Multi-mode optical fiber is a type of
optical fiber
An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means to ...
mostly used for communication over short distances, such as within a building or on a campus. Multi-mode links can be used for data rates up to 100 Gbit/s. Multi-mode fiber has a fairly
large core diameter that enables multiple light
modes
Mode ( la, modus meaning "manner, tune, measure, due measure, rhythm, melody") may refer to:
Arts and entertainment
* '' MO''D''E (magazine)'', a defunct U.S. women's fashion magazine
* ''Mode'' magazine, a fictional fashion magazine which is ...
to be propagated and limits the maximum length of a transmission link because of
modal dispersion
Modal dispersion is a distortion mechanism occurring in multimode fibers and other waveguides, in which the signal is spread in time because the propagation velocity of the optical signal is not the same for all modes. Other names for this phenom ...
. The standard
G.651.1
G.651.1 is an international standard developed by the Standardization Sector of the International Telecommunication Union (ITU-T) that specifies multi-mode optical fiber
Multi-mode optical fiber is a type of optical fiber mostly used for ...
defines the most widely used forms of multi-mode optical fiber.
Applications
The equipment used for communications over multi-mode optical fiber is less expensive than that for
single-mode optical fiber
In fiber-optic communication, a single-mode optical fiber (SMF), also known as fundamental- or mono-mode, is an optical fiber designed to carry only a single mode of light - the transverse mode. Modes are the possible solutions of the Helmholtz ...
. Typical transmission speed and distance limits are 100 Mbit/s for distances up to 2 km (
100BASE-FX
In computer networking, Fast Ethernet physical layers carry traffic at the nominal rate of 100 Mbit/s. The prior Ethernet speed was 10 Mbit/s. Of the Fast Ethernet physical layers, 100BASE-TX is by far the most common.
Fast Ethern ...
), 1 Gbit/s up to 1000 m, and 10 Gbit/s up to 550 m.
Because of its high capacity and reliability, multi-mode optical fiber generally is used for backbone applications in buildings. An increasing number of users are taking the benefits of fiber closer to the user by running fiber to the desktop or to the zone. Standards-compliant architectures such as Centralized Cabling and
fiber to the telecom enclosure
Fiber to the telecom enclosure (FTTTE), also sometimes called fiber to the zone (FTTZ), or fibre to the cabinet (FTTC) in the UK, is a standards-compliant structured cabling system architecture that extends the optical fiber backbone network ...
offer users the ability to leverage the distance capabilities of fiber by centralizing electronics in telecommunications rooms, rather than having active electronics on each floor.
Multi-mode fiber is used for transporting light signals to and from miniature fiber optic spectroscopy equipment (spectrometers, sources, and sampling accessories) and was instrumental in the development of the first portable spectrometer.
Multi-mode fiber is also used when high optical powers are to be carried through an optical fiber, such as in
laser welding
Laser beam welding (LBW) is a welding technique used to join pieces of metal or thermoplastics through the use of a laser. The beam provides a concentrated heat source, allowing for narrow, deep welds and high welding rates. The process is frequen ...
.
Comparison with single-mode fiber
![Optical fibres modes vs wavelength](https://upload.wikimedia.org/wikipedia/commons/f/f3/Optical_fibres_modes_vs_wavelength.gif)
The main difference between multi-mode and
single-mode optical fiber
In fiber-optic communication, a single-mode optical fiber (SMF), also known as fundamental- or mono-mode, is an optical fiber designed to carry only a single mode of light - the transverse mode. Modes are the possible solutions of the Helmholtz ...
is that the former has much larger core diameter, typically 50–100 micrometers; much larger than the wavelength of the light carried in it. Because of the large core and also the possibility of large
numerical aperture, multi-mode fiber has higher "light-gathering" capacity than single-mode fiber. In practical terms, the larger
core
Core or cores may refer to:
Science and technology
* Core (anatomy), everything except the appendages
* Core (manufacturing), used in casting and molding
* Core (optical fiber), the signal-carrying portion of an optical fiber
* Core, the centra ...
size simplifies connections and also allows the use of lower-cost electronics such as
light-emitting diodes
A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (cor ...
(LEDs) and
vertical-cavity surface-emitting laser
The vertical-cavity surface-emitting laser, or VCSEL , is a type of semiconductor laser diode with laser beam emission perpendicular from the top surface, contrary to conventional edge-emitting semiconductor lasers (also ''in-plane'' lasers) which ...
s (VCSELs) which operate at the 850
nm and 1300 nm wavelength (single-mode fibers used in telecommunications typically operate at 1310 or 1550 nm
). However, compared to single-mode fibers, the multi-mode fiber
bandwidth–distance product limit is lower. Because multi-mode fiber has a larger core-size than single-mode fiber, it supports more than one
propagation mode
Radio propagation is the behavior of radio waves as they travel, or are propagated, from one point to another in vacuum, or into various parts of the atmosphere.
As a form of electromagnetic radiation, like light waves, radio waves are affected ...
; hence it is limited by
modal dispersion
Modal dispersion is a distortion mechanism occurring in multimode fibers and other waveguides, in which the signal is spread in time because the propagation velocity of the optical signal is not the same for all modes. Other names for this phenom ...
, while single mode is not.
The LED light sources sometimes used with multi-mode fiber produce a range of wavelengths and these each propagate at different speeds. This
chromatic dispersion
In optics, and by analogy other branches of physics dealing with wave propagation, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency; sometimes the term chromatic dispersion is used for specificity to o ...
is another limit to the useful length for multi-mode fiber optic cable. In contrast, the lasers used to drive single-mode fibers produce
coherent light
In physics, two wave sources are coherent if their frequency and waveform are identical. Coherence is an ideal property of waves that enables stationary (i.e., temporally or spatially constant) interference. It contains several distinct concepts ...
of a single wavelength. Because of the modal dispersion, multi-mode fiber has higher pulse spreading rates than single mode fiber, limiting multi-mode fiber's information transmission capacity.
Single-mode fibers are often used in high-precision scientific research because restricting the light to only one propagation mode allows it to be focused to an intense,
diffraction-limited
The resolution of an optical imaging system a microscope, telescope, or camera can be limited by factors such as imperfections in the lenses or misalignment. However, there is a principal limit to the resolution of any optical system, due to t ...
spot.
Jacket color is sometimes used to distinguish multi-mode
cables from single-mode ones. The standard TIA-598C recommends, for non-military applications, the use of a yellow jacket for single-mode fiber, and orange or aqua for multi-mode fiber, depending on type.
Some vendors use violet to distinguish higher performance OM4 communications fiber from other types.
Types
Multi-mode fibers are described by their core and
cladding
Cladding is an outer layer of material covering another. It may refer to the following:
*Cladding (boiler), the layer of insulation and outer wrapping around a boiler shell
*Cladding (construction), materials applied to the exterior of buildings
...
diameters. Thus, 62.5/125 μm multi-mode fiber has a core size of 62.5 micrometres (μm) and a cladding diameter of 125 μm. The transition between the core and cladding can be sharp, which is called a
step-index profile
For an optical fiber, a step-index profile is a refractive index profile characterized by a uniform refractive index within the core and a sharp decrease in refractive index at the core- cladding interface so that the cladding is of a lower refrac ...
, or a gradual transition, which is called a
graded-index profile
A graded-index or gradient-index fiber is an optical fiber whose core has a refractive index that decreases with increasing radial distance from the optical axis of the fiber.
Because parts of the core closer to the fiber axis have a higher ref ...
. The two types have different dispersion characteristics and thus different effective propagation distance. Multi-mode fibers may be constructed with either
graded or
step-index profile
For an optical fiber, a step-index profile is a refractive index profile characterized by a uniform refractive index within the core and a sharp decrease in refractive index at the core- cladding interface so that the cladding is of a lower refrac ...
.
In addition, multi-mode fibers are described using a system of classification determined by the
ISO 11801 standard — OM1, OM2, and OM3 — which is based on the
modal bandwidth
Modal bandwidth, in the discipline of telecommunications
Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of ...
of the multi-mode fiber. OM4 (defined in TIA-492-AAAD) was finalized in August 2009, and was published by the end of 2009 by the
TIA
TIA or Tia may refer to:
Aviation
* Tampa International Airport, US, IATA code TPA
* Texas International Airlines, US, ICAO code
* Tirana International Airport Nënë Tereza, Albania, IATA code
* Trans International Airlines, former U.S. airl ...
. OM4 cable will support 125m links at 40 and 100 Gbit/s. The letters "OM" stand for ''optical multi-mode''.
For many years 62.5/125 μm (OM1) and conventional 50/125 μm multi-mode fiber (OM2) were widely deployed in premises applications. These fibers easily support applications ranging from
Ethernet
Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...
(10 Mbit/s) to
gigabit Ethernet
In computer networking, Gigabit Ethernet (GbE or 1 GigE) is the term applied to transmitting Ethernet frames at a rate of a gigabit per second. The most popular variant, 1000BASE-T, is defined by the IEEE 802.3ab standard. It came into use i ...
(1 Gbit/s) and, because of their relatively large core size, were ideal for use with LED transmitters. Newer deployments often use laser-optimized 50/125 μm multi-mode fiber (OM3). Fibers that meet this designation provide sufficient bandwidth to support
10 Gigabit Ethernet up to 300 meters. Optical fiber manufacturers have greatly refined their manufacturing process since that standard was issued and cables can be made that support 10 GbE up to 400 meters. Laser optimized multi-mode fiber (LOMMF) is designed for use with 850 nm VCSELs.
Older FDDI grade, OM1, and OM2 fiber can be used for 10 Gigabit ethernet through 10GBASE-LRM. This requires the SFP+ interface to support electronic dispersion compensation (EDC) however, so not all switches, routers and other equipment can use these SFP+ modules.
The migration to LOMMF/OM3 has occurred as users upgrade to higher speed networks. LEDs have a maximum modulation rate of 622 Mbit/s because they cannot be turned on/off fast enough to support higher bandwidth applications. VCSELs are capable of modulation over 10 Gbit/s and are used in many high speed networks.
Some 200 and 400 Gigabit Ethernet speeds (e.g.
400GBASE-SR4.2) use
wavelength-division multiplexing (WDM) even for multi-mode fiber which is outside the specification for OM4 and lower. In 2017, OM5 has been standardized by TIA and ISO for WDM MMF, specifying not only a minimum modal bandwidth for 850 nm but a curve spanning from 850 to 953 nm.
Cables can sometimes be distinguished by jacket color: for 62.5/125 μm (OM1) and 50/125 μm (OM2), orange jackets are recommended, while
aqua
Aqua is the Latin word for water. It is used in many words which relate to water, such as aquatic life. In English, it may also refer to:
Arts
* Aqua (color), a greenish-blue color
Business
* Aqua (skyscraper), an 82-story residential skysc ...
is recommended for 50/125 μm "laser optimized" OM3 and OM4 fiber.
[ Some fiber vendors use violet for "OM4+". OM5 is officially colored ]lime green
Lime commonly refers to:
* Lime (fruit), a green citrus fruit
* Lime (material), inorganic materials containing calcium, usually calcium oxide or calcium hydroxide
* Lime (color), a color between yellow and green
Lime may also refer to:
Botany
...
.
VCSEL power profiles, along with variations in fiber uniformity, can cause modal dispersion which is measured by differential modal delay (DMD). Modal dispersion is caused by the different speeds of the individual modes in a light pulse. The net effect causes the light pulse to spread over distance, introducing intersymbol interference
In telecommunication, intersymbol interference (ISI) is a form of distortion of a signal in which one symbol interferes with subsequent symbols. This is an unwanted phenomenon as the previous symbols have a similar effect as noise, thus making ...
. The greater the length, the greater the modal dispersion. To combat modal dispersion, LOMMF is manufactured in a way that eliminates variations in the fiber which could affect the speed that a light pulse can travel. The refractive index profile is enhanced for VCSEL transmission and to prevent pulse spreading. As a result, the fibers maintain signal integrity over longer distances, thereby maximizing the bandwidth.
Comparison
Encircled flux
The IEC 61280-4-1 (now TIA-526-14-B) standard defines ''encircled flux'' which specifies test light injection sizes (for various fiber diameters) to make sure the fiber core is not over-filled or under-filled to allow more reproducible (and less variable) link-loss measurements.
See also
*Fiber-optic communication
Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of infrared light through an optical fiber. The light is a form of carrier wave that is modulated to carry information. Fiber is pref ...
*Graded-index fiber
A graded-index or gradient-index fiber is an optical fiber whose core has a refractive index that decreases with increasing radial distance from the optical axis of the fiber.
Because parts of the core closer to the fiber axis have a higher re ...
*ISO/IEC 11801
International standard ISO/IEC 11801 ''Information technology — Generic cabling for customer premises'' specifies general-purpose telecommunication cabling systems (structured cabling) that are suitable for a wide range of applications (analog an ...
* IEEE 802.3
*Optical fiber connector
An optical fiber connector joins optical fibers, and enables quicker connection and disconnection than splicing. The connectors mechanically couple and align the cores of fibers so light can pass. Better connectors lose very little light due ...
References
*
*
*
*
*
* {{cite web , url = http://www.hp.com/rnd/pdfs/100FXtechbrief.pdf , title = 100BASE-FX Technical Brief , author = Hewlett-Packard Development Company, L.P. , year = 2007 , access-date = Nov 20, 2012 , url-status = dead , archive-url = https://web.archive.org/web/20121009212203/http://www.hp.com/rnd/pdfs/100FXtechbrief.pdf , archive-date = 2012-10-09
External links
Optics: Single mode fiber , MIT Video Demonstrations in Lasers and Optics
Optics: Multi-mode fiber , MIT Video Demonstrations in Lasers and Optics
Optical fiber