TLDR: In communications fiber optic cables, light is introduced into the core of an optical fiber via a light source (LED, Laser Diode).  The light rays reflect off the outer walls of the core (the cladding layer) until they reach the other end of the fiber where a light-sensing receiver converts the pulses into digital ones and zeros.

How do fiber optic communications cables really work?

At its most basic, a communications optical fiber cable is composed of glass strands, like threads, about the diameter of human hair, each of which can transmit messages modulated onto light waves at the speed of light. They offer greater bandwidth than copper wire cable and have become the go-to option to meet the demands of the age of the internet where large amounts of data (e.g., streaming apps) must be distributed to thousands of subscribers, miles away and instantaneously.   Fiber optic cables are not only found in communications systems, they are also used in industrial networks, in sensing and in avionics applications.

The first step to understanding how fiber optic works is to understand what happens when you send light through air or water. Light travels as a wave. When it passes through air, the wave loses some energy and becomes more spread out. The result is that the light beam gets wider and less intense. This loss of intensity is called attenuation.

When light enters water, however, it does not lose any energy.  Instead, it bends around the water molecules, making it easier for the light to pass through. Water also slows down the light’s velocity by a factor of 1/v2 where v is the speed of light in water. This means that light traveling through water will travel farther than if it were traveling through air.  Optical fibers use these principles to carry data from one point to another.

Optical Fiber Cable

Most optical fibers in use today consist of glass strands (the core) made of pure silica surrounded by cladding material made of doped silica. The core is so small that only a single ray of light at a particular wavelength can travel through to the end.  These are called single-mode fibers.  In this design, the cladding layer has a lower refractive index and acts like a mirror to keep the mode inside the core.  This phenomenon is known as total internal reflection.

The performance of optical fibers depends on how well they can transmit light. One way to measure this is by measuring the return loss (also called insertion loss) of the fiber. Return loss is defined as the ratio between the power in the forward direction and the power in the reverse direction. If the return loss is high, more light will be lost when traveling through the fiber than if the return loss was low.

Optical fibers have many advantages over traditional copper wires: they’re smaller, lighter-weight, and are immune from magnetic and electrical interference.  They can transmit much greater volumes of information faster and over much longer distances.  For example, copper wires can move data at a maximum rate of 100 megabits per second, while optical fibers today can move data at rates up to 10 gigabits per second.

Types of Fiber Optic Cable
Types of Communication Fiber Optic Cable

There are 2 basic types of fibers, single mode and multimode.  Single-mode optical fiber is smaller in core diameter (8.3-10 microns) and holds advantages in terms of bandwidth and reach for longer distances, while multimode optical fibers have larger core diameters (50 microns or larger) and easily support most distances required in enterprise and data center networks, at a cost typically less than single-mode installations.

Optical fiber technology is used in many ways today. It is used for transmitting voice and video signals, for carrying computer data, and for sending information across long distances.

Optical fibers are used to manufacture endoscopes which allow doctors to view inside the human body and perform surgery without the need of invasive scalpel procedures.  Large core fibers can carry laser energy to facilitate the removal of tattoos, the cleaning of historical monuments, and the powering of laser-directed defense systems.

Distributed fiber optic sensing (DFOS) allows for the entire length of an optical fiber to be used as a sensing device.  Structures like fuel pipelines, bridges, aircraft wings can have optical fibers embedded into them to detect such parameters as strain, temperature or sound and help ensure their structural integrity.

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