Single-mode vs multimode fiber

Single-mode fiber (SMF) guides light in one transverse mode through a core about 9 µm across, and multimode fiber (MMF) guides hundreds of modes through a core 50 or 62.5 µm across; both have the same 125 µm cladding. The difference decides what each is used for. With one mode there is no modal dispersion, so single-mode fiber carries a signal for kilometers, limited by chromatic dispersion and loss, and across oceans with amplifiers. In multimode fiber the modes arrive at slightly different times, which limits Ethernet links to between 70 and 400 m depending on the data rate and the grade of fiber, but the large core accepts light from low-cost 850 nm VCSELs with loose alignment, which keeps multimode the cheaper choice for short links inside buildings and data centers. Both kinds of optical fiber look alike in a cable; the jacket color and the printed legend tell them apart.

Single-mode and multimode compared

Why the number of modes matters

The number of modes a core guides is set by its V number, $V=2\pi a\text{NA}\mathrm{/}\lambda$ for a core of radius $a$. A step-index core is single mode when $V<2.405$. Standard single-mode fiber has $V\approx 2.0$ at 1550 nm and is single mode above its cable cutoff wavelength of 1260 nm; at 850 nm its $V$ rises to about 3.7 and it guides the LP₁₁ mode as well. A 50 µm graded-index core with an NA of 0.20 has $V=37$ at 850 nm and guides about $V^2\mathrm{/}4$, or 340 modes, and a 62.5 µm core about 1,000.

Each mode travels at its own group velocity, so a pulse launched into many modes arrives spread in time. In a step-index multimode core the spread is about $n_1\mathrm{\Delta }\mathrm{/}c$ per unit length, where $\mathrm{\Delta }$ is the relative index difference: 65 ns/km for a 50 µm core with $\mathrm{\Delta }=0.013$, which limits a kilometer of such fiber to a few megahertz. Graded-index fiber lowers the index away from the axis, so that the modes that travel furthest do so in faster glass, and an ideal profile reduces the spread to about $n_1{\mathrm{\Delta }}^2\mathrm{/}8c$ per unit length (Olshansky and Keck). All current multimode grades are graded-index, and they are rated by the bandwidth-distance product that their residual modal dispersion allows: 2000 MHz·km for OM3 and 4700 MHz·km for OM4, as effective modal bandwidths at 850 nm.

A single-mode fiber has no modal spread. Its pulses broaden through chromatic dispersion, about 17 ps/(nm·km) at 1550 nm and near zero at 1310 nm, which a narrow-linewidth laser keeps small and a coherent receiver removes in its signal processing, and through polarization mode dispersion, which matters only at high rates over long distances. The single mode is also why interferometers, most fiber lasers and many fiber sensors use single-mode fiber: the light leaving it has a fixed, nearly Gaussian profile, 9.2 µm in mode field diameter at 1310 nm and 10.4 µm at 1550 nm.

Distance and data rate

The conformance reaches that IEEE 802.3 sets for some common Ethernet optics:

The multimode reach shrinks as the rate per lane rises, because a given length of multimode fiber has a fixed bandwidth, set mainly by its modal dispersion: 300 m of OM3 at 10 Gb/s per lane, 70 m at 25 Gb/s. Single-mode reach is set by the transceiver rather than the fiber, and the same installed fiber serves later generations of optics, which is why some data-center builds install single-mode fiber even for short runs. The multimode grades, their bandwidths and the reach of each Ethernet standard are tabulated in Types of Optical Fiber.

Light sources, alignment and cost

The mode of a single-mode fiber is about 9 µm across, so the laser and every connector must be aligned to about a micrometer: two modes of 9.2 µm diameter offset sideways by 1 µm couple with a loss of 0.21 dB, and by 2 µm with 0.82 dB, in the Gaussian approximation. The Fiber Coupling Efficiency Calculator gives the tolerances for other offsets and tilts. A 50 µm multimode core with an NA of 0.20 accepts light over a much larger area and range of angles, so a VCSEL's beam can be placed several micrometers off center with little loss, and VCSELs are made and tested on the wafer in large numbers. Multimode transceivers cost less for these reasons; the price of the fiber itself is not the main difference. Light-emitting diodes, whose broad emission cannot be coupled efficiently into a single mode, are used with multimode fiber and served the early fiber LANs.

Connecting single-mode and multimode fiber

• Multimode into single-mode. The single-mode fiber accepts only the part of the light that matches its one mode, so most of the power is lost. If all the multimode fiber's modes carry equal power, the fraction coupled is $2\mathrm{/}N$, where $N$ is the number of modes counting both polarizations: 340 modes in a 50 µm core at 850 nm give a loss of 22 dB. The loss is lower when the power is concentrated in the low-order modes.
• Single-mode into multimode. The small mode fits well inside the large core and passes with little loss, but it excites only a few modes, which exposes the link to the delay differences between those modes. Single-mode transceivers used on legacy multimode cable, such as 1000BASE-LX, therefore launch through a mode-conditioning patch cord, which offsets the single-mode fiber from the center of the multimode core so that a controlled set of modes is excited.
• Transceivers on the wrong fiber. An 850 nm multimode transceiver does not work on single-mode fiber, since the small core accepts little of the VCSEL's light and the fiber is not single mode at 850 nm.
• 62.5 µm and 50 µm multimode. Light passing from a 62.5 µm core into a 50 µm core loses the ratios of the core diameters and of the numerical apertures, squared: about 4.7 dB for a fully filled launch, 1.9 dB from the cores and 2.8 dB from the apertures, while the opposite direction loses little. OM3, OM4 and OM5 share the same core and numerical aperture and connect without extra loss, but a mixed link performs as its lowest grade.

Telling them apart

Cable jackets follow the colors of TIA-598: yellow for single-mode, orange for OM1 and OM2, aqua for OM3 and OM4, erika violet for OM4 in much of Europe, and lime green for OM5. The jacket is also printed with the core and cladding diameters, 9/125 for single-mode and 50/125 or 62.5/125 for multimode, usually with the grade. Connectors and adapters are color coded under TIA-568: blue for single-mode with a UPC polish, green for single-mode with an angled (APC) polish, beige for 62.5 µm multimode and aqua for laser-optimized 50 µm multimode.

Fibers between the two

Few-mode fibers guide a handful of modes on purpose, as separate channels for mode-division multiplexing, one of the routes to space-division multiplexing. Double-clad fibers combine a single-mode core, which carries the laser light, with a multimode inner cladding that carries the pump light of a fiber laser or amplifier. Large step-index multimode fibers, with cores from 100 µm to more than 1 mm, deliver high laser power and light for illumination and spectroscopy, where their large core and numerical aperture collect and carry more light.

Common questions

What is the difference between single-mode and multimode fiber?

Single-mode fiber has a core about 9 µm across that guides one mode, and carries signals for kilometers, limited by chromatic dispersion and loss. Multimode fiber has a 50 or 62.5 µm core that guides hundreds of modes, whose different speeds limit links to a few hundred meters, but it works with low-cost 850 nm VCSEL transceivers.

Which is better, single-mode or multimode fiber?

It depends on the distance and the budget. Multimode OM4 fiber with VCSEL transceivers is usually the cheaper choice up to about 100 m at 25 Gb/s per lane and above, or 400 m at 10 Gb/s. Beyond that, and where the cable plant must serve future higher-rate optics, single-mode fiber is the choice, since its reach is set by the transceiver rather than the fiber.

How far can multimode fiber carry a signal?

Under IEEE 802.3, 10GBASE-SR reaches 300 m over OM3 and 400 m over OM4, and 100GBASE-SR4 reaches 70 m over OM3 and 100 m over OM4. Older OM1 fiber carries 10GBASE-SR only 33 m.

Can single-mode and multimode fiber be connected?

Physically yes, but light passing from multimode into single-mode fiber loses most of its power, about 22 dB when all the modes of a 50 µm core are filled. Light from single-mode into multimode fiber passes with little loss, and mode-conditioning patch cords exist for running single-mode transceivers over legacy multimode cable.

How can single-mode and multimode fiber be told apart?

By the jacket color and the printed legend: single-mode fiber is yellow and marked 9/125; multimode is orange (OM1, OM2), aqua (OM3, OM4), erika violet (OM4 in Europe) or lime green (OM5), and marked 50/125 or 62.5/125. Single-mode connectors are usually blue or green, multimode connectors beige or aqua.

References: B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics, 3rd ed. (Wiley, 2019). G. P. Agrawal, Fiber-Optic Communication Systems, 4th ed. (Wiley, 2010). R. Olshansky and D. B. Keck, "Pulse broadening in graded-index optical fibers," Applied Optics 15, 483 (1976). ITU-T Recommendation G.652, Characteristics of a single-mode optical fibre and cable. ISO/IEC 11801-1, Information technology: Generic cabling for customer premises. IEC 60793-2-10, Optical fibres: Part 2-10: Product specifications: Sectional specification for category A1 multimode fibres. IEEE Std 802.3, IEEE Standard for Ethernet. TIA-598-D, Optical Fiber Cable Color Coding. ANSI/TIA-568.3-D, Optical Fiber Cabling and Components Standard.