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Single-Mode Or Multi-Mode Fibre Optic Cable?

When you choose to install fibre optic cabling, you have some choices. Let’s understand both the single-mode and multi-mode fibre optic cable types.

1. Single-mode: single path through the fibre

Singlemode cable is a single strand of glass fibre with a diameter of 8.3 to 10 microns that has one mode of transmission. It has a relatively narrow diameter, through which only one mode will propagate (typically 1310 or 1550 nm). It carries a higher bandwidth than multi-mode fibre but requires a light source with a narrow spectral width.

Single-mode fibre is used in many applications where data is sent at multi-frequency (WDM Wave-Division-Multiplexing) so only one cable is needed- (single-mode on one single fibre). It gives you a higher transmission rate and up to 50 times more distance than multi-mode, but it also costs more. Single-mode fibre has a much smaller core than multi-mode.

The small core and single light-wave virtually eliminate any distortion that could result from overlapping light pulses, providing the least signal attenuation and the highest transmission speeds of any fibre cable type.

2. Multi-mode: multiple paths through the fibre

Multi-mode fibre has a little bit bigger diameter, with common diameters in the 50-to-100 micron range for the light carry component. In most applications in which Multi-mode fibre is used, 2 fibres are used (WDM is not normally used on multi-mode fibre).

Multi-mode fibre gives you high bandwidth at high speeds (10 to 100MBS – to 275m to 2km) over medium distances. Light waves are dispersed into numerous paths, or modes, as they travel through the cable’s core typically 850 or 1300nm.

Typical multi-mode fibre core diameters are 50, 62.5, and 100 micrometres. However, in long cable runs (greater than 3000 feet [914.4 meters), multiple paths of light can cause signal distortion at the receiving end, resulting in an unclear and incomplete data transmission, so designers now call for single-mode fibre in new applications using Gigabit and beyond.

What option should I choose?

Single-mode fibre and multi-mode fibre are not compatible. You cannot mix multi-mode and single-mode fibre between two endpoints and the optics are not compatible either.

In most circumstances, multi-mode fibre is a more cost-effective choice for data centre applications where cable runs are less than 550 meters. Single-mode fibre is best used for distances greater than 550 meters. Besides the transmission distance, the overall cost should also be taken into consideration.

Even though single-mode fibre cable is less expensive than multi-mode cable, the cost of transmission devices for single-mode is much higher. Over the last number of years, we have seen an increase in the use of single-mode fibre where multi-mode would have been the preferred choice. It seems that communications consultants, engineers and equipment manufacturers are looking ahead and addressing the need for higher bandwidth and speeds in preparation for future technologies.

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