Fibre

Many decisions come into play when installing fiber optic cabling.  By far, one of the most important questions is whether to install single mode or multimode.  This decision has huge implications for your network’s distance, bandwidth, and budget, so it’s vital to understand the differences between these two types of fiber optic glass.

Before we discuss each type of fiber, here are some definitions:

Optical fiber: The glass portion of a fiber optic cable – no jacketing or strength members included.  An optical fiber is made up of a light carrying core surrounded by cladding.  The cladding prevents light from escaping the core, effectively keeping the signal moving down the glass.

Single mode fiber: a fiber featuring a small light-carrying core of about 9 micrometers (µm) in diameter.  For reference, a human hair is closer to 100 µm. The core is surrounded by a cladding that brings the overall diameter of the optical fiber to 125 µm.

Multimode fiber: a fiber with a core of 50 µm or above.  A larger core means multiple modes (or rays of light) can travel down the core simultaneously. Just like single mode, the core is surrounded by a cladding that brings the overall diameter of the optical fiber to 125 µm.

Common Misconceptions

It’s important to remember that (without the visual acuity of Superman) there is no way to distinguish between single mode and multimode optical fibers with the naked eye.  As noted above, standard optical fibers have cladding around the core that brings the diameter of the optical fiber itself to 125 µm.  When you put a connector on an optical fiber, you are primarily seeing the cladding and any integral protective coating, like SSF™ polymer.

The terms “single mode” and “multimode” also have no relation to the number of optical fibers in the fiber optic cable you are running.  It’s possible to have a cable containing 144 single mode optical fibers, and it’s also possible to have a cable containing 144 multimode optical fibers.

Is Multimode Better?

To installers new to fiber, multimode fiber may seem appealing because the name implies that more can be sent over the cable.  However, “multimode” refers to multiple rays of light simultaneously taking different tracks down the core of the fiber.   This characteristic, enabled by multimode’s larger core, actually creates some limitations.