optical fiber practical 3 - otdr

8/3/2019 Optical Fiber Practical 3 - OTDR

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Optical Time Domain Reflectometer

(OTDR)

EN 4580: Practical 2

Name: N. M. Ellawala

Index No: 070120F

Field: ENTC


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Introduction

An Optical Time DomainReflectometer(OTDR) is used in fiber optics to measure the time and

intensity of the light reflected on anoptical fiber. It is used as a troubleshooting device to findfaults, splices, and bends in fiber optic cables, with an eye toward identifying light loss. Light

loss is especially important in fiber optic cables because it can interfere with thetransmissionofdata. An OTDR can detect such light loss and pinpoint trouble areas, making repairs easy.

Procedure

1. 50m optical fiber

= 1310nm

Pulse Width = 10nm

2. 50m optical fiber = 1310nm

Pulse Width = 10nm
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Pulse Width = 20nm

Pulse Width = 100nm

Wavelength : (nm) Pulse Width (ns) X1 (m) X2 (m) Fiber Loss (dB/km)

1310 10 13.10 51.18 72.303

1310 20 13.82 51.18 72.0301310 100 20.98 51.18 74.981

1550 10 14.33 51.18 67.060

1550 20 15.35 51.18 66.408

1550 100 (50) 20.47 51.18 72.103


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3. 50m optical fiber

= 1310nm

= 1550nm

Wavelength:

(nm)

Pulse

Width (ns)

X1 (m) X2 (m) X1 (m) X2 (m) Fiber loss

(X1-X2) dB/km

Fiber loss

(X3-X4) dB/km

1310 10 13.31 51.18 63.46 102.35 72.508 19.694

1550 10 13.31 51.18 63.46 102.35 69.763 19.669


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4.

100m Cable 1310 nm Laser 100m Cable 1550 nm Laser

5. After changing IOR from 1.4655 to 1.6655;

50m Cable 1310 nm Laser IOR = 1.4655 50m Cable 1310 nm Laser IOR = 1.6655

Wavelength:

(nm)

Pulse

Width

(ns)

IOR X1 (m) X2 (m) X1 (m) X2 (m) Fiber loss

(X1-X2)

dB/km

Fiber loss

(X3-X4)

dB/km

1310 10 1.4655 13.31 51.18 63.46 102.35 72.508 19.694

1310 10 1.5655 13.41 47.91 59.41 95.82 75.202 21.230

1310 10 1.6655 12.61 45.03 55.84 90.06 80.808 22.440


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Discussion

1. Operation of the OTDRAn optical time-domain reflectometer (OTDR) is

an optoelectronic instrument used to characterize an optical fiber.An OTDR combines a laser source and a detector to provide an

inside view of the fiber link. The laser source sends a signal into

the fiber, where the detector receives a signal, due to the

light that is scattered (Rayleigh backscatter) or the light reflected

from the different elements of the fiber, from the same end of the fiber. This produces a trace on

a graph made in accordance with the signal received, and a post-analysis event table that contains

complete information on each network component is then generated. The signal sent is a short

pulse that carries a certain amount of energy. A clock then precisely calculates the time of flight

of the pulse, and time is converted into distanceknowing the properties of this fiber. As the

pulse travels along the fiber, a small portion of the pulses energy returns to the detector due tothe reflection of the connections and the fiber itself. When the pulse has entirely returned to the

detector, another pulse is sentuntil the acquisition time is complete. Therefore, many

acquisitions will be performed and averaged in a second to provide a clear picture of the links

components. After the acquisition has been completed, signal processing is performed to

calculate the distance, loss and reflection of each event, in addition to calculating the total link

length, total link loss, optical return loss and fiber attenuation.

Figure 1: Scattring in an optical fiber

Figure 2: Block diagram of an OTDR


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Relationship between the pulse-width and the resolution

The ability for an OTDR to pinpoint the right distance of an event relies sampling resolution.Sampling resolution is the minimum distance between two consecutive sampling points acquired

by the instrument. This parameter is crucial, as it defines the ultimate distance accuracy and

fault-finding capability of the OTDR. Having a high number of points results in a higher

resolution (short distance between points), which is the ultimate condition for finding faults. This

is illustrated in the figure below. To have high number of sampling points pulse width should be

short. That means to identify two separate faults with close proximity (to increase resolution),

pulse width should be reduced.

The pulse width is actually the time during which the laser is on. This time is then converted into

distance so that the pulse width has a length. In an OTDR, the pulse carries the energy requiredto create the back reflection for link characterization. Therefore the reduction in pulse width

reduces the energy transmitted and the shorter the distance it travels due to the loss along the

link. So it cannot use to measure longer fiber links.

Figure 4: Resolution vs. fault-finding efficiency: (a) 5-meter resolution

(higher resolution). (b) 15-meter resolution

Figure 3: Long and short Pulse width


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2. Possible errors that could be with OTDR measurements and ways to overcome thoseerrors.

The OTDR 's distance resolution is limited by the transmitted pulse width. The transmitted pulse

will cause crosstalk and reflections in the receiver if the event is too close to the instrument. The

output signal simply blinds the receiver at the very high speeds (the speed of light) which thesemeasurements are taking place. In order to limit this effect, a launch cable is commonly utilized

between the OTDR and the cable to be tested. This launch length is chosen such that the receiver

has time to recover from the aforementioned crosstalk and reflections before any "important"

reflections from the cable to be tested are received. The minimum length of the launch cable is

dependent upon the wavelength and pulse width selected. However, an absolute minimum at the

present level of technology should be about 20-25m.

Closely spaced events are also problematic for the OTDR. An "event dead zone" is present

immediately after a detectable event on a fiber optic trace. While the first event is located

correctly, any event occurring within this "dead zone" is masked by the backscatter from the first

event. They are thus not resolvable into multiple events. Event dead zones are generally on the

order of 1m-5m for high resolution OTDR devices.

Two sources of error are present in OTDR measurements that are not instrumented related. The

velocity of the light pulse in the fiber is not constant. Both manufacturing processes and

temperature differences along the length of the cable can affect the velocity of light propagation.

In addition, more fiber is enclosed in the insulation than the insulation is long. About 1 to 2%

extra fiber length is allowed during fabrication to compensate for twists, kinks, bends, and turns

as compared to the insulation length. This must be accounted for when conducting base line testsfor any installation.

3.We can identify following losses as important factors on optical fiber communication system that

can be measure using OTDR

Scattering loss Reflection loss

Absorption loss

Attenuation

Attenuation is the gradual reduction of light intensity along the optical fiber as a function of

distance from the source. The attenuation characteristics of an optical fiber are a result of twofactors, absorption and scattering.


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Absorption within the travel path is mainly caused by impurities within the fiber core. Thelargest cause of attenuation is light scattering. Scattering occurs when light collides with

individual atoms within the fiber optic cable. As such, it is anisotropic in nature. Scattered lightthat that impinges on the fiber at angles outside the numerical aperture of the fiber will be

absorbed into the cladding or transmitted backtoward the source.

Bending Losses

Fiber optic cables are also subject to losses as a result of stress and bending. Macro bends, largebends on the order of centimeters, because deflections of the core/cladding interface such that

absorption takes place. As the light attempts to negotiate the bend, some light exceeds thereflection angle of the cladding and is absorbed. It is this property which is utilized in OTDR

geotechnical monitoring for shear dislocation prior to complete cable shear. Micro bending dueto tiny imperfections of the core, or due to mechanical stress, can result in changes in geometry

sufficient to allow light to escape the core as well. In both cases, some reflection also occurs.

Measuring Losses from OTDR

The slope of the fiber trace shows the attenuation coefficient of the fiber and is calibrated in

dB/km by the OTDR. In order to measure fiber attenuation, we need a long length of fiber with

no distortions on either end from the OTDR resolution or overloading due to large reflections. If

the fiber looks nonlinear at either end, especially near a reflective event like a connector, avoid

that section when measuring loss.

Connectors and splices are called "events". Both should show a loss, but connectors andmechanical splices will also show a reflective peak. The height of that peak will indicate the

amount of reflection at the event, unless it is so large that it saturates the OTDR receiver. Then

peak will have a flat top and tail on the far end, indicating the receiver was overloaded.

Sometimes, the loss of a good fusion splice will be too small to be seen by the OTDR. Even

though it is good for the system, it can be confusing to the operator. It is very important to know


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the lengths of all fibers in the network, so you know where to look for events and won't get

confused when unusual events show up.

Reflective pulses can show the resolution of the OTDR. But we cannot see two events closer

than is allowed by the pulse width. Generally longer pulse widths are used to be able to see

farther along the cable plant and narrower pulses are used when high resolution is needed,although it limits the distance the OTDR can see.

Fiber Attenuation by Two Point Method

The OTDR measures distance and loss between the two markers. This can be used for measuring

loss of a length of fiber, where the OTDR will calculate the attenuation coefficient of the fiber,

or the loss of a connector or splice.

To measure the length and attenuation of the fiber, we place the markers on either end of the

section of fiber we wish to measure. The OTDR will calculate the distance difference between

the two markers and give the distance. It will also read the difference between the power levels

of the two points where the markers cross the trace and calculate the loss, or difference in thetwo power levels in dB. Finally, it will calculate the attenuation coefficient of the fiber by

dividing loss by distance and present the result in dB/km, the normal units for attenuation.

In order to get a good measurement, it is necessary to find a relatively long section of fiber to

give a good baseline for the measurement. Short distances will mean small amounts of loss, and

the uncertainty of the measurement will be higher than if the distance is longer. It is also

advisable to stay away from events like splices or connectors, as the OTDR may have some

settling time after these events, especially if they are reflective, causing the trace to have

nonlinearities caused by the instrument itself.

Splice Loss by Two Point Method


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The OTDR measures distance to the event and loss at an event - a connector or splice - between

the two markers.

To measure splice loss, move the two markers close to the splice to be measured, having each

about the same distance from the center of the splice. The splice wont look as neat as this, with

the instrument resolution and noise making the trace less sharp looking, as you will see later on.The OTDR will calculate the dB loss between the two markers, giving you a loss reading in dB.

Measurements of connector loss or splices with some reflectance will look very similar, except

you will see a peak at the connector, caused by the back reflection of the connector.

ReflectanceThe OTDR measures the amount of light that's returned from both backscatter in the fiber and

reflected from a connector or splice. The amount of light reflected is determined by the

differences in the index of refraction of the two fibers joined, a function of the composition of

the glass in the fiber, or any air in the gap between the fibers, common with terminations andmechanical splices.

This is a complicated process involving the baseline of the OTDR, backscatter level and power

in the reflected peak. Like all backscatter measurements, it has a fairly high measurement

uncertainty, but has the advantage of showing where reflective events are located so they can be

corrected if necessary.

By choosing the reflectance measurement and putting the right (blue) cursor on the peak of the

reflection and the left (red) cursor just to the left of the reflection, the OTDR will measure thereflectance


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References

http://en.wikipedia.org/wiki/Optical_time-domain_reflectometer

http://www.techoptics.com/pages/OTDR/How%20it%20works.html
http://en.wikipedia.org/wiki/Optical_time-domain_reflectometerhttp://en.wikipedia.org/wiki/Optical_time-domain_reflectometerhttp://www.techoptics.com/pages/OTDR/How%20it%20works.htmlhttp://www.techoptics.com/pages/OTDR/How%20it%20works.htmlhttp://www.techoptics.com/pages/OTDR/How%20it%20works.htmlhttp://en.wikipedia.org/wiki/Optical_time-domain_reflectometer

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