In our previous articles, we have discussed the extensive use of FOCs in building and enhancing newer and innovative convergent telecommunication networks. The fibers are predominantly used to link sites that are geographically set apart from each other- for example, intercity networks- they are also deployed within campuses and multi-purpose buildings. The fiber cables can be classified according to their use; we have the following classifications –

Outside Plant (OSP)

The OSP cables are basically used in the telecommunications industry to enhance telecom/internet services for intra-city and intercity connections. The cables can be laid out for use in two ways. They can either be blown into ducts or laid in trenches that have been dug to depths specified by the State/Central governments or the municipal corporations. The government bodies support the activities of the network providers for the greater good of the societies, it is important for the cables to be deeply buried to prevent damage from rodents and other means. The cables can also be strung on poles overhead, in areas where the terrain will not permit trenches to be dug. There are also special submarine cables that are passed underwater to link international sites. The single mode fiber cables are preferably used for outside plant installations (as discussed in the previous sections); these cables have a remarkably high fiber count. They have been reinforced to withstand damage from rodents and they are moisture proof.

Inside Plant (ISP)

The ISP cables are different from the OSP cables in their characteristic shorter length. They are used in installations within a campus or a building. Their deployment will cover an area anywhere within a hundred feet and maybe more, used with 2 to 48 fibers per cable. The fiber used can be a single mode, multi-mode or a hybrid cable that has the features of a single mode and the multi-mode fiber. They are usually made of glass or plastic.

In communication networks, there are two basic types of FOCs. They are the single mode fibers and the multimode fibers. The distinguishing features that can be used to differentiate these two fibers include the attenuation characteristics, material composition, and manufacturing technique.

Attenuation is a condition that is experienced as a result of the double effect of absorption and scattering. The wavelength transmitted determines the attenuation of light within the fiber. This occurrence can be managed by using an optimal window within the optical spectrum. The following wavelengths indicate areas where the attenuation losses are minimal- 780 nm, 850 nm, 1310 nm, 1550 nm, and 1625 nm. You can check more on this in an article we discuss Transmission Wavelenghts, or Windows.

Single Mode Fibers

SMF can be described as a strand of fiber made of glass; they usually measure about 8.3 to 10 microns in diameter with a single transmission mode. This fiber permits only a single mode of light measured at 1310 or 1550 nm due to the very narrow diameter if the core.

These fibers have been optimized to support a remarkably higher bandwidth than the capacity of the multimode fiber over longer distances. Its use is however limited due to the small size of the core, which requires the support of a convergent source of light with a characteristic constricted spectral width. The single mode fibers are widely used for telecom/data applications which include Wave Length Division (WDM) systems. There is a preference for the 1310 nm wavelength due to the zero dispersion characteristics, but there are observed losses in the 1550 nm region. The SMF are also profiled as step index or graded index. The step index profile shows a sudden change in the refractive index (RI) at the core-cladding (core RI is greater than cladding) intersection, the RI for the graded index profile is observed to gradually reduce from the core to the cladding. There are three typical classes of the single mode fiber commonly used in modern telecommunication systems:

1. Non dispersion-shifted fiber (NDSF) – this is a very common fiber that was introduced with the advent of optical communication. They are designed to be used with the 1310 nm wavelengths in cases where the dispersion loss is zero. The NSDF has also been enhanced to work with the 1550 nm which is currently available. The limitations of the 1550 nm NSDF is the high rate of dispersion loss experienced during transmission.
2. Dispersion-Shifted Fibers (DSF) was developed as a solution to the limitations faced with the NSDF fiber at 1550 nm. When the DSF was used, the zero dispersion point is moved to the region of 1550 nm with the right dopants. They have also been observed to show severe non-linear characteristics when multiple wavelengths in the 1550 nm band are transmitted in cases related to the WDM systems.
3. Non-Zero-Dispersion-shifted fibers (NZ-DSF) is the product of the research carried out to develop a fiber that overcomes the limitations faced when using the DSF. The NZ DSF is made to have positive and negative characteristics. It is widely used in the modern day telecommunication operations.

Multi-Mode Fibers

The MMF fibers permit the passage of multiple modes of light through it. They have a larger diameter than the SMF which is usually in the order of 50, 62.5 or 100 micrometers. It is also important to note that the wavelength for the MMF is remarkably high, reaching values between 3000 to 3500 feet with significant distortions that occur at the receiving end leading to failure in transmission. For many cases of short range operations, a 2-fiber (2F) MMF will be used. The MMF cables are classified into the following –

1. Step-Index Multimode Fiber– The Step index core fiber comprises a core that has a uniform RI and a sharp decrease in RI observed at the core-cladding interface. The result is a lower RI recorded for the cladding. The fiber was named step-index due to the characteristic sharp change in the RI
2. Graded-Index Multimode Fiber- The experience with the Graded Index MMF shows an RI of the core that steadily reduces as it progresses from the central axis outwards to the cladding. The consequence of this is the light rays which are made to travel in a helical curve pattern within the core. Multiple modes of propagation can be observed in the multimode fiber. It is also observed that specific modes can progress through the helical path, this is much longer but effectively quicker as a result of the changing RI when the other modes travel in a shorter path along the central axis at a much slower pace. This feature is responsible for the simultaneous arrival of the modes at the receiving end. The attenuation features, dispersion characteristics and the cost of a GI MMF are observed in between the values of SMF and the SI SMF. The GI fibers are produced with core diameters in the order of 50, 62.5 and 85 microns also with a cladding diameter of 125 microns. The GI MMF is produced in glass and plastic.

More on differences between Single and Multimode fiber in general, you can read in this article.