What is Fiber Optic Cable?
Fiber Optic CableThe world of telecommunications is rapidly moving from copper wire networks to fiber optics. Optical fiber is a very thin strand of pure glass which acts as a waveguide for light over long distances. It uses a principle known as total internal reflection. Fiber optic cable is actually composed of two layers of glass: The core, which carries the actual light signal, and the cladding, which is a layer of glass surrounding the core. The cladding has a lower refractive index than the core. This causes Total Internal Reflection within the core. Most fibers operate in duplex pairs: one fiber is used to transmit and the other is used to receive. But it is possible to send both signals over a single strand. There are two main types of fiber optic cables: Single Mode Fiber (SMF) and Multi-Mode Fiber (MMF). The difference is basically in the size of the core. MMF has a much wider core, allowing multiple modes (or “rays”) of light to propagate. SMF has a very narrow core which allows only a single mode of light to propagate. Each type of fiber has different properties with its own advantages and disadvantages
Optical fibre types
There is a variety of different types of fibre optic cable that can be used, and there are a number of ways in which types may be differentiated. There are two major categories:
Step index fibre optic cabling
Graded index fibre optic cabling
The step index cable refers to cable in which there is a step change in the refractive index between the core and the cladding. This type is the more commonly used. The other type, as indicated by the name, changes more gradually over the diameter of the fibre. Using this type of cable, the light is refracted towards the centre of the cable.
Optical fibres or optical fibers can also be split into single mode fibre, and multimode fibre. Mention of both single mode fiber and multi-mode fiber is often seen in the literature.
Single-Mode Fiber
Single-mode fiber is a common type of optical fiber that is used to transmit over longer distances. It is one of the two optical fiber types, the other being multi-mode fiber. A single-mode fiber is a single glass fiber strand used to transmit a single mode or ray of light.
Single-mode fiber features only one transmission mode. Compared with multi-mode fiber, it can carry higher bandwidths; however, it needs to have a light source having a narrow spectral width.
Single-mode fiber is also known as a single-mode optical fiber, uni-mode fiber, mono-mode optical fiber and single-mode optical waveguide.
Single-mode fiber provides users with a greater transmission rate in addition to nearly 50 times longer distance as opposed to multi-mode fiber. However, single-mode fiber is more expensive than multi-mode fiber. Among all the differences between single-mode and multi-mode fibers, the most basic is the size difference in the fibers’ core as well as the associated loss or attenuation and fiber bandwidth.
The optical fiber itself includes three fundamental parts: the core, the cladding and the coating or buffer. The most central part of the optical fiber is the core. This is the place through which the light travels. The core of single-mode fiber is much smaller than that of multi-mode fiber. There are three fundamental diameter sizes for fiber cores.
Single-mode fiber features a core diameter of nominally 9 µm, whereas multi-mode fiber often features a 62.5-µm or 50-µm core diameter. The small core, coupled with a single light wave, eradicates any distortion caused by overlapping light pulses, offering a minimal signal attenuation plus the highest transmission speed.
In contrast, multi-mode fiber provides users with high bandwidth at high speeds across moderate distances.
It is not advisable to mix and match fibers. If you try to connect a single-mode fiber with a multi-mode fiber, it may result in a 20-dB loss, which is 99% of the total power.
Multi-Mode Fiber
Multi-mode fiber is a type of optical fiber designed to carry multiple light rays or modes simultaneously, each at a marginally different reflection angle inside the optical fiber core.
Multi-mode fiber is mainly used to transmit across comparatively shorter distances, as the modes are more likely to disperse over longer extents. This phenomenon is known as modal dispersion. Another common type of optical fiber is the single-mode fiber, which is used mainly for longer distances.
Multi-mode fiber is also known as multi-mode optical fiber.
Multi-mode cable consists of glass fibers with a common diameter in the range of 50 to 100 microns for the light-carrying element. The most prevalent size is 62.5 microns. Plastic optical fiber (POF) is a modern plastic-based cable that ensures performance much like glass cable for brief runs, but economically.
On the contrary, single-mode fibers include a small glass core, usually close to 9 microns. With single-mode fibers, data can be transmitted at high speed over longer distances. Multi-mode fibers are more vulnerable to attenuation compared with single-mode fibers.
Multi-mode fiber provides users with high bandwidth at high speeds across moderate distances. Light waves are spread into various modes, or paths, as they travel via the core of the cable, usually at 850 or 1300 nm.
On the other hand, in long cable runs (e.g., more than 3000 feet), various paths of light may lead to signal distortion on the receiving end. This ultimately results in an ambiguous and incomplete transmission of data.
Multi-mode fibers may not be suitable for high-speed data transmission. It is not advisable to mix and match fibers either. Attempting to connect a single-mode fiber with a multi-mode fiber may result in a 20-dB loss, which is 99% of the total power.
Attenuation within an optical fibre
Although fibre optic cables offer a far superior performance to that which can be achieved with other forms of cable, they nevertheless suffer from some levels of attenuation. This is caused by several effects:
Loss associated with the impurities
There will always be some level of impurity in the core of the optical fibre. This will cause some absorption of the light within the fibre. One major impurity is water that remains in the fibre.
Loss associated with the cladding
When light reflects off the interface between the cladding and the core, the light will actually travel into the core a small distance before being reflected back. This process causes a small but significant level of loss and is one of the main contributors to the overall attenuation of a signal along an fibre optic cable.
Loss associated with the wavelength
It is found that the level of signal attenuation in the optical fibre depends the wavelength used. The level increases at certain wavelengths as a result of certain impurities.
Despite the fact that attenuation is an issue, it is nevertheless possible to transmit data along single mode fibres for considerable distances. Lines carrying data rates up to 50 Gbps are able to cover distances of 100 km without the need for amplification.
Materials used for optical fibres
There are two main types of material used for optical fibres. These are glass and plastic. They offer widely different characteristics and therefore fibres made from the two different substances find uses in very different applications.
Optical fibre sizes
One of the major ways of specifying optical fibre cables is by the diameters of the inner core and the external cladding. As may be expected there are industry standards for these and this helps in reducing the variety of fittings needed for connectors, splices and the tools needed for fitting.
The standard for most optical fibres is 125 microns (um) for the cladding and 245 microns (um) for the outer protective coating. Multimode optical fibres have core sizes of either 50 or 62.5 microns whereas the standards for single mode fibres is approximately 8 to 10 microns.
When specifying optical fibre cables, the diameters usually form the major part of the cable specification. A multimode fibre with a core diameter of 50 microns and a cladding diameter of 125 microns would be referred to as a 50/125 fibre.
In addition to the specification of the diameter, other parameters such as the loss, etc are also required, but these elements do not form part of the cable type in the same way as the diameter.
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