Written by Ben Hamlitsch, trueCABLE Technical and Product Innovation Manager RCDD, FOI

One of the most confusing aspects around fiber optic cabling technology is the difference between Single mode Fiber (SMF) and Multimode Fiber (MMF). When shopping around and choosing the right fiber optic cable for your application, there is a huge variety of fiber counts and connector methods with little to no explanation to help. This blog will focus on the basics. In future blogs we will get into more advanced topics such as jacket types, multi-fiber count cables, and more.

What is the Difference Between Singlemode and Multimode Fiber?

The difference between SMF and MMF comes down to how light behaves as it is transmitted down the fiber core. This behavior is driven by the construction of the fiber optic cable. How the light travels down the core of the fiber impacts possible transmission distance and speed (or both) that each type of fiber optic cable can achieve. We will talk about:

• Modality differences. This simply means exactly what the differences are in the light transmission characteristics.
• Physical construction differences. Multi mode fiber optic and Single Mode fiber optic cable appear similar but there are significant differences in construction.
• We will present some high level differences in speed and distance capability between the two.
• Integrating fiber into the modern structured cabling system

Modality Differences

Well, a picture will go a long way to help explain this part!

Multi-mode fiber optic has a core that exceeds the cut-off wavelength of the light pulse, resulting in modal dispersion. Think of modal dispersion as signal degradation caused by light bouncing off the walls as it travels through the fiber cable. This bouncing effect scatters the light and results in more modes of light going down the cable than you would like…hence “multi-mode”. This is NOT a feature, and is not technically desirable. As improvements were made to MMF core and cladding, the OM type increased. For example, OM3 is better than OM2 due to better reduction of modal dispersion giving higher OM types of multi-mode fiber longer reach with better bandwidth. That said, only so much can be achieved by optimizing the core and cladding. You have to make a bigger (actually, smaller) change.

Single mode fiber has a core that is physically sized to carry a single wavelength of light, nearly eliminating the modal dispersion and scattering effect. This was done by reducing the core diameter and tuning the cladding to not exceed the cut-off wavelength. You heard that correctly: The size of the core literally defines what modes the fiber optic cable can carry! The benefits are greatly increased speeds and distances. Of course there is always a tradeoff, and this comes at the need for more advanced and powerful lasers to transmit the data. This all seems rather counterintuitive doesn’t it? One would think a bigger pipe would allow for more data and therefore more speed. Well, with fiber optic cable the reverse is true. A bigger pipe just allows for more garbage to flow down it, and this results in crowding out the actual light you want.

Physical Construction Differences

We now know that multi-mode fiber optic cable has a larger core. Single mode fiber has a smaller core. What exactly does this look like? Well, a picture is needed. Just for reference, if you want to see the core of a multimode fiber cable (assuming you could remove the cladding) you would see a core that is as thin as fine human hair. Single mode fiber cores are so small you need a microscope!

The above image shows a cross section of the actual core and cladding of fiber optic cable, sans all the additional stuff like color coatings, buffering, and cable jackets. This is the beating heart of fiber optic cable. The core and cladding are fused together, and no matter what kind of fiber cable you are talking about the diameter will be 125 microns (µm). Multi-mode fiber has a larger 50 micron core and operates at 850nm (nanometer) light wavelength. Single mode fiber has a teeny 9 micron core and operates at 1310nm or 1550nm light wavelengths.

Just for reference, how does the 125 micron core/cladding look in relation to the rest of the cable? Let’s take a tight buffered fiber patch cord as an example:

It should be noted that in the above picture the overall diameter of that assembly is only 2mm! Compare this with the average Category copper cable like Cat6A which is typically 7.0mm and above, and you start to appreciate just how fiber optic cable is a huge benefit to installation. The advantages do not stop with physical size, they also come in speed and distance!

Fiber Optic Speed and Distance Comparison Chart

Fiber optic cable, regardless of whether it is single or multi-mode, confers huge advantages in bandwidth and distance over copper Ethernet cable. The differences between single mode and multi-mode fiber can be just as pronounced. What it all comes down to is bandwidth over distance. As your speed needs increase, the permitted distances get shorter. If you need 1G (1Gb/s) then you can go really far with fiber. With copper Cat6A cable you hit a hard limit of 328 feet from powered device to powered device, whether it be 1G or 10G. Let’s compare a few examples and then toss in Cat6A just for kicks.

*Note that these distances are “up to”. The type of light emitter, number of splices, and connectors affects this too!

**Cat6A distances are at 68℉ or less. Higher ambient temperature will shorten permitted run length.

Don’t you just love how everything in life comes with a caveat of some sort? Well, except for death and taxes.

I note that distances for fiber optic runs can vary depending on the type of transmitter (light source). Light sources come in three primary types:

• LEDs (light emitting diode). This is bargain basement technology and not typically used with fiber any longer.
• VCSELs (Vertical Cavity Surface Emitting Lasers). This is a mass produced and lower powered laser. This is the type of laser typically used with multi-mode fiber that is “laser optimized” (OM3, OM4, OM5) and required to run at 10Gb/s and above or up to the maximum 1,800 feet with 1Gb/s.
• Fabry-Perot (FP) or Distributed Feedback (DFB) type lasers. These are the more pricey and powerful lasers typically seen used with single mode fiber. These are the type of lasers associated with transmission of 10Gb/s at nearly 25 miles!

Technological Convergence

Given the stark differences between how far and fast you can go with various types of networking cable, a typical installation is going to be using a mix of technologies. This mix also includes copper Category cable too, as there is one big thing that copper Ethernet cable can do that fiber optic cannot do: Power over Ethernet (PoE).

Quite frankly the number of different types of installations is equal to the number of actual installations out there. No two are precisely the same due to a huge number of factors.

Single mode fiber optic cable has been typically associated with long haul runs from service providers to business and residential areas. This would make sense, as this type of fiber can run for miles between transmission nodes. Multi-mode fiber is typically seen in businesses where the run requirements don’t exceed 1,000 feet or so. This is often referred to as a “campus” installation. Going deeper inside that campus structure, copper Ethernet technology is still used for various reasons and will be for some time to come. For our purposes, “campus” = anything from a residence to a large business or perhaps even a real college campus.

Single mode fiber is replacing multi-mode fiber, however. The price differences between the actual cable are not large and the equipment costs are coming down. This lessens the worry of how far a fiber optic cable can run inside any one campus or large business. Prior to that, careful planning was required. Given higher bandwidths are associated with single mode fiber up to those longer distances and the choice is starting to become clear. Single mode fiber won out.

So now we have brought some clarity to the difference between single and multi-mode fiber and how this technology is of benefit. Choices are a good thing, and understanding the differences between those choices is key!

Common Questions and Answers

What are the differences between singlemode and multimode fiber?

A: Single-mode fiber uses solid lasers as the light source, while multi-mode fiber uses light-emitting diodes or VCSEL's as the light source. Single-mode fiber optic transmission has the characteristics of wideband and long transmission distance, but because it requires laser sources, the cost is higher, while multi-mode fiber is characterized by low transmission speed and short distance, but its cost is relatively lower. Single-mode fiber core diameter and dispersion is small, 8 -10 microns allowing only one mode of transmission, while multi-mode fiber core diameter 50 micron dispersion is large, allowing many modes of transmission.

What is the difference between singlemode and multimode optical transceiver modules?

A: multimode optical transceiver module works at 850nm, singlemode optical transceiver module works at 1310nm, 1550nm. The devices used in singlemode optical modules are twice as many as those used in multimode optical modules, so the overall cost of singlemode optical module is much higher than that of multimode optical transceiver modules. The transmission distance of singlemode optical transceiver module can reach 150 to 200km, and the transmission distance of multimode optical transceiver module can reach 5km.

Where are singlemode/multimode fiber and singlemode/multimode optical transceiver modules used?

A: Singlemode fiber enables the fiber to be launched directly to the data center, ISP location which is generally used for long distance data transmission, while in multimode fiber the optical signal is propagated through multiple paths. Therefore, multimode fiber is commonly used in short distance data transmission. Single-mode optical modules are often used in metro networks over long distances and at relatively high transmission rates.

Can singlemode/multimode fiber be mixed with singlemode/multimode optical module?

A: No! they can’t be mixed, we have to match the fiber and optical module to the type of fiber used. Multimode with Multimode and Singlemode with Singlemode.

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