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

2023-03-17

I recently wrote about using 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 in-depth 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. On the other hand, you may have some multi-mode fiber in your data center or at your desk. That is, multi-mode 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 can transmit 100Gb at shorter distances.

The 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, 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 functions of very complex mathematics.

Since multi-mode fiber is more significant, it can use less expensive LEDs (for OM1 and OM2) and lasers (OM3 and OM4) than 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 many television channels, for example.

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

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.

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This piece was originally posted on Apr 9, 2019, and has been refreshed with updated styling.

Written by John McDermott

John McDermott, CPTD, started his work in computer security in 1981 when he caught an intruder in a system he was managing. In recent years his consulting has included security consulting for small businesses. He is Security+ and CCP certified. In his 30 years with Learning Tree John has written and taught courses in programming, networking and computer security. He is the co-author of Learning Tree’s course System and Network Security: A Comprehensive Introduction. John is currently a learning and development consultant in northern New Mexico. He lives in a house made of earth with his wife, who is an artist.