Whilst browsing our products (such as our SFP transceivers) you may have come across the term singlemode fibre and multimode fibre (or SMF and MMF to use their abbreviated terms). These terms refer to the different types of fibre cable available. We’ll explore each of them below:
Firstly, let’s look at Singlemode fibre (SMF). The core (centre of the fibre) size of singlemode fibre is much smaller than multimode, because it’s only a single wavelength of light that’s travelling down the cable. Due to this, the number of light reflections that are created within the core decreases, which means that the signal can travel further. Singlemode cables usually have much more cladding within them.
In comparison, Multimode fibre (MMF) has a much larger core, which means that multiple modes of light are able to ‘propagate’. This in turn increases the number of reflections created, causing more light to pass through the core, which means that more data is able to pass through at any given time. The cladding to core ratio is much closer with multimode fibre due to the larger core diameter.
For long distances, such as between buildings etc., singlemode fibre is necessary. This is because, as mentioned, there is less attenuation (which is the rate at which the signal light decreases in intensity). You can liken this to a passenger train that starts at one end of a country and only has one stop at the other end of the country.
Conversely, multimode fibre can only operate over short distances (such as within local networks) because of the much higher attenuation. If we look again at the passenger train metaphor, we can imagine a train that stops at every station in the country before arriving at the final stop. If both trains had the same amount of fuel, the ‘multimode train’ would be much further away from its final stop when it’s fuel ran out than its singlemode counterpart would be.
Singlemode fibre operates at 1310nm and 1550nm, whereas Multimode fibre is designed to operate at 850nm and 1300nm. It’s also possible to use wavelength-division multiplexing (WDM) systems that are able to use the full range of wavelengths between 1260nm and 1670nm simultaneously.
In terms of colour, as a general rule, singlemode fibre is coated with a yellow outer sheath, whereas multimode fibre is coated with an orange or aqua jacket, depending on the type (OM stands for 'Optical Multimode' and OS stands for 'Optical Singlemode' - based on the construction of the cable, there are different types. You'll often see OS1, OS2, OM1, OM2 etc). You may also see violet sheaths that are sometimes used to denote OM4 fibre.
In summary, whether you choose singlemode or multimode fibre completely depends on the requirements of your project, and how far you need to cable to go. For example, in a datacentre, multimode cable is fine (and cheaper). Just be aware, that you can’t mix transceivers and cables with different modes (such as an MMF transceiver with SMF fibre), though that shouldn’t be a problem if you buy from Cable Intelligence as we stock transceivers and fibre of both types!