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What is a single mode fiber?


Single-mode optical fiber (SMF) is an optical fiber designed to carry light only directly down the fiber - the transverse mode. Modes are the possible solutions of the Helmholtz equation for waves, which is obtained by combining Maxwell's equations and the boundary conditions. These modes define the way the wave travels through space, i.e. how the wave is distributed in space. Waves can have the same mode but have different frequencies. This is the case in single-mode fibres, where we can have waves with different frequencies, but of the same mode, which means that they are distributed in space in the same way, and that gives us a single ray of light.

Since there is only one mode, there is no problem with modal dispersion and the choice of core material can reduce chromatic dispersion (see below) which increases the bandwidth to almost infinity - but it's practically limited to about 100,000 gigahertz - that's still a lot! Single mode fiber has a core diameter of 8-10 microns, specified as "mode field diameter," the effective size of the core, and a cladding diameter of 125 microns.


Single mode fiber is commonly used for high-speed, long-distance application. They are used by many telecommunications companies to transmit telephone signals, Internet communication, and cable television signals. Due to much lower attenuation and interference, Single mode fiber has large advantages over existing copper wire in long-distance and high-demand applications.

Single-mode fiber

Single mode fiber types:

There are several designations used to describe various types of SM fiber that are often confusing. Here are the ones in common use today.



ITU Spec

TIA Spec

Standard Single mode Fiber




Cutoff Shifted Fiber




Low Water Peak Fiber




Dispersion Shifted Fiber




Non-Zero Dispersion Shifted Fiber



TIA-492E000 / TIA-492EA00

Bend-Insensitive Fiber


G .657


Single mode VS Multi-mode fiber

Single mode fiber has a small light carrying core of 8 to 10 microns in diameter. It is normally used for long distance transmissions with laser diode based fiber optic transmission equipment. Multimode fiber has a relatively large light carrying core, usually 62.5 microns or larger in diameter. It is usually used for short distance transmissions with LED based fiber optic equipment. 


The optical communication is now coming into its own and is becoming the preferred technology for many applications. In this way, integrated electronic circuits have defined the 20th century as the century of electronics, whilst the 21st century is emerging as the century of optoelectronics – as a result of light wave technology. The success of this evolution will lead to the development of new optoelectronic devices, a field where innovations are progressing at "light velocity".

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Type of fiber mode


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