Why can single-mode fiber support more bandwith than multimode fiber?

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Clarification of Question by dantemm-ga on 03 Feb 2004 10:42 PST

Please do not respond with something along the lines of ?because more
precise optical transceivers are used?. Assume that all things are
equal except the fiber itself.

Dear dantemm,

The reason for the higher bandwidth of single mode fibers in
comparison with multimode fibers lies within the concepts behind each
of those fiber types.

Multimode fibers have a relatively large core. The large diameter of
the core allows multiple paths for the light to flow. This means that
some light travels straight down the fiber while other light bounces
back and forth from the sides of the fiber. As a result, the modes of
a light pulse that enter the fiber at one time exit the fiber a
different times. This effect, known as "modal dispersion", delays the
transmission; light pulses tend to spread more as the length of the
cable and increases modal dispersion which limits its bandwidth or
data rate capabilities. However, the relatively large core of a
multi-mode fiber allows good coupling from inexpensive LED light
sources, and the use of inexpensive couplers and connectors.

Single mode fiber, in contrast,  has a core diameter that is so small
(on the order of 8 to 10 microns) that only one single lightwave ray
can flow straight down the fiber, so there is no modal dispersion.
Therefore single mode fiber has a much higher bandwidth and data rate
capability. Because modal dispersion and the resulting delays in
transmitting are completely eliminated, this fiber has much greater
bandwidths which can exceed several hundred gigahertz-kilometer
(GHz-km). A drawback to single-mode fiber is that, because of its
small size, it is more demanding to work with when installing
connectors and splicing.


If you would like to learn more about the modal dispersion effect in
multimode fibers and how it is avoided in single mode fibers, you may
want to read the following sources I used:

Symbex: Fiber Optics Technology Overview
http://www.symbex.com.au/techover.htm

Professional Academy of Stuttgart: Fiber Distributed Data Interface
http://www.ba-stuttgart.de/~schulte/htme/55773.htm

Belden.com: Fiber Optics - Bandwidth
http://bwcecom.belden.com/Catalog/TechInfo/TechBandwidth.htm

Graybar: Frequently Asked Questions (*.doc file)
http://www.graybar.com/automation/ga_manuals/Phoenix%20Contact/FAQ%20-%20100Base%20T%20fiber%20converters.doc

Integrated Publishing: Fiber Optics - Dispersion
http://www.tpub.com/neets/tm/106-15.htm

Integrated Publishing: Fiber Optics - Bandwidth
http://www.tpub.com/neets/tm/109-5.htm

The Optical Networks Project: Fiber Optic Cable Design
http://www.ari.vt.edu/ece5516/opticalnetworks/cable_design.htm


Search terms used:
"single mode fiber" "multimode fiber" bandwidth dispersion
://www.google.de/search?hl=de&ie=UTF-8&oe=UTF-8&newwindow=1&q=%22single+mode+fiber%22+%22multimode+fiber%22+bandwidth+dispersion&btnG=Google+Suche
bandwidth "modal dispersion" fiber multimode "single mode"
://www.google.de/search?q=bandwidth+%22modal+dispersion%22+fiber+multimode+%22single+mode%22&hl=de&lr=&ie=UTF-8&oe=UTF-8&newwindow=1&start=0&sa=N
"single mode fiber" "multimode fiber" higher bandwidth
://www.google.de/search?hl=de&ie=UTF-8&oe=UTF-8&newwindow=1&q=%22single+mode+fiber%22+%22multimode+fiber%22+higher+bandwidth&btnG=Google+Suche

Hope this answers your question!
Regards,
Scriptor

Quick and excellent. Thank You.