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Three Basic Components of a Fiber Optic Cable


When choosing a fiber optic jumper cable, other than the connectors, such as SC to ST fiber cable, LC SC fiber patch cable, or SC to SC patch cord, we also need to focus on the cable. There are also multiple choices of fiber optic cables, like LC LC multimode fiber patch cord, or LC LC multimode duplex fiber cable. Typically, a fiber optic cable contains three basic components: the core, which carries the light signals; the cladding, which surrounds the core with a lower refractive index and contains the light; and the coating, which protects the fragile core and cladding within it. This artical will introduce these three components in details.


The core, which carries the light, is the smallest and the most important part of the optical fiber. The optical fiber core is usually made of glass, although some are made of plastic. The glass used in the core is extremely pure silicon dioxide (SiO2), a material so clear that you could look through 5 miles of it as though you were looking through a household window. In the manufacturing process, dopants such as germania, phosphorous pentoxide, or alumina are used to raise the refractive index under controlled conditions.

Optical fiber cores are manufactured in different diameters for different applications. Typical glass cores range from as small as 3.7um up to 200um. Core sizes commonly used in telecommunications are 9um, 50um and 62.5um. Based on the core size, there are mainly two types of optical fiber, single-mode fiber and multimode fiber. Single-mode fiber has a core of 8 to 10 microns and light travels toward the center of the core in a single wavelength. Multimode fiber has a core of either 50 or 62.5 microns. Plastic optical fiber cores can be much larger than glass. A popular plastic core size is 980um.


The cladding surrounds the core and has lower refractive index to make the optical fiber work. When glass cladding is used, the cladding and the core are manufactured together from the same silicon dioxide-based material in a permanently fused state. The manufacturing process adds different amounts of dopants to the core and the cladding to maintain a difference in refractive indexes of about 1%. A typical core may have a refractive index of 1.49 at 1300nm while the cladding may have a refractive index of 1.47. These numbers, however, are wavelength dependent. The core of the same fiber will have a different refractive index at a different wavelength.

The cladding is also manufactured in standard diameters. Two commonly used diameters are 125µm and 140µm. The 125µm cladding typically supports core sizes of 9µm, 50µm, and 62.5µm, and the 140um cladding typically has a 100µm core. The following picture shows the structure of a fiber optic cable.

fiber optic cable structure


The coating is the ture protective layer of the optical fiber. It absorbs the shocks, nicks, scrapes, and even moisture that could damage the cladding. An optical fiber is very fragile without the coating. A single microscopic nick in the cladding could cause the optical fiber to break when it's bent. Coating is essential for all-glass fibers, and they are not sold without it. The coating does not contribute to the light-carrying ability of the optical fiber. The outside diameter of the coating is typically either 250µm or 500µm. Generally the coating is colorless. In some applications it is colored to be identified easily.

The coating is selected for a specific type of performance or environment. One common coating type is acrylate. This coating is typically applied in two layers. The primary coating is applied directly on the cladding. This coating is soft and provides a cushion for the optical fiber when it is bent. The secondary coating is harder than the primary coating and provides a hard outer surface. However, Acrylate is limited in temperature performance. Silicone, carbon, and polyimide are coatings used in harsh environments associated with avionics, aerospace, and space, and applications like mining, or oil and gas drilling.


Many combinations of core and cladding sizes are possible. To ensure that connectors and equipment can be matched properly, it is necessary to have some standards, which is especially important when dealing with components as small as those used in fiber optics, where even slight misalignments can jeopardize the entire system. TIA and ITU published many standards on optical fiber. Key standards that you should be familiar with are ANSI/TIA-568-C.3, ITU-TG.653, ITU-TG.655 and ITU-T G.657. The ANSI/TIA-568-C.3 standard is applicable to premises optical fiber cabling components. The ITU standards are applicable to single mode optical fiber and cable.


Fiber optic cables are commonly made with a glass core and glass cladding, but other materials may be used if the fiber's performance must be balanced with the cost of installing the fiber, fitting it with connectors, and ensuring that it is properly protected from damage. In many cases, fibers must run only a short distance, and the benefits of high quality all glass fibers become less important than simply saving money. There are also circumstances in which the fibers are exposed to harsh conditions, such as extreme temperature, repeated handling, or constant movement. Different fiber classifications have evolved to suit different conditions, cost factors, and performance requirements.


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