Types of Optical Fiber: OM1 to OM5 Multimode, Single-Mode Grades and Specialty Fibers
Reference to the optical fiber types in use: the OM1 to OM5 multimode grades with their modal bandwidths, Ethernet reaches and jacket colors, the ITU-T single-mode fiber families from G.652 to G.657, typical parameters of standard single-mode fiber, specialty fibers, and how to choose between them.
Scope
This article is a reference to the fiber types an engineer meets in data centers, telecom networks and the lab. It covers the multimode grades OM1 to OM5 first, because the choice between OM3, OM4 and OM5 is the one most often made, then the single-mode families defined by ITU-T, the typical parameters of standard single-mode fiber, the specialty fibers, and the practical rules for choosing and mixing them. The physics of the single-mode and multimode distinction is in single-mode vs multimode fiber, and the index profiles in step-index fiber and graded-index fiber.
Multimode grades: OM1 to OM5
All current multimode grades are graded-index fibers with a 125 μm cladding. Their grade is set by their modal bandwidth, the bandwidth-distance product that modal dispersion allows, which is specified in MHz·km. Two figures are used. The overfilled-launch (OFL) bandwidth assumes every mode is excited equally, as by an LED. The effective modal bandwidth (EMB) is the figure that applies to a VCSEL, which excites a smaller set of modes; it is guaranteed for the laser-optimized grades by measuring the fiber's differential mode delay.
| Grade | Core | EMB at 850 nm | OFL at 850 / 1300 nm | Jacket color | Typical use |
|---|---|---|---|---|---|
| OM1 | 62.5 μm | not specified | 200 / 500 MHz·km | Orange | Legacy LAN, LED sources |
| OM2 | 50 μm | not specified | 500 / 500 MHz·km | Orange | Legacy LAN, 1 Gb/s |
| OM3 | 50 μm, laser-optimized | 2000 MHz·km | 1500 / 500 MHz·km | Aqua | VCSEL links, 10 to 100 Gb/s |
| OM4 | 50 μm, laser-optimized | 4700 MHz·km | 3500 / 500 MHz·km | Aqua, or erika violet in much of Europe | VCSEL links, longer reach than OM3 |
| OM5 | 50 μm, wideband | 4700 MHz·km at 850 nm, 2470 MHz·km at 953 nm | 3500 / 500 MHz·km | Lime green | Short-wavelength WDM over 850 to 950 nm |
OM4 is the same fiber as OM3 made to a tighter index profile, so that the delays of the modes a VCSEL excites are more nearly equal. The gain appears directly as reach. OM5 has OM4's performance at 850 nm and adds a bandwidth specification out to 953 nm, which makes it useful for transceivers that multiplex several VCSEL wavelengths in that range onto one fiber; at a single 850 nm wavelength it behaves as OM4.
Reach by Ethernet standard
The reaches below are the conformance distances set by IEEE 802.3 for each grade. Transceiver names explains the SR, SR4 and SR8 labels.
| Standard | Lanes | OM3 | OM4 | OM5 |
|---|---|---|---|---|
| 10GBASE-SR | 1 × 10 Gb/s | 300 m | 400 m | – |
| 25GBASE-SR | 1 × 25 Gb/s | 70 m | 100 m | – |
| 40GBASE-SR4 | 4 × 10 Gb/s | 100 m | 150 m | – |
| 100GBASE-SR4 | 4 × 25 Gb/s | 70 m | 100 m | – |
| 400GBASE-SR8 | 8 × 50 Gb/s | 70 m | 100 m | 100 m |
| 400GBASE-SR4.2 | 4 fibers × 2 wavelengths | 70 m | 100 m | 150 m |
For comparison, 10GBASE-SR reaches 33 m over OM1 and 82 m over OM2. The standards written before OM5 existed do not list it; at 850 nm it performs as OM4. Across the table, OM4 gives a third to a half more reach than OM3, and OM5 goes further than OM4 only where more than one wavelength shares a fiber, as in SR4.2.
Attenuation
Multimode fiber loses a few dB per kilometre at 850 nm, much more than single-mode fiber at 1310 or 1550 nm, but over 100 m that is a few tenths of a dB. Ethernet link budgets for multimode links assume 3.5 dB/km at 850 nm, and in a data-center link the connector losses usually exceed the fiber loss; the link budget entry shows the allocation.
Single-mode fiber families
Single-mode fibers are specified by ITU-T recommendations that set their mode field diameter, cutoff wavelength, dispersion and loss. The families differ mainly in where they put the zero-dispersion wavelength and how they handle bending.
| Recommendation | Common name | Defining property | Typical use |
|---|---|---|---|
| G.652 | Standard single-mode fiber (SMF-28 class) | Zero dispersion at 1300 to 1324 nm; G.652.D adds low water peak | Nearly all terrestrial networks and data-center single-mode links |
| G.653 | Dispersion-shifted fiber | Zero dispersion moved into the C-band | Largely obsolete; four-wave mixing makes it unsuitable for WDM |
| G.654 | Cutoff-shifted fiber | Larger effective area and lower loss, with cutoff moved up | Submarine and long-haul terrestrial links |
| G.655 | Non-zero dispersion-shifted fiber | Small but nonzero dispersion across the C-band | Long-haul DWDM, particularly systems designed before coherent detection |
| G.656 | Wideband NZDSF | NZDSF with dispersion specified over 1460 to 1625 nm | CWDM and DWDM over a wider band |
| G.657 | Bend-insensitive fiber | Low bend loss at small radii | Access networks, in-building cabling, tight routing |
G.657 is divided into category A, which is fully compatible with G.652.D and can be spliced into the same network, and category B, which gives up some of that compatibility for tighter bends. The subcategories are named by their minimum bend radius: A1 for 10 mm, A2 and B2 for 7.5 mm, B3 for 5 mm. Much of the standard single-mode fiber sold today meets both G.652.D and G.657.A1.
Cabling standards (ISO/IEC 11801 and TIA-568) name single-mode cable grades OS1a and OS2. Both use G.652.D fiber; the grades differ in the cabled attenuation they allow, and OS2 is the grade for longer and outside-plant runs.
Typical parameters of standard single-mode fiber
| Parameter | Typical value |
|---|---|
| Mode field diameter | 9.2 μm at 1310 nm; 10.4 μm at 1550 nm |
| Numerical aperture | About 0.14 |
| Cable cutoff wavelength | Below 1260 nm |
| Attenuation | About 0.32 to 0.35 dB/km at 1310 nm; 0.18 to 0.20 dB/km at 1550 nm |
| Chromatic dispersion | About 17 ps/(nm·km) at 1550 nm; zero near 1310 nm |
| Cladding and coating diameter | 125 μm cladding; 250 μm coating, with 200 μm coatings used in high-density cables |
The attenuation minimum near 1550 nm is set by Rayleigh scattering on the short-wavelength side and by infrared absorption of silica on the long side. The dispersion at any wavelength follows from the zero-dispersion wavelength and slope, which the Dispersion and Pulse Broadening Calculator computes with the G.652 formula.
Specialty fibers
| Fiber | What differs | Used for |
|---|---|---|
| Polarization-maintaining fiber | Stress rods or an elliptical core make the fiber strongly birefringent, so light launched on one axis stays there | Lasers, modulators and interferometric sensors that need a fixed polarization |
| Photonic crystal fiber | A pattern of air holes along the length replaces the doped core | Endlessly single-mode fiber, nonlinear and supercontinuum sources |
| Hollow-core fiber | Light travels in air, guided by an antiresonant or bandgap structure | Low-latency links, high-power delivery, low-nonlinearity transmission |
| Few-mode fiber | Supports a small, controlled number of modes | Mode-division multiplexing research |
| Multi-core fiber | Several cores in one cladding | Space-division multiplexing, dense interconnects |
| Double-clad fiber | A doped single-mode core inside a larger multimode inner cladding that carries pump light | Fiber lasers and amplifiers pumped by multimode diodes |
| Rare-earth-doped fiber | Erbium or ytterbium in the core | EDFAs and ytterbium fiber lasers |
Choosing a fiber
Data-center links up to about 100 m. Multimode OM4 with VCSEL-based SR transceivers is the established choice where the reach allows it; OM3 is adequate to 70 m at 25 Gb/s per lane and above. OM5 pays only where the transceivers use more than one wavelength per fiber.
Links longer than the multimode reach, or planned for later generations. Standard single-mode fiber, G.652.D or G.652.D that is also G.657.A1. Single-mode reach is set by the transceiver rather than the fiber, which is why some data-center builds install single-mode even for short runs.
Tight routing and in-building runs. G.657 fiber, with category A where the fiber must join a G.652 network.
Lab work. Match the fiber to the component: the mode field diameter of a pigtail sets the coupling loss to anything it meets, as calculated in Coupling a Free-Space Beam into Single-Mode Fiber, and a polarization-sensitive setup needs PM fiber throughout.
Mixing fiber types
Multimode grades. OM3, OM4 and OM5 have the same core and numerical aperture, so they connect with no extra loss, but the link then performs as its lowest grade. Connecting 50 μm fiber to 62.5 μm fiber is different: light passing from the 62.5 μm core into the 50 μm core loses the ratio of core areas and of numerical apertures squared, which for a fully filled launch is about 4.7 dB by a geometric estimate, while the opposite direction loses little.
Single-mode to multimode. Light from single-mode fiber enters multimode fiber with little loss, but light from multimode fiber into single-mode fiber loses most of its power, and a single-mode transceiver on multimode fiber gives a link that works poorly or not at all. Special mode-conditioning patch cords exist for launching single-mode transceivers into multimode cable plant.
Single-mode families. G.652 and G.657.A fibers splice to each other with only the small loss from any difference in mode field diameter. Connector types and polish, which matter more in practice than the fiber family, are covered in PC, UPC and APC connectors and Fiber Connector Inspection and Cleaning.
References: ITU-T Recommendations G.652, G.653, G.654, G.655, G.656 and G.657; TIA-492AAAC and TIA-492AAAD (OM3 and OM4 fiber); ISO/IEC 11801 (cabling grades); IEEE 802.3 (Ethernet physical layers); TIA-598 (color coding).