What is the difference between Singlemode and Multimode Fiber Optic

A lot of you already are familiar with fiber optic cable, but it does no harm to have a refresher from time-to-time. So let's do some basic Fiber 101.

Optical Fiber

In case some of you thought optical fiber has been in use for a relatively short period of time, say the last 10 to 20 years that would incorrect. Optical fiber dates way back, but the first patent application for this technology was in 1966. In fact, NASA used fiber optics in the television cameras sent to the moon. At the time, the use in the cameras was classified confidential, and only those with the right security clearance or those accompanied by someone with the right security clearance were permitted to handle the cameras. General Telephone and Electronics deployed the first commercial telecommunications traffic in 1977, and fiber has continued to replace copper cabling ever since. Oh, and in case you were wondering, the first transatlantic telephone cable to use optical fiber was TAT-8 that went into operation in 1988.

Fiber Types

Fiber sizes are measured in microns also known as micrometers with the symbol "μm." Multi-mode fiber is the most common in two sizes: 50/125μm (a 50μm core with a 125μm cladding) and 62.5/125μm (a 62.5μm core with a125μm cladding). The differences between the two are relatively minor, but they have different strengths and weakness depending on the type of optical transmitter coupling to it. As the name implies, each fiber has multiple-modes or “paths” through it, even though it’s a single piece. 62.5μm fiber has more pathways than a 50μm, simply because of its larger diameter. Depending on the type of transmitter that's connected to it, the two sizes can have different strengths that affect link speed and distance.

When a transmitter injects light into a multi-mode fiber, most of the light bounces down the fiber at slight angles with some loss as the light gets reflected. These multiple paths (modes) through the fiber all have slightly different characteristics that lower the amount of light and add delay. At the end of the fiber, these paths eventually hit the optical receiver. As long as there isn't much accumulated delay difference between these paths and there are sufficient levels of light, the receiver will properly recognize the pulses of light. However, as the distance increases the light through these pathways will start to arrive at slightly different times. These time differences will slightly alter the pulse width of the original signal. At slow speeds this distortion is negligible in relation to the actual duration time of light on or off, so it has little effect. At very high speeds, since the pulses are now very short in time, these multiple paths arriving at different times may start to cause receive errors. This phenomenon is called modal dispersion. Multi-mode transmitters will generally work with either size multi-mode fiber, but at very high speeds or long distances, one type may be better suited than the other.

Single-mode fiber (9µm/125μm), at 9 microns (again, with a 125 micron cladding , single mode fiber is much smaller than multimode fiber and because it's so small there is essentially a single path down the fiber. Single mode fiber will give you longer distances than multimode, but they will also require a stronger transmitter level since not as much light is coupled into the fiber.

Four factors come into play for a fiber link that will limit the combination of speed and distance at which a link can operate. They are; transmitter source type, power aggregate optical rate, cable type, and wavelength of the light.

And there you have it, a basic understanding of fiber. Sorry if this put any of you to sleep or made your head explode. After all, you now have the weekend to recover.

Doug Schwartz

Western US Client Relations Executive