Editing Transverse mode (section)

====Optical fibers====
{{see also|Equilibrium mode distribution|Mode volume|Cladding mode}}

The number of modes in an optical fiber distinguishes [[multi-mode optical fiber]] from [[single-mode optical fiber]].  To determine the number of modes in a step-index fiber, the [[Normalized frequency (fiber optics)|V number]] needs to be determined: <math display="inline">V = k_0 a \sqrt{n_1^2 - n_2^2}</math> where <math>k_0</math> is the [[wavenumber]], <math>a</math> is the fiber's core radius, and <math>n_1</math> and <math>n_2</math> are the [[Refractive index|refractive indices]] of the core and [[Cladding (fiber optics)|cladding]], respectively.  Fiber with a V-parameter of less than 2.405 only supports the fundamental mode (a hybrid mode), and is therefore a single-mode fiber whereas fiber with a higher V-parameter has multiple modes.<ref>{{cite web |publisher=Stanford University |work=EE 247: Introduction to Optical Fiber Communications, Lecture Notes |date=Sep 21, 2006 |first=Joseph M. |last=Kahn |title=Lecture 3: Wave Optics Description of Optical Fibers |page=8 |archive-url=https://web.archive.org/web/20070614013059/http://eeclass.stanford.edu/cgi-bin/handouts.cgi?s=&t=1181774370&cc=ee247&action=handout_download&handout_id=ID112717331418248&viewfile=lecture_3.pdf |url=http://eeclass.stanford.edu/cgi-bin/handouts.cgi?s=&t=1181774370&cc=ee247&action=handout_download&handout_id=ID112717331418248&viewfile=lecture_3.pdf |archive-date=June 14, 2007 |access-date=27 Jan 2015}}</ref>

Decomposition of field distributions into modes is useful because a large number of field amplitudes readings can be simplified into a much smaller number of mode amplitudes.  Because these modes change over time according to a simple set of rules, it is also possible to anticipate future behavior of the field distribution.  These simplifications of complex field distributions ease the [[signal processing]] requirements of [[Optical communications#Optical fiber communication|fiber-optic communication]] systems.<ref>{{cite encyclopedia |title=Modes |url=https://www.rp-photonics.com/modes.html |encyclopedia=Encyclopedia of Laser Physics and Technology |publisher=RP Photonics |first=Rüdiger |last=Paschotta |access-date=Jan 26, 2015}}</ref>

The modes in typical low refractive index contrast fibers are usually referred to as ''LP'' (linear polarization) modes, which refers to a [[scalar (physics)|scalar]] approximation for the field solution, treating it as if it contains only one transverse field component.<ref>K. Okamoto, ''Fundamentals of Optical Waveguides'', pp. 71–79, Elsevier Academic Press, 2006, {{ISBN|0-12-525096-7}}.</ref>