Fiber Facts – Single-mode vs. Multi-mode Fiber

Copper cabling is still widely used in most data centres, but over the last few years, many data centres have made the switch to optical fiber. Optic fiber is still a relatively new technology and not everyone fully understands the fundamental aspects of optic fiber. In this article, we clear some of the misconceptions you may have about optic fiber.

Optic fiber cabling is categorised into single-mode fiber and multi-mode fiber. Single-mode fiber is costlier than multi-mode fiber, but there are other factors involved apart from the price which have to be considered as well, before you make a decision on which cabling system to procure for your data centre. By this, we mean factors such as attenuation.

What is attenuation? Attenuation refers to the higher weakness of fiber optic signals as the distance covered by the cable increases. This is what one called dB loss. Single-mode fiber cables suffer a much higher dB loss compared to multi-mode fiber, which explains why they are much pricier.

Single-mode fiber cables perform better because their fiber cores are only 9 mm in diameter. This means the light that passed through these cables is not reflected too many times, which keeps the attenuation to a minimum. So you can use the single-mode fiber for long range connections and for applications that require higher speeds.

Multi-mode fiber cables have a 50 mm or 62.5 mm diameter. The attenuation suffered by the light that passes through these cables is kept to a minimum at short distances, but increases over long distances. That’s because the core has a bigger diameter, and this leads to more light reflections.

The wavelength is another important factor to be considered when you are choosing between a single-mode fiber and a multi-mode fiber. We discuss that next.

Fiber Wavelengths and How They Affect Attenuation

There are 3 types of wavelengths used in fiber optics. Single-mode fiber cables make use of the 1550 nm wavelength, while multi-mode cables make use of 850 nm and 1300 nm wavelengths. The 3 wavelengths listed here have close to 0% absorption even when water vapor is collected in the glass, which leads to attenuation.

Scattering is an important factor to be considered when discussing wavelengths and how they affect the attenuation of the light. Scattering refers to how the light bounces off the atoms within the glass. It is much higher at shorter wavelengths and decreases at higher wavelengths.

So single-mode fibers, which have higher wavelengths of 1550 nm have less scattering and hence less attenuation of the light signals. This ensures a superior quality of communication over long distances.

Loss of Light in Passive Optical Network

Passive Optical Networks are those that are built on a low budget. Generally, there is a certain amount of light lost when it passes through a passive optical network – this cannot be avoided. There is no specific industry standard for measuring this, but there are three areas in which you can measure the loss of fiber optic light as it passes through a passive optical network.

#1: The loss measured through splitters.
#2: The loss measured through splitters and the loss measured through one mated pair.
#3: The loss measured through splitters and the loss measured through two mated pairs.