Single vs multi mode fiber

A fiber-optic cable, also known as an optical-fiber cable, is a cable that is one or more optical fibers that are used to carry light. The optical fiber elements (typically made of glass) are typically individually coated with plastic layers and contained in a protective tube suitable for the environment where the cable is used. Different types of cable[1] are used for fiber-optic communication: single mode, multi mode and POF (plastic optical fiber).

For most fiber optic applications single mode and multi mode fiber optic cable made of glass is most suitable. Single mode and multi mode are the two transmission methods for fiber optics- having to do with how they get the signal to the other end of the glass. They are not interchangeable, so it’s important to know which you have. Single-mode fibers provide a single pathway for light to travel and are defined by their small core size of approximately 8.3 µm. Multimode fibers, on the other hand, have various paths, or modes, in which light can travel through optical fiber. These core sizes are larger, ranging from 50 µm to 62.5 µm.

Single Mode Fiber Optic Cable
Core Diameter: Approximately 8-10 micrometers.
Light Propagation: Uses a single light path (mode) to transmit data.
Wavelengths: Typically operates at 1310 nm and 1550 nm.
Distance: Ideal for long-distance communication (up to 100 km or more).
Bandwidth: Higher bandwidth and lower attenuation, providing high data transfer rates over long distances.
Applications: Long-haul telecommunications, cable television, and Internet backbones.

Multi Mode Fiber Optic Cable
Core Diameter: Approximately 50-62.5 micrometers.
Light Propagation: Uses multiple light paths (modes) to transmit data.
Wavelengths: Typically operates at 850 nm and 1300 nm.
Distance: Suitable for shorter distances (up to 2 km for slower speeds and up to 550 meters for high-speed networks).
Bandwidth: Lower bandwidth compared to single mode, with higher attenuation over long distances.
Applications: Local area networks (LANs), data centers, and intra-building networks.

Single mode fiber has very thin center core. You need a laser source that is very accurately shooting signal to it and all cable splices need to be very accurate. In typical telecommunications fiber, single mode operation is obtained with core diameters of 2–10 microns with a standard outer diameter of 125 microns. Once you get signal to single mode fiber, it will go through it with very little loss and other signal quality issues. Single-mode fibre are used almost universally in telecommunications over 1 km or so and are generally used at the 1300 nm and 1550 nm wavelengths. (Below 1100 nm, the Rayleigh-scattering dominates, while above 1600 nm the infrared absorption dominates). Single-mode fibers used in telecommunications typically operate at 1310 or 1550 nm and require laser sources (that some were expensive and many are still are expensive). OS1 and OS2 are standard single-mode optical fiber used with wavelengths 1310 nm and 1550 nm (size 9/125 μm) with a maximum attenuation of 1 dB/km (OS1) and 0.4 dB/km (OS2). Single-mode fibres are capable of wide bandwidths (e.g. >40 GHz) and are, therefore, ideally suited for long-haul and high capacity circuits.

Multimode fiber has core diameters considerably larger, typically 50, 62.5, 85, and 110 microns, again with a cladded diameter of 125 microns. Because of their larger core size, multi-mode fibers have higher numerical apertures which means they are better at collecting light than single-mode fibers. Multi mode fiber has thicker core, so sending light to it is easier (cheaper led technology will work and does not necessarily need laser transmitter). Multi-mode fiber has higher “light-gathering” capacity than single-mode optical fiber. In practical terms, the larger core size simplifies connections and also allows the use of lower-cost electronics such as light-emitting diodes (LEDs) and vertical-cavity surface-emitting lasers (VCSELs) which operate at the 850 nm and 1300 nm wavelength. Also cable splices do not need to be micrometer accurate to work OK. Multi mode is easier to work with and hardware for it is cheaper. As the name implies, multimode fibres are capable of propagating more than one mode at a time and they are ideally sited for high bandwidth (i.e. a few GHz) and medium haul applications. The disadvantages are much higher signal loss and more signal distortion – meaning supports less distance and less data bandwidth compared to single mode. Multimode fibers tend to have higher attenuation than single-mode fibers since the intrinsic loss of the multimode fiber is higher due to the natural loss of the fiber in the operating wavelengths of 850 nm and 1300 nm. The LED light sources sometimes used with multi-mode fiber produce a range of wavelengths and these each propagate at different speeds, which limits the available bandwidth over distance.

When transporting data over fiber optic cable, the transceivers have to match the cable type used. This means that the connector type in the transceiver and fiber optic type need to match. LC is the connector type tat is pretty common to find those when using SFP’s in network gear. Other connectors you may run into would be LC, SC, ST, and FC.

You need to select the transceivers that are designed to work with fiber type you have. Generally if you try to use multi mode transceivers with single mode cable, it will not work. Also trying to use single mode transceivers with multi mode cable does not give good results or work at all. Mixing different cable types on one fiber optic link is a recipe for a disaster.

Jacket color is sometimes used to distinguish multi-mode cables from single-mode, but it cannot always be relied upon to distinguish types of cable. The standard TIA-598C recommends, for civilian applications, the use of a yellow jacket for single-mode fiber, and orange for 50/125 µm (OM2) and 62.5/125 µm (OM1) multi-mode fiber.[3] Aqua is recommended for 50/125 µm “laser optimized” OM3 fiber.

Can you mix and match single mode and multi mode components / fiber? In some rare cases yes.

There are some special adapters that allow some mixing:
“The mode conditioning cables have allowed us to succesfully run Gb ehternet over our multimode cable using Single mode transcievers, but only at distances that would have worked using multimode transcievers. It did not work where we were exceeding the distance spec for multimode, must have been too much loss in the cable…”
“we just successfully ran from a SM Finisar FTLF1323P1BTR over a 400’ run of MM fiber to a media converter and it worked. I was trying to research this at this site so I posted our results.”
“I’m using 1000Base-LX/LH singlemode transceivers over 62.5 multimode fiber in several places, including a link over 1000’ long between two buildings. I do use mode-conditioning patch cables.”
Mode Conditioning Cable Selector: Launching a single-mode laser into the center of a multimode fiber can cause multiple signals to be generated that confuse the receiver at the other end of the fiber. A mode conditioning patch cord eliminates these multiple signals by allowing the single-mode launch to be offset away from the center of a multimode fiber. This offset point creates a launch that is similar to typical multimode LED launches. “The launch of the light coming out of the equipment begins on a Singlemode fiber. The Singlemode fiber is precision fusion spliced to the multimode fiber to a precise core alignment”

In a research laboratory environment I have run setup where a single mode transmitter sense signal to single or multi mode cable, and the signal was received with multi mode receiver. Also setup where single mode transmitter, single mode fiber, multimode fiber and multimode receiver has worked. Generally the direction that light goes from single mode to multi mode cable works somewhat OK, while the direction from multi mode to single mode causes a very high attenuation.