My TN Connection

When your vendor asks you if you want single-mode or multimode fiber, is your response “Whatever is cheaper” or “Whatever you think is best”? Or maybe you ask yourself, “Who would want a single strand of fiber?” If you answered yes to any of these questions or have wondered, just what is the difference between multimode and single-mode fiber, then this edition of Fiber FAQs is the one for you.

Before we get too deep into a discussion of multimode fiber and single-mode fiber, we need to define the word “mode” as it pertains to fiber optics. Think of a mode as a single wave or ray of light moving through a strand of fiber. From this, we can infer that single-mode fiber is made up of one mode and multimode fiber is made up of more than one mode. Take a look at the illustrations below, which show how light travels within each type of fiber.

In its simplest form, a fiber optic strand is made up of a core, cladding, and an outer protective coating. The core is the central part of the fiber through which light is transmitted. The cladding surrounds the core and is what keeps the transmitted light traveling down the core, instead of escaping from the glass altogether. And the coating, or buffer tubing, is a plastic covering that is meant to protect the fiber strands until each is terminated.

Single-mode and multimode fibers have different characteristics which make them better choices for different applications. Single-mode fiber is made up of a core that is 8.3 microns in diameter. The smaller core combined with the fact that the fiber is passing only one mode, allows for information to be transmitted longer distances with less signal degradation or attenuation. It should be noted that lasers are used to send light down single-mode fiber, which also has an impact on the transmission distances that can be reached. Single-mode fiber is a great fit for backbones within campus network topologies, as well as long distance telephony applications.

Most multimode fiber strands are made of cores that are 50 microns or 62.5 microns in diameter. The larger core diameter allows for multiple waves of light to be transmitted down the fiber at once. This transmission is accomplished through the use of LEDs and less powerful lasers, called VCSELs. Multimode fiber is better suited for shorter distances and applications that require less bandwidth, usually LAN applications where the electronics being connected are within close proximity of each other.

Next up for Fiber FAQs: I will outline the terms index of refraction and dispersion, both as they relate to Optical Networks - while trying to use as little Physics as humanly possible.

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