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When we send a light signal through a fiber optic cable, the ray of light can travel in one or more methods of
transmission. Methods in this context refer to modes. These modes are always an integer as there is only a certain
whole number of ways of transmitting light. When there is one mode in the fiber, it is called a singlemode fiber. When
there is more than one mode in the fiber, it is called a multimode fiber.
The physical structure of singlemode and multimode fibers are different in terms of the diameter of the fiber core. For
singlemode fibers, the typical core diameter is between 8 to 10µm. For multimode fibers, the typical core diameter is
either 50 or 62.5µm. In general, the larger the diameter of the fiber core, the more modes the fiber can carry.
Intermodal Dispersion
When different modes of light travel at different speeds or at different light paths along the same fiber, they will not be
received at the same time. This would cause the signal to become distorted, a phenomenon known as intermodal
dispersion.
Intermodal dispersion is minimized in the design of fibers. The design of fiber eliminates up to 99% of intermodal
dispersion. They do this by varying the speed of each mode so that they are received at the same time. For example in
Diagram 3, Ray A travels a longer distance than Ray B. Intermodal dispersion is minimized when Ray A travels faster
than Ray B so that both rays arrive at the same time.
Since intermodal dispersion is applicable only to multimode fibers, singlemode fibers have better performances with
lower signal loss.
Standards
Various standard bodies have aimed to categorize and standardized various types of single mode and multi mode fibers.
The following list is some of the most internationally recognized standard bodies governing optical fibers:
- IEC (International Electrotechnical Commission)
- ISO (International Organization for Standardization)
- ITU (International Telecommunication Union)
- TIA (Telecommunications Industry Association)
Understanding OS/OM Fiber Designations
The OS/OM standards were introduced in the ISO/IEC 11801 standard where fiber types specified were OS1 for single
mode, OM1, OM2 and OM3 for multi mode. OS2 single mode fiber was introduced in 2006 in ISO/IEC 24702. OM4
multi mode fiber is the recent addition in 2009, as specified in ISO/IEC 60793:ID and TIA:492AAAD.
These standards identify the cable transmission performance of each fiber type. Each fiber type is given a specification
of maximum attenuation (i.e. signal loss) of a cabled fiber. They are given according to the wavelength in decibels per
kilometer. For multi mode fibers, there is an additional metric of the minimum bandwidth for each fiber type. The
difference between overfilled launch and laser launch is from the type of signal launch used in transmission. For
overfilled launch, it is generically referring to LED as the launching device. OM3 and OM4 have an additional laser
launch modal bandwidth as OM3 and OM4 are designed to be laser optimized. |
