Modal Propagation In Fibers
Optical fibers are classified according to the number of rays of light that can be carried down the fiber at one time. This is referred to as the 'Mode of Operation' of the fiber. Therefore a mode of light is simply a ray of light.
The following section discusses the various modes of propagation in optical fibers and the effects of modal dispersion.
Modal Dispersion. It is important firstly to examine the nature and effects of modal transmission. A fiber that has a high NA and/or diameter will have a large number of modes (rays of light) operating along the length of that fiber.
Multimode Step and Graded Index Fibers. The term 'Multimode' generally applies to fibers with a diameter of 50 micrometers or greater. Because of the relatively wide diameter of the core, multiple modes of light are able to travel down the core. As was previously discussed, allowing multiple modes of light to travel down a fiber causes modal dispersion.
The modal dispersion that occurs in a multimode fiber affects or is affected by a number of the operating parameters of the fiber.
Attenuation. Multimode fibers have a maximum operating distance of approximately 5 km.
Bandwidth. Multimode fibers have a maximum operating data speed of approximately 2-300 Mbits/s.
Wavelength. They generally operate at wavelengths of 850 nm or 1300 nm. Some fibers are available that will operate at both wavelengths. (Different physical communications standards use different operating wavelengths).
The wide diameter of the multimode fiber makes it suitable for using with LED light sources. This in turn makes the complete transmission system a lot cheaper than compared to fibers that have a thinner diameter and which require the use of lasers. A further advantage with using multimode fibers is that the wider diameter makes them easier to splice and to terminate, which makes the final installed system cheaper.
Multimode fibers are constructed in three main sizes.
1. 50 micron cores
2. 62.5 micron cores
3. 100 micron cores
Monomode Fibers. A monomode fiber (or sometimes referred to as a single mode fiber) is basically a step index fiber with a very small core diameter. In theory because the cores are so small only a few modes of light can travel down the fiber. To further reduce the number of modes the fiber is constructed with very little difference between the refractive indices of the core and the cladding. This is illustrated in Figure 7.
Figure 7. Monomode
Optic Fiber Transmission
For the transmission of light down a monomode fiber to operate as described above the pulse of light that is injected into the core must be very precisely aimed down the centre of the core or the majority of the light will be lost in the cladding. If the system is implemented correctly the input signal pulse into the fiber will appear at the output of the fiber as a signal pulse with almost exactly the same shape. With only the fundamental mode travelling down the fiber there can theoretically be no modal dispersion in monomode fiber.
The core diameter of a monomode fiber is generally in the region of 8 to 9 µm. A typical measurement specification for a monomode fiber is:
8.5/125/250 µm