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OPTICAL TIME DOMAIN REFLECTOMETER

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OTDR BLOCK DIAGRAM

CONTROLLERCRT OR

LCDDISPLAY

LASER

DETECTOR

COUPLER

FIBER UNDER TEST

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THE OTDR SENDS OUT A PULSE OF LIGHT AND MEASURES THE LEVEL OF LIGHT THAT IS REFLECTED BACK. AN OPTICAL COUPLER ALLOWS BOTH OPTICAL SOURCE AND OPTICALRECEIVER TO BE CONNECTED TO THE SAMEFIBER.

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HOW AN OTDR WORKS

The modern OTDR knows how fast light will travel throughthe core of the fiber under test because of the index of refraction (IOR) setting. The OTDR knows how far it needs to measure because of the fiber length setting. With this information, the OTDR will repeatedly sample the level of reflected light.

GN Nettest’s CMA4000 makes up to 16,384 samples of reflected light per pulse of transmitted light this means that if the fiber length setting was 128km, sampling would occur every 8 meters.(128km/16384)

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OTDR

COURSE OBJECTIVES:

1. Principal of OTDR.

2. OTDR Specifications

3. Using an OTDR.

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1. PRINCIPLES OF OTDR

An OTDR is a fiber optic tester characterizing fibers and optical networks.

The aim of this instrument is to detect, locate and measureevents at any location in the fiber optic link.

An OTDR can test a fiber from only one end that is if operates as a one dimensional Radar System.

The OTDR technique produces geographic information with regard to localized loss and reflective eventsprovide a pictorial and permanent record which may beused as a permanent baseline.

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Principles of OTDR(Contd..)

The OTDR’s ability to characterize a fiber is base on detecting small signals returned to OTDR in responseto the injection of a large signal.

OTDR depends on two types of Optical Phenomena:

A. Rayleigh BackscatteringB. Fresnel Reflections

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A. Rayleigh Scattering

Rayleigh scattering is intrinsic to the fiber material itselfand is present all along the length of fiber.

If Rayleigh scattering is uniform along the length of fiber,than discontinuities in the back scatter can be used toidentify anomalies in transmission along the length of fiber.

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B. Fresnel Reflections

Fresnel Reflections are only point events.

Fresnel reflections occur only where the fiber comes incontact with air or any other media such as a mechanicalconnections/splice or joint.

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2. OTDR SPECIFICATIONS

A. Dynamic RangeB. Dead ZoneC. ResolutionD. Accuracy E. Wavelength

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A. Dynamic Range

Dynamic Range determines maximum observable length of a fiber and therefore OTDR suitability for analyzing anyperticular network.

The higher the signal of noise ratio and the batter the trace will be with a better event detection.

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B. Dead Zone

OTDR is designed to detect the back scattering level all along the fiber link, it measured back scattered signalswhich are much smaller than the signal sent to the fiber.

The device that receives these back scattered signals is anOTDR, which is designed to receive a given level range.

When there is a strong reflection, than the power receivedby the photodiode can be more than 4000times higher thanthe back scattered power and can saturate the photodiode.

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Dead Zone

The photodiode requires time to recover from the saturatedconditions,during this time it will not detect any signal accurately.

The length of the fiber which is not characterized during recovery is termed the Dead Zone.

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DEAD ZONE

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C. RESOLUTION

1. Sampling resolution

Sampling resolution is the minimum distance between two acquisition points.

This data resolution can go down to centimetersdepending on pulse width and range.

The more data points an OTDR can acquire and process, the more the resolution.

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C. Resolution

2. Distance Resolution

Distance resolution is very similar to sampling resolution,if OTDR samples acquisition points every 1 meter, thanonly it can locate a fiber within +/- 10meter.

The distance resolution is than like sampling resolution,a function of pulse, width and range.

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D. Accuracy

The accuracy of measurement is the capacity ofmeasurment to be compared with a referance value.

Linearity Accuracy : Determines how close an Opticallevel corresponds to an electrical level across the wholerange.

Distance Accuracy : Depends on the accuracy of groupindex(Index of refraction refers to a single ray in a fiber ,while group index refers to propogation of all the light pulse in the fiber.

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E. Wavelength

OTDR’s measure according to wavelength.

The measure wavelengths are :850nm, 1310nm and 1550nmthe fourth wavelength is now appearing for the monitoring live systems which is 1625nm.

The wavelength is usually specified with central wavelength and central width.

The attenuation of wavelength varies with wavelength, andthe measurement should be corrected to transmissionwavelength or to the central wavelength.

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3. USING AN OTDR

We can broadly define the use of OTDR in two process.

A. Acquisition step : Where the unit acquires data and displays it graphically or numerically.

B. Measurment step : Where the operator analyzes the datamakes a decision based on the results to either store, printor go to the next acquisition.

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A. Acquisition

There are three major approaches to configure and acceptacquisition parameter selected by OTDR(Automatic).

A user may allow the OTDR unit to auto configure,analyze the results and change one or more parameters (accordingly).

A more experienced user may choose not to use autoconfiguration feature altogether and enter the acquisitionparameters based on his experience(Manual).

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Acquisition parameters

Given below are various acquisition parameters and theireffect on the resulting trace.

Injection Level.

Wavelength.

Pulse Width.

Range.

Averaging.

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Injection Level

Injection level is defined as the power injected into the fiber under test, the higher this level the higher the powerlevel.

The presence of dirt on connector faces and damaged or low quality pigtails or patch cords are the primary cause of low injection levels

Mating a dirt connector with a OTDR connector may scratchthe OTDR connector, degrading the OTDR launch condition.

Some OTDR’s will display the measured injection levelduring real time acquisition or just prior to averaging.

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OTDR Wavelength

A fiber must be tested with same wavelength as that used for transmission.

For a given dynamic range 1550nm will see more distance than 1310nm.

Single mode fiber has more mode field diameter at 1550nm that at 1310nm.

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Pulse Width

The OTDR pulse width controls the amount of light that will be injected into the fiber(it is the time for which theLesser is on and determines the resolution of waveform).

Longer the pulse width, more light is injected into the fiber.

Longer pulse widths also produce longer dead zones in theOTDR trace waveform where the measurements areimpossible.

Short pulse widths inject lower power level of light butreduce dead zone.

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Pulse width

By reducing the pulse width, there is a reduction in thedead zone of the fiber, compared to that of a larger pulsewidth and also an increase.

But the reduction in the pulse width, there is areduction in the dynamic range, a reduction in thesensitivity of the receiver and also the distance.

By proper selection of pulse with we can optimize the useof OTDR for making fiber measurements.

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Range

Range of an OTDR is the distance over which it can acquire data samples.

The longer this parameter the more distance OTDR willshoot the pulses.

This parameter is generally set to twice the distance of theend of fiber.

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Averaging

The OTDR detector works with extremely low opticalpower levels(as low as 100 photons per meter of fiber).

Average is the process by which each acquisition point is sampled repeatedly and results averaged to improve signal to noise ratio.

Averaging can be done by selecting the time of acquisitionor the number of averages, the longer the time or higherthe number of averages, the more signal the trace waveformwill display in random noise conditions.

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Modes of operation of OTDR

Free Run Mode(Real Time) :

It continually sends lesser pulses down the fiber under testand obtains back scatter.

This mode is useful for optimizing fiber alignment.

The waveform obtained in free run mode containunacceptable amounts of noise making it impossible to determine small attenuation changes such as non-reflectivesplices.

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Modes of operation of OTDR

Averaging mode :In the averaging mode each pulse are averaged from that of preceding pulses which makes the trace appear clearfor each of the succeeding pulses.

The number of samples that are to be averaged is predefined for an OTDR.

The larger the number,the longer the OTDR takes for displaying the results.

Recent OTDR specifies their averaging in terms of time taken for display , instead of number of samples.

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Tests performed using OTDR

Acceptance test:

Acceptance of fiber using OTDR(To measure loss per km):

This loss measurement is wavelength dependent, so theOTDR is set to the wavelength which matches with the fiber system operating wavelength.

When using an OTDR to make any measurement it is critical to correctly place reference markers so that theOTDR can display the loss & distance between them.

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Loss and span length

This test has to be conducted in averaging mode, when ever we choose averaging mode a trace will be displayed.

To make any measurements it is critical to correctly place reference markers so that OTDR can display loss and distance between them.

For making the measurement, a trace is obtained on OTDRin real time mode.

Place the reference markers accurately, first referencemarker is placed exactly where the back scatter starts, that isbeyond dead zone(correct point is on the trailing edge of fiber

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Span Loss and span length

Than move the cursor to the end of the trace and place the second marker before the refractive fiber end, the correct point is where the slope starts increasing faster than the normal slope of the trace.

To exactly locate these reference markers use the horizontal and vertical zoom controls.

Now choose the averaging mode and the display gives usthe loss per span and the span length.

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Attenuation of splice or connector

OTDR can be used to measure splice or connector loss,inorder to do this a marker is placed on either of the abettation of the OTDR trace.

OTDR will than display the attenuation between the two points.

The vertical separation of the two marker points is theattenuation of the splice or the connector

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Attenuation of Fusion Splice

Fusion splice has a loss value which is very negligible, so to accurately measure this value the OTDR is used in averaging mode.

To measure the loss value, first amply the slope of the OTDR trace and than place the two reference points oneither side of the aberration.

To accurately place the markers use horizontal and verticalzoom controls.

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Automatic operation

In two cursor method, sometime the cursor might nothave been placed properly and the OTDR also adds some looses and thereby increasing the loss value.

For short distance applications the effect is negligible, butbecomes highly pronounced for long haul.

Fortunately, most OTDR’s have the provision to perform automaticaly.

That is in averaging mode the OTDR displays the spliceloss as well as the connector loss systematically on the trace.

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Summary

OTDR can thus be used to determine the loss per span of the fiber , splice loss connector loss and also the end of thefiber.

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What is splicing?

Splicing is method of joining two properly aligned fibers so that the two fibers are held together and the transmission oflight continues.

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DIFFERENT TECHNIQUS FOR JOINING OF FIBER.

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Splicing/Jointing…….

1. Why splicing is necessary?

2. Types of splicing.

3. Pros and Cons.

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Why Splicing is necessary?

Long cable runs.

Crowded conduits.

Fire-coderestrictions.

Building or Campusenvironments.

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Types of Splicing.

Temporary splicing :

V-Grove SplicingConnectorization

Permanent Splicing :

Mechanical Splicing.Fusion Splicing.

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Pros and Cons of Splicing

Flexibility for future system reconfiguring.

Easy in testing.

Trade-offs are increased signal loss.

Large space requirements.

Expensive- increase system cost.

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Types of Splicing

Fusion splicing :

A fusion splice is a junction of two (or more) optical fibers that have been melted together. This is accomplishedwith a machine that performs two basic functions : aligningthe fibers and melting them together typically using anelectric arc.

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Pros and Cons of Fusion Splicing

Low Loss (<0.05db for SM fiber).Very fast and fully automated process.

Expensive.Less safer than mechanical splicing.

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Five steps ahead for fusion splicing

1. Fiber and preparation.

2. Cleave the fiber.

3. Alignment of the two(or more) fibers.

4. Fuse the fiber.

5. Protect the fiber.

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Fiber and preparation

It mainly concerns with removing bare fiberfrom OFC and cleaning the fiber.

Required accessories are…… 1. Sheath Cutter 2. Jacket stripper 3. Primary coat stripper 4. Alcohol (>99% pure) 5. Lint-free tissue paper 6. Cotton swat.

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Cleave the fiber

Good cleaving is key for good splicing.

Actually, cleaving is same as cutting a window pane to size, only on a much finer scale. The cleaver first nicksthe fiber, and than pulls or flexes it to cause a cleanebreak.

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Alignment of two fibers

1. Manually.

2. Automated.

3. Misalignment causes bad splicing.

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Fuse the fiber

For better Fusion Splicing

Set the……….

Observe & try to maintain…….

1. Current supply to electrodes2. Splicing time.

1. Weather condition2. Temperature and Humidity.

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Some observations about fusion splicing….

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Splice loss

SPLICE LOSS DUE TO CORE MISMATCH :

Off-center core in second fiber does not receive all the light from the first fiber. The amount of light lost is the splice loss.

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Protect the fiber

Spliced fiber has to protect by using protection sleeve.

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HOW TO USE OPTICAL FIBER:

1. Always make sure you have clean hands and clean equipment.

2. Always clean all fiber ENDFACES couplers and interfaces before use.

3. Always use clean tissues and cleaning sticks.

4. Always keep the can, contain the dry air in the upright position.

5. Always check if you are using the same fibers before coupling them (fiber type SM or MM & core diameter).

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HOW TO USE OPTICAL FIBER:

6. Always check if the fiber end faces are angled or not.

7. Always keep fiber away from extreme heat(>85c).

8. Always store not used optical cables, couplers and interfaces with protection caps and in a dry, clean place.

9. Always use a microscope when you are using cleaning fluid.

10. Always check the bending radius (>30mm) and prevent torsion (<90degree/meter). (Never apply mechanical stress to optical fibers)

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HOW TO USE OPTICAL FIBER:

11. In case of doubt while using optical fibers and/ or components, consult an expert.

12. Always check if the optical input power applied to the optical components or optical (measuring ) equipment is within the dynamic range.

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How fiber works

The operation of an optical fiber is based on the principle oftotal internal reflection. Light reflects(bounce back) or refracts(alters its direction while penetrating a differentmedium),depends on the angle at which it strikes a surface.

One way of thinking about this concept is to envision aperson looking a t a lake down at a steep angle, the personwill see fish, rocks or whatever is below the surface of thewater(in a somewhat distorted location due to reflection),assuming that the water is relatively clear and calm. Howeverby casting a glance farther out, this making the angle of lightless steep, the individual is likely to see a reflection of treesor other objects on an opposite shore. Contin…..

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How fiber works

Because air and water have different indices of reflection, theangle at which a person looks into or across the water influences the image seen.

This principle is at the heart of how optical fiber works. Light waves are guided through the core of the optical fiber inmuch the same way that radio frequency(RF) signals are guided through co-axial cable. The light waves are guided to the other end of the fiber by being reflected within the core.Controlling the angle at which the light waves are transmittedmakes it possible to control how efficiently they reach there destination. The composition of the cladding glass relative tothe core glass determines the fibers ability to reflect light.

Contin..

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How fiber works

The difference in the index of refraction of the core and the cladding cause most of the transmitted light to bounce off the cladding glass and stay within the core. In this way, the fiber core acts as wave guide for the transmitted light.

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The design of fiber

An optical fiber consist of two different types of highly pure,solid glass,composed to form the core and cladding. A protection acrylate coating than surrounds the cladding. Inmost cases the protective coating is a dual layer composition.

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n1

n2

Reflected light ray

Incidentlight ray

Coating

Cladding

Core Reflected light ray

Index of refraction = nn1>n2 permits total internal reflection

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The design of fiber

A protective coating is applied to the glass fiber as the final step in the manufacturing process. This coating protects theglass from dust and scratches that can affect fiber strength.This protective coating can be comprised of two layers:a soft inner layer that cushions the fiber and allows the coating to be stripped from the glass mechanically and aharder outer layer that protects the fiber during handling,particularly the cabling, installations and terminations processes.

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TYPES OF FIBER

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SINGLE-MODE AND MULTIMODE

Lightsignal

Cladding glass

Core glass

Single mode fiber

Multi-mode fiber

Cladding glassLightsignals

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SINGLE-MODE AND MULTIMODE

Multi-mode fiber was the first type of fiber to be commercialized. It has a much larger core than single-modefiber, allowing hundreds of modes of light to propagatethrough the fiber simultaneously. Additionally, the largercore diameter of multi-mode fiber facilitates the use oflower-cost optical transmitters(such as light emitting diodes (LEDs) or vertical cavity surface emitting lasers(VCSELs) and connectors.Multi-mode fibers are used primarily in systems with short transmission distance(under2km.), such as premises communications, private data networks.

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SINGLE-MODE AND MULTIMODE

Single-mode fiber on the other hand ,has a much smaller core and allows only one mode of light at a time to propagatethrough the core. While it might appear that multi-mode fibers have higher capacity, in fact the opposite is true. Single-mode fibers are designed to maintain spatial and spectral integrity of each optical signal over longer distances,allowing more information to be transmitted.

Its tremendous information carrying capacity and low losshave made single mode fiber the ideal transmission mediumfor a multitude of applications. Single-mode fiber is typically used for longer-distance and higher bandwidth applications.

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Optical fiber sizes

The international standard for outer cladding diameterof most single-mode optical fibers is 125 microns for the glass and 245 microns for the coating. This standard is important because it ensures compatibility among connectors, splices, and tools used throughout theindustry.

Standard single mode fibers are manufactured with a smallcore size, approximately 8 to 10 microns in diameter.Multi-mode fibers have core size of 50 to 62.5microns in diameter.

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Optical fiber sizes

125 microns 125 microns 125 microns

8-10microns 50 microns 62.5 microns

Single-mode Multi-mode

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FUSION SPLICING

Fusion splicing provides a fast , reliable , low -loss , fiberto fiber connection by creating a homogenous joint betweenthe two fiber ends, typically using an electric arc. Fusion splices provide a high quality joint with the lowest loss. (inthe range of 0.01db to 0.10db for single mode fibers) and are practically non reflective.

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MECHANICAL SPLICING

Mechanical splicing is an alternative method of making apermanent connection between fibers. In the past,the disadvantages of mechanical splicing have been slightlyhigher losses, less reliable performance and a cost associatedwith each splice. However, advances in technology have significantly improved performance. System operators typically use mechanical splicing for emergencyrestoration because it is fast, inexpensive, and easy. (Mechanical splice losses typically range from 0.05-0.2dbfor single mode fiber.)

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OPTICAL FIBER CABLE

Basically we can used this cable for transmission media in our telecommunication field as this cable is capable of carrying a large amount of data or voice traffic.

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OPTICAL FIBER CABLE

CAPACITY WISE THE CAPACITY OF A FIBER IS13000 TIMES MORE THAN THAT OF A SAME DIAMETER COPPER.

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OPTICAL FIBER CABLE

USED FOR LONG DISTNCE PROJECT

FOR MAINTAINANCE NEEDS DOCUMENTATIONSHOULD BE 100%.

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OPTICAL FIBER CABLE

HOW TO DO THE DOCUMENTATION OF OFC.

1. Fiber laying reports.

2. Fiber termination reports.

3. Splicing reports.

4. As-build drawings.

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Attenuation

As light is guided through the core four properties cancause attenuation.

1. ABSORPTION :

Occurs when light strikes impurities in the core glassand absorbed.

Impurity(Absorption)

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Attenuation

2. SCATTERING :

Occurs when light strikes an area where the material density changes.

Density change(scattering)

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Attenuation

3. MACROBENDING :

Macro bending is large scale bending of the fiber which exceeds the fiber bend radius and causes light to leavethe core and travel in the cladding (usually an installationproblem).

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Attenuation

4. MICROBENDING :

Micro-bending is microscopic distortion of the fiber which causes light to leave the core and travel in the cladding (created during manufacturing).

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Dispersion

DISPERSION IS THE SPREAD OF A PULSE OF LIGHT AS IT IS GUIDED THROUGH THE FIBER.

There are 3 types of dispersion :

1. MODAL

2. MATERIAL

3. WAVEGUIDE

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Dispersion

1. MODAL :

Modal dispersion occurs when light follows different pathsthrough the fiber and arrives at the far end at different times. It occurs only in Multi-mode fibers.

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Dispersion

2. MATERIAL :

Material dispersion occurs because different wavelengths(colors) of light travel at different velocities through the fiber.

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Dispersion

3. WAVEGUIDE :

Wave-guide dispersion occurs because light travels in both core and cladding at slightly different speeds.It is most significant in single-mode fibers.