Introduction to Optical Fibers
By Gabriel Torres e Cássio Lima on June 21, 2005


Introduction

In 1952 physicist Narinder Singh Kapany, based on studies conducted by english physicist John Tyndall that the light could travel in curve inside a material (in Tyndall's experiment this material was water), could conclude his experiments that led to the invention of the optical fiber. Optical fiber is an excellent transmition medium used by systems that require a high bandwidth, like the telephony systems, videoconference, local networks (LANs), etc.

There are two main advantages on optical fibers over metallic cables. Optical fiber is totally imune to electromagnetic interference, which means that data isn't corrupted during their transmition. The second main advantage is that optical fiber doesn't conduct electrical current, thus no electricity-related issue is found by using optical fibers, like electrical potential difference between devices or problems with lightnings.

As the name implies, optical fibers use light to transmit data. At one end of the cable, a LED (Light Emitting Diode) or a semiconductor laser is used as the light source. LEDs can transmit data up to 300 Mbps and is used on short-distance fibers, while with laser the transfer rate can easily reach the Gbps range and is used in long-distance fibers.

The light used in optical fibers is near the infrared range, so it is invisible to the human eye. Actually optical fibers can use light from different wavelenghts, as you can in the table below. Recently ITU classified the wavelenghts that can be used in optical fibers into "bands". So an optical fiber operating on the O band means that the wavelenght of the light used in the cable is between 1260 and 1360 nm.

Band

Descriptor 

Range (nm) 

O band

Original

1260 to 1360

E band

Extended

1360 to 1460

S band

Short wavelength

1460 to 1530

C band

Conventional

1530 to 1565

L band

Long wavelength

1565 to 1625

U band

Ultralong wavelength

1625 to 1675

Anatomy of Optical Fibers

The fundamental principle behind optical fibers is a physic phenomena called total internal reflection. In order to have total internal reflection, light has to get out from a more refringent (refractive) medium to a less refrigent one and the angle of incidence must be equal or greater than the limit angle (also known as Brewster angle).

Optical Fiber
Figure 1: Example of an optical fiber.

Optical fibers are basically made of dielectric (insulating) materials that, as we already mentioned, allow complete imunity to electromagnetic interference, having two areas, a center region called core, where the light pass through, and an external region called cladding which covers the core. The refracting index of the material used on the core is higher than the refracting index from the material used on the cladding.

In Figure 2, you can see the anatomy of an optical fiber.

Optical Fiber
Figure 2: Anatomy of an optical fiber.

Here is the description of each part of the optical fiber:

Types

There are two types of optical fibers: multimode and single-mode (or monomode). These types define how light travels inside the fiber core.

Optical Fiber

Figure 3: Difference between graded-index multimode, step-index multimode and single-mode optical fibers.

 
ITU released a series of standards in order to classify the properties of multimode and single-mode fibers:
Important: Optical fibers trasmit light in a wave lenght invisible to the human eye. So, we can never look directly to the end of an optical fiber while it is connected to a system, because we can go blind looking at it.

Originally at http://www.hardwaresecrets.com/article/Introduction-to-Optical-Fibers/154


© 2004-13, Hardware Secrets, LLC. All Rights Reserved.

Total or partial reproduction of the contents of this site, as well as that of the texts available for downloading, be this in the electronic media, in print, or any other form of distribution, is expressly forbidden. Those who do not comply with these copyright laws will be indicted and punished according to the International Copyrights Law.

We do not take responsibility for material damage of any kind caused by the use of information contained in Hardware Secrets.