What is the major advantage of a single mode
optical fiber？
Single
Mode Fiber is an optical fiber that can only transmit one mode at a specified
wavelength. The central glass core is very thin (the core diameter is generally
9 or 10 μm), so its inter-mode dispersion is small, suitable for remote
communication, but it still exists due to material dispersion and waveguide
dispersion, single-mode fibers have higher requirements on the spectral width
and stability of the light source, that is, the spectral width should be narrow
and the stability should be better. Later, it was discovered that at the
wavelength of 1.31μm, the material dispersion and waveguide dispersion of the
single-mode fiber were positive and negative, and the magnitudes were exactly
the same. In this way, the 1.31μm wavelength region has become a very ideal
working window for optical fiber communication, and it is also the main working
band of practical optical fiber communication systems. The main parameters of
1.31μm conventional single-mode optical fiber are recommended by the
International Telecommunication Union ITU-T in G652. Certainly, so this fiber
is also called G652 fiber. Single-mode fiber can be divided into 652
single-mode fiber, 653 single-mode fiber and 655 single-mode fiber.
Interpretation
of "single-mode fiber" in academic literature: Generally, when V is
less than 2.405, only one wave peak passes through the fiber, so it is called
single-mode fiber. Its core is very thin, about 8-10 microns, and the mode
dispersion is very small. The main factors affecting the width of the optical
fiber transmission band are various dispersions, and the modal dispersion is
the most important. The dispersion of single-mode fibers is small, so it can
transmit light in a wide frequency band for a long distance.
Single-mode fiber has a
core diameter of 10 micron, which can allow single-mode beam transmission, and
can reduce the bandwidth and modal dispersion limitations. However, because the
core diameter of single-mode fiber is too small, it is difficult to control the
beam transmission, so it is necessary expensive lasers are used as the light
source. The main limitation of single-mode optical cables is material
dispersion. Single-mode optical cables mainly use lasers to obtain
high-frequency bandwidth. Since LEDs emit a large number of light sources with
different bandwidths, material dispersion requirements are very important.
Compared with multi-mode fiber, single-mode fiber can support longer
transmission distance. From 100Mbps Ethernet to 1G Gigabit network, single-mode
fiber can support transmission distance of more than 5000m from a cost
perspective. From the perspective of cost, since the optical transceiver is
very expensive, the cost of using single-mode fiber will be higher than that of
multi-mode fiber optic cable.
The refractive
index distribution is similar to that of abrupt fiber, the core diameter is
only 8~10μm, and the light propagates in the direction of the core axis in a
straight line. Because this fiber can only transmit one mode (the two
polarization states are degenerate), it is called a single-mode fiber, and its
signal distortion is very small.
Summary
The equipment for
single-mode fiber is relatively expensive, and the coupling loss is large. The
bending loss is large. But the advantage is that the transmission distance is
long.
Multimode fiber
has short transmission distance and low price of supporting equipment. The
coupling is relatively lossless. The bending loss is small.
So in the end, the
choice must be made according to the specific purpose. In the long-distance
backbone network, single-mode must be used.
However, if it is
in a local area network or a small metropolitan area network, multi-mode is
almost always used. Especially fiber to the home now also uses multi-mode
fiber.