Patch cables are indispensable in today’s fiber cabling system, as they provide significant flexibility, therefore spare loss of time, energy & cost. However, as they’re very complicated, some novices are still bewildered about which mode to choose. Here we’ll focus on comparing single-mode patch cords and multimode patch cords to be helpful to newbies.
What is the mode?
An optical fiber is indeed a cylindrical waveguide, and as the famous wave-particle duality, light is really an electromagnetic wave. Mode describes the distinct pattern of the distribution of light energy across the fiber. The number of modes depends on the fiber’s acceptance angle, core diameter, and the wavelength of the light. Among them, the dominant factor is the core diameter.
Figure 1: Propagation modes
What is single-mode?
Single-mode fiber could transmit light in only a single mode due to its small core size, varying from 6 to 10µm, generally around 9µm. There is one and only one path for light rays to travel, which is directly along the axis of the fiber cable.
Long-distance fibers are always single-mode fibers, as this well-controlled light can carry signals at the highest speed with little attenuation.
Standard single-mode fibers have a cladding index only about 0.5% lower than the core index. The main limitation of single-mode fiber lies in that its core size must be small enough to confine the light into a single-mode, while it has to be large enough to collect most of the input optic signal.
In the early 1980s, the first long-distance transatlantic backbone networks were developed for telecommunication purposes using single-mode fiber as the communication medium and optical sources at 1300 nm wavelength.
Single-mode fibers are standard for long-distance, high-performance systems.
Figure 2: Single-mode propagation
What is multimode?
Large core fibers carry multiple modes. The “multimode” refers to “multiple light rays” from an optical source that propagates through the fiber core following different paths.
Various modes travel down the fiber at different speeds, generating the modal dispersion effect. Multimode propagation causes more dispersion, whereas single-mode fiber has much less dispersion.
However, multimode transmission is much better than single-mode transmission, as large cores are designed to collect as much light as possible. Typically, multimode fibers have 50µm or 62.5µm core diameters.
Concerning the traditional short supply of bandwidth, the technicians have developed the graded-index multimode fiber to significantly improve the transmission capacity than step-index multimode fibers. The modal dispersion of graded-index fibers is resolved. Therefore, the graded-index multimode fibers can carry higher-speed signals, yet their usage is limited to shorter reach, no farther than a few kilometers.
Figure 3: Multimode propagation
The trade-off of the single-mode
Single-mode fiber is more expensive than multimode fiber, as the coupling light into single-mode fiber inevitably requires much tighter tolerances. After all, the smaller the core diameter, the harder it is to couple light into the fiber.
But the single-mode transmission is cleaner and more straightforward. From the maintenance point of view, a single-mode cabling system requires less time & energy than a multimode cabling system.
In a word, single-mode fiber is always the preferred one in long-haul telecommunications.
The trade-off of the multimode
The multimode fibers are preferred when the installation budget is tight and the deployment distance is modest. Because within short reach, it offers much higher transmission capacity at high speed and costs inexpensively because the larger core size, the easier input coupling, also the numerical aperture is larger than single-mode fiber.
Comparison between single-mode and multimode
Next, we’ll compare the two types in detail, including wavelength, light source, color code, working distance, etc.
Wavelength & Light Source
Single-mode fiber usually uses a laser diode, typically widely used in telecommunications systems operating at 1310 and 1550 nm.
Multimode fiber often uses low-cost light sources, such as Light-Emitting Diodes, aka LEDs, or Vertical-Cavity Surface-Emitting Lasers, aka VCSELs, which work at the 850 and 1300 nm wavelength. Many short-range systems use a wavelength of about 820 nm, as LEDs are less costly than those designed for longer wavelengths.
Single-mode fiber has no limit in bandwidth, while multimode fiber is limited to about 28000 MHz*km of OM5 fiber for now.
The TIA-598C regulates that the outer jacket of the single-mode cable is yellow-colored while the outer coat of the multimode cable is orange- or aqua-colored.
Single-mode has no limit in transmission distance, yet multimode only applies for short reach, generally within 550m.
Single-mode optical transceiver is much more expensive than multimode transceivers due to their difficulty in coupling light.
While in deployment, single-mode fiber often costs less than multimode fiber.
The longer the operating wavelength is, the better the system performance is, but at a much higher cost.
Generally speaking, when choosing between single-mode and multimode fiber, if your budget is adequate, always select single-mode fiber unless your cabling system is within 550m. If so, multimode fiber would be the best option for its transmission capacity and cost-effectiveness.
Hecht, Jeff. Understanding Fiber Optics. Auburndale, MA: Laser Light Press, 2015.
Singal, Tarsem Lal. 2016. Optical fiber communications: principles and applications. Delhi: Cambridge University Press.