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What’s Inside a Fiber Optic Cable?



Fiber optic cable is a technology that’s increasingly being used to transfer data between network devices. While it’s a much faster option than copper wire, it’s still not without its challenges.

Optical fiber cables consist of strands of glass that carry light signals. The strands have an inner core and cladding that bend the incoming light at certain angles. The cladding also reflects the light signal back to the core when it reaches it.

Optical Fibers

Optical Fibers are a type of wire used for telecommunications. They consist of incredibly thin glass strands that can transmit light pulses at a speed comparable to lightning.

They also have a low rate of bit error as they are not susceptible to electromagnetic interference, like electrical copper cables.

Typically, an optical fiber is made of glass strands (called the core) with a cladding layer that has a lower index of refraction than the core. This causes a process called total internal reflection to occur, which allows light to travel down the core without leaking out of it.


The core of fiber optic cables consists of a glass or plastic made cylinder running along the cable’s length. It is surrounded by a medium with a lower index of refraction (cladding) that enables more light to be transmitted into the core.

The size of the core and cladding of optical fibers are usually quoted in microns. A micron is one millionth of a meter, or about the diameter of a human hair.

The core of single-mode fibers has a very small light-carrying area, about 8 microns. This keeps the path of light narrow so it can travel further distances before having to be regenerated. The cladding of multimode fibers is larger, about 50 microns or more. This allows multiple paths for light to be transmitted, but modal dispersion can create limitations over longer distances.


The cladding is the glass or plastic covering of a fiber optic cable. This is an important layer for keeping light signals inside the core, and it also protects the fiber from environmental hazards like moisture and chemicals.

There are many types of cladding, some of which are stronger than others. For example, some manufacturers use a special coating that provides extra protection against extreme cable bends.


The coating of Fiber Optic Cables plays an important role in maintaining the glass fiber’s tensile strength. Coatings help prevent flaws from growing large enough to break the glass during proof testing, allowing the fiber to pass the test without breaking.

Coatings also help to maintain the fiber’s optical properties, especially attenuation and polarization properties, by helping it keep its refractive index low. The refractive index is the measure of how fast light travels through a material.

Typically, communication fibers have a secondary coating that is only a few angstroms thick. This coating extends the fiber’s lifetime significantly.

Strengthening Fibers

With their high speeds, large bandwidth and immunity to interference, fibers have become the communications medium of choice in today’s highly connected world. They also require far less maintenance than copper wires, making them a smart choice for long-term connectivity.

The glass fibers inside a single-mode cable have a narrow core, typically 8.3 to 10 microns in diameter, that allows only one mode of light to travel through the core. This reduces the number of light reflections between the core and cladding, lowering attenuation and allowing signals to travel further.

Multimode fibers have a much larger core, ranging in size from 100 microns to 1000 microns, that allow light to travel through a variety of different rays as it passes through the core. The dispersion of these different rays, or modes, increases the length of time it takes for light to arrive at its destination.


The outer jacket of a Fiber Optic Cable protects the core conductor and shielding from environmental hazards such as moisture, chemical, flame and fire. This is accomplished by coating the cable with a polyethylene (PE) material that provides excellent weather resistance, good electrical properties, and abrasion resistance.

Depending on the type of fiber used, there are many different types of cable jackets. These include yellow for single mode, orange for OM1 and OM2 multimode, aqua for OM3 multimode, purple for OM4 cables, and black for outside plant (OFP) cables.