In fiber-optic communication, a single-mode optical fiber is an optical fiber designed
to carry light only directly down the fibre - 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 fibers, 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. Although the ray travels parallel to
the length of the fiber, it is often called transverse mode since its electromagnetic
vibrations occur perpendicular to the length of the fiber. The 2009 Nobel Prize in Physics
was awarded to Charles K. Kao for his theoretical work on the single-mode optical fiber.
History Professor Huang Hongjia of the Chinese Academy
of Sciences, developed coupling wave theory in the field of microwave theory. He led a
research team that successfully developed Single-mode optical fiber in 1980.
Characteristics Like multi-mode optical fibers, single mode
fibers do exhibit modal dispersion resulting from multiple spatial modes but with narrower
modal dispersion. Single mode fibers are therefore better at retaining the fidelity of each light
pulse over longer distances than multi-mode fibers. For these reasons, single-mode fibers
can have a higher bandwidth than multi-mode fibers. Equipment for single mode fiber is
more expensive than equipment for multi-mode optical fiber, but the single mode fiber itself
is usually cheaper in bulk. A typical single mode optical fiber has a
core diameter between 8 and 10.5 µm and a cladding diameter of 125 µm. There are a
number of special types of single-mode optical fiber which have been chemically or physically
altered to give special properties, such as dispersion-shifted fiber and nonzero dispersion-shifted
fiber. Data rates are limited by polarization mode dispersion and chromatic dispersion.
As of 2005, data rates of up to 10 gigabits per second were possible at distances of over
80 km with commercially available transceivers. By using optical amplifiers and dispersion-compensating
devices, state-of-the-art DWDM optical systems can span thousands of kilometers at 10 Gbit/s,
and several hundred kilometers at 40 Gbit/s. The lowest-order bounds mode is ascertained
for the wavelength of interest by solving Maxwell's equations for the boundary conditions
imposed by the fiber, which are determined by the core diameter and the refractive indices
of the core and cladding. The solution of Maxwell's equations for the lowest order bound
mode will permit a pair of orthogonally polarized fields in the fiber, and this is the usual
case in a communication fiber. In step-index guides, single-mode operation
occurs when the normalized frequency, V, is less than or equal to 2.405. For power-law
profiles, single-mode operation occurs for a normalized frequency, V, less than approximately
, where g is the profile parameter.
In practice, the orthogonal polarizations may not be associated with degenerate modes.
OS1 and OS2 are standard single-mode optical fiber used with wavelengths 1310 nm and 1550 nm.
OS1 is defined in ISO/IEC 11801, and OS2 is defined in ISO/IEC 24702.
Connectors Optical fiber connectors are used to join
optical fibers where a connect/disconnect capability is required. The basic connector
unit is a connector assembly. A connector assembly consists of an adapter and two connector
plugs. Due to the sophisticated polishing and tuning procedures that may be incorporated
into optical connector manufacturing, connectors are generally assembled onto optical fiber
in a supplier’s manufacturing facility. However, the assembly and polishing operations
involved can be performed in the field, for example to make cross-connect jumpers to size.
Optical fiber connectors are used in telephone company central offices, at installations
on customer premises, and in outside plant applications. Their uses include:
Making the connection between equipment and the telephone plant in the central office
Connecting fibers to remote and outside plant electronics such as Optical Network Units
and Digital Loop Carrier systems Optical cross connects in the central office
Patching panels in the outside plant to provide architectural flexibility and to interconnect
fibers belonging to different service providers Connecting couplers, splitters, and Wavelength
Division Multiplexers to optical fibers Connecting optical test equipment to fibers
for testing and maintenance. Outside plant applications may involve locating
connectors underground in subsurface enclosures that may be subject to flooding, on outdoor
walls, or on utility poles. The closures that enclose them may be hermetic, or may be “free-breathing.”
Hermetic closures will prevent subjection of the connectors within to temperature swings
unless they are breached. Free-breathing enclosures will subject them to temperature and humidity
swings, and possibly to condensation and biological action from airborne bacteria, insects, etc.
Connectors in the underground plant may be subjected to groundwater immersion if the
closures containing them are breached or improperly assembled.
The latest industry requirements for optical fiber connectors are in Telcordia GR-326,
Generic Requirements for Singlemode Optical Connectors and Jumper Assemblies.
A multi-fiber optical connector is designed to simultaneously join multiple optical fibers
together, with each optical fiber being joined to only one other optical fiber.
The last part of the definition is included so as not to confuse multi-fiber connectors
with a branching component, such as a coupler. The latter joins one optical fiber to two
or more other optical fibers. Multi-fiber optical connectors are designed
to be used wherever quick and/or repetitive connects and disconnects of a group of fibers
are needed. Applications include telecommunications companies’ Central Offices, installations
on customer premises, and Outside Plant applications. The multi-fiber optical connector can be used
in the creation of a low-cost switch for use in fiber optical testing. Another application
is in cables delivered to a user with pre-terminated multi-fiber jumpers. This would reduce the
need for field splicing, which could greatly reduce the amount of hours necessary for placing
an optical fiber cable in a telecommunications network. This, in turn, would result in savings
for the installer of such cable. Industry requirements for multi-fiber optical
connectors are covered in GR-1435, Generic Requirements for Multi-Fiber Optical Connectors.
Fiber Optic Switches An optical switch is a component with two
or more ports that selectively transmits, redirects, or blocks an optical signal in
a transmission medium. According to Telcordia GR-1073, an optical switch must be actuated
to select or change between states. The actuating signal is usually electrical, but in principle,
could be optical or mechanical. See Optical switch for more detailed information.
See also Optical waveguide
Multi-mode optical fiber Graded-index fiber
Notes ^ Nobel Prize Citation http:www.nobelprize.orgphysics2009/kao-facts.html
^ Hongjia Huang. Microwave approach to highly-irregular fiber optics. Wiley. ISBN 978-0-471-31023-5.
Retrieved 12 January 2011. ^ "Hong-Jia HUANG". The Electromagnetics Academy
of Zhejiang Univserity ^ Hongjia Huang. Coupled mode theory: As applied
to microwave and optical transmission. VSP. ISBN 978-90-6764-033-6
^ ARC Electronics. "Fiber Optic Cable Tutorial". ^ ISO/IEC 11801:2002, Information technology
-- Generic cabling for customer premises. ^ ISO/IEC 24702:2006, Information technology
-- Generic cabling -- Industrial premises ^ GR-1073-CORE, Generic Requirements for Single-mode
Fiber Optic Switches, Telcordia. References
This article incorporates public domain material from the General Services Administration
document "Federal Standard 1037C". "Types of Optical Fiber". 2005-14-14.
External links The Basics of Fiber Optic Cable