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.

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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:

1. The core or the centre of the optical fiber is a very thin strand of glass that carries the light signal.
2. 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.
3. 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:

1. Glass fibers, which are more common, because they allow longer distance transmission and they are more efficient.
2. 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:

1. 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.

2. 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

3. 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 t­he 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:

1. 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.

2. 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-by-side. 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

ADVANTAGES OF FIBER OPTICS

The use of fiber optics is fast becoming the medium of choice for telecommunication systems, television transmission and data networks. Fiber optic cables have a multitude of advantages and benefits over the more traditional methods of information systems, such as copper or coaxial cables.

Speed

One of the greatest benefits to using fiber optic systems is the capacity and speed of such a system. Light travels faster than an electrical system which allows faster delivery and reception of information. Fiber optic cables also have a much higher capacity for bandwidth than the more traditional copper cables.

Immunity to electromagnetic interference

Coaxial cables have a tendency for electromagnetic interference, which renders them less effective. Fiber optics is not affected by external electrical signals, because the data is transmitted with light.

Security

Optical systems are more secure than traditional mediums. Electromagnetic interference causes coaxial cables to leak information. Optical fiber makes it impossible to remotely detect the signal being transmitted within the cable. The only way to do so is by actually accessing the optical fiber itself. Accessing the fiber requires intervention that is easily detectable by security surveillance. These circumstances make fiber extremely attractive to governments, banks and companies requiring increased security of data.

Fire prevention

Copper wire transmission can generate sparks, causing shortages and even fire. Because fiber optical strands use light instead of electricity to carry signals, the chance of an electrical fire is eliminated. This makes fiber optics an exceptionally safe form of wiring and one of the safest forms of data transmission.

Data signalling

Fiber optic systems are much more effective than coaxial or copper systems, because there is minimal loss of data. This can be credited to the design of optical fibers, because of the principle of total internal reflection. The cladding increases the effectiveness of data transmission significantly. There is no crosstalk between cables, e.g. telephone signals from overseas using a signal bounced off a communications satellite, will result in an echo being heard. With undersea fiber optic cables, you have a direct connection with no echoes.

Unlike electrical signals in copper wires the light signals from one fiber do not interfere with those of other fibers in the same cable. This means clearer phone conversations or TV reception.

Less expensive

Several kilometers of optical cable can be made far cheaper than equivalent lengths of copper wire. Service, such as the internet is often cheaper because fiber optic signals stay strong longer, requiring less power over time to transmit signals than copper-wire systems, which need high-voltage transmitters.

Large Bandwidth, Light Weight and Small Diameter

Modern applications require increased amounts of bandwidth or data capacity, fiber optics can carry much larger bandwidth through a much smaller cable and they aren’t prone to the loss of information. With the rapid increase of bandwidth demand, fiber optics will continue to play a vital role in the long-term success of telecommunications.

Space constraints of many end-users are easily overcome because new cabling can be installed within existing duct systems. The relatively small diameter and light weight of optical cables makes such installations easy and practical.

Easy Installation and Upgrades

Long lengths of optical cable make installation much easier and less expensive. Fiber optic cables can be installed with the same equipment that is used to install copper and coaxial cables.

Long Distance Signal Transmission

The low attenuation and superior signal capacity found in optical systems allow much longer intervals of signal transmission than metallic-based systems. Metal based systems require signal repeaters to perform satisfactory. Fiber optic cables can transmit over 100km with no active or passive processing. Even greater distances are being investigated for the future.

To use fiber optics in data systems have proven to be a far better alternative to copper wire and coaxial cables. As new technologies are developed, transmission will become even more efficient, assuring the expansion of telecommunication, television and data network industries.

DISADVANTAGES OF FIBER OPTICS

Despite the many advantages of fiber optic systems, there are some disadvantages.

The relative new technology of fiber optic makes the components expensive. Fiber optic transmitters and receivers are still somewhat expensive compared to electrical components. The absence of standardisation in the industry has also limited the acceptance of fiber optics. Many industries are more comfortable with the use of electrical systems and are reluctant to switch to fiber optics.

The cost to install fiber optic systems is falling because of an increase in the use of fiber optic technology. As more information about fiber optics is made available to educate managers and technicians, the use of fiber optics in the industry will increase over time.

The advantages and the need for more capacity and information will also increase the use of fiber optics.

APPLICATIONS OF FIBER OPTICS

As the popularity of optical fibers continue to grow, so does their applications and practical uses. Fiber optic cables became more and more popular in a variety of industries and applications.

Communications / Data Storage

Since fiber optics are resistant to electronic noise, fiber optics has made significant advances in the field of communications. The use of light as its source of data transmission has improved the sound quality in voice communications. It is also being used for transmitting and receiving purposes.

Military

Optical systems offer more security than traditional metal-based systems. The magnetic interference allows the leak of information in the coaxial cables. Fiber optics is not sensitive to electrical interference; therefore fiber optics is suitable for military application and communications, where signal quality and security of data transmission are important.

The increased interest of the military in this technology caused the development of stronger fibers, tactical cables and high quality components. It was also applied in more varied areas such as hydrophones for seismic and SONAR, aircrafts, submarines and other underwater applications.

Medical

Fiber optic are used as light guides, imaging tools and as lasers for surgeries. Another popular use of fiber-optic cable is in an endoscope, which is a diagnostic instrument that enables users to see through small holes in the body. Medical endoscopes are used for minimally invasive exploratory or surgical procedures. Fiber optics is also used in bronchoscopes and laparoscopes.

All versions of endoscopes look like a long thin tube, with a lens or camera at one end through which light is emitted from the bundle of optical fibers banded together inside the enclosure.

Mechanical or Industrial

Industrial endoscopes also called a borescope or fiberscope, enables the user to observe areas that are difficult to reach or see under normal circumstances, such as jet engine interiors, inspecting mechanical welds in pipes and engines, inspecting space shuttles and rockets. Inspection of sewer lines and pipes.

Networking

Fiber optic is used to connect servers and users in a variety of network settings. It increases the speed, quality and accuracy of data transmission. Computer and Internet technology has improved due to the enhanced transmission of digital signals through optical fibers.

Industrial/Commercial

Fiber optics are used for imaging in areas which are difficult to reach. It is also used in wiring where electromagnetic interference is an problem. It gets used often as sensory devices to make temperature, pressure and other measurements as well as in the wiring of motorcars and in industrial settings.

Spectroscopy

Optical fiber bundles are used to transmit light from a spectrometer to a substance which cannot be placed inside the spectrometer itself, in order to analyse its composition. A spectrometer analyses substances by bouncing light off of and through them. By using optical fibers, a spectrometer can be used to study objects that are too large to fit inside, or gasses, or reactions which occur in pressure vessels

Broadcast/CATV/Cable Television

Broadcast or cable companies use fiber optic cables for wiring CATV, HDTV, internet, video and other applications.

Usage of fiber-optic cables in the cable-television industry began in 1976 and quickly spread because of the superiority of fiber optic cable over traditional coaxial cable. Fiber optic systems became less expensive and capable of transmitting clearer signals further away from the source signal. It also reduced signal losses and decreased the number of amplifiers required for each customer. Fiber optic cable allows cable providers to offer better service, because only one optical line is needed for every ± 500 households.

Lighting and Imaging

Fiber optic cables are used for lighting and imaging and as sensors to measure and monitor a vast range of variables. It is also used in research, development and testing in the medical, technological and industrial fields.

Fiber optics are used as light guides in medical and other applications where bright light needs to shine on a target without a clear line-of-sight path. In some buildings, optical fibers are used to route sunlight from the roof to other parts of the building. Optical fiber illumination is also used for decorative applications, including signs, art and artificial Christmas trees.

Optical fiber is an essential part of the light-transmitting concrete building product, LiTraCon which is a translucent concrete building material.

Conclusion

With the introduction of highly transparent fiber-optic cable in the 1970s, very high-frequency laser signals now carry phenomenal loads of telephone conversations and data across the country and around the world.

From surgical procedures to worldwide communication via the internet, fiber optic has revolutionised our world. Fiber optics has made important contributions to the medical field, especially with regards to surgery. 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. But perhaps the greatest contribution of the 20th century is the combination of fiber optics and electronics to transformed telecommunications.

References / Bibliography

• Why is fiber optics importany
• http://library.thinkquest.org/TQ0312432/lessons_part-04.html
• http://www.greatachievements.org/?id=3706