10.8.8 How Does Light Remain Trapped Inside an Optical Fiber?
- What Is an Optical Fiber?
- What Is Meant by the Refractive Index?
- Why Does Light Bend When It Enters Another Material?
- What Is Total Internal Reflection?
- Why Doesn't the Light Escape After Many Reflections?
- What Is the Numerical Aperture?
- What Happens If the Fibre Is Bent?
- Why Can Optical Fibres Carry Signals Over Such Long Distances?
- Where Are Optical Fibres Used?
- Why Has Optical Fiber Revolutionised Communications?
- What Should You Remember?
Short Answer
An optical fiber guides light by exploiting the principle of total internal reflection. The fibre consists of a glass or plastic core surrounded by a cladding having a slightly lower refractive index. When light travelling within the core strikes the boundary at an angle greater than the critical angle, it is reflected back into the core rather than escaping into the cladding. By undergoing repeated total internal reflections, the light remains confined within the fibre, allowing information to be transmitted over distances of hundreds or even thousands of kilometres with remarkably low attenuation.
What Is an Optical Fiber?
An optical fiber is a very thin strand of transparent material designed to guide light from one point to another.
Although it resembles an ordinary piece of glass, an optical fibre is carefully engineered to control the propagation of light.
A typical optical fibre consists of three layers:
- a transparent core through which the light propagates;
- a surrounding cladding with a slightly lower refractive index; and
- a protective outer coating that provides mechanical strength and environmental protection.
The core is usually manufactured from extremely pure silica glass, although specialised fibres may use other materials for particular applications.
What Is Meant by the Refractive Index?
Light travels at different speeds in different materials.
The refractive index is a measure of how much a material slows the propagation of light compared with its speed in free space. Air has a refractive index very close to 1.0. Glass typically has a refractive index of approximately 1.45. This difference in propagation speed causes light to change direction whenever it passes from one material into another.
This bending of light is known as refraction.
Why Does Light Bend When It Enters Another Material?
When light passes from one material to another having a different refractive index, its speed changes.
Because one part of the wavefront changes speed before the remainder, the wave changes direction. This familiar phenomenon explains why a straight object appears bent when partially immersed in water. The relationship between the angles of incidence and refraction is described by Snell's Law, one of the fundamental principles of geometrical optics.
Although Snell's Law predicts how light refracts under most circumstances, something even more interesting happens when light attempts to leave a material having a higher refractive index.
What Is Total Internal Reflection?
Suppose light is travelling from the glass core towards the lower-index cladding.
For small angles of incidence, some of the light passes into the cladding while the remainder is reflected. As the angle increases, progressively less light escapes. Eventually a particular angle is reached at which the refracted ray travels exactly along the boundary. This angle is called the critical angle.
For angles greater than the critical angle, no light escapes into the cladding. Instead, the entire wave is reflected back into the core. This phenomenon is known as total internal reflection.
Unlike an ordinary mirror, virtually all of the optical energy remains within the fibre.
Why Doesn't the Light Escape After Many Reflections?
A common misconception is that repeated reflections should gradually allow light to leak from the fibre.
In an ideal optical fibre, however, total internal reflection is essentially lossless. As long as the angle of incidence remains greater than the critical angle, the light continues to reflect from the core-cladding boundary with very little loss. Modern optical fibres are manufactured with extraordinary precision, allowing light to undergo millions of reflections while travelling many kilometres.
The small losses that do occur arise primarily from material absorption, microscopic scattering, connector losses, and fibre bending rather than from failures of total internal reflection itself.
What Is the Numerical Aperture?
Not every ray entering an optical fibre will remain trapped.
Only those entering within a certain range of angles satisfy the conditions for total internal reflection. This range is described by the numerical aperture (NA). The numerical aperture determines the fibre's acceptance angle—the maximum angle at which light may enter while still remaining guided within the core.
Fibres having larger numerical apertures accept light more easily, simplifying alignment between the transmitter and the fibre.
What Happens If the Fibre Is Bent?
Although optical fibres are flexible, excessive bending causes problems.
When the fibre is bent sharply, some light rays strike the core-cladding boundary at angles smaller than the critical angle. Instead of undergoing total internal reflection, part of the light escapes into the cladding. This phenomenon is known as bend loss.
Modern fibres are remarkably tolerant of gentle bending, but installers still observe specified minimum bend radii to minimise attenuation.
Why Can Optical Fibres Carry Signals Over Such Long Distances?
Optical fibres exhibit extraordinarily low attenuation.
Modern single-mode fibres typically lose only around 0.2 dB of signal per kilometre at the commonly used wavelength of 1550 nm. This is several orders of magnitude lower than the attenuation of equivalent electrical transmission lines operating at comparable information rates.
Combined with erbium-doped fibre amplifiers (EDFAs) and wavelength-division multiplexing (WDM), this low attenuation allows optical communication systems to span entire continents and oceans with relatively few repeaters.
Where Are Optical Fibres Used?
Optical fibres now form the backbone of the world's communication infrastructure.
Typical applications include:
- Internet backbone networks;
- submarine communication cables;
- metropolitan fibre networks;
- fibre-to-the-home (FTTH) systems;
- mobile phone backhaul;
- cable television distribution;
- data centres;
- industrial sensing;
- medical endoscopes; and
- scientific instrumentation.
Virtually every international communication network now depends upon optical fibre.
Why Has Optical Fiber Revolutionised Communications?
The invention of low-loss optical fibre transformed global communications.
Compared with copper cables, optical fibres offer:
- vastly greater bandwidth;
- extremely low attenuation;
- complete immunity to electromagnetic interference;
- excellent electrical isolation;
- improved security against interception; and
- much smaller size and weight.
These advantages have enabled the explosive growth of the Internet, cloud computing, video streaming, and international telecommunications.
Without optical fibre, today's global digital society would simply not be possible.
What Should You Remember?
- Optical fibres guide light using total internal reflection.
- The fibre consists of a high-index core surrounded by a slightly lower-index cladding.
- Light remains trapped when it strikes the boundary at angles greater than the critical angle.
- The numerical aperture determines the range of angles over which light can enter the fibre successfully.
- Excessive bending causes bend loss because some light no longer satisfies the conditions for total internal reflection.
- Modern optical fibres exhibit extremely low attenuation, allowing communication over hundreds or even thousands of kilometres.
- Optical fibre has become the dominant transmission medium for long-distance and high-capacity communication networks because of its enormous bandwidth and exceptionally low loss.