Garth Naar - Fiber optics and its applications

Garth Naar - Fiber optics and its applications INTRODUCTION Nothing in the world gives us more power and confidence than having information. The ability to communicate information is essential to achieve the successful advancement of humankind. Transmission of information is imperative to the expansion of our horizons.

What does this all have to do with fiber optics? This research paper will cover the basis of fiber optics in terms of its transmission, communication, origin, uses and applications.

Fiber optics transports light in a very directional way. Light is focused into and guided through a cylindrical glass fiber. Inside the core of the fiber light bounces back and forth at angles to the side walls, making its way to the end of the fiber where it eventually escapes.

The light does not escape through the side walls because of total internal reflection.

Why is fiber optics so important? Besides being a flexible conduit that is used to illuminate microscopic objects, fiber optics can also transmit information similarly to the way a copper wire can transmit electricity. However, copper transmits only a few million electrical pulses per second, compared to an optical fiber that carries up to a 20 billion light pulses per second. This means telephone, cable and computer companies can handle huge amounts of data transfers at once, much more than conventional wires can carry. Fiber optic cable was developed because of the incredible increase in the quantity of data over the past 20 years.

Without fiber optic cable, the modern Internet and World Wide Web would not be possible.

WHAT IS FIBER OPTICS? Fiber optics is extremely thin strands of purified glass that carry information from one point to another in the form of light. Unlike copper wire, fiber optics does not use electricity during transmission. Optical fibers can be either glass or plastic tubing capable of transmitting light, which is then converted into sound, speech or information. Fiber optic cables transmit a digital signal via pulses of light through the very thin strands of glass.

A basic fiber optic system consists of: a transmitting device, which generates the light signal, an optical fiber cable, which carries the light, and a receiver, which accepts the light signal that was transmitted.

A fiber optic strand is about the thickness of a human hair, about 120 micrometers in diameter and can carry as many as 20 billion light pulses per second. The fibers are bundled together to form optical bundles, which transmit the light signals over long distances up to 50 km without the need for repeaters.

Each optic fiber is made up of three main parts: The core or the centre of the optical fiber is a very thin strand of glass that carries the light signal.

The cladding is the optical material which reflects the light signals back into the core. This prevents the light from escaping and allows it to travel through the fiber.

The outside jacket or buffer coating is made of a plastic material that protects the optical fiber from any moisture, corrosion and external damage.

There are only two types of fiber optic cable: Glass fibers, which are more common, because they allow longer distance transmission and they are more efficient.

Plastic optical fibbers are used in less technical applications and are normally used in very short-length transmissions.

HOW ARE OPTICAL FIBERS MADE? Optical fibers are made of very pure glass. The glass core or centre is made of silica and is purified to minimise the loss of signal. It then gets coated to protect the fibers and to contain the light signals. The light signals carried by the optical cable consist of electrical signals that have been converted or changed into light energy.

The following process is followed to manufacture the optical fibers: The Manufacturing of the Preform Blank The silica must first be purified before it can be spun into glass fibers. This process takes a long time and the silica is heated to very high temperatures and then distilled to purification. The sand is heated to a temperature that will change the silica into a gaseous state. The silica will then be combined with other materials called dopants, which will react with the silica (in its gaseous state) to form the fibers. All the solid impurities are removed and the gas is cooled to form the fiber material.

A process called modified chemical vapour deposition (MCVD) is used to change the glass into the preform blank. During this process oxygen is bubbled through solutions of silicon chloride (SiCl4), germanium chloride (GeCl4) and other chemicals. The gas vapours are channelled to the inside of a synthetic silica quartz tube in a special lathe to form the cladding. While the lathe rotates a burning flame is moved back and forth on the outside of the tube.

The extreme heat from the burner causes the following: The silicon and the germanium react with oxygen to form silicon dioxide (SiO2) and germanium dioxide (GeO2).

The silicon dioxide and the germanium dioxide settles on the inside of the tube and it fuses together to form glass.

The lathe turns continuously to allow the preform blank to be coated evenly. To maintain the purity of the glass a corrosion resistant plastic is used to accurately control the flow and the structure of the mixture. This process of manufacturing the preform blank takes a couple of hours. The preform blank is cooled and is inspected for quality through an inspection and control process.

Drawing fibers from the Preform Blank After testing the preform, it is placed into a fiber “drawing tower.” The preform blank gets lowered into a furnace and is heated between 1,900°C to 2,200°C until the tip starts to melt an a molten blob starts to fall down. As it drops down, it cools and forms a strand. This strand is pulled through a sequence of coating cups (buffer applicators) and curing ovens using ultraviolet light, and then coiled onto a tractor-controlled reel. This process is accurately controlled using a laser micrometer to measure the thickness of the fiber. This information is then sent back to the tractor mechanism. The tractor mechanism pulls the fibers at a rate of 10 to 20m/sec and the finished product is wound onto a spool. A spool can contain more than 2,2km of optical fiber.

Testing the Finished Optical Fiber Once the optical fiber is manufactured it goes through a process of testing. The following tests are done: Tensile strength – The fibers must withstand 100,000 lb/in2 or more Refractive index profile – Determine that the core diameter, cladding dimensions and coating diameter are uniform. Screen also for optical defects.

Attenuation – Determine the extent that light signals of various wavelengths degrade or reduce over certain distances.

Information carrying capacity (bandwidth) – the number of signals that can be carried at one time (multi-mode fibers) Chromatic dispersion – Spread of various wavelengths of light through the core, this is very important for bandwidth.

Operating temperature/humidity range – Determines the temperature and humidity that the fiber can withstand.

Ability to conduct light underwater – Important for undersea cables Once t-he fibers have passed the quality control process, they are sold to telephone companies, cable companies and network providers. Currently many companies are replacing their old copper-wire-based systems with new fiber-optic-based systems to improve speed, capacity and clarity.

TYPES OF OPTICAL FIBERS There are two types of optical fibers: Single Mode Fiber Single mode fibers transmit a single data stream. The core of the glass fiber is much finer than in multi-mode fibers. Light thus travels parallel to the axis, creating little pulse dispersion. Data transmission modes are higher, and the distances that single mode fiber can cover can be over 50 times longer than multi-mode fibers. Telephone and cable television networks install millions of kilometers of this fiber every year.

Multi-Mode Fiber Multi-mode fibers allow different data streams to be sent simultaneously over a particular fiber. The glass fiber has a slightly larger diameter to allow light to be sent through the fiber at different angles. “An LED or laser light source is used in the 50 micron and 62.5 micron fiber optic cables. They are also used in the same networking applications. The main

difference between the two is that 50 micron fiber can support 3 times the bandwidth of 62.5 micron fiber. The 50 micron fiber also supports longer cable runs than 62.5 micron cable.

Simplex cable consists of only one single fiber optic strand. The data can only be transmitted in one direction. The duplex cable is made up of two fiber optic strands that run side-byside. One strand runs from transmit to receive and the other strand joins receive to transmit. This allows communication in both directions (bi-directional) between devices.

Some optical fibers can be made from plastic. These fibers have a large core (0.04 inches or 1 mm diameter) and transmit visible red light (wavelength = 650 nm) from LEDs. Due to their inferior optical properties, plastic fiber optic (POF) strands and cables are not suitable for extended data transmission.

HOW DOES A FIBER OPTIC CABLE WORK? Traditionally when we sent data transmissions over copper cables we transmit electrons over a copper conductor. “Fiber optic cables transmit a digital signal via pulses of light through a very thin strand of glass.� The fiber strands are extremely thin, not much thicker than a human hair.

The basic fiber optic transmission system consists of three basic components:

Transmitter fiber optic cable receiver A transmitter is connected to the one end of the fiber cable. Electronic pulses are converted by the transmitter into light pulses and the optical signal gets sent through the fiber cable. A receiver on the other end decodes the optical signal into digital pulses.

The core of the cable is surrounded by a cladding which reflects the light back into the core and eliminates light from escaping the cable. This is called total internal reflection.

When light is sent through the core of a fiber optic cable, the light constantly bounces off the cladding, which is highly reflective, like a mirror-lined wall. The cladding does not absorb any light allowing complete internal reflection and allowing the light to travel far distances without losing its intensity.

The discovery of lasers influenced the development of fiber optics. Lasers and LED’s can generate an enormous amount of light in a very small area, which can successfully used in fiber optics.

Laser diodes are complex semiconductors that convert an electrical current into light. The process of converting the electrical signal into light is far more efficient because it generates less heat than an ordinary light bulb.

Reasons for using laser diodes in fiber optics: laser diodes are very small laser diodes are highly reliable and have a long life laser diodes have high radiance laser diodes emit light into a very small area laser diodes can be turned on and off at very high speeds