Editorial Feature

What Are Fiber Optics?

Modern high speed voice and data communications have outgrown the traditional copper wire and now rely on fiber optic cables used for the bulk transfer of information.

Fiber optics or optical fibers are long, thin strands with the diameter of a human hair. They are made from either high purity glass or plastic. Fiber optics are arranged in bundles called optical cables where light signal is transmitted along the center of the fiber from one end to the other.

How Do Optical Fibers Work?

In order to understand the way in which fiber optics work, we need to define what the optical fiber is.

An optical fiber consists of three basic elements, i.e. the core, the cladding and the outer coating called the jacket (Figure 1).

Fig 1. Basic optical fiber scheme.

The core is a single continuous strand of high-purity glass (SiO2) or plastic whose diameter is measured in microns (less than the diameter of a human hair). The fiber core transports optical signals from an attached light source to a receiving device. The core is surrounded by a cladding material that reflects light back into the core. This cladding layer is surrounded by a buffer material (coating) that reinforces the fiber core, helps absorb shocks and protects the fiber from moisture and damage. The coating usually comprises one or more coats of a plastic material.

Optical fibers usually are specified by their size, given as the outer diameter of the core, cladding and coating. For example, a 62.5/125/250 refers to a fiber with a 62.5µm diameter core, a 125µm diameter cladding and a 250µm outer coating diameter. There are two types of optical fibers, i.e. single-mode fibers and multi-mode fibers. Single-mode fibers have small cores of about 9 microns in diameter and transmit infrared laser light (wavelength = 1,300 to 1,550 nanometers). Multi-mode fibers have larger cores of about 62.5 microns in diameter and transmit infrared light (wavelength = 850 to 1,300 nm) from light-emitting diodes (LEDs). Plastic optical fibers (POF) have a large core of 1 mm diameter and transmit visible red light (wavelength = 650 nm) from LEDs.

How Does Light Travel Down a Fiber Optic Cable?

Fiber optics work is based on the principle of total internal reflection (TIR) phenomenon that occurs at the interface between the core and the cladding. The light in a fiber-optic cable travels through the core (hallway) by constantly bouncing from the cladding (mirror-lined walls). In other words, the light is contained within the glass core and cladding by precise design of their refractive indices. The cladding layer has a lower refractive index than a core, thus the cladding keeps the light signals inside the core. Moreover, the cladding does not absorb any light from the core, the light wave can travel great distances.

In other words, when a fiber is bent a beam of light will eventually hit the wall. If it was a flashlight beam in a hallway, the light would stop when it hit the wall. If the wall was lined with mirrors, the light beam would bounce off and continue. This is basically what happens in a fiber optic cable. All this bouncing around gradually degrades the light beam and at certain intervals the signal must be regenerated. This makes an optic fiber communications system more complex than just a transmitter and receiver joined by a fiber.

Image Credits: Studi8Neosiam/shutterstock.com

What are the main Components of a Fiber Optic System?

Fiber optic systems consist of four main parts:

  • Transmitter
  • Optical Fiber
  • Optical Regenerator
  • Optical Receiver

Transmitter generates and encodes the signals of light. It is typically a laser diode or light-emitting diode (LED) based system.

Optical fiber conducts the light signals over a distance. It is an optically pure glass or plastic as mentioned above. Nowadays, the researchers are focused on developing glasses with the highest possible purity. For example, glasses with a high fluoride content (fluorozirconate, fluoroaluminate) are applied for improving optical fiber performance because they are transparent to almost the entire range of visible light frequencies.

Optical Regenerator is a form of laser amplifier to boost a light signal. As the light signal degrades with distance, any number from one to numerous regenerators are required.

Optical Receiver uses a photodiode or photocell to detect the incoming light signal. The receiver decodes the light signals.

Advantages of Fiber Optics

Fiber optics have numerous advantages over conventional metal wire systems or coaxial transmission, e.g.:

  • Improved system performance
  • Optical loss reduced to a minimum
  • No interference
  • Non-flammability
  • Safety
  • Greater bandwidth and high data-carrying capacity
  • Availability of long distance light-weight cables
  • Lower cost in installation and maintenance

The main disadvantages of fiber optics are relatively high material cost comparing to copper cables and high-skilled manpower needed for optical fiber mounting.


Optical fiber technology has been used in many areas of telecommunication, photonics, medical and engineering. Fiber optics are associated with voice, data and video transmission, the Internet, cable TV and telephone connection. In addition to the broad communications and data applications, fiber optics are also used in medical imaging, sensing, military and space applications or mechanical inspection.

Sources and Further Reading

  • Yeh, Chai. Handbook of Fiber Optics. Academic Press, 1990.
  • S. AddankiaI, S. Amiribc, P. Yupapin, Results in Physics, Review of optical fibers-introduction and applications in fiber lasers, Results in Physics 10, 743-750, 2018.

This article was updated on 3rd December, 2018.

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