Single-mode vs. Multimode Fiber

Today, along with the surging technological development of Internet networks, people are used to watching 4K videos, and VR and even 8K monitors are increasing in popularity. Yet, we often ignore the infrastructure of cabling systems. How many people know the difference between single-mode and multimode fiber? Or put in another way, do you want to know their distinctive features? If you do, then this article is for you.

Classification of Optical Fibers

Based on different characteristics, such as the refractive index profiles of the fiber core, modes of propagation of light in the fiber, and the improvement in the propagation properties, the optical fibers can be classified as Step-index optical fiber vs. Graded-index optical fiber, Single-mode optical fiber vs. Multimode optical fiber, and Polarization-maintaining optical fiber vs. Dispersion-shifted optical fiber vs. Dispersion-flattened optical fiber. Here we only deal with the modes of propagation.

What is the mode?

Mode, in telecommunication terms, refers to the specific pattern a light travels along the fiber. As T. L. Singal defines it, the mode is the path traversed by light propagation within the fiber. ¹ These propagation modes are called modes of the optical fiber.

Modes differ in propagating angles, i.e., incidence or reflected angles.

What is single-mode?

Singlemode is exactly the fundamental mode, also called the zero-order mode, meaning the propagating mode of the light travels precisely along the central axis of the optical fiber. When the diameter of optical fiber is small enough that there is only one possible path for light rays to travel through, then it is singlemode fiber.

The figure below shows the single-mode propagation.

Source: Tarsem Lal Singal, Optical Fiber Communications: Principles and Applications. All figures, unless otherwise indicated, are credited from this book.

Obviously, in singlemode fiber, there is one and only one path for light rays to travel directly along the axis of the fiber cable.

What is Multimode?

Conversely, if the diameter of optical fiber is large enough to allow more than one path for light rays to travel through, then it is a multimode fiber.

As the name implies, the multimode signifies “multiple light rays” from an optical light source propagating through the fiber core via different paths, as described in the figure below. The different ways of light rays are owing to the core structure. The light rays bounce between the cladding, following separate paths, propagating in a zigzag manner.

[1]  Tarsem Lal Singal, Optical Fiber Communications: Principles and Applications (Delhi, India: Cambridge University Press is part of the University of Cambridge, 2016), p. 59.

What is dispersion?

The big difference between singlemode and multimode propagation lies in the dispersion. Dispersion, also called pulse spreading, refers to the spreading out of an optical pulse of light energy in time as it propagates down a fiber. The time lapse of the propagation time taken by the light rays that traverse different propagation paths within the fiber results in dispersion. More considerable dispersion means a more substantial possibility of interference between pulses, leading to worse performance.

Then we’ll discuss the dispersion in singlemode and multimode fiber, respectively.

Dispersion in a single-mode step-index fiber

As shown in the figure, there is only one single transmission path that all rays must follow. Each light ray travels the same distance in a given time; thus, there is virtually no intermodal dispersion. Due to the intramodal group velocity dispersion (GVD), a light pulse propagating as a fundamental mode still causes the broadening of the transmitted optical pulse.

Dispersion in a multimode step-index fiber

In contrast, in a multimode step-index fiber, the propagated light energy would spread out over time, as the different distances that rays travels imply a considerably longer traveling time. This is known as intermodal dispersion, or modal dispersion, which results in a stretched and reduced-in-amplitude-pulse at the output of the fiber.

Dispersion in a multimode graded-index fiber

As shown in the figure above, in a multimode graded-index fiber, although each light ray traverses a different path, they spend virtually identical time propagating through the optical fiber, thanks to the index of refraction, which decreases with radial distance from the center axis of the fiber core toward the cladding.

Compared to the singlemode optical fiber, multimode fiber exhibits a large intermodal dispersion(nearly nonexistent in the singlemode fiber). In multimode fiber, choosing an appropriate multimode graded-index profile fiber helps to reduce the intermodal dispersion considerably.

International standards for optical fibers

The practical meaning of the difference between singlemode and multimode fiber

We’ve introduced the technique aspect of the difference between singlemode and multimode fiber. But what matters most in real life?

We need to discuss several other aspects to answer this question, like the core diameter, light source, bandwidth, color sheath, distance, and cost.

7~9 µm vs. 50/62.5 µm core diameter

As stated previously, singlemode fiber core is much smaller than multimode fiber, typically around 7~9 µm vs. 50 µm(legacy OM1 core diameter 62.5 µm excluded).

Laser or laser diode vs. LED optical source

Meanwhile, a smaller singlemode core diameter requires a tighter tolerance than a large multimode core to couple light inside; thus, it demands a much more expensive optical source like a laser or laser diode working at 1310nm and 1550nm wavelength to inject light from the transceiver into the core. In comparison, a large multimode core only needs a less expensive light source, like LED(light-emitting diodes) and VCSELs (vertical-cavity surface-emitting lasers) working at 850nm and 1300nm wavelength, to inject light into the core.

Long haul vs. short reach

Due to its fundamental mode feature, the singlemode fiber can propagate long haul with low signal attenuation, so it is ideally suitable for transmitting data over long distances such as 100 km (60 miles). By comparison, the multimode has significant attenuation over distance because of its large core, so it can only support transmission reach less than several hundred meters.

Bandwidth and data rate

Though limited in bandwidth, multimode fiber can support a higher data rate due to its large core diameter; for instance, OM3/OM4/OM5 can support 40/100Gb within 400m. The maximum bandwidth at present is 28000MHz*km of OM5 fiber. However, OS2 singlemode fiber could support up to 200km distances in 40G and 100G links. Theoretically, singlemode fiber bandwidth is unlimited.

Source: Single Mode Fiber Optical Cable VS Multimode Fiber Optical Cable

Color Sheath

Singlemode fiber is coated with yellow, while multimode fiber is coated with orange or aqua.

Cost

The singlemode fiber, per se, is cheaper than the multimode fiber. However, due to its more stringent demands, its related optics are much more costly than multimode fiber; for example, the price of singlemode transceivers is nearly two or three times higher than that of multimode transceivers.

Generally speaking, the multimode fiber system offers the lowest system cost and upgrades path to 100G for standard-based premises applications using parallel-optic based interconnects.

Summary

The single-mode fiber is preferable for long-haul data transmission applications, like carrier networks, MANs and PONs. On the contrary, the multimode fiber is suitable for enterprises, data centers, and LANs because of its short reach and higher data rate.