Multimode fiber (MMF) is a common fiber for short-haul, low bandwidth applications. The primary difference between MMF and single-mode fiber (SMF) is the diameter of the core; the MMF core diameter is relatively large (typically between 50 and 100 micrometers), which means it is cheaper to manufacture than SMF and has excellent light gathering characteristics (i.e., a large numerical aperture).
However, because the core diameter is so large, it is possible for many light rays to traverse the fiber. This is illustrated in the graphic to the right. Each of these rays of light is referred to as a mode, hence the term, multimode. Although many light rays sounds like a good idea, it seriously diminishes performance of the fiber due to a phenomenon called dispersion. As shown in the graphic, some of the rays have relatively large critical angles, traveling almost parallel to the core boundary, while others have smaller critical angles. If you imagine a person walking each of these paths, you can immediately see the net effect; some of the rays traverse the fiber in record-setting times, while others meander along enjoying the sights. If you can imagine the pulse of light to be a collection of photons traveling at differing angles, you can also imagine that pulse of photons spreading out as it travels down the cable. This means two successive pulses, if they travel far enough, will bleed into one another, causing problems for the receiver. This is dispersion.
There are two basic types of MMF, they are known as graded-index multimode fiber (GRIN) and stepped-index multimode fiber. These two types differ primarily in the way light is focused in the core, which gives them differing dispersion properties.
|<mp3>http://podcast.hill-vt.com/podsnacks/2007q3/mmf-smf.mp3%7Cdownload</mp3> | Multimode fiber (MMF) vs. Single-mode fiber (SMF)|
|<mp3>http://podcast.hill-vt.com/podsnacks/2008q2/grin-stin.mp3%7Cdownload</mp3> | Graded-index multimode fiber vs. Stepped-index multimode fiber|