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

The main difference between multi-mode and

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 In optics, the numerical aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light. By incorporating index of refraction in its definition, NA has the proper ...

, 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

, 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 ...

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 Cable may refer to: Mechanical * Nautical cable, an assembly of three or more ropes woven against the weave of the ropes, rendering it virtually waterproof * Wire rope, a type of rope that consists of several strands of metal wire laid into a hel ...

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 refracti ...

, 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

. 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, refers to the maximal signaling rate for a given distance or – the other way around – the maximal distance for a given signaling rate. The signaling rate can typically be measured in ...

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 10 Gigabit Ethernet (10GE, 10GbE, or 10 GigE) is a group of computer networking technologies for transmitting Ethernet frames at a rate of 10 gigabits per second. It was first defined by the IEEE 802.3ae-2002 standard. Unlike previous Eth ...

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 In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (i.e., colors) of laser light. This techniq ...

(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.

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 OpticsOptics: Multi-mode fiber , MIT Video Demonstrations in Lasers and Optics
Optical fiber