What Is The Difference Between Single-mode and Multi-mode Fiber?

I recently wrote about the use of fiber optic cable for physical intrusion detection. In that post, I mentioned single- and multi-mode fiber. Here, I’d like to discuss the two types in more detail. This is a bit of a technical overview, so if you are looking for deep details or a non-technical explanation, this may not be for you.

If you have fiber providing data to your home or office from some provider, it is more than likely single-mode fiber or SMF for short. You may have some multi-mode fiber in your data center or at your desk. That is, multimode fiber is primarily for local area networks.

What are the fiber characteristics?

There are four types of multi-mode fiber summarized in the table below

Designation Typical Jacket color Max distance at 1Gb Max Distance at 10Gb Max Distance at 100Gb
OM1 Orange 550m 33m
OM2 Orange 550m 82m
OM3 Aqua 550m 300m 100m
OM4 Aqua 550m 400m 150m

OM1 has a core (the actual fiber) size of 62.5 micrometers while the others have a core size of 50 micrometers. It seems counterintuitive that a smaller core means higher speeds, but it does. We’ll talk more about that in a bit.

Single-mode fiber has a core size of 8.5-10 micrometers and can send 10 Gb Ethernet up to around 200km. There are two types of SMF: one for indoor use and one for outdoor use called OS1 and OS2 respectively. OS1 is much less expensive than OS2 but has a distance limit of about 10km. Some single-mode systems have the ability to transmit 100Gb at shorter distances.

Beam profile of a single-mode (left) and a multi-mode (right) fiber

What is the difference between single- and multi-mode fiber?

The actual difference relates to how the light is carried down the fiber. In SMF light is carried only down the fiber which is called the “transverse mode”. In MMF the light can be carried in other modes. Modes relate effectively to the pattern of the light transmitted but are actually functions of very complex mathematics.

Since multi-mode fiber is larger, it can use less expensive LEDs (for OM1 and OM2) and lasers (OM3 and OM4) compared to SMF.

A technique called “Wave Division Multiplexing” uses light of different wavelengths (colors) to increase the capacity of (generally SMF) fiber. Depending on the number of wavelengths used this can increase the fiber speed well over 100 times! This is how providers can offer large numbers of television channels, for example.

Things can get complicated, though. Some SMF cable uses multiple cores to increase the maximum rate, for instance.

The world of fiber optics is complex and changing. Hopefully, this short introduction will help you see some of the differences between the two types.

image sources

  • 256px-Beamprofile_SM+MM-Fiber: Kirnehkrib [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)] https://commons.wikimedia.org/wiki/File:Beamprofile_SM%2BMM-Fiber.jpg

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