Definition of Fibre Optic
Transcript of Definition of Fibre Optic
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Definition of Fibre Optic
Fibre Optic is a thin strand of highly transparent glass or sometimes plastic that guide light.
It is used as a medium for carrying information from one point to another in the form of
light. A basic fibre optic system consists of a transmitting device, which generates the light
signal; an optical fibre cable, which carries the light; and a receiver, which accepts the light
signal transmitted. The fibre itself is passive and does not contain any active properties
Basic Construction of Optical Fibre.
1.Core:
The centre of the fibre through which the light is transmitted.
2. Cladding:
The outside optical layer of the fibre that traps the light in the core and guides it
along and even through curves.
3. Buffer coating or primary coating:
A hard plastic coating on the outside of the fibre that protects the glass from
moisture or physical damage.
Fiber optic cable functions as a "light guide," guiding the light introduced at one end of the
cable through to the other end. The core and cladding are manufactured together as a single
piece of silica glass. The core regions refractive index (or optical density) is greater than the
Core
Cladding
Coating
Core8.3m
Cladding125m
Coating250m
Cross Sectional View of a Single Mode Fiber Side View of a Single Mode Fiber
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cladding layer. The glass does not have a hole in the core, but is completely solid throughout.
The light is "guided" down through the core. The cladding traps the light in the core using an
optical technique called "total internal reflection. The third section of an optical fibre is the
outer protective coating called the "primary buffer coating". This coating is typically an
ultraviolet (UV) light-curedacryl ateapplied during the manufacturing process to provide
physical and environmental protection for the fibre. During the installation process, thiscoating is stripped away from the cladding to allow properterminationto an optical
transmission system.
Constructional Details of Fibre Optic Cable
Rip Cord
Water Blocking Material
(Jelly)
Outer Sheath (Jacket)Double Layer for Direct
Buried Single Layer
for Duct Cable
Steel for !rounding
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Dielectric Strength$le%ent (&e'lar)
Central Strength Me%ber
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Optical fibre
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Types of Fiber Optic Cables
There are two types of fiber optic cable commonly used:
1. Multi Mode Cables
2. Single Mode Cables
1. Multi Mode Cables
Over the years a variety of core sizes have been produced but these days there are only two
main sizes for Multimode fibers. These cables are most widely used in data networks. The
numbers 50/125 & 62.5/125 represent the diameters of the fibre core and cladding, these are
measured in microns which are millionths of a metre.
2. Single Mode Cable
Single Mode cable has a core diameter of 8.3 to 10 microns. It is the most commonly
used cable in Telecommunication for transmission systems. The numbers 8.3/125
represent the diameters of the fiber core and cladding,
Note:Both multimode and single mode fibres have an outside diameter of
125 microns - about 5 thousandths of an inch - just slightly larger than
a human hair.
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Definitions of Terms used in Fibre Optic Cable
1. Terminations
Patch panels:-
Provides a centralized location for patching fibres, testing, monitoring and restoring
cables.
Connector:
A non-permanent device for connecting two fibres or fibres to equipment where they
are expected to be disconnected occasionally for testing or rerouting. It also provides
protection to both fibres.
Ferrule:
A tube which holds a fibre for alignment, usually part of a connector
LC Connector
LC stands for Latched Connector and its interconnect is based upon the RJ-45
telephone interface. The LC Connector uses Zirconia ceramic ferrules in a free-
floating and pull proof design.
SC connector
SC Stands for Single Coupling. It is Square shaped snap-in connector that latches
with a simple push-pull.The SC connector has the advantage (over ST) of being
duplexed into a single connector clip with both transmit/receive fibres.
MU Connector
The MU stands for Miniature Unit fibre-optic connector, which features compact size,high packaging density, and high performance and a simple push-pull design. The
MU connector ferrules are half the size of the standard FC, SC connectors and are
excellent for high density installations.
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For long cable runs outside, the point where cables are spliced, sealed up and buried
in the ground, put in a vault of some kind or hung off a pole.
Splice panels
Connect individual fibres from cables to pigtails
Splice:
A permanent joint between two fibres
Mechanical Splice:
A splice where the fibres are aligned by mechanical means
Fusion Splice:
A splice created by fusing two fibres together
Fusion Splicer:
An instrument that splices fibres by fusing them, typically by electrical arc.
3. Measurements
Attenuation:
The reduction in optical power as it passes along a fibre, usually expressed in decibels
(dB).
Bandwidth:
The range of signal frequencies or bit rate within which a fibre optic link or network
will operate.
Chromatic dispersion:
A property of optical fibre due to which different wavelengths travel at different
speeds and arrive at different times, resulting in spreading of a pulse in an optical
waveguide.
Decibels (dB):
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A unit of measurement for optical power which indicates relative power. A -10 dB
means a reduction in power by 10 times.
dBm:
Absolute Power, Optical power referenced to 1 milliwatt.
Nanometer (nm):
A unit of measure used to measure the wavelength of light (meaning one one-
billilonth of a meter)
Optical Loss:
The amount of optical power lost during transmission of through fiber, splices,
couplers, etc. expressed in dB.
Optical Power:
It is measured in "dBm", or decibels referenced to one miliwatt of power. While loss is
a relative reading, optical power is an absolute measurement, referenced to
standards. Absolute power is measured to test transmitters or receivers and relative
power to test loss.
Optical Return Loss, back reflection:
Light reflected from the cleaved or polished end of a fibre caused by the difference ofrefractive indices of air and glass.
Power budget:
The total amount of power lost in the link. Often used in terms of the maximum
amount of loss that can be tolerated by a given link.
Polarization Mode Dispersion:
The spreading of a pulse in an optical waveguide by virtue of different light paths
lengths is called Modal dispersion.
Refractive index:
A measure of the speed of light in a material, a property of optical materials that
relates to the velocity of light in the material
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Scattering:
The change of direction of light after striking small particles that causes loss in
optical fibres and is used to make measurements by an OTDR
Wavelength:
A term for the color of light, usually expressed in nanometers (nm) or microns (m).
Fiber is mostly used in the infrared region where the light is invisible to the human
eye.
4. Test Equipment
Optical Power Meter:
An instrument that measures optical power from the end of a fibre
Laser Source:
An instrument that uses a laser or LED to send an optical signal into fibre for testing
loss of the fibre
Optical Loss Test Set (OLTS):
A measurement instrument for optical loss that includes both a power meter and
laser source
Reference Test Cables:
Short, single fibre cables with connectors on both ends, used to test unknown cables.
Mating Adapter:
Also called couplers, allow two cables with connectors to mate.
Optical Microscope:
Used to inspect the end surface of a connector for dirt.
Fibre Optic Splicing
There are two methods of fibre optic splicing, fusion splicing & mechanical splicing.
Mechanical splicing is usually carried out for emergency restorations whereas fusion splicing
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is done for permanent repairs of damaged cable or to connect the reels of cable during
installation
Mechanical Splicing:
Mechanical splices are simply alignment devices, designed tohold the two fibre ends in a precisely aligned position thus enabling light
to pass from one fibre into the other. (Typical loss: 0.3 dB)
Fusion Splicing:
Fusion splicing is the joining and fusing of two fibres by placing them between two electrodes,
and discharging an electric arc over the fibres. This splice technique is non-reflective.Fusion
splicing machine is used to precisely align the two fibre ends then the glass ends are "fused"
together using electric arc. This produces a continuous connection between the fibres
enabling lower loss and less back reflection than mechanical splicing because the resulting
fusion splice points are almost seamless. (Typical loss: 0.1 dB)
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+o*+ik Stripper
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2.Cleaning
After the coating is removed, clean the fibre withspecially designedisopropyl alcohol
wipesso that the fibre squeaks.
3.Cleaving
A good cleave is the key to obtaining a good splice. Use cleaver to cut the fibre. After
cleaving do not touch or clean the fibre.
4.Splicing
The fibre is now ready to be spliced mechanically or Fusion. Insert the fibre carefully
in the mechanical splice or in the fusion splicer for splicing. While inserting in the
mechanical splice make sure that fibre is inserted directly in the groove and do not
touch any other surface. Fusion splicer will automatically align and fuse the fibre.
5.Protection.
In case of fusion splicing cover the splice with heat shrink sleeve and place it in the
heater, for mechanical splice carefully close the mechanical splice.
6.Organizing.
Organize the fibre in the enclosure properly Make sure that organising do not cause
Micro-bending.
Testing
Cables need to be tested forcontinuity, end-to-endlossand any other potential problems.
For long outside plant cables with intermediate splices, all individual splices need to be
verified with an OTDR, since that's the only way to make sure that each one is good. Within
the network testing forpoweris necessary as power is the measurement that tells whether
the system is operating properly.
Tools and test equipment for the job.
1.Source and power meter, optical loss test.
2.Reference test cables.
3.Cleaning materials - lint free cleaning wipes and pure alcohol.
4.OTDR and launch cable for outside plant jobs.
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Measurement of Optical Power & Loss
There is a difference between the power coupled into a component like a cable or a connector
and the power that is transmitted through it. This difference is what we call optical loss and
defines the performance of a cable, connector, splice, etc.
Measuring power
Power in a fibre optic system is like voltage in an electrical circuit. To measure power,
attach the meter to the cable that has the output you want to measure. Turn on the
transmitter/source and note the power the meter measures.
Testing Loss
Following two methods are used to measure loss. Optical Loss Test Sets houses a light source
and power meter in the same unit. For both methods two units of loss test sets (one at each
end of the fibre under test) are required.
Single Ended Loss
This test is initiated from one end and results are displayed at both units.
Double Ended Loss
In this test Laser source is initiated from one end and the result is displayed at the
other end.
Both test method measure the loss of two ODF connectors (one on each end), the loss of cable
and splices in between. Most commonly FASTTEST setup method is used for loss testing.
Fiber Optic Linkunder Test
Optical
Loss
Test Set
Optical
Loss
Test Set
Transmitter Receiver
Test
Patch
Cord
Test
Patch
Cord
FASTTEST SETUP
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Prior to perform Loss test measurement, a reference measurement must be stored in both
units. The reference measurement includes the loss caused by the test setup components
including test jumpers. The unit will store a reference reading of power level at the end of
test jumper. This reference measurement is subtracted from the overall loss so the final lossresult represents the loss of system under test alone.
Referencing Optical Loss Test Unit
There are two referencing methods in practice for Loss test sets and both results in accurate
loss measurement.
1.Loop-back Method with only one test jumper
2.Side-by-Side Method with two test patch cords and a mating adapter.
1. Loop back Method
The main advantage of the loop back referencing method is that there is no need to bring
both units at same location. This is performed by connecting a single test patch cord from theunits Source Port (FASTTEST Port) to Detector Port.
After performing the loop-back reference, simply disconnect the test patch cord from
the Detector Port and connect it to the ODF of Fiber link Under Test.
Optical
Loss
Test Set
Source of FASTEST port
not to be disrupted once
the reference is set.
etector Port! isconnect
this end and connect to OF
of FO lin" under Test
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It is very important not to disconnect it from the source port (FASTTEST Port)
because the amount of light coupled or injected into the test patch cord varies from
one connection to another.
If the test patch cord is disconnected from the source port, it is required to repeat the
references.
The loop-back test is performed individually on each of the two units.
An important advantage of the loop-back method is that it automatically takes into
account the loss of the test patch cord and Mating adapters, allowing a true
measurement of the fiber itself.
2. Side-by-Side Method
To perform the side-by-side reference procedure,two test patch cord are connected via a
Mating adapters and then connect the test patch cord ends to the Source Port (FASTTEST
Port) on one unit and the Power Meter (Detector Port) on the other.
When using the side-by-side reference method, both units must be brought to a
common site to take the appropriate references.
Optical
Loss
Test Set
FastTest Port
#ot to be disrupted once
the reference is set
Optical
Loss
Test Set
etector Port $
Po%er &eter Port
&atin' Adapter
isconnect here to connect to
OF of FO lin" Under Test
ReceiverTransmitter
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Once the side-by-side reference is performed, disconnect the test jumpers at the
Mating Adopter and connect both test jumpers to the ODF of Fiber Link Under Test.
Much like the loopback reference, it is very important not to disconnect the test
jumper from the source port as the amount of light coupled or injected into the test
patch cord will vary from one connection to another. If the test patch cord isdisconnected from the source port (FASTTEST Port), it is required to repeat the
reference.
Note: Before measuring optical loss with an automated OLTS, referencing is a
crucial procedure that should be performed before every test session.
Single Ended Loss Measurement
To measure the Single-ended loss only launch cable is used and the loss is measured by
mating the FO cable under test to the reference launch cable and measuring the power out atthe far end with the meter. Following losses are measured.
1.The loss of the connector mated to the launch cable.
2. The loss of fibre, splices or other connectors in the cable you are testing.
Double Ended Loss Measurement
To measure the double-ended loss in addition to the reference launch cable a receive cable is
also attached to the meter. In a double-ended loss test, we attach the FO Cable Under Test
between two reference cables, one attached to the source (FASTTEST Port) and other to the
power meter (Detector Port). This way, we measure two connectors' losses, one on each end,
plus the loss of all the cable or cables in between.
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
For single mode fiber cable with two most commonly used wavelengths 1310 nm and 1550
nm
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The attenuation measurement will vary depending upon which wavelength is in use.
Attenuation is measured in dB and is quoted as attenuation in dB/km.
Fiber Loss Variables
Attenuation:
All fiber has losses from absorption and back reflection of the light caused by
impurities in the glass. Attenuation is a function of wavelength and needs to be
specified or measured at the wavelength in use.
Modal Dispersion:
The higher the data rate, the shorter the distance the signal can travel before modal
dispersion creates an inability to accurately detect the signal (i.e. a "1" from a "0").
Dispersive Losses:
Another dispersion effect, which causes pulse spreading, and limits distance is
chromatic dispersion, where the broader spectrum of light can result in varying
travel times for different parts of a light pulse.
Splices:
Although small and often insignificant, there is no perfect loss-less splice. Many
errors in loss calculations are made due to a failure to include splices. Average splice
loss is usually less than 0.1 dB.
Connectors:
Like splices, there is no perfect loss-less connector. It is important to note that even
the highest quality connectors can get dirty. Dirt and dust can completely obscure a
fiber light wave and create huge losses. A 0.5 dB loss per connector is commonly the
worst case scenario assuming a cleaned and polished connector is used. There will
always be a minimum of two connectors per fiber segment, so remember to multiply
connector loss by two.
Safety Buffer:
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It is common to add a loss as a design margin. Allowing 2 - 3 dB of loss can take fiber
aging, poor splices, temperature and humidity, etc., into account and ensure a solid
system.
NOTE:
To determine minimum/maximum losses and maximum distances you need to
identify all of the above variables. Failure to identify even one of these variables can
create potential problems
Fiber Terminology
Calculating Power Budget
There are commonly two different calculations you require with fiber. Each assumes you have
known values for different sets of variables.
One calculation determines the maximum signal loss across a piece of pre-existing
fiber (Link Loss)
The other calculation determines the power budget of a fiber link.
The first calculation below will calculate link loss through a known length of fiber.
Second calculation below Calculate the power budget of the link (i.e. maximum signal loss is
simply the sum of all worst-case variables within your fiber segment.)
Table( Attenuation Criteria
Optical
Fiber Type
Loss/Km Loss
in dB Connector Splice
1310nm 1550nmIn dB
Single Mode 0.35 0.23 0.50 0.09
1. Link Loss Calculation
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The measured value of attenuation of a FO link should not exceed the sum of
allowable attenuation of each component.
These components are: -
The Fiber Optic cable The FO connectors
The Splices
Link Loss (dB)=Cable Loss+Connector Loss+Splice Loss
Cable Loss (dB) = Cable length (Km) x Attenuation Coefficient
(db/Km)*
Connector Loss (dB) = Number of Connector Pairs x Connector Loss
(dB)*
Splice Loss (dB) = Number of Splices x Splice Loss (dB)*
(*) : from above table for Attenuation Criteria
). Optical Po%er *ud'et Calculation
Optical Power Budget=(Fiber Attenuation x km)+(Splice Attenuation x No
of splices)+(Connector Attenuation x No of connectors)+(Safety Margin)
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OTDR Trace Analysis
Non-Refective
Event(Fusion Splice,Bend)
Non-RefectiveEvent
(Micro Bending)
Noise
Out ut End-F!ceRefection
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Reflective
Event Loss
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Launc
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Non-
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Non-
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Event Loss
caused by
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Bad Splice
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There are four main settings that the technician must set on the OTDR beforeOTDR testing.
Those are Wavelength, Index Of Refraction, Pulse Width and Distance.
1. Wavelength-
The behaviour of an Optical system is directly related to the wavelength of transmission. Not
only Optical fibre will exhibit different loss characteristics at different Wavelengths, but
splice loss value also differ at different wavelengths. In general fibre should be tested with
both wavelengths i.e. 1310 and 1550nm for single mode fibres. If testing is only to be
performed at one wavelength it should be done with 1550nm considering the following points
1550nm will see longer distances down the fibre due to the lower attenuation as
compared t0 1310nm.
1550nm is more sensitive to losses incurred by bending during installation and
organising of fibres in the splice enclosures after splicing.
2. Index Of Refraction-
The index of refraction sets the OTDR to the proper speed of light for a particular fibre link
being tested.
Changing the IOR value will change the distances to events on the OTDR trace, andalso the overall length of the fibre.
The IOR of a particular fibre is usually provided by the manufacturer.
3. Pulse Width or Duration-
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This is another setting that must be selected to receive the clearest information from the
OTDR trace. The length of time that the OTDR's laser is turned on is called the "pulse
width". As the OTDR turns the laser on and off, the duration of the laser being on results in
a pulse of a certain length.
Shorter pulse widths provide better traces of events that are close together, as theshorter pulse widths will have shorter dead zones after reflective events. However,
short pulse widths will result in a noisy, hard to interpret trace for long distance fibre
link, as the OTDR process weaker returned signals.
Long pulse widths means more light energy is injected in the fiber. The more light
injected means the more light is reflected back from the fiber to OTDR. It causes
longer dead zones, and reduces resolution of events that are close to each other.
Long Pulse width is therefore used to see long-distance down a cable.
The General Rule to set Pulse width is:
Short Fibre Link = Short Pulse Width
Long Fibre Link = Long Pulse Width
Shorter pulse widths can be used on longer fibre links to give greater detail to events
close to the OTDR and for fault analysis.
4. Range or Distance-
The range on an OTDR is the maximum distance that OTDR will acquire data samples. This
parameter is generally set at twice the distance of of the end of the fibre.
Note:
Neglecting to set any of these parameters properly can result in erroneous reporting by
the OTDR.
Dead Zone
The OTDR is designed to detect the back scattering level all along the fibre link. It measures
the back scattered signals which are much smaller than the signal sent to the fibre. When
there is a strong reflection then the power received at the OTDR is much higher than the
backscattered power which saturates the OTDR. OTDR requires time to recover from the
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saturated condition. During this time OTDR cannot detect the backscattered signal
accurately. The length of fibre which is not fully characterized during the recovery period is
termed as dead zone.
This affect is similar to the one when we are driving a car at night and that another cars
headlight dazzle our vision momentarily.
The dead zone depends on the pulse width, the reflectance, the loss and the location.
Cleaning of Connectors
Proper cleaning of connectors is very important. The core diameter of a single-mode fiber is
only about 9um. This generally means you cannot see streaks or scratches on the surface.
Follow the under mentioned procedure to clean the connector.
.
1.Clean the connector by rubbing it on cleaning tape or a new, dry cotton swab using a
small circular movement.
2.Blow away any remaining lint with compressed air.
If the connector has greasy dirt on its tip follow the following procedure.
1.Take a new Moisten cotton swab with isopropyl alcohol.
2.Clean the connector by rubbing the cotton swab over the surface using a
small circular movement.
3.Take cleaning tape and rub it in small circular motion to remove the alcohol
dissolved sediment and dust.
4.4. Blow away any remaining lint with compressed air.
Do Not Forget to clean the connector with cleaning tape
after cleaning it with isopropyl alcohol swab.
L
Specifications and procedures for Cable Installation
1. Cable Depth:
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The depth at which buried cable can be placed will vary with local conditions i.e
Type of soil. However fiber optic cable must be buried at a minimum depth of 80
cm.
Table showing depth of Buried Fiber Optic Cables*
Location Depth
Soft Soil 80 ~ 120 Cm
Hard Soil / Rock Soil Minimum 80 cm
Road Way crossing Minimum 110 cm
Under mentioned diagram shows the typical layout of Direct Buried cable.
12 c%
12 c%Soft Sand
Soft Sand
32 452 c%
52 4612 C%
Min 6Meter
Back
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ront 7ie.
iber OpticCable
Soft Sand
Soft Sand
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In certain installation areas, for example, rights-of-way with limited access (public highways,
private property boundaries, water ways, Culverts and under the bridges, cable must be
buried in a duct and if such installations are done after the installation of cable, Fiber Optic
cable must be protected in the affected area with PVC pipe, iron barring and concrete.
Cable must be protected at all constructions site such as unimproved roads, streets and alleys
that may later be paved or hard surfaced9*Deviations from desired depthshould be noted on drawings
CAUTION:
Depths less than those specified may expose the cable to erosion or excavation
damage.
In conditions where these depths are not feasible or permitted Iesser depth is
permissible provided additional protection inthe form of concrete casements
or sub duct is provided.
Splice Points
Splice point locations must be chosen carefully for easy access for future maintenance.
Splicing must always be done in the car and in order to reach splicing vehicle, ensure a
minimum of 10 ~ 15meters of extra cable on both cable ends at each splice point
At Hand Holes and Man Holes place the cable slack vertically (in line with the cable
route)
In the case of a buried splice point, coil and bury the slack Horizentally .Figure 5
Hand hole
20 Cm
Warning Tape
10~15M slack
Splice Pit2 x 2 Meter
80~120 cm
Splicing Van
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Points To Remember
Safety First!
Small scraps of glass i.e. cleaved-off ends of the fibres being terminated or
spliced is very dangerous! They are extremely sharp and are basically glass
needles that will easily penetrate flesh then break off and become nearly
impossible to remove. Once in the body it will likely become infected. If they
get into the eyes, they are very hard to flush out. Don't even think about what
happens if you eat one. Always follow these rules when working with fibre.
Find and dispose-off all cut fibre fragments immediately after cutting.
Buried Splice #ointMan ,ole,and ,ole
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20 cm
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Dispose-off all scraps properly.
Handle cut fibre fragments with tweezers only
Do not drop them on the floor where they will stick in carpets or shoes and be carried
elsewhere.
It is your responsibility to ensure that no fibre fragments escape and injure
someone. If you lose a fibre fragment you must look until you find it.
5.Fiber fragments can stick to the cover of the cleaver. Move slowly when opening the
cover. Always look on the inside of the cover if you dont see your fragment on the
shelf of the cleaver.
If you cant find your fragment, get more light on the subject and work area.
Do not move the cleaver until the fragment has been found.
Use a magnifying glass if you need to butFIND THAT FRAGMENT.
6.Do not eat or drink anywhere near the work area.
The light in Transmission system is infrared and you can't see it therefore always be
careful with your eyes.
When using a fibre optic microscope.NEVERlook into a fibre unless you personaly
confirm nolight is present. Use a power meter to check it.
Zero Tolerance for Dirt
With fibre optics, our tolerance to dirt is near zero. Airborne particles are about the size of
the core of SM fibre- they absorb lots of light and may scratch connectors if not removed! Dirt
on connectors is the biggest cause of scratches on polished connectors and high loss
measurements!
Try to work in a clean area.
Always keep dust caps on connectors & patch panels when not in use. Keep them
covered to keep them clean.
Use lint free pads and isopropyl alcohol to clean the connectors.
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After cleaning with isopropyl alcohol swab do not forget to clean it with the Cleaning
Tape.
Tools and Materials
Make sure to have the proper tools for the job.
Confirm that all tools are in good shape before you head out for the job. This includes
all the cable tools and test equipment.
Make sure that your test cables are good? Without that, good terminations are tested
as bad every time.
Make sure that your test equipment is fully charged and you have sparebattery
backup.
Documentation and Record Maintenance
It is very hard to troubleshoot cables when you don't know how long they are, where is the
route or how they were tested originally! So keep good records. It is recommended that the
following records be maintained and kept current on a daily basis:
schematic drawings to include "as-built" information for street maps records
splice loss data
end-to-end optical loss measurements
end-to-end OTDR signature traces
end-to-end power meter tests