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Modulation instability only happens under certain circumstances. The most important condition is ''anomalous group velocity [[dispersion relation|dispersion]]'', whereby pulses with shorter [[wavelength]]s travel with higher [[group velocity]] than pulses with longer wavelength.<ref name="agrawal" /> (This condition assumes a ''focusing'' [[Kerr nonlinearity]], whereby refractive index increases with optical intensity.)<ref name="agrawal" />
The instability is strongly dependent on the frequency of the perturbation. At certain frequencies, a perturbation will have little effect,
The tendency of a perturbing signal to grow makes modulation instability a form of [[amplifier|amplification]]. By tuning an input signal to a peak of the gain spectrum, it is possible to create an [[optical amplifier]].
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== Modulation instability in soft systems ==
Modulation instability of optical fields has been observed in photo-chemical systems, namely, photopolymerizable medium.<ref>{{Cite journal|last1=Burgess|first1=Ian B.|last2=Shimmell|first2=Whitney E.|last3=Saravanamuttu|first3=Kalaichelvi|date=2007-04-01|title=Spontaneous Pattern Formation Due to Modulation Instability of Incoherent White Light in a Photopolymerizable Medium|journal=Journal of the American Chemical Society|volume=129|issue=15|pages=4738–4746|doi=10.1021/ja068967b|pmid=17378567|bibcode=2007JAChS.129.4738B |issn=0002-7863}}</ref><ref>{{Cite journal|last1=Basker|first1=Dinesh K.|last2=Brook|first2=Michael A.|last3=Saravanamuttu|first3=Kalaichelvi|title=Spontaneous Emergence of Nonlinear Light Waves and Self-Inscribed Waveguide Microstructure during the Cationic Polymerization of Epoxides|journal=The Journal of Physical Chemistry C|language=en|volume=119|issue=35|pages=20606–20617|doi=10.1021/acs.jpcc.5b07117|year=2015}}</ref><ref>{{Cite journal|last1=Biria|first1=Saeid|last2=Malley|first2=Philip P. A.|last3=Kahan|first3=Tara F.|last4=Hosein|first4=Ian D.|date=2016-03-03|title=Tunable Nonlinear Optical Pattern Formation and Microstructure in Cross-Linking Acrylate Systems during Free-Radical Polymerization|journal=The Journal of Physical Chemistry C|volume=120|issue=8|pages=4517–4528|doi=10.1021/acs.jpcc.5b11377|issn=1932-7447}}</ref><ref>{{Cite journal|last1=Biria|first1=Saeid|last2=Malley|first2=Phillip P. A.|last3=Kahan|first3=Tara F.|last4=Hosein|first4=Ian D.|date=2016-11-15|title=Optical Autocatalysis Establishes Novel Spatial Dynamics in Phase Separation of Polymer Blends during Photocuring|journal=ACS Macro Letters|volume=5|issue=11|pages=1237–1241|doi=10.1021/acsmacrolett.6b00659|pmid=35614732 }}</ref> Modulation instability occurs owing to inherent optical nonlinearity of the systems due to photoreaction-induced changes in the refractive index.<ref>{{Cite journal|last1=Kewitsch|first1=Anthony S.|last2=Yariv|first2=Amnon|date=1996-01-01|title=Self-focusing and self-trapping of optical beams upon photopolymerization|journal=Optics Letters|language=EN|volume=21|issue=1|pages=24–6|doi=10.1364/ol.21.000024|issn=1539-4794|bibcode=1996OptL...21...24K|url=https://authors.library.caltech.edu/2845/1/KEWol96.pdf|pmid=19865292}}</ref> Modulation instability of spatially and temporally incoherent light is possible owing to the non-instantaneous response of photoreactive systems, which consequently responds to the time-average intensity of light, in which the femto-second fluctuations cancel out.<ref>{{Cite book|url=https://www.springer.com/us/book/9783540416531|title=Spatial Solitons {{!}} Stefano Trillo {{!}} Springer|language=en}}</ref>
==References==
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