How do fiber optic cables carry light?

Nowadays, nearly everyone is talking about fiber-optic cables. These cables are now commonly used in telephone systems, cable TV systems and the Internet. One of the main advantages with optics cables is their huge bandwidth. That means fiber optics cables can carry far more signals than copper wires can. Usually made of optically pure glass, these cables are very thin – nearly as thin as human hair. Because of their high signal carrying capacity, optical fiber cables are also used for mechanical engineering inspection and in medical imaging. Optical fiber cables are made of long, thin strands of extremely pure glass. With a diameter close to that of human hair, several strands are bundled together, to form cables that are used to transmit light signals over long distances. When examined closely, each single fiber can be seen to consist of three parts.

The central core of the fiber is made of glass and this is where the light travels. The core is covered with a cladding, which effectively reflects light back into the core. The core is surrounded by a buffer coating, mainly for protecting the fiber from moisture ingress and physical damage. Optical cables contain hundreds or even thousands of such optical fibers arranged in bundles. On the outside, a jacket, also called the cable’s outer covering, protects the cable.
In general, there are two major types of optical fibers – single-mode and multi-mode. With a small core of about 9 microns in diameter, single-mode fibers can transmit infrared laser light having wavelengths of 1,300 to 1550 nanometers. On the other hand, multi-mode fibers have core diameters of about 62.5 microns, capable of transmitting infrared light of wavelengths from 850 to 1300 nanometers.

Other types of optical fibers can be made of plastic as well. These usually have a larger core of about 1 mm diameter, capable of transmitting visible red light of wavelength 650 nm, such as from LEDs.

Light always travels in straight lines. This is easily seen when a flashlight beam is shown down a straight long hallway. You can see the entire length of the hallway until the next bend, but beyond which nothing is visible. However, placing a mirror at the corner will allow you to see round the bend. This is possible because light from around the bend strikes the mirror and reflects down the hallway. If the hallway were to be very winding with multiple bends, lining the walls with mirrors will help. Light bounces from side-to-side and travels down the hallway making the entire path visible. This is exactly how an optical fiber works.

Light travels through the core of the fiber-optic cable, constantly bouncing off the cladding. This follows a well-known principle of optics known as total internal reflection. Very little light is lost in total internal reflection from the cladding, allowing light to travel long distances within the cable.

Although the core is made from optically pure glass, some impurities remain. These degrade the light signal as it travels down the core. The extent of signal degradation depends both on the impurities present in the glass used for the core and the wavelength of the light traveling through it.