FIBER - OPTIC TECHNOLOGY

Definition

Fiber-optic communications is based on the principle that light in a glass medium can carry information over longer distances than electrical signals can carry in a copper or coaxial medium. The glass purity of today's fiber, combined with improved system electronics, enables fiber to transmit digitized light signals well beyond 100 km (60 miles) without amplification. With few transmission losses, low interference, and high bandwidth potential, optical fiber is an almost ideal transmission medium.

Overview

The advantages provided by optical fiber systems are the result of a continuous stream of product innovations and process improvements. As the requirements and emerging opportunities of optical fiber systems are better understood, fiber is improved to address them. This tutorial provides an extensive overview of the history, construction, operation, and benefits of optical fiber, with particular emphasis on outside vapor deposition (OVD) process.

1. From Theory to Practical Application: A Quick History

An important principle in physics became the theoretical foundation for optical fiber communications: light in a glass medium can carry more information over longer distances than electrical signals can carry in a copper or coaxial medium.

The first challenge undertaken by scientists was to develop a glass so pure that one percent of the light would be retained at the end of one kilometer (km), the existing unrepeated transmission distance for copper-based telephone systems. In terms of attenuation, this one percent of light retention translated to 20 decibels per kilometer (dB per km) glass material.

Glass researchers all over the world worked on the challenge in the 1960s, but the breakthrough came in 1970, when Corning scientists Drs. Robert Maurer, Donald Keck, and Peter Schultz created a fiber with a measured attenuation of less than 20 dB per km. It was the purest glass ever made.

The three scientists' work is recognized as the discovery that led the way to the commercialization of optical fiber technology. Since then, the technology has advanced tremendously in terms of performance, quality, consistency, and applications.

Working closely with customers has made it possible for scientists to understand what modifications are required, to improve the product accordingly through design and manufacturing, and to develop industry-wide standards for fiber.

The commitment to optical fiber technology has spanned more than 20 years and continues today with the endeavor to determine how fiber is currently used and how it can meet the challenges of future applications. As a result of research and development efforts to improve fiber, a high level of glass purity has been achieved. Today, fiber's optical performance is approaching the theoretical limits of silica-based glass materials. This purity, combined with improved system electronics, enables fiber to transmit digitized light signals well beyond 100 km (more than 60 miles) without amplification. When compared with early attenuation levels of 20 dB per km, today's achievable levels of 0.35 dB per km at 1310 nanometers (nm) and 0.25 dB per km at 1550 nm, testify to the incredible drive for improvement.

2. How Fiber Works

The operation of an optical fiber is based on the principle of total internal reflection. Light reflects (bounces back) or refracts (alters its course while penetrating a different medium), depending on the angle at which it strikes a surface. This occurs because different interfaces between materials refract light in different ways.

One way of thinking about this concept is to envision a person looking at a lake. By looking down at a steep angle, the person will see fish, rocks, vegetation, or whatever is below the surface of the water, assuming that the water is relatively clear and calm. However, by casting a glance farther out, thus making the angle of sight less steep, the individual is likely to see a reflection of trees or other objects on an opposite shore. Because air and water have different indices of refraction, the angle at which a person looks into or across the water influences the image seen.

This principle is at the heart of how optical fiber works. Lightwaves are guided through the core of the optical fiber in much the same way that radio frequency (RF) signals are guided through coaxial cable. The lightwaves are guided to the other end of the fiber by being reflected within the core. Controlling the angle at which the light waves are transmitted makes it possible to control how efficiently they reach their destination. The composition of the cladding glass relative to the core glass determines the fiber's ability to refract light. The difference in the index of refraction of the core and the cladding causes most of the transmitted light to bounce off the cladding glass and stay within the core. In this way, the fiber core acts as a waveguide for the transmitted light.