What is the difference between single mode and multimode?

There are two main types of fiber for telecommunications: multimode and single mode. What are the differences between these, and when should you use single mode or multimode?

The structure of a fiber optic cable is the same for both single mode and multimode. The fiber consists of two parts, a core and a cladding, both made of glass. These two parts are doped with substances that give them slightly different properties (refractive index). This allows light to stay confined within the fiber, as long as it is not bent too sharply. A primary coating of plastic is applied to the fiber for protection.

Multimode fiber has a core diameter of either 62.5 micrometers (µm) or 50 micrometers. Single mode fiber has a core diameter of 9 micrometers.

MULTIMODE

The light in a multimode fiber consists of multiple different light beams, or modes, hence the name multimode. The disadvantage of this is that the different beams travel at different distances. The signals transmitted through the cables are in the form of pulses, and when these pulses have traveled a certain distance, they no longer arrive simultaneously, making the pulses unreadable. Therefore, multimode fiber has clear limitations in terms of transmission distance, especially at high data rates.

Despite this, multimode is still used to a certain extent in environments such as office buildings or data centers where the signals do not need to be transmitted over long distances. Another advantage is that transceivers (optical modules) consume less power and generate less heat, which affects the cooling requirements. Another valid argument could be that there is already an existing multimode network that one wishes to expand.

SINGLE MODE

In a single mode fiber, the core is so small that only one light beam is transmitted. This eliminates the limitations of multimode fiber, allowing signals to be transmitted over very long distances. Additionally, it is possible to send light in parallel with different wavelengths, creating multiple separate channels within the same fiber.

Single mode fiber is used in all scenarios where signals need to be transmitted over long distances. It is also increasingly used in applications where multimode traditionally was used, such as indoor cabling. Single mode is preferred for high data rates (10 Gbit/s and above).

Transmission Distance MM, OM2: 1 Gbit/s = 550 meters, 10 Gbit/s = 300 meters
Transmission Distance SM, OS2: 1 Gbit/s = over 160 km, 10 Gbit/s = over 80 km

Note that these transmission distances are without signal amplification along the way.