1.0 MULTIMODE FIBRE
Multimode cablehas a little bit bigger diameter, with a common diameters in the 50 to 100 micron range for the light carry component.Most applications in which Multimode fibrE is used, 2 fibres are used (WDM is not normally used on multi-mode fibre). POF is a newer plastic based cable which promises performance similar to glass cable on very short runs, but at a lower cost.
Multimode fibre gives high bandwidth at high speeds (10 to 100MBS Gigabit to 275m to 2km) over medium distances. Light waves are dispersed into numerous paths, or modes, as they travel through the cable's core typically 850 or 1300nm. Typical multimode fibre core diameters are 50, 62.5, and 100 micrometers. However, in long cable runs (greater than 3000 feet [914.4 meters), multiple paths of light can cause signal distortion at the receiving end, resulting in an unclear and incomplete data transmission so designers now call for single mode fibre in new applications using Gigabit and beyond.
The Corning Glass Works was able to produce a fibre with a loss of 20dB/km. It was recognized that optical fibre would be feasible for telecommunication transmission only if glass could be developed so pure that attenuation would be 20dB/km or less. That is, 1% of the light would remain after traveling 1 km. Today's optical fibre attenuation ranges from 0.5dB/km to 1000dB/km depending on the optical fibre used. Attenuation limits are based on intended application. 
1.1 Comparison with single-mode fibre
Multimode fibre has higher "light-gathering" capacity thansingle mode optical fibre. In practical terms, the larger core size simplifies connections and also allows the use of lower-cost electronics such aslight-emitting diodes(LEDs) andvertical cavity surface emitting lasers(VCSELs) which operate at the 850 nm and 1300 nm wavelength (single-mode fibres used in telecommunications operate at 1310 or 1550nmand require more expensive laser sources. Single mode fibres exist for nearly all visible wavelengths of light).However, compared to single-mode fibres, the limit on speed times distance is lower. Because multi-mode fibre has a larger core-size than single-mode fibre, it supports more than onepropagation mode; hence it is limited bymodal dispersion, while single mode is not. The LED light sources sometimes used with multimode fibre produce a range of wavelengths and these each propagate at different speeds. In contrast, the lasers used to drive single-mode fibres producecoherent lightof a single wavelength. Thischromatic dispersionis another limit to the useful length for multimode fibre optic cable. Because of their larger core size, multi-mode fibres have higher numerical apertureswhich means they are better at collecting light than single-mode fibres. Due to the model dispersion in the fibre, multi-mode fibre has higher pulse spreading rates than single mode fibre, limiting multi-mode fibres information transmission capacity.
Single-mode fibres are most often used in high-precision scientific research because the allowance of only one propagation mode of the light makes the light easier to focus properly.
Jacket colour is sometimes used to distinguish multi-modecablesfrom single-mode, but it cannot always be relied upon to distinguish types of cable. The standard TIA-598C recommends, for civilian applications, the use of a yellow jacket for single-mode fibre, and orange for 50/125m (OM2) and 62.5/125m (OM1) multimode fibre.Aquais recommended for 50/125m "laser optimized" OM3 fibre.
1.3 Multimode Advantages
Multimode fibre optic cable and components are less expensive and easier to work with than their single mode counterparts. This is due largely to the fact that the multimode fibre core is larger, and alignment tolerances are much less critical than they are for single mode fibre.
Like single mode, multimode fibre provides high bandwidth at high speeds, but transmission is limited to shorter distances than single mode. (In longer cable runs, the multiple paths of light in a multimode fibre tend to create signal distortion).
Standard multimode cable is made of glass fibres, usually 50-to-100 micron in diameter (most common is 62.5). Multimode cable is also available as low-cost Plastic Optical Fibre (POF), which offers performance similar to glass cable for very short runs.
2.0 SINGLEMODE FIBRE
Single Mode cableis a single stand (most applications use 2 fibres) of glass fibre with a diameter of 8.3 to 10 microns that has one mode of transmission. Single Mode Fibre with a relatively narrow diameter, through which only one mode will propagate typically 1310 or 1550nm. Carries higher bandwidth than multimode fibre, but requires a light source with a narrow spectral width. Synonyms mono-mode optical fibre, single-mode fibre, single-mode optical waveguide, uni mode fibre.
Single Modem fibre is used in many applications where data is sent at multi-frequency (WDM Wave-Division-Multiplexing) so only one cable is needed - (single-mode on one single fibre)
Single-mode fibre gives you a higher transmission rate and up to 50 times more distance than multimode, but it also costs more. Single-mode fibre has a much smaller core than multimode. The small core and single light-wave virtually eliminate any distortion that could result from overlapping light pulses, providing the least signal attenuation and the highest transmission speeds of any fibre cable type.
Single-mode optical fibre is an optical fibre in which only the lowest order bound mode can propagate at the wavelength of interest typically 1300 to 1320nm.
2.1 Conditions for Single Mode Transmission
To calculate the number of modesNmin a step-index fibre,Nmcan be simplified as:
Optical Fiber Single Mode Condition
Dis core diameter of the fibre
γis the operating wavelength
nfis refractive index of the fibre core
ncis refractive index of the fibre cladding
Reducing the core diameter sufficiently can limit transmission to a single mode. The following formula defines the maximum core diameter,D, which limits transmission to a single mode at a particular wavelength,γ:
Maximum Core Diamater for Single Mode Fiber
If the core is any larger, the fibre can carry two modes     .
2.2 Advantages of Single Mode Fibre
Single mode fibre doesnt have modal dispersion, modal noise, and other effects that come with multimode transmission; single mode fibre can carry signals at much higher speeds than multimode fibres. They are standard choice for high data rates or long distance span (longer than a couple of kilometres) telecommunications which use laser diode based fibre optic transmission equipment    
2.3 Disadvantages of Single Mode Fibre
Since single mode fibres core is so much smaller than a multimode fibres core, coupling light into single mode fibre requires much tighter tolerances than coupling light into the larger cores of multimode fibre. However, those tighter tolerances have proved achievable.
Single mode fibre components and equipment are also more expensive than their multimode counterparts, so multimode fibres are widely used in systems where connections must be made inexpensively and transmission distances and speeds are modest      .
3.0 Calculating Fibre Optic Loss Budget
3.1 Criteria & Calculation Factors
Design of a fibre optic system is a balancing act. As with any system, you need to set criteria for performance and then determine how to meet those criteria. It's important to remember that we are talking about a system that is the sum of its parts.
Calculation of a system's capability to perform is based upon a long list of elements. Following is a list of basic items used to determine general transmission system performance:
- Fibre Loss Factor Fibre loss generally has the greatest impact on overall system performance. The fibre strand manufacturer provides a loss factor in terms of dB per kilometre. A total fibre loss calculation is made based on the distance x the loss factor. Distance in this case the total length of the fibre cable, not just the map distance.
- Type of fibre Most single mode fibres have a loss factor of between 0.25 (@ 1550nm)
- and 0.35 (@ 1310nm) dB/km. Multimode fibres have a loss factor of about 2.5 (@850nm) and 0.8 (@ 1300nm) dB/km. The type of fibre used is very important. Multimode fibres are used with L.E.D. transmitters which generally don't have enough power to travel more than 1km. Single mode fibres are used with LASER transmitters that come in various power outputs for "long reach" or "short reach" criteria.
- Transmitter There are two basic type of transmitters used in a fibre optic systems. LASER which come in three varieties: high, medium, and low (long beach, medium reach and short reach). Overall system design will determine which type is used. L.E.D. transmitters are used with multimode fibres, however, there is a "high power" L.E.D. which can be used with Single mode fibre. Transmitters are rated in terms of light output at the connector, such as -5dB. A transmitter is typically referred to as an "emitter. Receiver Sensitivity The ability of a fibre optic receiver to see a light source. A receiving device needs a certain minimum amount of received light to function within specification. Receivers are rated in terms of required minimum level of received light such as -28dB. A receiver is also referred to as a "detector. Number and type of splices There are two types of splices. Mechanical, which use a set of connectors on the ends of the fibres, and fusion, which is a physical direct mating of the fibre ends? Mechanical splice loss is generally calculated in a range of 0.7 to 1.5 dB per connector. Fusion splices are calculated at between 0.1 and 0.5 dB per splice. Because of their limited loss factor, fusion splices are preferred.
- Margin This is an important factor. A system can't be designed based on simply reaching a receiver with the minimum amount of required light. The light power budget margin accounts for aging of the fibre, aging of the transmitter and receiver components, addition of devices along the cable path, incidental twisting and bending of the fibre cable, additional splices to repair cable breaks, etc. Most system designers will add a loss budget margin of 3 to 10 dB
3.2 Calculating a "Loss Budget"
Let's take a look at typical scenario where a fibre optic transmission system would be used.
Two operation centres are located about 8 miles apart based on map distance. Assume that the primary communication devices at each centre are a wide area network capable router with fibre optic communication link modules, and that the centres are connected by a fibre optic cable. The actual measured distance based on walking the route , is a total measured length (including slack coils) of 9 miles. There are no additional devices installed along the cable path. Future planning provides for the inclusion of a freeway management system communication link within 5 years.
Note: All distance measurements must be converted to kilometres. Fibre cable is normally shipped with a maximum reel length of 15,000 feet (or 4.5km). 9 miles is about 46,000 feet or 14.5km. Assume that this system will have at least 4 mid-span fusion splices.
Because a loss margin of 5.0dB was included in the fibre loss calculation, the short reach option will provide sufficient capability for this system. In fact, the total margin is 8.0dbbecause the difference between the loss budget and receiver sensitivity is 3.0db
Inoptics, thenumerical aperture(NA) of an optical system is adimensionless numberthat characterizes the range of angles over which the system can accept or emit light. The exact definition of the term varies slightly between different areas of optics.
The numerical aperture with respect to a pointPdepends on the half-angle?of the maximum cone of light that can enter or exit the lens
4.1 DERIVIATION OF NUMERICAL APRETURE
Multimode optical fibrewill only propagate light that enters the fibre within a certain cone, known as theacceptance coneof the fibre. The half-angle of this cone is called theacceptance angle,mix. Forstep-indexmultimode fibre, the acceptance angle is determined only by the indices of refraction:
wheren1is the refractive index along the central axis of the fibre. Note that when this definition is used, the connection between the NA and the acceptance angle of the fibre becomes only an approximation. In particular, manufacturers often quote "NA" forsingle-mode fibrebased on this formula, even though the acceptance angle for single-mode fibre is quite different and cannot be determined from the indices of refraction alone.
The number of boundmodes, themode volume, is related to thenormalized frequencyand thus to the NA.
In multimode fibres, the termequilibrium numerical apertureis sometimes used. This refers to the numerical aperture with respect to the extreme exit angle of arayemerging from a fibre in whichequilibrium mode distributionhas been established.
5.0 OPTICAL FIBRE USES
5.1 Optical fibre communication
Optical fibre can be used as a medium for telecommunication and [Computer network because it is flexible and can be bundled as cables. It is especially advantageous for long-distance communications, because light propagates through the fibre with little attenuation compared to electrical cables. This allows long distances to be spanned with fewrepeaters. Additionally, the per-channel light signals propagating in the fibre have been modulated at rates as high as 111gigabits per secondbyNTT,although 10 or 40Gb/s is typical in deployed systems.Each fibre can carry many independent channels, each using a different wavelength of light (wavelength-division multiplexing(WDM)). The net data rate (data rate without overhead bytes) per fibre is the per-channel data rate reduced by the FEC overhead, multiplied by the number of channels (usually up to eighty in commercialdense WDMsystems as of 2008). The current laboratory fibre optic data rate record, held by Bell Labs in Villarceaux, France, is multiplexing 155 channels, each carrying 100 Gb/s over a 7000km fibre.
For short distance applications, such as creating a network within an office building, fibre-optic cabling can be used to save space in cable ducts. This is because a single fibre can often carry much more data than many electrical cables, such as 4 pairCat-5Ethernet cabling.[vague]Fibre is also immune to electrical interference; there is no cross-talk between signals in different cables and no pickup of environmental noise. Non-armoured fibre cables do not conduct electricity, which makes fibre a good solution for protecting communications equipment located inhigh voltageenvironments such aspower generationfacilities, or metal communication structures prone tolightningstrikes. They can also be used in environments where explosive fumes are present, without danger of ignition.Wiretappingis more difficult compared to electrical connections, and there are concentric dual core fibres that are said to be tap-proof.
Although fibres can be made out of transparentplastic,glass, or acombination of the two, the fibres used in long-distance telecommunications applications are always glass, because of the lower optical attenuation. Both multi-mode and single-mode fibres are used in communications, with multi-mode fibre used mostly for short distances, up to 550m (600 yards), and single-mode fibre used for longer distance links. Because of the tighter tolerances required to couple light into and between single-mode fibres (core diameter about 10micrometers), single-mode transmitters, receivers, amplifiers and other components are generally more expensive than multi-mode components.
Examples of applications areTOSLINK,Fibre,Synchronous optical networking.[jargon]
5.2 Fibre optic sensors
Fibres have many uses in remote sensing. In some applications, the sensor is itself an optical fibre. In other cases, fibre is used to connect a non-fibrotic sensor to a measurement system. Depending on the application, fibre may be used because of its small size, or the fact that noelectrical poweris needed at the remote location, or because many sensors can be multiplexedalong the length of a fibre by using different wavelengths of light for each sensor, or by sensing the time delay as light passes along the fibre through each sensor. Time delay can be determined using a device such as anoptical time-domain reflect meter.
Optical fibres can be used as sensors to measurestrain,temperature,pressureand other quantities by modifying a fibre so that the quantity to be measured modulates theintensity,phase,polarization,wavelengthor transit time of light in the fibre. Sensors that vary the intensity of light are the simplest, since only a simple source and detector are required. A particularly useful feature of such fibre optic sensors is that they can, if required, provide distributed sensing over distances of up to one meter.
Extrinsic fibre optic sensors use anoptical fibre cable, normally a multi-mode one, to transmitmodulatedlight from either a non-fibre optical sensor, or an electronic sensor connected to an optical transmitter. A major benefit of extrinsic sensors is their ability to reach places which are otherwise inaccessible. An example is the measurement of temperature insideaircraftjet enginesby using a fibre to transmitradiationinto a radiationpyrometerlocated outside the engine. Extrinsic sensors can also be used in the same way to measure the internal temperature ofelectrical transformers, where the extremeelectromagnetic fieldspresent make other measurement techniques impossible. Extrinsic sensors are used to measure vibration, rotation, displacement, velocity, acceleration, torque, and twisting.
5.3 Other uses of optical fibre
AFrisbeeilluminated by fibre optics Light reflected from optical fibre illuminates exhibited model
Fibres are widely used in illumination applications. They are used aslight guidesin medical and other applications where bright light needs to be shown on a target without a clear line-of-sight path. In some buildings, optical fibres are used to route sunlight from the roof to other parts of the building (seenon-imaging optics). Optical fibre illumination is also used for decorative applications, includingsigns,art, and artificialtrees. Swarovskiboutiques use optical fibres to illuminate their crystal showcases from many different angles while only employing one light source. Optical fibre is an intrinsic part of the light-transmitting concrete building product,LiTraCon.
Optical fibre is also used in imaging optics. A coherent bundle of fibres is used, sometimes along with lenses, for a long, thin imaging device called anendoscope, which is used to view objects through a small hole. Medical endoscopes are used for minimally invasive exploratory or surgical procedures (endoscopy). Industrial endoscopes (seefiberscopeorbore scope) are used for inspecting anything hard to reach, such as jet engine interiors.
Inspectroscopy, optical fibre bundles are used to transmit light from a spectrometer to a substance which cannot be placed inside the spectrometer itself, in order to analyze its composition. A spectrometer analyzes substances by bouncing light off of and through them. By using fibres, a spectrometer can be used to study objects that are too large to fit inside, or gasses, or reactions which occur in pressure vessels.
An optical fibredopedwith certainrare earth elementssuch aserbiumcan be used as thegain mediumof alaseroroptical amplifier. Rare-earth doped optical fibres can be used to provide signalamplificationby splicing a short section of doped fibre into a regular optical fibre line. The doped fibre isoptically pumpedwith a second laser wavelength that is coupled into the line in addition to the signal wave. Both wavelengths of light are transmitted through the doped fibre, which transfers energy from the second pump wavelength to the signal wave. The process that causes the amplification isstimulated emission.
Optical fibres doped with awavelength shifterare used to collectscintillationlight inphysicsexperiments.
Optical fibre can be used to supply a low level of power (around one watt) to electronics situated in a difficult electrical environment. Examples of this are electronics in high-powered antenna elements and measurement devices used in high voltage transmission equipment.
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