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Single-Mode vs Multimode Fiber: Key Differences Explained

Single-mode fiber has a thin core about 9 µm in diameter, and light travels in only one mode, so there is no difference in speed between modes (modal dispersion). It is used for long-distance links of tens of km or more and for high-speed communications. Multimode fiber has a thicker core of 50 µm or 62.5 µm, and many modes travel at once, so modal dispersion limits its reach to roughly tens to hundreds of m. In return, it can use inexpensive light sources such as 850 nm VCSELs, and alignment is easy, so it suits short connections inside data centers and buildings.

AspectSingle-mode fiberMultimode fiber
Core diameter (typical)About 9 µm (cladding 125 µm)50 µm or 62.5 µm (cladding 125 µm)
Number of modes carriedOnly oneHundreds to around a thousand modes travel at once
Main wavelengths1310 nm (O-band) and 1550 nm (C-band)Mainly around 850 nm
Main light sourcesEdge-emitting semiconductor lasers (such as DFB lasers), with light focused into the thin coreVCSELs (vertical-cavity surface-emitting lasers). Older generations also used LEDs
Modal dispersionNone. However, chromatic dispersion becomes significant with distance (nearly zero around 1310 nm)Present. Graded-index fiber, whose core refractive index changes gradually, reduces it, but it still limits bandwidth and reach
Loss (typical)About 0.35 dB/km at 1310 nm and about 0.2 dB/km at 1550 nmAbout 2.5 to 3.5 dB/km at 850 nm
Reach (typical)Hundreds of m to tens of km. Thousands of km or more with optical amplifiersAbout hundreds of m at 10 Gbit/s, and about 100 m or less for lanes of 25 Gbit/s or more
AlignmentTight. Sub-µm precision is required, and active alignment is often usedRelaxed. The thick core makes coupling to light sources and photodetectors easy
CostThe fiber itself is cheap, but lasers, optical transceivers and assembly tend to be expensiveThe fiber is somewhat more expensive, but light sources and assembly are cheap, so the total tends to be lower over short distances
Main usesLong-distance and intercity communications, access lines (FTTH), connections of hundreds of m to km scale between buildings and within data centersShort connections such as between racks in data centers, in-building LANs and storage connections

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

Single-mode fiber
Cabling for distances over a few hundred m or in anticipation of future speed increases. Long-distance communications, access lines, and connections between data-center buildings or between switches. Also when you want to carry many signals on one fiber with wavelength-division multiplexing (WDM).
Multimode fiber
When you want to build short connections of roughly 100 m or less cheaply, such as within the same room or building. Combined with optical transceivers that use 850 nm VCSELs.
Trend
As the speed per lane rises, the reach of multimode fiber gets shorter, so single-mode fiber is being used more widely even inside data centers.

FAQ

Why can't multimode fiber reach far?

Because the core is thick and many modes (light paths) travel at the same time. Each mode travels a slightly different distance at a slightly different speed, so a pulse that started out as one spreads out in time by the time it arrives (modal dispersion). Faster signals have shorter intervals between pulses, so their reach is shorter.

Is single-mode fiber more expensive?

The fiber itself is, if anything, cheaper for single-mode. The cost tends to be in the optical transceivers: coupling light into a thin core of about 9 µm requires a laser with stable output and alignment with sub-µm precision. Over short distances, multimode, which can use VCSELs, is often cheaper for the link as a whole.

Why is 850 nm light commonly used with multimode fiber?

VCSELs around 850 nm can be made cheaply in volume, tested at the wafer level, and coupled easily to multimode fiber. Multimode fiber is also designed to have wide bandwidth at this wavelength.

What limits the distance of single-mode fiber besides modal dispersion?

Loss and chromatic dispersion. Loss is at its minimum around 1550 nm (about 0.2 dB/km), and the chromatic dispersion of standard single-mode fiber is nearly zero around 1310 nm. Over long distances, optical amplifiers make up for loss, and dispersion is handled by compensation or signal processing.

Check your understanding

Which correctly describes optical links combining VCSELs at around 850 nm with multimode fiber?

  1. AThe components are inexpensive and alignment is easy, but reach is limited to short distances of tens to hundreds of m
  2. BThey use the wavelength band with the lowest fiber loss and are mainstream for long-haul communication exceeding several hundred km
  3. CSilicon transmits 850 nm light well, making them ideal for direct connection to silicon waveguides
Show answer and explanation

Answer: A. The components are inexpensive and alignment is easy, but reach is limited to short distances of tens to hundreds of m

VCSELs are surface-emitting lasers that can be tested on the wafer, mass-produced inexpensively and consume little power. Multimode fiber has a thick core, such as 50 µm in diameter, so alignment is easy. As a result, they have been widely used for short links within and between racks connecting servers and switches. On the other hand, multimode fiber suffers modal dispersion, in which the multiple propagation paths (modes) arrive at different times, so reach shortens as speed rises (at the 100 Gb/s-per-lane class, about 50–100 m over OM4 fiber and even shorter over OM3), and fiber loss at 850 nm is also relatively high, around 3 dB/km. Silicon absorbs 850 nm light, so it cannot be used for waveguides.

For relatively short-reach optical links inside data centers, which is a correct reason wavelengths around 1.31 µm (O-band) are often used?

  1. AFiber loss is the lowest of all wavelengths, so signals reach thousands of km without amplifiers
  2. BChromatic dispersion in standard single-mode fiber is nearly zero, so high-speed signal waveforms are less distorted
  3. CThe photon energy equals silicon's band gap, so silicon can be used as-is for the photodetector
Show answer and explanation

Answer: B. Chromatic dispersion in standard single-mode fiber is nearly zero, so high-speed signal waveforms are less distorted

Chromatic dispersion is the spreading of optical pulses caused by slight differences in propagation speed through the fiber at different wavelengths. In standard single-mode fiber, dispersion is nearly zero around 1.31 µm, so high-speed signals can easily be sent a few km to about 10 km without dispersion compensation, and many data center optical transceivers use this band. Loss is lowest around 1.55 µm (about 0.2 dB/km); at 1.31 µm it is somewhat higher, about 0.3–0.35 dB/km. The photon energy at 1.31 µm is about 0.95 eV, which silicon does not absorb.

In long-haul optical fiber communication, what is the main reason wavelengths around 1.55 µm (C-band) are widely used?

  1. AIt is the band where loss in silica optical fiber is lowest
  2. BIt is the band where silicon absorbs light most strongly, making photodetectors easy to build
  3. CIt is the band where chromatic dispersion is zero, so pulses do not spread at all
Show answer and explanation

Answer: A. It is the band where loss in silica optical fiber is lowest

Loss in silica (SiO2) fiber increases from Rayleigh scattering at short wavelengths and from infrared absorption at long wavelengths, so it is lowest in between, around 1.55 µm (about 0.2 dB/km). Even after 100 km the loss is only about 20 dB, and the ability to amplify the light directly in the optical domain with erbium-doped fiber amplifiers (EDFAs) is another reason the C-band is used for long-haul communication. In standard single-mode fiber, chromatic dispersion is nearly zero around 1.31 µm (O-band); dispersion remains at 1.55 µm. Silicon barely absorbs this wavelength.

  1. Which is a characteristic of a VCSEL (vertical-cavity surface-emitting laser)?
  2. Why is active alignment—aligning while actually passing laser light through and measuring the output—often used when attaching single-mode fiber to a silicon photonic chip?

Practice Silicon photonics questions

Key terms explained

Single-mode fiber
Optical fiber that carries only one propagation mode, with a core diameter of about 9 µm. With no pulse spreading from different modes traveling at different speeds (modal dispersion), it is used for long-distance and high-speed links.
VCSEL (vertical-cavity surface-emitting laser)
A semiconductor laser that emits light perpendicular to the substrate. It can be tested on the wafer and easily arranged in 2D arrays, so it is used in data links and as the light source for face recognition.
Chromatic dispersion
The broadening of light pulses in an optical fiber because light of different wavelengths travels at slightly different speeds. In standard single-mode fiber it is nearly zero near 1.31 µm and about 17 ps/(nm·km) at 1.55 µm.
O-band
The optical fiber communication band around 1260 to 1360 nm. It includes the region near 1.31 µm where chromatic dispersion in standard single-mode fiber is nearly zero, and is widely used for short links such as those inside data centers.
C-band
The optical fiber communication band around 1530 to 1565 nm. Silica fiber loss is lowest there (about 0.2 dB/km) and erbium-doped fiber amplifiers (EDFAs) can amplify it, so it is used for long-haul links and dense wavelength division multiplexing (DWDM).

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