Fiber optic technology allows telephone companies, radio and TV stations, major corporations, and a variety of other businesses to send lots of information (data) over greater distances and at faster speeds than existing transmission systems will allow. The type of information that can be sent on fiber optic cable includes video, audio, written material, and computer data. Fiber optic technology works by using long strands of glass to send and receive laser-generated light impulses that contain digitized data. Unlike traditional copper cable that sends information in the form of electrons, fiber optic technology requires the electrons to be converted to photons (light). The information that is sent on fiber optic cable must be coded (placed) onto the light pulses. At the site where the fiber optic information is being sent, a decoder converts the light information into a form (pictures, audio, or written material) that we can understand. Fiber optic technology requires the use of ultra-pure glass. Ultra-pure glass is required because it allows the light pulses to flow through the long strands of fiber optic cable without being distorted. A tiny impurity in the fiber optic cable could cause the light pulse to lose some of the information that it is carrying.
Fiber optics is an ideal medium for transmitting information. The fibers allows extremely fast data transmission (even without compression) as well as offering the convenient availability that conventional wire offers. For these reasons, fiber optic technology has been employed in numerous applications ranging from lighting and sending images through medical instruments to uniting not only the nation, but possibly the world, with a high-speed data network. Some of the common applications include:-
Fiber wiring that is seen in medical equipment where surgeons must navigate through the body using surgical tools. The problem with these tools is that they need light to operate (orthoscopic surgery), which is where the flexibility of fiber comes into play. Optical fibers can be used to transmit light into narrow passageways (for example, within the human body). The extremely small size of the wiring allows surgeons to have vast amounts of flexibility when examining patients under limited conditions.
Another common example which one may not recognize is the newer highway signs. Each sign is equipped with a halogen tube which emits a large amount of light. The light is sent through fibers to the actual display area where computer-driven shutters decided which pixels should be open or closed therefore making up letters and words on the signs. The job of transporting the light is all based upon fiber technology.
There is mass communication through fiber optics. For example, a business office in SYDNEY may be linked to a business office in MELBOURNE to privately share data between central offices. Many of these applications may seem as if they are not really using the power of fiber optics to their advantage. This method of thought is brought up by the media stressing the future of interactive television and the like. The truth is that more applications based upon fiber optic wiring are still years down the road when fiber optics are more commonplace.
Telecommunication's
Optical fibres are now the standard point to point cable link between telephone substations.
Optical Fibre Sensors
Many advances have been made in recent years in the use of Optical Fibres as sensors. Gas concentration, chemical concentration, pressure, temperature, and rate of rotation can all be sensed using optical fibre.
Choice of Fibre
The energy carried by a single mode fibre, however, is much less than that carried by a multimode fibre. For this reason single mode fibre is made from extremely low loss, very pure, glass.
Graded Index Fibre
In graded index fibre rays of light follow sinusoidal paths. This means that low order modes, i.e. oblique rays, stay close to the centre of the fibre, high order modes spend more time near the edge of core. Low order modes travel in the high index part of the core and so travel slowly, whereas high order modes spend predominantly more time in the low index part of the core and so travel faster. This way, although the paths are different lengths, all the modes travel the length of the fibre in tandem, i.e., they all reach the end of the fibre at the same time. This eliminates multimode dispersion and reduces pulse spreading.
Graded Index fibre has the advantage that it can carry the same amount of energy as multimode fibre. The disadvantage is that this effect takes place at only one wavelength, so the light source has to be a laser diode which has a narrow linewidth.
In designing an fibre optic system there two main areas of crucial importance to consider. These are:-
We have to calculate both of these to see if our system will carry out the task required of it. But often there are compromises which we must make on the basis of cost.
Power budget Losses occur at many points in a fibre optic system. We have to ensure that the light source launches enough power into the fibre to provide enough power at the receiver. The receiver has limited sensitivity.
Transmitter output - Receiver input = Losses + Margin (All calculations are done in dB)
Types of Loss:-
couplers, connectors and splices
Simply multiply either the measured loss or the manufacturers specifications by the number of these devices in the system. For small numbers of devices use the maximum loss quoted per device. For large numbers of devices use the average loss quoted per device.
Fibre Loss Multiply the dB.km~1 loss figure for the fibre by the length of the fibre.
So called "transient" losses occur in the first few 100 m of MM fibre coupled to an LED. So for short lengths of fibre the loss/km is greater than the manufacturer's figure.
Fibre / receiver coupling loss This is not usually a problem since the area of the detector and its numerical aperture are larger than those of fibre.
In addition to the above known losses it is usual to allow a margin, in case some of the losses turn out to be higher than expected, but mainly to compensate for any future degradation of the system which may happen with time.
Bandwidth Budget The bandwidth budget is a series of calculations which allows us to work out whether the fibre system can support the data rate which we require. We do this by calculating the overall Response Time of the system. This overall time response of a fibre system must be less than the bit time of the signal.
There are a number of coding systems for digital information. The simplest to use, from the point of view of calculating response time is the Non Return to Zero coding (NRZ).
Response time of a system is defined as longer of the rise time or the fall time of a bit leaving the system.
A system which can transmit 1 Mbit/s, for example, must have a response time less than 1ms, then one bit will be trying to rise while the previous bit is still falling. As a result bits of information will merge together.
Calculation of overall response time The overall response time is affected by only 3 individual response times.
Components such as splices and connectors have a negligible effect on response time. The light can pass through them without any delaying effect.
Light Sources
There are two main light sources used in the field of fibre optics.-
A LED is a p-n junction diode in a transparent capsule usually with a lens to let the light escape and to focus it. LED's can be manufactured to operate at 850 nm, 1300 nm, or 1500 nm. These wavelengths are all in the infrared region. LED's have a typical response time of 8 ns, a linewidth of 40 nm, and an output power of tens of microwatts.
There are two main stages to the manufacture of optical fibres. These are:-
1) the making of the preform
2) the extrusion of the preform
Preform Manufacture
The most common method of making fibre preforms is known as Modified Chemical Vapour Dispersion (MCVD). An outer glass "bait tube" is heated by a traversing burner. Through this tube a mixture of gases is passed at a steady rate, which when heated undergoes a chemical reaction. The gas mix contains compounds of silicon, metal halides, oxygen and dopant materials which will determine the refractive index of the glass of the core. The solid end products of the reaction are deposited on the interior of the bait tube as "soot". This soot will eventually form the core of the fibre while the bait tube will form the cladding. When enough soot has been deposited the gas flow is stopped and the heat is turned up so that the soot melts to form a sintered glass.
Finally the tube is heated up enough to soften the bait tube and the sintered glass so that the whole tube collapses to form a solid rod.
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