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Ask RP Photonics to numerically simulate supercontinuum generation in a fiber in order to optimize the parameters before you start with expensive experiments. It is possible not only to predict the resulting intensity spectra but also to simulate the noise performance. RP Photonics has the powerful numerical software RP ProPulse for the simulation of pulse propagation.
Definition: a nonlinear process for strong spectral broadening of light
Supercontinuum generation is a process where laser light is converted to light with very broad spectral bandwidth (i.e., low temporal coherence), whereas the spatial coherence usually remains high. The spectral broadening is usually accomplished by propagating optical pulses through a strongly nonlinear device, such as an optical fiber. 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 quite moderate input powers, very broad spectra are achieved (→ a kind of "laser rainbow"). In some cases, tapered fibers can also be used.
Applications of supercontinua include coherence tomography, fluorescence microscopy, flow cytometry, the characterization of optical devices, the generation of multiple carrier waves in optical fiber communications systems, and the measurement of the carrier-envelope offset frequency of frequency combs.
Figure 1: Supercontinuum, simulated with the program ProPulse by R. Paschotta, assuming that 20-fs laser pulses propagate through a 2-mm long photonic crystal fiber. The time domain (upper graph) exhibits a complicated multi-peak structure, whereas the spectrum (lower graph, with logarithmic scale) has a significant power spectral density over more than one optical octave. The simulation took into account chromatic dispersion, the Kerr nonlinearity (leading to self-phase modulation and four-wave mixing) with self-steepening, and Raman scattering.
Figure 2: Evolution of the spectrum along the fiber (vertical axis). After ∼1 mm propagation distance, further spectral broadening is weak because the peak power has decreased a lot. Note that the intensity color scale (bar on the right-hand side) is logarithmic.
The Physics of Supercontinuum Generation
The physical processes behind supercontinuum generation in fibers can be very different, depending particularly 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). For pumping with picosecond or nanosecond pulses, Raman scattering and four-wave mixing can be rather important. Supercontinuum generation is even possible with continuous-wave beams, when using multi-watt laser beams in rather 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 higher-order soliton effects), 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 quite substantially from pulse to pulse.
The strongly nonlinear nature of supercontinuum generation makes it hard to intuitively understand 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.
Coherence Properties
It is worth spending some thoughts on the coherence properties of supercontinua. The spatial coherence is usually very high, particularly when the source involves a single-mode fiber. 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. This kind of coherence is in fact very important 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.
Bibliography
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| [3] | J. K. Ranka et al., "Visible continuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nm", Opt. Lett. 25 (1), 25 (2000) |
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| [8] | W. J. Wadsworth et al., "Supercontinuum generation in photonic crystal fibers and optical fiber tapers: a novel light source", J. Opt. Soc. Am. B 19 (9), 2148 (2002) |
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| [13] | S. G. Leon-Saval et al., "Supercontinuum generation in submicron fibre waveguides", Opt. Express 12 (13), 2864 (2004) |
| [14] | B. Schenkel et al., "Pulse compression with supercontinuum generation in microstructure fibers", J. Opt. Soc. Am. B 22 (3), 687 (2005) |
| [15] | F. Vanholsbeeck et al., "The role of pump incoherence in continuous-wave supercontinuum generation", Opt. Express 13 (17), 6615 (2005) |
| [16] | C. Xia et al., "Mid-infrared supercontinuum generation to 4.5 μm in ZBLAN fluoride fibers by nanosecond diode pumping", Opt. Lett. 31 (17), 2553 (2006) |
| [17] | J. Dudley et al., "Supercontinuum generation in photonic crystal fiber", Rev. Mod. Phys. 78, 1135 (2006) |
| [18] | J. H. V. Price et al., "Mid-IR supercontinuum generation from nonsilica microstructured optical fibers" (contains numerical simulations with RP ProPulse), IEEE J. Sel. Top. Quantum Electron. 13 (3), 738 (2007) |
| [19] | A. V. Gorbach and D. V. Skryabin, "Light trapping in gravity-like potentials and expansion of supercontinuum spectra in photonic-crystal fibres", Nat. Photonics 1, 653 (2007) |
| [20] | D. R. Solli et al., "Optical rogue waves", Nature 450, 1054 (2007) |
| [21] | R. R. Alfano, "The Supercontinuum Laser Source", Springer, New York (1989) |
(For additional references, see the articles on frequency combs and frequency metrology.)
See also: coherence, amplified spontaneous emission, photonic crystal fibers, tapered fibers, frequency combs, frequency metrology, spectrograms
This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics Consulting GmbH. Contact this distinguished expert in laser technology, nonlinear optics and fiber optics, and find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, or staff training) could become very valuable for your business!
