Abstracts

The communications medium in most electronic communications systems is either wire conductor cable or free space. Recently, a new medium is growing in popularity, the fibreoptic cable. A fiberoptic cable is essentially a light pipe that is used to carry light beam from one place to another. Light is an electromagnetic signal like radio waves. It can be modulated by information and sent toa destination. Because the frequency of the light is extremely high, it can accomodate very wide bandwidth of information and extremely high data r ates can be achieved with excellent reliability. This article will introduce to the concepts and characteristic of fibreoptic cable .

Introduction

One of the main limitation of communication systems is their restricted information -carrying capabilities. In more specific terms what this means is that the communications medium can only carry a certain amount of messages. This information handling ability is directly proportional to the bandwidth of the communication channel. In telephone systems, the bandwidth is limited by the characteristic of the cable used to carry the signals. As the demand for telephones has increase, better cables and wiring systems have been developed to transmit multiple telephone conversations over asingle cable. These techniques have the same effect as if the number of cables or channels of communications were greatly multiplied.The high frequency of light waves can just fullfill these requirement.

Light transmitter and receiver can be setup to form a communication system between two distant places. However this is not practical to implement. Since light can only travels in straight line, any opaque objects in the light path will immediately stop the light. Mirrors can always be use to "bent" the light so as to contineu it's journey, but that is very inpractical. Luckily, the creation of fiberoptic overcome this problem.

In a fiberoptic communication system, light-beam pulses from a transmitter are fed into a fiberoptic cable where they are transmitted over long distances. At the receiving end, a light sensitive device, known as photocell is used to detect the light pulses. This photocell converts the light pulses into an electrical signals. The electrical signal are amplified and reshaped back into digital form. They are fed to a decoder, such as a D/A converter, where the original information is recovered.

What are Fiber Optics

Fiber optics (optical fibers) are long, thin strands of very pure glass about the diameter of a human hair. They are arranged in bundles called optical cables and used to transmit light signals over long distances.

Fiber optic cable consist of the folowing parts:

plastic coating that protects the fiber from damage and buffer coating moisture

Hundreds or thousands of these optical fibers are arranged in bundles in optical cables. The bundles are protected by the cable's outer covering called a jacket.

Optical fibers come in two types:

single-mode fibers used to transmit one signal per fiber (used in telephones and cable TV)

multi-mode fibers used to transmit many signals per fiber (used in computer networks, Local Area Networks (LAN)

Single-mode fibers have small cores (about 3.5 x 10-4 inches or 9 microns in diameter) and use to carry infra-red laser light (wavelength = 1,300 to 1,550 nanometers [nm]). Multi-mode fibers have larger cores (about 2.5 x 10-3 inches or 62.5 microns in diameter) and transmit infra-red light (wavelength = 850 to 1,300 nm) from light-emitting diodes (LED). Some optical fibers can be made from plastic. These fibers have a large core (0.04 in or 1 mm diameter) and transmit visible red light (wavelength = 650 nm) from LEDs.

How Does an Optical Fiber Transmit Light

The light in a fiber optic cable travels through the core by constantly bouncing from the cladding, a principle called total internal reflection. Because the cladding does not absorb any light from the core, the light wave can travel great distances. However, some of the light signal degrades within the fiber, mostly due to impurities in the glass. The extent that the signal degrades depends on the purity of the glass and the wavelength of the transmitted light (for example, 850 nm = 60 to 75 percent/km, 1,300 nm = 50 to 60 percent/km, 1,550 nm is greater than 50 percent/km). Some premium optical fibers show much less signal degradation, less than 10 percent/km at 1,550 nm.

How Does an Optical Fiber Transmit Light?

Suppose you want to shine a flashlight beam down a long, straight hallway. Just point the beam straight down the hallway light travels in straight lines, so it is no problem. What if the hallway has a bend in it? You could place a mirror at the bend to reflect the light beam around the corner. What if the hallway was very winding with multiple bends? You might line the walls with mirrors and angle the beam so that it bounces from side-to-side all along the hallway. This is exactly what happens in an optical fiber.

Advantages of Fiber Optics

Why are fiber optic systems revolutionizing telecommunications? Compared to conventional metal wire (copper wire), optical fibers are

Less expensive

several miles (or kilometers) of optical cable can be made cheaper than equivalent sizes of copper wire.

Thinner

optical fibers can be drawn to smaller diameters than copper wire.

Higher carrying capacity

because optical fibers are thinner than copper wires, more fibers can be bundled into a given diameter cable than copper wires. This allows more phone lines to go over the same cable or more channels to come through the cable into your cable TV box.

Less signal degradation

the loss of signal in optical fiber is less than in copper wire.

Light signals

unlike electrical signals in copper wires, 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.

Low power

because signals in optical fibers degrade less, lower power transmitters can be used instead of high voltage electrical transmitters for copper wires. Again, this saves your provider and your money.

Digital signals

optical fibers are ideally suited for carrying digital information, especially useful in computer networks.

Non-flammable

because no electricity is passed through optical fibers, there is no fire hazard.

Lightweight

an optical cable weighs less than a comparable copper wire cable. Fiber optic cables take up less space in the ground.

Flexible -

because fiber optics are so flexible and can transmit and receive light, they are used in many flexible digital cameras for the following purposes: medical imaging - in bronchoscopes, endoscopes, laparoscopes mechanical imaging - inspecting mechanical welds in pipes and engines (in plumbing - to inspect sewer lines

Because of these advantages, you see fiber optics in many industries, most notably telecommunications and computer networks. For example, if you have ever telephoned Europe from the United States or vice versa and the signal has been bounced off a communications satellite, you often hear an echo on the line. However, with transatlantic fiber optic cables, you have a direct connection with no echoes.

How are Optical Fibers Made?

Now that we know how fiber optic systems work and why they are useful, how do they make them? Optical fibers are made of extremely pure optical glass. We think of a glass window as transparent, but the thicker the glass gets, the less transparent it becomes due to impurities in the glass. However, the glass in an optical fiber has far less impurities than window pane glass. One company's description of the quality of glass is as follows: If you were on top of an ocean that is miles (or kilometers) of solid core optical fiber glass, you could see the bottom clearly.

Making optical fibers requires the following steps:

making a preform glass cylinder

drawing the fibers from the preform testing the fibers

Making the Preform Blank

The glass for the preform is made by a process called modified chemical vapor deposition (MCVD). In MCVD, oxygen is bubbled through solutions of silicon chloride (SiCl4), germanium chloride (GeCl4), and/or other chemicals. The precise mixture governs the various physical and optical properties (index of refraction, coefficient of expansion, melting point, etc.). The gas vapors are then conducted to the inside of a synthetic silica or quartz tube (cladding) in a special lathe. As the lathe turns, a torch is moved up and down the outside of the tube. The extreme heat from the torch causes two things to happen:

the silicon and germanium react with oxygen forming silicon dioxide (SiO2) and germanium dioxide (GeO2)

the silicon dioxide and germanium dioxide deposit on the inside of the tube and fuse together to form glass

The lathe turns continuously to make an even coating and consistent blank. The purity of the glass is maintained by using corrosion-resistant plastic in the gas delivery system (valve blocks, pipes, seals) and by precisely controlling the flow and composition of the mixture. The process of making the preform blank is highly automated and takes several hours. After the preform blank cools, it is tested for quality control index of refraction).

Drawing Fibers from the Preform Blank

the preform blank has been tested, it gets loaded into a fiber drawing tower. The blank gets lowered into a graphite furnace (3452 - 3992 degrees Fahrenheit or 1900 - 2200 degrees Celsius) and the tip gets melted until a molten glob falls down by gravity. As it drops, it cools and forms a thread. The operator threads the strand through a series of coating cups (buffer coatings) and ultraviolet light (UV)-curing ovens onto a tractor-controlled spool. The tractor mechanism slowly pulls the fiber from the heated preform blank and is precisely controlled by using a laser micrometer to measure the diameter of the fiber and feed the information back to the tractor mechanism. Fibers are pulled from the blank at a rate of 33 - 66 ft/s (10 - 20 m/s) and the finished product is wound onto the spool. It is not uncommon for spools to contain more than 1.4 miles (2.2 km) of optical fiber.

Testing the Finished Optical Fiber

The finished optical fiber is tested for the following: tensile strength - must withstand 100,000 lb/in2 or more × refractive index profile determine numerical aperture as well as screen for optical defects fiber geometry core diameter, cladding dimensions and coating diameter are uniform attenuation determine the extent that light signals of various wavelengths degrade over distance information carrying capacity (bandwidth) - number of signals that can be carried at one time (multi-mode fibers) chromatic dispersion - spread of various wavelengths of light through the core (important for bandwidth) operating temperature/humidity range temperature dependence of attenuation×

ability to conduct light underwater - important for undersea cables

once the fibers have passed the quality control, they are sold to telephone companies, cable companies and network providers. Many companies are currently replacing their old copper wire-based systems with new fiber optic-based systems to improve speed, capacity, and clarity.

Total Internal Reflection

When light passes from a medium with one index of refraction (m1) to another medium with a lower index of refraction (m2), it bends or "refracts away from an imaginary line perpendicular to the surface (normal line). As the angle of the beam through m1 becomes greater with respect to the normal line, the refracted light through m2 bends further away from the line. At one particular angle (critical angle), the refracted light will not go into m2, but instead will travel along the surface between the two media (sin [critical angle] = n2/n1 where n1 and n2 are the indices of refraction [n1 is less than n2]). If the beam through m1 is greater than the critical angle, then the refracted beam will be reflected entirely back into m1 (total internal reflection), even though m2 may be transparent! In physics, the critical angle is described with respect to the normal line. In fiber optics, the critical angle is described with respect to the parallel axis running down the middle of the fiber. Therefore, fiber optic critical angle = (90 degrees - physics critical angle).

In an optical fiber, the light travels through the core (m1, high index of refraction) by constantly reflecting from the cladding (m2, lower index of refraction) because the angle of the light is always greater than the critical angle. Light reflects from the cladding no matter what angle the fiber itself gets bent, even a full circle! Furthermore, because the cladding does not absorb any light from the core, the light wave can travel great distances. However, some of the light signal degrades within the fiber, mostly due to impurities in the glass. The extent that the signal degrades depends upon the purity of the glass and the wavelength of the transmitted light (for example, 850 nm = 60 to 75 percent/km, 1,300 nm = 50 to 60 percent/km, 1,550 nm is greater than 50 percent/km). Some premium optical fibers show much less signal degradation, less than 10 percent/km at 1,550 nm.