Supercontinuum generation is a process where laser light is converted to light with a very broad spectral bandwidth (i.e., low temporal coherence), i.e., a super-wide continuous optical spectrum. This means that the temporal coherence is very low (but with important restrictions – see below!), whereas the spatial coherence usually remains high.
The spectral broadening is usually accomplished by propagating light pulses through a strongly nonlinear device. For example, one may send an intense (amplified) ultrashort pulse through a piece of bulk glass. Alternatively, one can send pulses with much lower pulse energy through an optical fiber, having a waveguide structure which allows for a long propagation length with small effective mode area. Of special interest are photonic crystal fibers, mainly due to their unusual chromatic dispersion characteristics, which can allow a strong nonlinear interaction over a significant length of fiber. Even with fairly moderate input powers, very broad spectra are achieved; this leads to a kind of “laser rainbow”.
In many cases, optical fibers are used for supercontinuum generation. Frequently, one uses photonic crystal fibers, which can be made with tailored chromatic dispersion properties and often also exhibit increased nonlinearity due to strong mode confinement. Some special solutions, which are less widely used, are briefly mentioned in the following:
- In some cases, tapered fibers are used , which provide a very strong nonlinear interaction over a short length.
- There have been demonstrations where the air holes of a photonic crystal fiber was filled either with a gas (which may e.g. be Raman-active) or with a highly nonlinear liquid such as carbon tetrachloride  or toluene .
Figures 1 and 2 show numerically simulated results for supercontinuum generation in a very short piece of fiber. In many cases, however, much longer fibers are used – then usually in conjunction with much lower peak powers.
The Physics of Supercontinuum Generation
The physical mechanisms behind supercontinuum generation in fibers depend very much on the chromatic dispersion and length of the fiber (or other nonlinear medium), the pulse duration, the initial peak power and the pump wavelength.
When femtosecond pulses are used, the spectral broadening can be dominantly caused by self-phase modulation. In the anomalous dispersion regime, the combination of self-phase modulation and dispersion can lead to complicated soliton dynamics, including the split-up of higher-order solitons into multiple fundamental solitons (soliton fission).
Supercontinuum generation is even possible with continuous-wave beams, when using multi-watt laser beams in long fibers; Raman scattering and four-wave mixing are very important in that regime.
The noise properties of the generated continua can also be very different in different parameter regions. In some cases, e.g. with self-phase modulation being the dominant mechanism and the dispersion being normal, the process is very deterministic, and the phase coherence of the generated supercontinuum pulses can be very high, even under conditions of strong spectral broadening. In other cases (e.g. involving a modulational instability or high Raman gain), the process can be extremely sensitive to the slightest fluctuations (including quantum noise) e.g. in the input pulses, so that the properties of the spectrally broadened pulses vary substantially from pulse to pulse. Substantial noise can also arise from strong Raman gain in spectral regions where the power spectral density is not yet substantial; that often happens for relatively long pump pulses.
Generally, it is desirable to use a highly nonlinear fiber, usually having a particularly small effective mode area. However, suitable chromatic dispersion properties are usually most important; inappropriate dispersion properties can hardly be compensated with a smaller mode area.
The strongly nonlinear nature of supercontinuum generation makes it difficult to understand intuitively all the details of the interaction, or to predict relations with analytical tools. Therefore, numerical pulse propagation modeling (often with special precautions due to the extreme optical bandwidth) is required for the analysis of such processes. Intuitive pictures or analytical guidelines can be tested by comparison with results from such numerical models.
It is worth spending some thoughts on the coherence properties of supercontinua. The spatial coherence (considering the cross-spectral density) is usually very high, particularly when the source involves a single-mode fiber, which is often the case. On the other hand, the high spectral bandwidth suggests a very low temporal coherence. However, supercontinua generated from periodic pulse trains can still have a high temporal coherence in the sense that there can be strong correlations between the electric fields corresponding to different pulses, if the spectral broadening mechanism is highly reproducible. That kind of coherence is in fact essential for the generation of frequency combs in photonic crystal fibers, and it may or may not be achieved depending on parameters such as the seed pulse duration and energy, fiber length, and fiber dispersion.
The initially surprising discrepancy between high bandwidth and high temporal coherence can be resolved by realizing the shape of the field correlation function: it has a very narrow peak around zero time delay (with a width of e.g. a few femtoseconds), but there are also additional peaks with comparable height at time delays corresponding to integer multiples of the pulse period. Hence there is low temporal coherence in the sense of vanishing correlations for most time delays, but high temporal coherence in the sense of strong correlations for some large time delays.
Supercontinuum light sources are generally used for purposes where one requires light with a broad optical bandwidth (i.e., low temporal coherence) but at the same time a high degree of spatial coherence, so that the light can be well collimated and focused (with some limitations by chromatic aberrations). For example, one often uses such a source in conjunction with a monochromator as a tunable light source in spectroscopy. In comparison with a tunable laser, a supercontinuum source can usually cover a much wider wavelength range. On the other hand, its power spectral density is far lower, i.e., one obtains only a low power e.g. transmitted through a narrowband monochromator.
Other applications like fluorescence microscopy, CARS microscopy, fluorescence lifetime imaging (e.g. for bio-imaging), flow cytometry, the characterization of optical devices, the generation of multiple carrier waves in optical fiber communications systems, and optical coherence tomography can similarly profit from supercontinuum sources.
Supercontinuum generation also has other applications in ultrafast laser physics, for example for the detection and stabilization of the carrier–envelope offset frequency. That is important in optical frequency metrology.
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See also: optical spectrum, coherence, amplified spontaneous emission, photonic crystal fibers, tapered fibers, frequency combs, frequency metrology, spectrograms
and other articles in the category nonlinear optics
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