Optics Lessons: Part 4 - Fiber Optics

What is Fiber Optics? Fiber optics is a new field that scientists and engineers have only recently discovered. The materials used are usually long strands of special glass called "optical fibers" bundled up into cables called "optical cables", which are capable of transmitting light across distances.

A single optical fiber consists of three parts: the thin glass core where the light travels, the outter cladding, which reflects light into the core, and the plastic buffer, which protects the inner core and cladding from moisture and damage. A group of these optical fibers bundled up into an "optical cable" is protected by the jacket, or outer covering.

There are usually two types of fiber optics used: single-mode fibers, which trasmit infrared laser light and have smaller inner cores, and multi-mode fibers, which trasmit infrared lights of shorter wave lengths from a diode, a semi-conductor device.

The way the system works is as follows: the light is bounced along the cladding and travels by way of the glass. This is known as a type of refraction called total internal reflection. The more pure the glass is, the farther the light will be able to travel without interference.

The History of Fiber Optics

Fiber optics is based on sending signals by light. Earlier, it was done so by transmitting the signals via air, a prehistoric "wireless communication".

The field of fiber optics first placed a foot into the limelight in the 1790’s, when French engineer Claude Chappe invented an “optical telegraph” that consisted of a chain of semaphores on towers, where human operators relayed messages. This was later rendered obsolete by the telegraph.

Alexander Graham Bell invented an optical telephone or "photophone", but this was quickly rendered useless when the telephone proved more efficient, cheap, and reliable, considering wires transmitted electric signals better than air transmits light.

It was later, in the mid-19th century, that the concept of total internal refraction was discovered. Water was the medium first experimented with, and scientists soon turned to prisms and glass. Breakthrough discoveries were made by people like John Logie Baird in England and Clarence W. Hansell in the United States, who made important contributions with regards to fiber optics and television. However, the first person who really showed the demonstration of transmitting images by light was Heinrich Lamm, a student living in Munich, who dreamed of exploring the human body by means of fiber optics. As engineers perfected fiber optics usage for medicine, they turned towards the communications sector.

Telecommunications engineers rushed to delve further and deeper into this hot, new field with vast possibilities. As more experiments were performed, scientists realized with dismay that transmitting by light seemed to be too flimsy for long-distance communication. A team from Standard Telecommunications Laboratories headed by Antoni E. Karbowiak, with a young Chinese engineer Charles K. Kao, did not give up hope. They submitted a proposal entailing the a fiber optics communication system based on single-mode fibers, which would serve to carry communication long-distance. With their success came break through upon breakthrough, as the field expanded into daily life (i.e. internet, cable television, telephones).

Today Fiber Optics is a serious competition for the more established communications systems. Compared to the commonly used copper wire, optical fibers are cheaper, thinner, more flexible, non-flammable, lightweight, low powered (and thus cheaper for the user), carries more information, and has less impurities and loss of signal.

A "Ray Relay"

A Fiber Optics system is like a relay of light rays. A fiber optics system usually consists of the following "relay runners": transmitter, optical fiber, optical regenerator, and optical receiver.

First, the transmitter turns the light in a series of "on"s and "off"s. Thus this creates an almost Morse code-like signal that is sent through the optical fibers, often with the aid of a lens. The optical regenerator is not mandatory, but is very useful when transmitting light long distances, since often the light gets weakened by constant internal reflection. The regenerator "dopes" the light rays, or absorbs them with special "doping" optical fibers and spits them out again as stronger and clearer rays. The optical receiver is the last participant in the relay. It takes in the light rays using sensors called photocells and translates it into electrical signals, to be used in televisions or computers.

Getting into the Nooks and Crannies

One of the most useful characteristics of optical fibers is their ability to enter the minute passageways and hard-to-reach areas of the human body. Fiber optics has made important contributions to the medical field, especially with regards to surgery. This has been accomplished by cutting and polishing the ends of a very slim bundle of fibers, to form a fiberscope. The light is sent to the site of inquiry, reflected off of the area that the doctor wishes to see, and sent back to a receiver. Afterwards, the image is magnified to be analyzed. Because optical fibers are so flexible, they are able to navigate around the curvy parts of the human body in areas such as the stomach, heart, blood vessals, and joints. It is now even becoming possible to do surgery with instruments attached to the optical fibers, such as joint surgeries. Fiber optics is becoming especially important in heart surgery, since it can be done without disturbing the functionings of this delicate organ.

The Physics of Fiber Optics

In physics, there is a phenomenon called refraction, where light "bends" upon entering medium with different indexes of refraction. There is a certain angle, called the critical angle, at which a light ray can enter the first medium and then travel between the two mediums, without crossing into the second medium. If the angle at which the ray passes through the first medium is greater than the critical angle, the ray will experience total internal reflection, where it will just reflect off the surface between the two mediums and reenter the first medium. This is the case with optical fibers.

The glass core is the first medium that the light travels through. The cladding is the second medium, whose tangent surface to the core the light bounces off of. To picture it better, imagine two mirrors facing each other. The light bounces continually off each of them, getting absorbed be neither. In this case, the air acts as the core instead of glass. For optical fibers, the angle at which the ray passes through is always greater than the critical angle, thus causing no absorbance of light by the cladding. Any error that occurs or loss of signal is mainly because of impurities within the glass, which continually proves to be a problem in the field.

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