonlinear media (also known as super continuum generation, SCG) 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. Results on SCG in PCFs have previously been presented with pumping in the anomalous dispersion regime or at the zero-dispersion wavelength in both the visible and the infrared wavelength range. Most experiments utilize femtosecond pumping as this results in spectacularly broad spectra. Picosecond pumping yields more narrow spectra but does so with far cheaper laser sources and is therefore commercially interesting.
Although SCG can be observed in a drop of water given enough pumping power, PCFs are ideal media for SCG 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 with relatively small holes 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.
Applications for supercontinuum are many, of which spectroscopy, sensors, optical coherence tomography and telecommunication WDM sources has generated most interest.
When selecting a fiber/pump combination for supercontinuum, the location of the zero-dispersion wavlength with respect to the pumping wavelength is the most important consideration. Below is a very simple guideline valid for femtosecond pumping (more detils can be found in our application note):
| Pumping wavelength | Output spectrum |
| Below the zero dispersion wavelength | Stable, smooth and narrow spectrum |
| Coinsiding with the zero dispersion wavelength | Irregular, medium-wide and with a dip at the zero-dispersion wavelength |
| Above the zero dispersion wavelength | Irregular and wide spectrum |
Movie of the development of supercontinuum in a fiber. The movie is created
by Karen Marie Hilligsøe from NKT
Academy in Århus.
Download
our application note on supercontinnum generation
Application
note on supercontinnum generation in SC-5.0-1040
