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| journal = Annual Review of Fluid Mechanics
| year = 1980
|bibcode = 1980AnRFM..12..303Y }}</ref> Therefore, it is also known as the '''Benjamin−Feir instability'''. However, spatial modulation instability of high-power lasers in organic solvents was observed by Russian scientists N. F. Piliptetskii and A. R. Rustamov in 1965,<ref>{{Cite journal|last1=Piliptetskii|first1=N. F.|last2=Rustamov|first2=A. R.|date=31 May 1965|title=Observation of Self-focusing of Light in Liquids|url=http://www.jetpletters.ac.ru/ps/1596/article_24469.shtml|journal=JETP Letters|volume=2|issue=2|pages=55–56}}</ref> and the mathematical derivation of modulation instability was published by V. I. Bespalov and V. I. Talanov in 1966.<ref>{{Cite journal|last1=Bespalov|first1=V. I.|last2=Talanov|first2=V. I.|date=15 June 1966|title=Filamentary Structure of Light Beams in Nonlinear Liquids|url=http://www.jetpletters.ac.ru/ps/1621/article_24803.shtml|journal=ZhETF Pisma Redaktsiiu|volume=3|issue=11|pages=471–476|bibcode=1966ZhPmR...3..471B|access-date=17 February 2021|archive-date=31 July 2020|archive-url=https://web.archive.org/web/20200731112029/http://www.jetpletters.ac.ru/ps/1621/article_24803.shtml|url-status=dead}}</ref> Modulation instability is a possible mechanism for the generation of [[rogue wave]]s.<ref>{{Cite journal
| doi = 10.1175/1520-0485(2003)33<863:NFIAFW>2.0.CO;2
| volume = 33
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| journal = Journal of Physical Oceanography
| year = 2003
|bibcode = 2003JPO....33..863J | doi-access = free
}}</ref><ref>{{Cite journal | doi = 10.1146/annurev.fluid.40.111406.102203
| volume = 40
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==Initial instability and gain==
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 ''
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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===Mathematical derivation of gain spectrum===
The gain spectrum can be derived <ref name="agrawal" /> by starting with a model of modulation instability based upon the [[nonlinear Schrödinger equation]]{{what|reason=Time and space reversed?|date=February 2024}}
: <math>\frac{\partial A}{\partial z} + i\beta_2\frac{\partial^2A}{\partial t^2} = i\gamma|A|^2A,</math>
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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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