Editing Optical isolator (section)

==Theory==
{{main|Faraday effect}}
[[File:Isolator diagram.GIF|thumbnail|right|200px|Figure 1: The optical circuit symbol for an isolator]]
[[File:Faraday-effect.svg|thumb|Faraday effect]]
The main component of the optical isolator is the Faraday rotator. The magnetic field, '''<math>B</math>''', applied to the Faraday rotator causes a rotation in the polarization of the light due to the Faraday effect. The angle of rotation, <math>\beta</math>, is given by,
:<math>\beta=\nu B d\,</math>,
where, <math>\nu</math> is the [[Verdet constant]] of the material<ref>{{cite journal |last1=Vojna |first1=David |last2=Slezák |first2=Ondřej |last3=Lucianetti |first3=Antonio |last4=Mocek |first4=Tomáš |title=Verdet Constant of Magneto-Active Materials Developed for High-Power Faraday Devices |journal=Applied Sciences |date=2019 |volume=9 |issue=15 |page=3160 |doi=10.3390/app9153160 |doi-access=free }}</ref><ref>{{cite journal |last1=Vojna |first1=David |last2=Slezák |first2=Ondřej |last3=Yasuhara |first3=Ryo |last4=Furuse |first4=Hiroaki |last5=Lucianetti |first5=Antonio |last6=Mocek |first6=Tomáš |title=Faraday Rotation of Dy2O3, CeF3 and Y3Fe5O12 at the Mid-Infrared Wavelengths |journal=Materials |date=2020 |volume=13 |issue=23 |page=5324 |doi=10.3390/ma13235324 |pmid=33255447 |pmc=7727863 |bibcode=2020Mate...13.5324V |doi-access=free }}</ref><ref>{{cite journal |last1=Vojna |first1=David |last2=Duda |first2=Martin |last3=Yasuhara |first3=Ryo |last4=Slezák |first4=Ondřej |last5=Schlichting |first5=Wolfgang |last6=Stevens |first6=Kevin |last7=Chen |first7=Hengjun |last8=Lucianetti |first8=Antonio |last9=Mocek |first9=Tomáš |title=Verdet constant of potassium terbium fluoride crystal as a function of wavelength and temperature |journal=Opt. Lett. |date=2020 |volume=45 |issue=7 |pages=1683–1686 |doi=10.1364/ol.387911 |pmid=32235973 |bibcode=2020OptL...45.1683V |s2cid=213599420 |url=https://www.osapublishing.org/ol/fulltext.cfm?uri=ol-45-7-1683&id=429076 |access-date=2021-04-10 |archive-date=2021-03-05 |archive-url=https://web.archive.org/web/20210305232749/https://www.osapublishing.org/ol/fulltext.cfm?uri=ol-45-7-1683&id=429076 |url-status=live |url-access=subscription }}</ref> (amorphous or crystalline solid, or liquid, or crystalline liquid, or vaprous, or gaseous) of which the rotator is made, and <math>d</math> is the length of the rotator. This is shown in Figure 2. Specifically for an optical isolator, the values are chosen to give a rotation of 45°.

It has been shown that a crucial requirement for any kind of optical isolator (not only the Faraday isolator) is some kind of non-[[Reciprocity (electromagnetism)|reciprocal]] optics <ref>{{cite journal|last1=Jalas|first1=Dirk|last2=Petrov|first2=Alexander|last3=Eich|first3=Manfred|last4=Freude|first4=Wolfgang|last5=Fan|first5=Shanhui|last6=Yu|first6=Zongfu|last7=Baets|first7=Roel|last8=Popović|first8=Miloš|last9=Melloni|first9=Andrea|last10=Joannopoulos|first10=John D.|last11=Vanwolleghem|first11=Mathias|last12=Doerr|first12=Christopher R.|last13=Renner|first13=Hagen|title=What is — and what is not — an optical isolator|journal=Nature Photonics|date=29 July 2013|volume=7|issue=8|pages=579–582|doi=10.1038/nphoton.2013.185|bibcode=2013NaPho...7..579J}}</ref>