Testing effectiveness of the splice through otdr and power meter tests

Optical Fiber Splicer Testing effectiveness of the splice through otdr and Power meter tests
BRBRAITT, Jabalpur Page 1 of 29
For Restricted Circulation
11 TESTING EFFECTIVENESS OF THE SPLICE THROUGH
OTDR AND POWER METER TESTS
STRUCTURE
11.1 INTRODUCTION
11.2 OBJECTIVE
11.3 OPTICAL TIME DOMAIN REFLECTOMETER
11.4 AN OTDR DISPLAY OF A TYPICAL SYSTEM
11.5 OTDR MEASUREMENTS
11.6 TESTING A FIBER OPTIC CABLE PLANT USING OTDR
11.7 OPTICAL POWER METER
11.8 DYNAMIC RANGE OF POWER METER
11.9 THE PROCEDURE TO MEASURE THE OPTICAL LOSS IN THE FIBER
OPTIC CABLE
11.10 OTHER OPTICAL INSTUMENTS
11.11 REPORT & RECORD
11.12 SUMMARY
11.13 REFERENCES AND SUGGESTED FURTHER READINGS
11.14 KEY LEARNINGS
11.15 WORK SHEET

11.1 INTRODUCTION
An optical time domain reflectometer (OTDR) is a fiber optic tester for the
characterization of fiber and optical networks. The purpose of an OTDR is to detect, locate,
and measure events at any location on the fiber link.
One of the main benefits of an OTDR is that it operates as a one-dimensional radar
system, allowing for complete fiber characterization from only one end of the fiber. An
OTDR generates geographic information regarding localized loss and reflective events,
providing technicians with a pictorial and permanent record of the fiber’s characteristics,
which may be used as the fiber’s performance baseline.
11.2 OBJECTIVE
After reading this unit, you should be able to:
 Ensure availability of test equipments like OTDR and Power meter for carrying out
optical tests
 Understand the Operation of OTDR
 Use of OTDR in fault localization
 Carryout the Testing of the splice through OTDR
 Measure the cable length and total loss
 Test the fiber joint with OTDR to confirm conformance to design requirements
 Ensure optical losses - reflectance, return and insertion are within the defined
specifications/ limits
 Interpret OTDR and power meter test results to identify and localize faults and/
 or measure optical losses
 Test the fiber at both ends for instances of cross fiber using power source and power
meter tests and ensure their elimination
 Ensure completion of OTDR register showing complete record of jointing tests
11.3 OPTICAL TIME DOMAIN REFLECTOMETER
OTDRs are powerful test instruments for fiber optic cable plants, if one understands
how to properly set the instrument up for the test and interpret the results. When used by a

skillful operator, OTDRs can locate faults, measure cable length and verify splice loss.
Within limits, they can also measure the loss of a cable plant. About the only fiber optic
parameters they don't measure is optical power at the transmitter or receiver.
OTDRs are almost always used on OSP cables to verify the loss of each splice and
pinpoint stress areas caused by installation. They are also widely used as OSP
troubleshooting tools since they can locate problems in the cables. Most ODTRs lack the
distance resolution for use on the shorter cables typical of premises networks. An OTDR
sends short pulses of light into a fiber. Light scattering occurs in the fiber due to
discontinuities such as connectors, splices, bends, and faults. An OTDR then detects and
analyzes the backscattered signals. The signal strength is measured for specific intervals of
time and is used to characterize events. n order to use an OTDR properly, it's necessary to
understand how it works, how to set the instrument up properly and how to analyze traces.
11.3.1 WORKING PRINCIPLE OF OTDR
OTDRs operate like radar. They generate short pulses of light and then sample the
light backscattered by fiber segments and reflected by connections and other events.
An OTDR uses the effects of Rayleigh scattering and Fresnel reflection to measure
the fiber's condition, but the Fresnel reflection is tens of thousands of times greater in power
level than the backscatter.
Rayleigh scattering occurs when a pulse travels down the fiber and small variations in
the material, such as variations and discontinuities in the index of refraction, cause light to be
scattered in all directions. However, the phenomenon of small amounts of light being
reflected directly back toward the transmitter is called backscattering.
Fresnel reflections occur when the light traveling down the fiber encounters abrupt
changes in material density that may occur at connections or breaks where an air gap exists.
A very large quantity of light is reflected, as compared with the Rayleigh scattering. The
strength of the reflection depends on the degree of change in the index of refraction.
The OTDR technique consists of sending impulses to the fiber and measuring the time
delay and intensity of the backscattered signal. The backscatter effect occurs because of the
same reasons that we have attenuation on optical fiber, What happens is that some of the light

gets reflected back due to changes in the molecular density of the glass. Measuring this light
is equivalent to measuring fiber attenuation.
The structure of an OTDR is basically a light source to emit signal pulses and an
optical receiver connected to a data processing unit. The emitted signal is sent directly into
the fiber and the incoming reflection directed to the receiver by a beamsplitter. The light
source is synchronized with the receiver so that time delay between outgoing and incoming
signals can be measured.
Figure 1: OTDR BLOCK DIAGRAM
A. Timer
The timer produces a voltage pulse which is used to start the timing process in the
display at the same moment as the laser is activated.
B. Pulsed Laser
The laser is switched on for a brief moment. The ‘on’ time being between 1ns and
10us. We will look at the significance of the choice of ‘on’ time or pulsewidth a little bit
later. The wavelength of the laser can be switched to suit the system to be investigated.
C. Directive Coupler
The directive coupler allows the laser light to pass straight through into the fiber
under test. The backscatter from the whole length of the fiber approaches the directive

coupler from the opposite direction. In this case the mirror surface reflects the light into the
avalanche photodiode (APD). The light has now been converted into an electrical signal.
D. Amplifying and Averaging
The electrical signal from the APD is very weak and requires amplification before it
can be displayed. The averaging feature is quite interesting and we will look at it separately
towards the end of this tutorial.
E. Display
The amplified signals are passed on to the display. The display is either a CRT like an
oscilloscope, or a LCD as in laptop computers. They display the returned signals on a simple
XY plot with the range across the bottom and the power level in dB up the side.
The following figure shows a typical display. The current parameter settings are
shown over the grid. They can be changed to suit the measurements being undertaken. The
range scale displayed shows a 50km length of fiber. In this case it is from 0 to 50km but it
could be any other 50km slice, for example, from 20km to 70km. It can also be expanded to
give a detailed view of a shorter length of fiber such as 0-5m, or 25-30m.
Figure 2: An OTDR display-no signal

The range can be read from the horizontal scale but for more precision, a variable
range marker is used. This is a movable line which can be switched on and positioned
anywhere on the trace. Its range is shown on the screen together with the power level of the
received signal at that point. To find the length of the fiber, the marker is simply positioned at
the end of the fiber and the distance is read off the screen. It is usual to provide up to five
markers so that several points can be measured simultaneously.
F. Data Handling
An internal memory or floppy disk can store the data for later analysis. The output is
also available via RS232 link for downloading to a computer. In addition, many OTDRs have
an onboard printer to provide hard copies of the information on the screen. This provides
useful ‘before and after’ images for fault repair as well as a record of the initial installation.
11.3.2 A SIMPLE MEASUREMENT
Whenever the light passes through a cleaved end of a piece of fiber, a Fresnel
reflection occurs. This is seen at the far end of the fiber and also at the launch connector.
Indeed, it is quite usual to obtain a Fresnel reflection from the end of the fiber without
actually cleaving it. Just breaking the fiber is usually enough.
Figure 3: Measurement through OTDR
The Fresnel at the launch connector occurs at the front panel of the OTDR and, since
the laser power is high at this point, the reflection is also high. The result of this is a relatively

high pulse of energy passing through the receiver amplifier. The amplifier output voltage
swings above and below the real level, in an effect called ringing. This is a normal amplifier
response to a sudden change of input level. The receiver takes a few nanoseconds to recover
from this sudden change of signal level.
11.3.3 DEAD ZONES
The Fresnel reflection and the subsequent amplifier recovery time results in a short
period during which the amplifier cannot respond to any further input signals. This period of
time is called a dead zone. It occurs to some extent whenever a sudden change of signal
amplitude occurs. The one at the start of the fiber where the signal is being launched is called
the launch dead zone and others are called event dead zones or just dead zones.
Figure 4: Dead Zone
11.4 AN OTDR DISPLAY OF A TYPICAL SYSTEM
The OTDR can ‘see’ Fresnel reflections and losses. With this information, we can
deduce the appearance of various events on an OTDR trace as seen in figure.
A. Connectors
A pair of connectors will give rise to a power loss and also a Fresnel reflection due to
the polished end of the fiber.
B. Fusion Splice
Fusion splices do not cause any Fresnel reflections as the cleaved ends of the fiber are
now fused into a single piece of fiber. They do, however, show a loss of power. A good
quality fusion splice will actually be difficult to spot owing to the low losses. Any signs of a
Fresnel reflection is a sure sign of a very poor fusion splice.

Figure 5: A typical OTDR Trace
C. Mechanical Splice
Mechanical splices appear similar to a poor quality fusion splice. The fibers do have
cleaved ends of course but the Fresnel reflection is avoided by the use of index marching gel
within the splice. The losses to be expected are similar to the least acceptable fusion splices.
D. Bend Loss
This is simply a loss of power in the area of the bend. If the loss is very localized, the
result is indistinguishable from a fusion or mechanic splice.
11.5 OTDR MEASUREMENTS
An OTDR can perform the following measurements:
 For each event: Distance location, loss, and reflectance
 For each section of fiber: Section length, section loss (in dB), section loss rate (in
dB/km), and optical return loss (ORL) of the section

 For the complete terminated system: Link length, total link loss (in dB), and ORL of
the link.
11.5.1 MEASUREMENT METHODS
The OTDR lets technicians perform measurements on the fiber span in several ways:
full-automatic, semi-automatic, and manual measurement functions. Technicians can also use
a combination of these methods.
11.5.1.1 Full-Automatic Function
Using the full-automatic function, the OTDR detects and measures all of the events,
sections, and fiber ends automatically, using an internal detection algorithm.
11.5.1.2 Semi-Automatic Function
Selecting the semi-automatic function, the OTDR measures and reports an event at
each location (distance) with a marker. Markers can be placed either automatically or
manually. The semi-automatic function is of high interest during span acceptance (after
splicing), when technicians completely characterize all events along the span in order to
establish baseline data. Because automatic detection will not detect and report a non-
reflective event with a zero loss, it places a marker at that location so that the semiautomatic
analysis will report the zero loss.
11.5.1.3 Manual Measurement Function
For even more detailed analysis or for special conditions, technicians completely
control the measurement function manually. In this case, technicians place two or more
cursors on the fiber in order to control the way the OTDR measures the event. Depending on
the parameter being measured, technicians may need to position up to five cursors to perform
a manual measurement. While this is the slowest and most cumbersome method of
measurement, it is important to have this capability available for fiber spans with unusual
designs and construction that are difficult to analyse accurately using automated algorithms.
11.6 TESTING A FIBER OPTIC CABLE PLANT USING OTDR
OTDR testing creates a snapshot of a fiber optic cable. This test is commonly used to
verify the quality of the installation and troubleshoot problems. OTDR testing requires

interpretation of the data acquired, called the trace or signature, by a skilled operator. This
test will acquire a trace of an installed fiber optic cable plant, single mode fiber, including the
loss of all fiber, splices and connectors.
Figure 6: Test setup
11.6.1 EQUIPMENT NEEDED TO PERFORM THIS TEST
 OTDR appropriate for the fiber being tested (Multimode: 850 and/or 1300nm,
singlemode, 1310, 1550 and/or 1625 nm)
 Launch and receive reference cables of the same fiber type and size as the cable plant
and with connectors compatible to those on the cable plant.
Notes:
 If you are only testing for length, you only need a launch reference cable. The
receive cable allows you to measure loss of the final connector on the cable.
 Reference cables must be long enough for the OTDR's initial test pulse to settle
down back to the baseline.
 Connectors on the launch and receive cables must be in good condition (low loss)
to properly test connectors on the cable under test.
 Mating adapters compatible to connectors
 Cleaning supplies

11.6.2 TEST PROCEDURE
 Turn on OTDR and allow time to warm-up
 Clean all connectors and mating adapters.
 Attach launch cable to OTDR. Attach receive cable (if used) to far end of cable.
 Set up test parameters on OTDR.
 Attach cable to test to end of launch cable. Attach receive cable (if used) to far end of
cable.
 Acquire trace.
Note: Most OTDRs have an "auto test" function, but these functions are not foolproof.
Most problems with OTDR tests occur when untrained users use the autotest function
without having an understanding of how the instrument works, what a good trace looks
like and, most importantly, what are the characteristics of the cable plant they are testing
(length, number and locations of splices and connectors). Refer to the next section on
reading OTDR traces. Once you are confident that the autotest function is giving valid
results, it is a major timesaver in OTDR testing.
11.6.3 READING OTDR TRACE OF A CABLE PLANT
Here is a schematic of what an idea OTDR trace should look like aligned to the cable
plant being tested showing how events will look in the trace.
The results of your test can be estimated by knowing typical component losses (fiber
attenuation, splice loss, connector loss) and calculating a loss budget for the cable under test.
Note:
 Clean all connectors regularly before and while testing.
 Set OTDR parameters for the highest resolution that will allow covering the length of
cable being tested by reducing the test pulse width or duration. Use longer averaging
times if necessary to reduce noise.

 Do not try to analyze traces where the segment being analyzed is nonlinear, usually
caused by a high reflectance event or inadequate OTDR resolution.
 Repair or replace launch and receive reference cables when their connector losses or
reflectance become unacceptable.
Figure 7: OTDR Trace Information
Record the following data to document your tests and keep copies for future reference:
 Date of the test
 Cable being tested and fiber identification
 Operator
 Test equipment used
 Test wavelength(s)
 Store trace Include documentation on location of events and lengths as
appropriate.

11.6.4 MEASURING DISTANCE WITH THE OTDR
 Place one of the markers on the OTDR (usually called Marker A) just before the
reflectance peak from the connection between the launch cable and the cable under
test.
 Place the second marker (usually called Marker B) just before the reflectance peak
from the connection between the cable under test and the receive cable. (If no receive
cable is used, there should still be a reflectance peak from the final connector on the
cable under test.)
 The OTDR will calculate the length of the segment between the markers.
Note:
 Put the actual value of the index of refraction; enter that in the OTDR setup.

 The OTDR measures the length of the fiber, not the length of the cable. The fiber is
usually 1-2% longer than the cable so you may need to multiply the correction factor
i.e 0. 985 into the length if you are testing a long length of cable.
11.6.5 MEASURING FIBER ATTENUATION COEFFICIENT
Figure 9: OTDR trace Information
 Place one of the markers on the OTDR (usually called Marker 1 or A) on the fiber
segment to be tested away from any splice or connection in the cable under test.
 Place the second marker (usually called Marker 2 or B) further away from the OTDR
on the same segment.
 The OTDR will calculate the loss of the segment between the markers and the
distance and show the results in dB/km.
Note:
 Ensure the markers are not placed on curved parts of the trace which will cause
erroneous readings.
11.6.6 MEASURING SPLICE OR CONNECTOR LOSS

OR
 Place one of the markers on the OTDR (usually called Marker 1 or A) just before the
splice or reflectance peak from the connection in the cable under test.
 Place the second marker (usually called Marker 2 or B) just after the splice or the
reflectance peak from the connection in the cable under test.
 The OTDR will calculate the loss of the segment between the markers.
Note:
erroneous readings.
 The loss of the fiber in the distance between the markers will be added to the
measured loss. To avoid this, use the "least squares" method for loss. Check your
OTDR manual for instructions .

11.6.7 MEASURING REFLECTANCE
Figure 11: OTDR trace Information
 Place one of the markers on the OTDR (usually called Marker 1 or A) just before the
reflectance peak from the connection in the cable under test.
 Place the second marker (usually called Marker 2 orB) at the top the reflectance peak
from the connection in the cable under test.
 The OTDR will calculate the reflectance of the peak chosen by the markers.
Note:
erroneous readings.
11.7 OPTICAL POWER METER
The power meter is the standard tester in a typical fiber optic technician’s tool kit. It is
an invaluable tool during installation and restoration. The power meter’s main function is to
display the incident power on the photodiode. Transmitted and received optical power is only
measured with an optical power meter. For transmitted power, the power meter is connected
directly to the optical transmitter’s output.

For received power, the optical transmitter is connected to the fiber system. Then, the
power level is read using the power meter at the point on the fiber cable where the optical
receiver would be.
The power meter, as it is commonly called, measures the optical power of light
present on a fiber optic cable. An optical power meter (OPM) is a device used to measure the
power in an optical signal. This light can be generated directly from the output of a fiber optic
transmitter device or from another common fiber optic testing device: a laser light source.
The optical power is measured in dBm or in mW.
A typical optical power meter consists of a calibrated sensor, measuring amplifier and
display. The sensor primarily consists of a photodiode selected for the appropriate range of
wavelengths and power levels. On the display unit, the measured optical power and set
wavelength is displayed.
A traditional optical power meter responds to a broad spectrum of light; however the
calibration is wavelength dependent. This is not normally an issue, since the test wavelength
is usually known, however it has a couple of drawbacks. Firstly, the user must set the meter
to the correct test wavelength, and secondly if there are other spurious wavelengths present,
then wrong readings will result.
11.8 DYNAMIC RANGE OF POWER METER
The requirements for a power meter vary depending on the application. Power meters
must have enough power handling capability to measure the output of the transmitter (to
verify operation). They must also be sensitive enough, though, to measure the received power
at the far (receive) end of the link. Long-haul telephony systems and cable TV systems use
transmitters with outputs as high as +16 dBm and amplifiers with outputs as high as +30
dBm. Receiver power levels can be as low as –36 dBm in systems that use an optical
preamplifier.
In local area networks (LANs), though, both receiver and transmitter power levels are
much lower. The difference between the maximum input and the minimum sensitivity of the
power meter is termed the dynamic range. While the dynamic range for a given meter has
limits, the useful power range can be extended beyond the dynamic range by placing an

attenuator in front of the power meter input. However, this limits the low-end sensitivity of
the power meter.
For high power mode, use an internal or external attenuator. If using an internal
attenuator, it can be either fixed or switched.
Typical dynamic range requirements for power meters are as follows:
 +20 to –70 dBm for standard power applications
 +26 to –55 dBm for high power applications such as Analog RF transmission
in cable TV (CATV) or video overlay in passive optical network (PON)
systems.
 –20 to –60 dBm for LAN applications
Sometimes optical power meters are combined with a different test function such as
an Optical Light Source (OLS) or Visual Fault Locator (VFL), or may be a sub-system is a
much larger instrument. When combined with a light source, the instrument is usually called
an Optical Loss Test Set.
Optical Loss Test Sets (OLTS) are available in dedicated hand held instruments and
platform-based modules to suit various network architectures and test requirements. They are
used to measure optical power and power loss, and reflectance and reflected power loss. The
products may also be used as optical sources or optical power meters, or to measure optical
return loss or event reflectance.
11.9 THE PROCEDURE TO MEASURE THE OPTICAL LOSS IN THE
FIBER OPTIC CABLE
Set the power meter to the wavelength of the light source you are using
 Connect a short fiber jumper cable between the light source and the power meter.
 Make note of the power level, in dBm. We will call this “Reading A”.
 Connect the fiber cable under test to the output of the light source

 Connect the power meter, set at the same wavelength as the power source, to the far
end of the fiber cable under test.
 Make note of the power level, in dBm. We will call this “Reading B”.
 The optical loss in the fiber cable is equal to “Reading A” minus “Reading B
Optical Loss = “Reading A” – “Reading B”
Figure 12: Connection of fiber jumper cable between the light source and the power meter
When multimode fiber is used, measurements should be made at 850nm or 1310nm.
It is preferred that measurements be made at both wavelengths, if possible, as the optical
loss can vary significantly as the wavelength varies when multimode fiber is used.
When single mode fiber is used, measurements should be made at 1310nm or 1550
nm as these are the most common wavelength used with single mode fiber. These
measurement procedures should be repeated for every fiber cable in the system.
Figure 13: Figure

11.10 OTHER OPTICAL INSTUMENTS
11.10.1 LIGHT SOURCES
A light source is a device that provides a continuous wave (CW) and stable source of
energy for attenuation measurements. It includes a source, either a light emitting diode (LED)
or laser that is stabilized using an automatic gain control mechanism. LEDs are typically used
for multimode fiber. On the other hand, lasers are used for single mode fiber applications.
The output of light from either an LED or laser source may also have the option of
modulation (or chopping) at a given frequency, which the power meter can then be set to
detect. This method improves ambient light rejection. In this case, a 2 kHz modulated light
source can be used with certain types of detectors to tone the fiber for fiber identification or
for confirmation of continuity
11.10.2 FIBER OPTIC TALKSETS
While technically not an measuring instrument, FO talksets are useful for FO
installation and testing. They transmit voice over fiber optic cables already installed, allowing
technicians splicing or testing the fiber to communicate effectively. Talksets are especially
useful when walkie-talkies and telephones are not available, such as in remote locations
where splicing is being done, or in buildings where radio waves will not penetrate.
The way to use talksets most effectively is to set up the talksets on one fiber (or pairs
appropriate) and leave them there while all testing or splicing work is done. Thus, there will
always be a communications link between the working crew, which facilitates deciding
which fibers to work with next. The continuous communications capability will greatly speed
the process.
11.10.3 ATTENUATORS
Attenuators are used to simulate the loss of long fiber runs for testing link margin in
network simulation in the laboratory or self-testing links in a loopback configuration. In
margin testing, variable attenuators are used to increase loss until the system has a high bit
error rate. For loopback testing, an attenuator is used between a single piece of equipment's
transmitter and receiver to test for operation under maximum specified fiber loss. If systems

work in loopback testing, they should work with a proper cable plant. Thus many
manufacturers of network equipment specify a loopback test as a diagnostic/troubleshooting
procedure.
Attenuators can be made by gap loss, or a physical separation of the ends of the fibers,
inducing bending losses or inserting calibrated optical filters. Both variable and fixed
attenuators are available, but variable attenuators are usually used for testing. Fixed
attenuators may be inserted in the system cables where distances in the fiber optic link are too
short and excess power at the receiver causes transmission problems.
11.10.4 VISUAL CABLE TRACERS AND FAULT LOCATORS
Many of the problems in connection of fiber optic networks are related to making
proper connections. Since the light used in systems is invisible, one cannot see the system
transmitter light. By injecting the light from a visible source, such as a LED or incandescent
bulb, one can visually trace the fiber from transmitter to receiver to insure correct orientation
and check continuity besides. The simple instruments that inject visible light are called visual
fault locators.
Figure 14: VisualCable Tracer and Fault Loacators
11.10.5 FIBER IDENTIFIERS
Optical Fiber technicians may need to identify a fiber in a splice closure or at a patch
panel. If one carefully bends a singlemode fiber enough to cause loss, the light that couples
out can also be detected by a large area detector. A fiber identifier uses this technique to
detect a signal in the fiber at normal transmission wavelengths. These instruments usually

function as receivers, able to discriminate between no signal, a high speed signal and a 2 kHz
tone. By specifically looking for a 2 kHz "tone" from a test source coupled into the fiber, the
instrument can identify a specific fiber in a large multifiber cable, especially useful to speed
up the splicing or restoration process.
Fiber identifiers can be used with both buffered fiber and jacketed single fiber cable.
With buffered fiber, one must be very careful to not damage the fiber, as any excess stress
here could result in stress cracks in the fiber which could cause a failure in the fiber anytime
in the future
Figure 15: Fiber Identifier
11.11 REPORT & RECORD
The fiber optic cable plant, therefore, must be documented as to the path of every
fiber, connection, and test. Perhaps the most important part of any installation is the
documentation. Good documentation is invaluable in upgrading, troubleshooting or restoring
a network. Documentation should include:
• Design data, e.g. GIS maps
• Component types and manufacturers
• The paths of each cable
• Types of cable (and where the excess is stored for restoration)
• Cable section lengths
• Locations of splices or terminations
• Calculated loss budget
• The optical loss of each fiber measured at installation

• Fiber numbers/colors connected to each communications device, noting transmitter
and receiver orientation
• Spare fibers available for expansion or use to replaced damaged fibers
• Types of communications equipment
• Wavelength of transmission
• Transmitter and receiver power for each transceiver (and attenuator values if used)
• OTDR traces
• Name and contact information for installers
• Copies of the documentation should be kept at each end of the link and backups
stored in a safe place.
Most of this data can be kept in a database that stores component, connection, and test
data. Long links may also have OTDR data that can be stored as printouts or in special file
formats for later viewing in case of problems. If the OTDR data is stored digitally, a database
of data files should be kept to allow finding specific OTDR traces more easily.
11.12 SUMMARY
The OTDR is a more sophisticated measurement instrument. It uses a technology that
injects a series of optical pulses into the fiber under test and analyses the light scattering and
the light reflection. This allows the instrument to measure the intensity of the return pulse in
functions of time and fiber length. The OTDR is used to measure the optical power loss and
the fiber length, as well as to locate all faults resulting from fiber breaks, splices or
connectors. OTDRs are also used for maintaining fiber plant performance. An OTDR allows
you to see more details on cable installation, termination quality and provides advanced
diagnostics to isolate a point of failure that may hinder network performance. An OTDR
allows discovery of features along the length of a fiber that may affect fiber reliability.
OTDRs characterize features such as attenuation uniformity and attenuation rate, segment
length, location and insertion loss of connectors and splices, and other events such as sharp
bends that may have been incurred during cable installation.
The most basic fiber optic measurement is optical power from the end of a fiber. This
measurement is the basis for loss measurements as well as the power from a source or
presented at a receiver. Fiber optic power meter is a test instrument used for absolute optical
fiber power measurement as well as fiber optic loss related measurement.

11.13 REFERENCES AND SUGGESTED FURTHER READINGS
 ITU-T manual on OF installation
 EI of BSNL
 EI on underground OF cable laying works by BBNL
 Fiber Optics Technician's Manual
 Understanding optical communication by Dutton
 Planning Fiber Optic Networks by Bob Chomycz
 www.timbercon.com
 http://www.ofsoptics.com
 http://www.thefoa.org/
 http://www.corning.com
 http://www.fiber-optics.info
 http://www.rp-photonics.com
 http://www.occfiber.com and other websites
11.14 KEY LEARNINGS
Qu.1 Fill in the Blanks
1. OTDR stands for ……………………………………
2. OTDR is based on the principle of………………….
3. Mechanical splices appear similar to a ……………..quality fusion splice in OTDR
trace.
4. Using the ……………………………. function, the OTDR detects and measures all
of the events, sections, and fiber ends automatically, using an internal detection
algorithm.
5. The optical power is measured in………………….
Qu. 2: State True or False
1. OTDRs are almost always used on OSP cables to verify the loss of each splice and
pinpoint stress areas caused by installation.
2. OTDRs operate like power meter.
3. A pair of connectors will give rise to a power loss and also a Fresnel reflection due to
the polished end of the fiber.
4. The OTDR can ‘see’ Fresnel reflections and losses.
5. For measuring the high power, use an internal or external attenuator.

Qu.3: Write down the principle of Working of OTDR?
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Qu. 4: What is the purpose of OTDR?
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Qu. 5 What is Dade Zone?
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Qu. 6 Draw the OTDR Trace for the following OSP?
Qu. 6 What are the data is to be documented of a OTDR trace?
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11.15 WORK SHEET
1. Draw the setup of OTDR for fault localization?
2. Conduct the test on fiber spool and find out the following
 Total loss
 No. of Splices
 Loss per splice

 Total fiber length
 Dead zone length
 Position of splice
 Break Point
3. Identify various useful measuring instruments used in OFC?
Name of Instruments Uses
OTDR
Power Meter
Talk set
Light Source
Fixed Attenuator
Variable Attenuator
Pair Identifier
4. Measure the optical power at different points of the OF system?
Name of the System Monitoring Point Measured Power

Safety Precautions
 Switch the OTDR off before you start to clean its connectors! Or at least disable
the laser.
 Invisible laser radiation! do not stare into beam or view directly with optical
instruments.
 Do Not Operate in an Explosive Atmosphere
 Do Not Remove Covers
Notes:
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Testing effectiveness of the splice through otdr and power meter tests

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Testing effectiveness of the splice through otdr and power meter tests