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Multi-mode optical fiber
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Multi-mode optical fiber is a type of optical fiber 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 800 Gbit/s. Multi-mode fiber has a fairly large core diameter that enables multiple light modes to be propagated and limits the maximum length of a transmission link because of modal dispersion. The standard G.651.1 defines the most widely used forms of multi-mode optical fiber.
Applications
[edit]The equipment used for communications over multi-mode optical fiber is less expensive than that for single-mode optical fiber.[1] Typical transmission speed and distance limits are 100 Mbit/s for distances up to 2 km (100BASE-FX), 1 Gbit/s up to 1000 m, and 10 Gbit/s up to 550 m.[2]
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 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.
Comparison with single-mode fiber
[edit]
Multi-mode optical fiber features a larger core diameter (typically 50–100 μm), allowing multiple light modes to propagate simultaneously.[3] This design simplifies alignment and installation, making MMF cost-effective and ideal for short- to medium-distance data transmission in enterprise networks, data centers, and campus environments.[4] MMF supports high data rates—up to 100 Gbps—over distances typically ranging from 300 to 550 meters, depending on fiber type (OM3, OM4, OM5).[5] Additionally, MMF can utilize lower-cost light sources such as light-emitting diodes (LEDs) and vertical-cavity surface-emitting lasers (VCSELs), reducing overall system cost while maintaining reliable performance.[6]
LEDs and VCSELs operate at the 850 nm and 1300 nm wavelength, whereas single-mode fibers used in telecommunications typically operate at 1310 or 1550 nm.[7] 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; hence it is limited by modal dispersion, 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 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 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 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.[8] Some vendors use violet to distinguish higher performance OM4 communications fiber from other types.[9]
Types
[edit]Multi-mode fibers are described by their core and cladding 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, or a gradual transition, which is called a graded-index profile. The two types have different dispersion characteristics and thus different effective propagation distances.[10] Multi-mode fibers may be constructed with either graded or step-index profile.[11]
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 of the multi-mode fiber. OM4 (defined in TIA-492-AAAD) was finalized in August 2009,[12] and was published by the end of 2009 by the TIA.[13] OM4 cable supports 125 m 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 (10 Mbit/s) to gigabit Ethernet (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 [citation needed] 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[14] 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 is recommended for 50/125 μm "laser optimized" OM3 and OM4 fiber.[8] Some fiber vendors use violet for "OM4+". OM5 is officially colored lime green.
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. 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
[edit]| Jacket color and category | Minimum modal bandwidth 850 / 953 / 1300 nm[b] |
Fast Ethernet 100BASE-FX | 1 Gb (1000 Mb) Ethernet 1000BASE-SX | 1 Gb (1000 Mb) Ethernet 1000BASE-LX | 10 Gb Ethernet 10GBASE-SR | 10 Gb Ethernet 10GBASE-LRM (requires EDC) | 25 Gb Ethernet 25GBASE-SR | 40 Gb Ethernet
40GBASE-SWDM4 |
40 Gb Ethernet 40GBASE-SR4 | 100 Gb Ethernet 100GBASE-SR10 | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| FDDI (62.5/125) | 160 / – / 500 MHz·km | 2000 m[15] | 220 m[16] | 550 m[17] (mode-conditioning patch cord required)[18][19] | 26 m[20] | 220 m[21] | Not supported | Not supported | Not supported | Not supported | |
| OM1 (62.5/125) | 200 / – / 500 MHz·km | 275 m[16] | 33 m[15] | 220 m | Not supported | Not supported | Not supported | Not supported | |||
| OM2 (50/125) | 500 / – / 500 MHz·km | 550 m[2] | 82 m[2] | 220 m | Not supported | Not supported | Not supported | Not supported | |||
| OM3 (50/125) *Laser Optimized* | 1500 / – / 500 MHz·km | 550 m (No mode-conditioning patch cord should be used.)[18] | 300 m[15] | 220 m | 70 m | 240m[22]
Duplex LC |
100 m[2]
(330 m QSFP+ eSR4[23]) |
100 m[2] | |||
| OM4 (50/125) *Laser Optimized* | 3500 / – / 500 MHz·km | 400 m[24] | >220 m | 100 m | 350m[22]
Duplex LC |
150 m[2]
(550 m QSFP+ eSR4[23]) |
150 m[2] | ||||
| OM5 (50/125) "Wideband multi-mode" for short-wave WDM[25] | 3500 / 1850 / 500 MHz·km | >220 m | 100 m | ||||||||
Encircled flux
[edit]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.[26]
See also
[edit]References
[edit]- ^ Telecommunications Industry Association. "Multimode Fiber for Enterprise Networks". Archived from the original on June 4, 2009. Retrieved June 4, 2008.
- ^ a b c d e f g Furukawa Electric North America. "OM4 - The next generation of multimode fiber" (PDF). Archived from the original (PDF) on April 22, 2014. Retrieved May 16, 2012.
- ^ "Multimode Fiber Optic Cable - Stanford Optics". www.stanfordoptics.com. Retrieved August 22, 2025.
- ^ "The FOA Reference For Fiber Optics - Optical Fiber". www.thefoa.org. Retrieved August 22, 2025.
- ^ "Digital communications". Open Learning. Retrieved August 22, 2025.
- ^ Pennsylvania Department of Homeland Security, Fiber Optic Data Communications, Retrieved 2025-08-22.
- ^ ARC Electronics (October 1, 2007). "Fiber Optic Cable Tutorial". Archived from the original on October 23, 2018. Retrieved March 4, 2015.
- ^ a b "Fiber optic cable color codes". Tech Topics. The Fiber Optic Association. Retrieved September 17, 2009.
- ^ Crawford, Dwayne (September 11, 2013). "Who is Erika Violet and what is she doing in my data center?". Tech Topics. Belden. Retrieved February 12, 2014.
- ^ British FibreOptic Industry Association. "Optical Fibers Explained" (PDF). Retrieved April 9, 2011.
- ^ "Fiber Optics Overview". Retrieved November 23, 2012.
- ^ "Meeting Report #14" (PDF). Telecommunications Industry Association.
- ^ Kish, Paul (January 1, 2010). "Next generation fiber arrives". # Cabling Networking Systems. Business Information Group.
- ^ IEEE 802.3 Clause 150
- ^ a b c Hewlett-Packard Development Company, L.P. (2007). "100BASE-FX Technical Brief" (PDF). Archived from the original (PDF) on October 9, 2012. Retrieved November 20, 2012.
- ^ a b IEEE 802.3-2012 Clause 38.3
- ^ IEEE 802.3 38.4 PMD to MDI optical specifications for 1000BASE-LX
- ^ a b Cisco Systems, Inc (2009). "Cisco Mode-Conditioning Patch Cord Installation Note". Retrieved February 20, 2015.
- ^ As with all multi-mode fiber connections, the MMF segment of the patch cord should match the type of fiber in the cable plant (Clause 38.11.4).
- ^ "Cisco 10GBASE X2 Modules Data Sheet". Cisco. Retrieved June 23, 2015.
- ^ "What is a 10GBASE-LRM transceiver and why do I need it?". CBO GmbH. Retrieved December 3, 2019.
- ^ a b "40GE SWDM4 QSFP+ Optical Transceiver | Finisar Corporation". www.finisar.com. Retrieved February 6, 2018.
- ^ a b "40G Extended Reach with Corning Cable Systems OM3/OM4 Connectivity with the Avago 40G QSFP+ eSR4 Transceiver" (PDF). Corning. 2013. Retrieved August 14, 2013.
- ^ "IEEE 802.3". Retrieved October 31, 2014.
- ^ "TIA Updates Data Center Cabling Standard to Keep Pace with Rapid Technology Advancements". TIA. August 9, 2017. Retrieved August 27, 2018.
- ^ Goldstein, Seymour (April 2010). "Encircled flux improves test equipment loss measurements". Cabling Installation & Maintenance. Retrieved June 1, 2017.
- Force, Inc. (April 14, 2005). "Types of Optical Fiber". Archived from the original on October 12, 2007. Retrieved April 17, 2008.
- Hayes, Jim; Karen Hayes (March 22, 2008). "Lennie Lightwave's Guide to Fiber Optics". Retrieved June 4, 2008.
- International Engineering Consortium. "Fiber Optic Technology". Archived from the original on February 13, 2009. Retrieved June 4, 2008.
- Telecommunications Industry Association. "Multimode Fiber for Enterprise Networks". Archived from the original on June 4, 2009. Retrieved June 4, 2008.
- Telecommunications Industry Association (September 2008). "Choosing the right multimode fiber for data communications" (PDF). Archived from the original (PDF) on January 6, 2009. Retrieved November 17, 2008.
- Hewlett-Packard Development Company, L.P. (2007). "100BASE-FX Technical Brief" (PDF). Archived from the original (PDF) on October 9, 2012. Retrieved November 20, 2012.
External links
[edit]Multi-mode optical fiber
View on Grokipediafrom Grokipedia
Fundamentals
Definition and Basic Characteristics
Multi-mode optical fiber is an optical fiber designed with a core diameter sufficiently large, typically ranging from 50 to 100 micrometers (μm), to support the simultaneous propagation of multiple transverse modes of light. This multimodality arises because the core size exceeds the wavelength of the light being transmitted, allowing multiple paths or modes for light to travel through the fiber, which contrasts with single-mode fibers that restrict propagation to one mode. Such fibers are primarily used in short-distance data transmission applications due to their ability to handle higher light power inputs without damage, though this comes at the cost of increased modal dispersion. The basic physical structure of multi-mode optical fiber consists of a central core, which serves as the light-carrying region made from a material like silica glass with a higher refractive index; a surrounding cladding layer of lower refractive index material, typically also silica but doped to reduce the index; and an outer protective jacket for mechanical strength and environmental protection. Common dimensions include a core diameter of 50 μm paired with a 125 μm cladding diameter (50/125 μm) or a 62.5 μm core with the same cladding size (62.5/125 μm), where the cladding diameter is standardized at 125 μm to facilitate compatibility with connectors and splices. The refractive index difference between core and cladding, often around 1-2%, enables total internal reflection to guide light along the fiber. Multi-mode optical fibers trace their origins to the early 1970s, when they were developed for short-haul telecommunications to address the limitations of early single-mode designs in terms of light source coupling efficiency. A pivotal advancement came from Corning Incorporated, which filed patents in 1970 for low-loss multimode fibers, leading to the first commercial demonstrations of practical optical fibers capable of transmitting light over distances exceeding 1 km with minimal attenuation. Key parameters defining multi-mode optical fibers include the core diameter, which directly influences the number of propagating modes; the refractive index profile, which can be step-index (abrupt change) or graded-index (gradual variation) to optimize performance; and operational wavelength ranges, predominantly 850 nm and 1310 nm, where multimode fibers exhibit low attenuation and are compatible with cost-effective light sources like vertical-cavity surface-emitting lasers (VCSELs). Manufacturing typically involves processes such as modified chemical vapor deposition (MCVD) or outside vapor deposition (OVD) to form the core and cladding layers by depositing doped silica particles, followed by drawing the preform into a fiber strand, or plasma chemical vapor deposition (PCVD) for precise index profiling.Principles of Light Propagation
In multi-mode optical fibers, light propagates through multiple transverse electromagnetic modes, primarily described as linearly polarized (LP) modes in the weakly guiding approximation where the core-cladding refractive index difference is small.[5] These LP modes, such as LP_{01}, LP_{11}, and higher-order variants, represent distinct field distributions across the fiber's cross-section that maintain their transverse profile while propagating longitudinally, allowing multiple independent paths for light rays and enabling higher data capacities at the cost of potential modal dispersion.[5][6] From a ray optics perspective, light guidance relies on total internal reflection (TIR) at the core-cladding interface, where the core refractive index exceeds the cladding index .[6] Rays incident on the interface at angles greater than the critical angle are reflected back into the core, confining the light within the fiber.[6] This model is particularly applicable to large-core multi-mode fibers, where rays can enter at various launch angles up to the fiber's numerical aperture (NA), defined as , determining the maximum acceptance angle for guided propagation.[6] Waveguide theory provides a more precise electromagnetic description, introducing the normalized frequency or V-number, , where is the core radius and is the operating wavelength.[6] Multi-mode operation occurs when , supporting numerous guided modes beyond the fundamental one, as this threshold marks the cutoff for higher-order modes in step-index fibers.[6] For step-index multi-mode fibers with large V, the approximate number of guided modes (including polarizations) is , scaling with the square of the V-number and thus increasing with larger core size, higher NA, or shorter wavelengths.[7] Light rays in multi-mode fibers can be classified as axial, meridional, or skew based on their paths. Axial rays travel parallel to the fiber axis with minimal reflections, while meridional rays pass through the axis and reflect directly across it. Skew rays, in contrast, follow helical trajectories around the axis without crossing it, undergoing more frequent reflections and contributing significantly to modal diversity in multi-mode propagation. To illustrate:-
Axial ray: Straight path along the center, no reflections.
Core: |-----| (fiber axis) Light: ------>Core: |-----| (fiber axis) Light: ------> -
Meridional ray: Zigzags through the axis.
Core: /\/\/\ (reflections at walls) Light: /\/\/\Core: /\/\/\ (reflections at walls) Light: /\/\/\ -
Skew ray: Helical wrap around axis.
Core: O (cross-section view) Light: ~ ~ ~ (circling path)Core: O (cross-section view) Light: ~ ~ ~ (circling path)
Types and Classifications
Step-Index and Graded-Index Profiles
Multi-mode optical fibers are characterized by two primary refractive index profiles: step-index and graded-index. The step-index profile features a uniform refractive index throughout the core, with a sharp abrupt drop to the lower cladding refractive index at the core-cladding interface.[9] This design represents the simplest construction for multi-mode fibers, but it results in significant intermodal dispersion because higher-order modes travel longer helical paths compared to lower-order axial modes, leading to pulse broadening.[10] In contrast, the graded-index profile exhibits a gradual variation in the core's refractive index, decreasing continuously from the center axis outward to match the cladding index at the boundary. This profile is typically described by the alpha law: where is the on-axis refractive index, is the relative index contrast, is the radial distance from the center, is the core radius, and for optimal performance in minimizing intermodal dispersion by equalizing the optical path lengths of different modes.[11] The parabolic shape () bends higher-order mode paths more sharply toward the core center, compensating for their longer geometric paths and reducing differential group delays.[11] Construction methods differ markedly between the two profiles. Step-index fibers are produced via uniform deposition techniques, such as chemical vapor deposition (CVD) with consistent dopant concentrations to maintain a constant core index. Graded-index fibers, however, require precise control over dopant distribution (e.g., germanium for index increase or fluorine for decrease) during fabrication; common methods include modified chemical vapor deposition (MCVD), where vapor-phase reactants are deposited in layers inside a rotating tube and sintered with varying compositions to achieve the index gradient, or plasma-activated CVD (PCVD) for finer radial control through plasma-enhanced reactions.[12][13] Step-index multi-mode fibers were among the earliest designs developed in the early 1970s, with low-loss versions (around 4 dB/km attenuation) demonstrated by Corning Glass Works in 1972.[14] Graded-index profiles were introduced around the same time to address the limitations of intermodal dispersion in step-index designs, enabling improved performance for longer-distance transmission.[15] Qualitatively, these profiles impact bandwidth significantly: step-index fibers typically achieve 20-50 MHz·km due to pronounced intermodal dispersion, while graded-index fibers can reach up to 500 MHz·km by mitigating path length differences among modes.[2][16]OM Standards and Specifications
The OM (optical multimode) classifications for multi-mode optical fibers are defined in the ISO/IEC 11801 standard for generic cabling systems, which specifies performance categories to ensure interoperability in structured cabling for data centers and local area networks.[4] These classifications evolved from earlier TIA-492 detail specifications, starting with legacy fibers in the 1980s to support increasing data rates, with each subsequent OM type offering improved modal bandwidth to accommodate higher-speed Ethernet applications while maintaining a 50/125 μm or 62.5/125 μm core/cladding diameter.[4] Jacket colors provide physical identification: orange for OM1 and OM2, aqua for OM3 and OM4, and lime green for OM5.[17] OM1, introduced in the 1980s as a legacy standard under TIA-492AAAA-A, features a 62.5/125 μm core/cladding diameter and overfilled launch (OFL) bandwidth of 200 MHz·km at 850 nm and 500 MHz·km at 1300 nm, designed primarily for LED sources in early networks like FDDI at 100 Mbps.[4][18] OM2, developed in the 1990s under TIA-492AAAB to support Gigabit Ethernet, uses a 50/125 μm core/cladding and OFL bandwidth of 500 MHz·km at both 850 nm and 1300 nm, extending the capabilities of legacy 50 μm fibers for 1 Gbit/s links up to 550 m.[4][18] OM3, standardized in 2002 under TIA-492AAAC as the first laser-optimized fiber (IEC 60793-2-10 Type A1a.2), retains the 50/125 μm core/cladding but achieves an effective modal bandwidth (EMB) of 2000 MHz·km at 850 nm (OFL 1500 MHz·km) and 500 MHz·km at 1300 nm, enabling 10 Gbit/s Ethernet over distances up to 300 m.[19][18] OM4, introduced in 2009 via TIA-492AAAD (IEC Type A1a.3), builds on OM3 with the same 50/125 μm dimensions and EMB of 4700 MHz·km at 850 nm (OFL 3500 MHz·km) and 500 MHz·km at 1300 nm, supporting 40/100 Gbit/s Ethernet up to 150 m and 10 Gbit/s up to 400 m.[19][18] OM5, defined in the 2017 edition of ISO/IEC 11801 as wideband multimode fiber (TIA-492AAAE), uses a 50/125 μm core/cladding with an EMB of ≥4700 MHz·km at 850 nm and ≥2470 MHz·km at 953 nm (OFL 3500 MHz·km at 850 nm and 500 MHz·km at 1300 nm), featuring a lime green jacket to distinguish it and enabling short-wavelength wavelength-division multiplexing (SWDM) for 40/100 Gbit/s Ethernet up to 150 m.[19][18][20]| OM Type | Core/Cladding (μm) | EMB at 850 nm (MHz·km) | OFL Bandwidth (MHz·km) at 850/1300 nm | Introduction Year | Typical Jacket Color | Key Supported Application |
|---|---|---|---|---|---|---|
| OM1 | 62.5/125 | N/A | 200/500 | 1980s | Orange | 100 Mbps FDDI |
| OM2 | 50/125 | N/A | 500/500 | 1990s | Orange | 1 Gbit/s Ethernet |
| OM3 | 50/125 | 2000 | 1500/500 | 2002 | Aqua | 10 Gbit/s up to 300 m |
| OM4 | 50/125 | 4700 | 3500/500 | 2009 | Aqua | 40/100 Gbit/s up to 150 m |
| OM5 | 50/125 | ≥4700 (850 nm), ≥2470 (953 nm) | 3500/500 | 2017 | Lime Green | SWDM 40/100 Gbit/s up to 150 m |
Optical Properties
Dispersion Mechanisms
In multi-mode optical fibers (MMFs), dispersion refers to the broadening of optical pulses as they propagate, primarily due to two mechanisms: modal dispersion and chromatic dispersion. These effects limit the bandwidth-length product and transmission distance by causing temporal spreading of the signal, leading to intersymbol interference at high data rates. Modal dispersion arises from differences in propagation velocities among the multiple guided modes, while chromatic dispersion results from the wavelength-dependent refractive index of the fiber material and waveguide structure, exacerbated by the spectral width of the light source.[21][22] Modal dispersion, also known as intermodal dispersion, is the dominant limitation in MMFs, particularly in step-index profiles where the refractive index changes abruptly between core and cladding. In a step-index MMF, the axial mode (propagating straight along the fiber axis) travels faster than higher-order modes that follow longer helical paths near the core-cladding boundary. The resulting delay spread between the fastest and slowest modes is approximated bywhere is the fiber length, is the core refractive index, is the relative index difference (with the cladding index), and is the speed of light in vacuum. This can be simplified to for small . Typical values of yield on the order of nanoseconds per kilometer, severely restricting bandwidth in step-index fibers to below 100 MHz·km. In graded-index MMFs, modal dispersion is significantly minimized by designing a parabolic refractive index profile that equalizes path lengths and group velocities across modes, reducing by factors of 50–100 compared to step-index designs.[23][24][25] Chromatic dispersion in MMFs consists of material dispersion (due to the wavelength dependence of the silica refractive index) and waveguide dispersion (from the fiber's geometric structure). The root-mean-square (RMS) pulse broadening due to chromatic dispersion is given by
where is the dispersion parameter (typically in ps/(nm·km)), is the fiber length, and is the spectral width of the source. For silica-based MMFs at common operating wavelengths, arises mainly from material effects, with values around -80 to -100 ps/(nm·km) near 850 nm and near zero at 1310 nm. Waveguide dispersion contributes a smaller positive offset, but the total remains dominated by material terms in standard MMF designs. Unlike single-mode fibers, MMF chromatic dispersion is more pronounced when using broadband sources, as the larger core supports multimode excitation that can couple with spectral variations.[26][27][28] The total pulse broadening in MMFs is the quadrature sum of modal and chromatic contributions, assuming independent Gaussian-distributed spreads:
This combined effect determines the overall temporal distortion, with modal dispersion often dominating in short-reach, high-mode-count scenarios, while chromatic becomes comparable or larger for low-modal-dispersion graded-index fibers or narrowband sources. For non-Gaussian profiles, more advanced models account for mode mixing and higher-order effects, but the RMS approximation provides a practical limit for system design.[29][30] The impact of total dispersion on system performance is quantified by the maximum achievable bit rate , which for non-return-to-zero (NRZ) signaling is limited by the condition that the bit duration exceeds four times the RMS broadening to maintain a low bit error rate (typically <10^{-12}):
For example, in a 500 m OM4 MMF link at 850 nm with ns, this yields Gb/s per channel, with practical limits approaching this dispersion-limited value in optimized systems. This relation underscores why MMFs are optimized for short distances (<500 m) in high-speed applications.[21][31] Dispersion effects are wavelength-dependent and more severe at 850 nm—the standard for short-reach MMF links—due to the higher magnitude of (peaking near the ultraviolet edge of silica's transparency) and larger from common sources like light-emitting diodes (LEDs, nm) or vertical-cavity surface-emitting lasers (VCSELs, nm). At 1310 nm, ps/(nm·km) minimizes chromatic contributions, but 850 nm operation persists for compatibility with low-cost VCSELs and reduced fiber attenuation in multimode designs.[32][33][34]
Attenuation and Bandwidth
Attenuation in multi-mode optical fibers refers to the reduction in optical power as light propagates through the fiber, primarily due to intrinsic and extrinsic mechanisms. Intrinsic attenuation arises from material properties, with Rayleigh scattering being the dominant source for wavelengths below approximately 1600 nm, contributing up to 90% of total losses.[35][36] This elastic scattering process follows an inverse fourth-power dependence on wavelength, expressed as α_R ∝ 1/λ^4, resulting in an approximate value of 2 dB/km at 850 nm, which decreases significantly at longer wavelengths.[37] Absorption by impurities, such as residual hydroxyl groups, adds a smaller component, typically less than 0.1 dB/km in high-quality silica fibers.[38] Extrinsic attenuation stems from installation and environmental factors, including bending losses, which occur when the fiber is curved beyond its minimum bend radius, causing light to leak from the core, and splicing or connector losses due to misalignment or air gaps.[38] Typical splicing losses in multi-mode fibers range from 0.1 to 0.5 dB per splice, while connector losses are around 0.25 to 0.75 dB per connection.[38][39] The total attenuation is the sum of intrinsic and extrinsic components, α_total = α_intrinsic + α_extrinsic, yielding typical values of 2 to 3 dB/km at 850 nm and about 0.7 to 1 dB/km at 1300 nm for standard multi-mode fibers.[40][38] Bandwidth in multi-mode fibers quantifies the fiber's ability to transmit high-speed signals without excessive modal distortion, often expressed as the bandwidth-length product (BL) in MHz·km.[19] Two key metrics are the overfilled launch (OFL) bandwidth, which applies to incoherent sources like LEDs and assumes uniform mode excitation, and the effective modal bandwidth (EMB), optimized for coherent laser sources such as vertical-cavity surface-emitting lasers (VCSELs).[41] EMB typically exceeds OFL values due to better mode selectivity in laser launches, enabling higher performance in modern systems.[41] Factors influencing bandwidth include the light source type, fiber refractive index profile, and transmission length; for instance, VCSELs achieve higher bandwidths than LEDs by exciting fewer modes, while higher-grade fibers like OM4 offer an EMB of 4700 MHz·km at 850 nm, supporting distances up to 550 m at 10 Gbit/s.[19][4] Bandwidth decreases with increasing length due to cumulative modal dispersion, limiting effective data rates over longer spans.[19] Long-term performance is affected by environmental factors such as temperature and aging, which can increase attenuation by 0.01 to 0.05 dB/km over typical operating temperature ranges through thermal expansion altering the fiber's geometry or inducing microbends.[42] Aging effects, including hydrogen diffusion or radiation exposure in harsh environments, may further elevate losses over time, though annealing at elevated temperatures can mitigate radiation-induced defects. These variations underscore the importance of specifying operational conditions for reliable multi-mode fiber deployments.[43]Measurement Techniques
Encircled Flux
Encircled flux (EF) is a standardized metric that quantifies the distribution of optical power launched into the core of a multimode optical fiber as a function of radial distance from the fiber's optical axis, expressed as the percentage of total launched power encircled within that radius. This approach ensures consistent and reproducible excitation of the fiber's propagation modes during performance testing, minimizing variations due to differences in light source characteristics. The method is formally defined in the international standard IEC 61280-4-1, which specifies EF for multimode attenuation and bandwidth measurements in installed cabling plants. The primary purpose of encircled flux is to provide a controlled launch condition that better simulates the restricted modal excitation produced by vertical-cavity surface-emitting lasers (VCSELs), which are commonly used in high-speed multimode links at wavelengths like 850 nm. It replaces the earlier overfilled launch (OFL) method, which relied on light-emitting diodes (LEDs) to uniformly excite all modes but often led to high variability in measured bandwidth and insertion loss due to inconsistent modal power distribution. By enforcing EF compliance, testing becomes more reliable, reducing discrepancies in results across different test equipment and operators.[44] Measurement of encircled flux involves capturing the near-field intensity profile of the launched light at the fiber input and integrating the power within successive radial distances. The encircled flux at radius is calculated as where is the integrated optical power within radius , and is the total launched power. Compliance is verified against predefined templates in the standards; for example, for a 50 μm core multimode fiber at 850 nm, the EF must fall between 86% and 100% at 18 μm radius to ensure appropriate modal filling without over- or under-excitation. These templates are derived from near-field measurements using calibrated imaging systems or mode conditioners, and launch devices must meet the bounds at multiple radii (e.g., 11.5 μm, 18 μm, 25 μm) to qualify as EF-compliant.[45][46] Encircled flux was formally adopted through the publication of TIA-526-14-B in October 2010, which updated optical power loss measurement procedures for installed multimode fiber cable plants to incorporate EF as the required launch condition. This standard, along with the second edition of IEC 61280-4-1 released around the same period, marked a shift from legacy methods and made EF mandatory for certifying higher-grade fibers such as OM3 and OM4, where precise modal control is essential for validating performance in 10 Gb/s and faster Ethernet applications. Prior to this, variability in launch conditions had hindered accurate inter-laboratory comparisons, but these standards established EF as the global benchmark.[47][48] The adoption of encircled flux has significantly improved the accuracy of link budget calculations in multimode systems by reducing measurement uncertainty in loss and bandwidth to within ±10%, representing up to a 75% decrease in variability compared to OFL methods. This enhanced precision is particularly beneficial for high-speed links, where even small errors in modal excitation can lead to overestimation of allowable loss margins, ensuring more reliable deployment of OM3+ fibers in data centers and enterprise networks.[44]Testing and Compliance Standards
The ITU-T G.651.1 recommendation specifies characteristics for 50/125 μm category A1a and A1b graded-index multimode optical fibers intended for optical access networks, including a maximum attenuation of ≤1.0 dB/km at 1300 nm to ensure reliable short-haul performance. This standard supports applications such as 1 Gbit/s Ethernet over distances up to 550 m at 850 nm, emphasizing compatibility with laser-optimized fibers for reduced modal dispersion. The ISO/IEC 11801 standard governs generic cabling systems, integrating OM classifications for multimode fibers with defined requirements for channel insertion loss and return loss (typically ≥20 dB for multimode links) to verify link performance in structured cabling environments. These parameters ensure interoperability across horizontal and backbone cabling, aligning with performance classes from OM1 to OM5 for varying bandwidth needs. Key testing procedures for multi-mode fibers include optical time-domain reflectometry (OTDR) to profile attenuation along the fiber length by analyzing backscattered light, enabling detection of splices, bends, and uniform loss (e.g., identifying events with resolution down to 1 m).[49] Optical loss test sets (OLTS) measure end-to-end insertion loss bidirectionally, providing accurate total link budget verification compliant with Tier 1 certification under ISO/IEC 11801.[50] Bandwidth assessment via differential mode delay (DMD) evaluates modal dispersion by launching controlled mode groups and measuring pulse broadening, with limits such as ≤0.5 ps/m for OM4 to confirm effective modal bandwidth.[51] Compliance certification for multi-mode fibers involves third-party verification through organizations like the Telecommunications Industry Association (TIA) and Electronic Industries Alliance (EIA), adhering to ANSI/TIA-568 standards for parameters including bandwidth and attenuation. This process extends to bend-insensitive variants introduced post-2010, which maintain OM specifications while reducing macrobend loss (e.g., ≤0.1 dB for 10 turns at 7.5 mm radius at 850 nm) for high-density deployments.[52] As of 2025, updates to multimode standards incorporate OM5 wideband multimode fiber (WBMMF) for shortwave wavelength division multiplexing (SWDM), supporting up to 100 Gbit/s over 100 m at wavelengths from 850 to 940 nm, while ensuring backward compatibility with OM4 infrastructure through equivalent effective modal bandwidth performance.[19]Applications
Telecommunications and Data Centers
Multi-mode optical fiber (MMF) plays a pivotal role in local area networks (LANs) and campus networks, where it supports high-speed Ethernet standards for short-haul connections. For instance, the 10GBASE-SR standard enables data transmission at 10 Gbit/s over distances up to 300 meters using OM3 fiber, leveraging its effective modal bandwidth to minimize dispersion in these environments.[53] Higher-speed applications, such as 400GBASE-SR8, achieve 400 Gbit/s over up to 100 meters on OM4 or OM5 fiber, utilizing parallel lanes to meet the demands of dense networking setups.[54] These capabilities make MMF ideal for interconnecting buildings or departments within campuses, where link lengths typically fall under 500 meters. In data centers, MMF serves as a cost-effective interconnect for switches and routers, particularly for intra-rack and inter-rack links shorter than 500 meters. It offers significant savings compared to single-mode fiber in these scenarios due to lower transceiver costs and simpler installation.[55] Multi-fiber push-on (MPO) connectors facilitate parallel optics in these environments, bundling 8 to 32 fibers into a single interface to support high-density cabling for 40G and 100G Ethernet.[56] For example, 100 Gbit/s transmission reaches up to 150 meters on OM4 fiber, enabling scalable architectures in hyperscale facilities that handle massive data volumes.[4] MMF's compatibility with vertical-cavity surface-emitting lasers (VCSELs) operating at 850 nm further enhances its suitability for these applications, providing low-cost, high-volume transceivers that align with the economic needs of LANs and data centers.[57] Recent advancements include 800 Gbit/s transmission over OM5 fiber using shortwave wavelength division multiplexing (SWDM); the IEEE 802.3df standard, approved in 2024, defines 800GBASE-SR8 for such multimode applications, extending bandwidth without increasing fiber count.[58][59] Additionally, MMF integrates into 5G backhaul for intra-facility links, supporting fronthaul connections in base stations with OM5's wideband properties for distances under 100 meters.[60]Industrial and Sensing Applications
Multi-mode optical fibers are widely employed in industrial sensing applications, particularly for distributed monitoring of temperature and strain in challenging environments. These systems often utilize Raman optical time-domain reflectometry (OTDR), where the multiple propagation modes in multi-mode fibers enhance the capture of backscattered Raman photons, thereby improving signal-to-noise ratios and enabling reliable measurements over extended distances.[61] For example, graded-index multi-mode fibers with 50/125 μm dimensions support Raman distributed temperature sensing (DTS) over spans up to 1 km, achieving spatial resolutions of approximately 8 m and temperature precisions of ±3°C through optimized pulse widths that exploit modal diversity.[62] Such configurations are advantageous for applications like pipeline integrity assessment and structural health monitoring, where the fibers' tolerance to modal dispersion allows for cost-effective deployment compared to single-mode alternatives.[63] In laser material processing, multi-mode optical fibers provide a durable medium for delivering high-power laser beams to workstations for welding, cutting, and cladding. Fibers featuring core diameters of 100–200 μm accommodate powers greater than 1 kW while resisting photodarkening, stimulated Raman scattering, and thermal mode instability, ensuring stable beam delivery without core damage under prolonged operation.[64] These large-core designs distribute heat effectively across modes, supporting industrial processes that require kilowatt-level intensities for precise metal joining and ablation, with coupling efficiencies exceeding 99% when matched with passive delivery fibers.[65] Multi-mode fibers also play a key role in medical imaging, endoscopy, and fluorescence spectroscopy by transmitting broadband light and collecting emitted signals with high efficiency. Their core sizes, typically ranging from 200 to 1000 μm in graded-index configurations, enable the propagation of multiple spatial modes that preserve image quality in microendoscopy while maximizing fluorescence capture from biological samples.[66] In fluorescence lifetime imaging, single multi-mode fibers with diameters around 350–1000 μm facilitate minimally invasive procedures, such as neural activity visualization, by guiding excitation light and returning diffuse emissions without the need for bulky lens systems.[67] This modal capacity supports applications in spectroscopy for analyzing molecular interactions in tissues, where numerical apertures up to 0.5 enhance light throughput for diagnostic accuracy.[68] Ruggedized multi-mode fibers are essential for sensing in harsh industrial settings, including oil and gas extraction, where they withstand high pressures up to 138 MPa and temperatures exceeding 200°C. In downhole monitoring, these fibers enable distributed temperature and acoustic sensing via Raman and Rayleigh scattering, providing real-time data over distances up to several kilometers for leak detection and reservoir optimization without electromagnetic interference.[69] For automotive applications, plastic optical fiber variants—operating as step-index multi-mode guides with 1000 μm cores—form the backbone of Media Oriented Systems Transport (MOST) networks, supporting multimedia data rates up to 150 Mbps over ring topologies spanning 40 m in vehicles.[70] Their chemical and mechanical resilience ensures reliable performance in vibrating, corrosive environments like engine compartments.[71] As of 2025, multi-mode plastic optical fibers are increasingly integrated into Internet of Things (IoT) ecosystems for factory automation, offering short-range, EMI-resistant links for sensor networks and machine-to-machine communication. These fibers, with their flexible, low-cost construction, support hybrid polymer optical fiber-visible light communication systems that achieve data rates suitable for real-time process control in smart manufacturing setups. Market projections indicate sustained growth in such applications, driven by the need for robust connectivity in automated production lines.[72]Comparisons and Performance
Advantages and Limitations
Multi-mode optical fiber offers several advantages in system design, particularly for short-reach applications. One key benefit is the use of lower-cost transceivers, such as light-emitting diodes (LEDs) or vertical-cavity surface-emitting lasers (VCSELs), which typically cost around $10 compared to over $100 for the lasers required in longer-reach systems.[73][74] The larger core diameter, usually 50 μm or 62.5 μm, facilitates easier alignment during coupling, with positional tolerances exceeding 10 μm, reducing the precision needed for connectors and transceivers.[1] Additionally, multi-mode fiber provides higher power handling capabilities due to its larger core, making it suitable for industrial applications involving high-power laser delivery, such as welding and cutting.[75] Installation advantages further enhance its practicality. Multi-mode fiber is less sensitive to bends, exhibiting macrobend losses below 0.5 dB for a 30 mm radius, which allows for more flexible routing in constrained environments.[76] Splicing is also simpler, with fusion splice losses typically around 0.1 dB, enabling reliable field repairs with minimal attenuation.[77] These features contribute to overall system cost savings for links under 2 km, where the initial fiber cost is comparable to other types, but transceiver and installation expenses are reduced.[55] Despite these strengths, multi-mode optical fiber has notable limitations. Transmission distances are capped at 400 m for standard OM4 fiber in high-speed applications like 10 Gbit/s, primarily due to modal and chromatic dispersion effects that broaden pulses over length.[78] It also experiences higher attenuation at short wavelengths, such as 850 nm, where losses can reach 3 dB/km compared to lower values at longer wavelengths.[79] Scalability beyond 400 Gbit/s requires parallel fiber configurations to maintain bandwidth, as single-fiber capacity is constrained by modal limitations.[80] In analog applications, multi-mode fiber is susceptible to modal noise, which arises from interference between propagating modes and can degrade signal-to-noise ratios in coherent systems.[81]Comparison with Single-Mode Fiber
Multi-mode optical fiber (MMF) features a larger core diameter, typically 50 μm or 62.5 μm, which supports the propagation of multiple light modes, in contrast to single-mode fiber (SMF), which has a core diameter of 8–10 μm and propagates only a single mode.[82][83] This larger core in MMF facilitates easier light coupling from sources like vertical-cavity surface-emitting lasers (VCSELs), reducing alignment precision requirements and simplifying installation compared to SMF, which demands precise coupling due to its smaller core.[82][1] However, the multiple modes in MMF lead to modal dispersion, where different paths cause signal broadening over distance, limiting its performance relative to SMF's lower dispersion.[82][1] MMF operates primarily at shorter wavelengths of 850 nm and 1300 nm, compatible with cost-effective light sources such as LEDs or VCSELs, whereas SMF uses longer wavelengths of 1310 nm and 1550 nm, requiring more expensive laser sources.[84][82] This difference contributes to MMF systems having lower overall costs for short links under 500 m, with transceivers typically 1.5 to 5 times less expensive than those for SMF, making MMF total link costs significantly lower for such applications.[82][83] In terms of transmission distance and capacity, MMF is optimized for shorter reaches with high bandwidth, while SMF excels in longer-haul scenarios. The following table summarizes representative performance metrics for Ethernet standards:| Data Rate | MMF (OM4) Distance | SMF Distance |
|---|---|---|
| 10 Gbit/s | 400 m | 40 km |
| 100 Gbit/s | 150 m | 10 km |