= Single-mode and multi-mode optical fiber
= Single-mode and multi-mode fiber
{synonym}
From a practical point of view <Single-mode optical fiber>[single-mode]:
* upside: can go further without a <#repeater>. In <multi-mode optical fiber>, different modes travel at different speeds, and start interfering with each other at some point
* downside: lower <#bandwitdh>, because we can fit less modes into it
As such, typical applications are:
* <single-mode optical fiber>: longer distance communications across buildings and cities
* <multi-mode optical fiber>: shorter distance communications e.g. within a single <#data center>
Then there are some more hardcore threads actually pondering about specific cost trade-offs:
* https://www.reddit.com/r/networking/comments/n2tgeu/why_use_multimode_fiber_when_you_can_use/
* https://www.reddit.com/r/networking/comments/ucm55k/single_mode_vs_multimode_fiber/
From a mathematical point of view:
* multi-mode: https://en.wikipedia.org/w/index.php?title=Optical_fiber&oldid=1229833804#Multi-mode_fiber[]:
> Fiber with large core diameter (greater than 10 micrometers) may be analyzed by <#geometrical optics>. Such fiber is called <multi-mode fiber>. In a step-index multi-mode fiber, rays of light are guided along the fiber core by <#total internal reflection>.
\Image[https://upload.wikimedia.org/wikipedia/commons/4/46/Optical-fibre.svg]
* single-mode: https://en.wikipedia.org/w/index.php?title=Optical_fiber&oldid=1229833804#Single-mode_fiber[]:
> Fiber with a core diameter less than about ten times the wavelength of the propagating light cannot be modeled using geometric optics. Instead, it must be analyzed as an electromagnetic waveguide structure, according to Maxwell's equations as reduced to the electromagnetic wave equation. As an optical waveguide, the fiber supports one or more confined transverse modes by which light can propagate along the fiber. Fiber supporting only one mode is called single-mode.
\Image[https://upload.wikimedia.org/wikipedia/commons/f/f3/Optical_fibres_modes_vs_wavelength.gif]
Another difference is that <single-mode fiber> usually uses <lasers> as the light soruce, while <multi-mode fiber> usually uses <LED>:
* https://www.quora.com/Do-fiber-optics-use-lasers-Why
* https://www.reddit.com/r/sysadmin/comments/7yzphi/multimode_beam_is_it_led_or_laser/
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