Optical Time Domain Reflectometer

Question 1

What does OTDR stand for?

Answer 1

Optical Time Domain Reflectometer (OTDR)

Question 2

What Pre-Use check are required on an OTDR prior to use?

Answer 2

• Ensure OTDR is in calibration
• Ensure you have all the correct accessories for the network you are about to test i.e. Launch leads, Tail Leads etc,
• Ensure you have the correct Input and Output modules on the OTDR, i.e. ST, FCPC etc.

Question 3

State the benefits of an OTDR?

Answer 3

• The complete characterisation of an installed fibre optic link.
• Is capable of measuring fibre length, fibre losses, connector losses, splice losses and fibre defects.
• Access required to only one end of the installed fibre link.
• A permanent record of the trace can be obtained on disk or paper.
• Accurate location of faults can be obtained.

Question 4

State the drawbacks of an OTDR?

Answer 4

• The technology used within an OTDR is expensive typically between £10k and £30k.
• A reasonably skill level is required to operate, carry out measurements and interpret results.
• Measurement errors have many possible causes.
• Mainframe OTDRs unlike their mini versions are large bulky units that usually require an external power source.
• OTDR’s are unable to see individual incidents if they are too close together.

Question 5

What aspects/events should an OTDR be able to measure?

Answer 5

• Length of the fibre.
• Loss of the fibre per Km.
• Splice loss.
• Insertion loss of connectors.
• Return loss of connectors.

Question 6

Explain how a Fresnel Reflection occurs?

Answer 6

A Fresnel reflection is like shining a flashlight at a window. Most of the light passes through the window, but some of it reflects back at you.

Question 7

State what events would cause a Fresnel Reflections?

Answer 7

Ends of fibres, at fibre breaks, and sometimes at splice points.

Question 8

Pulses of light make contact with microscopic particles known as dopants. The result of this occurrence causes the major loss within an optical fibre cable, what is this phenomenon known as?

Answer 8

Rayleigh Scattering

Question 9

Explain how would a Fresnel Reflection appear on an OTDR trace?

Answer 9

The Light represented by a continuos downward diagonal line on the OTDR is interrupted by a short upward spike and then returns to a normal downward diagonal line.

Question 10

Explain how Rayleigh Scattering is depicted on an OTDR trace?

Answer 10

The Light Source does not travel as far as it should and you would show attenuation even with nothing attached to the Fibre.

Question 11

After what event would you expect to find a “Dead Zone” on an OTDR trace?

Answer 11

Dead zone refers to the space on a fibre trace following a Fresnel reflection in which the high return level of the reflection covers up the lower level of backscatter.

Question 12

How can Dead Zones be reduced on an OTDR?

Answer 12

Since the dead zone is directly related to the pulse width, it can be reduced by decreasing the pulse width.

Question 13

Explain the difference between Loss Resolution and Spatial Resolution on an OTDR?

Answer 13

Loss Resolution is the ability of the sensor to distinguish between levels of power it receives.

Spatial Resolution is how close the individual data points that make up a trace are spaced in time (and corresponding distance).

Question 14

Explain what Dynamic Range is?

Answer 14

The dynamic range of an OTDR determines how long of a fibre can be measured.

Question 15

State why Saturation would occur on an OTDR trace?

Answer 15

This is a problem that is more likely to appear when conducting single mode tests on older OTDR’s. If a long launch pulse is used it is possible for the returning signal from the fibre close to the OTDR to saturate the detector making measurement of that portion of the fibre impossible.

Question 16

Explain how Saturation can be reduced or removed?

Answer 16

The saturation effects may usually be eliminated by selecting a shorter pulse width to limit the power put into the fibre.

Question 17

State when a optical fibre cable should be tested with an OTDR?

Answer 17

Testing Configurations;

• Check optical cable 25%*. (Before cable use)
• Check all system components
• Install optical cable.
• Test installed cable 25%*. (After Installation of cable)
• Terminate/Connectorized
• Full network test. *. (When Job is complete)

*Requires OTDR testing.

Question 18

What is the purpose of conducting a full Network test?

Answer 18

• Check continuity
• Identify complete characterisation
• Ensure acceptable loss budget.

Question 19

State four common problems that may be encountered when conducting OTDR measurements?

Answer 19

• List some of the common problems encountered when operating the OTDR.
• Describe some of the techniques for conducting OTDR measurements on unterminated fibres.
• Describe the maintenance procedures for launch leads.
• Describe how to eliminate disruptive ghosts.
• Describe the effect of fibre mismatches on an OTDR trace.
• Be able to recognise saturation effects on an OTDR trace and describe how to eliminate them.

Question 20

Explain why Ghost Reflections occur?

Answer 20

This usually happens when a large reflection occurs in a short fibre. The reflected light actually bounces back and forth within the fibre, causing one or more false reflections to show up at multiple distances from the initial large (true) reflection.

Question 21

State the techniques for removing Ghost Reflections from OTDR traces?

Answer 21

• Measure the distance to the suspect reflection. Then place a cursor half this distance on the fibre. If an expected reflection is at the half-way mark, then the suspect is probably a ghost.
• Suppress or reduce the known (true) reflection. By making the amount of returned power smaller, the ghost will also be reduced (or eliminated). To reduce the reflection, you can use index matching gel at the reflection, or reduce the amount of power going to the reflective point by selecting a shorter pulse width or by adding attenuation in the fibre before the reflection.
• Change the Distance Range (Display Range) of the OTDR. In some OTDRs, a ghost is caused when the Distance Range is too short. Increase the Range setting and the ghost may disappear.
• If a ghost seems to occur in the fibre, then measure the loss across the suspected reflection. A ghost will show no loss across it when you do a splice loss measurement.

Question 22

Explain how “Gain” or Fibre Mismatch would be depicted on an OTDR trace?

Answer 22

The slopes of the two fibre traces would be different. One fibre has a steeper slope than the other fibre, which indicates a higher backscatter level throughout the fibre. It would normally appear higher on the screen than the other fibre because it returns more light to the OTDR.

Question 23

What tests are required to be conducted on “Horizontal” cabling?

Answer 23

• Horizontal cabling: 100% Light Source & Power Meter (LS&PM).

Question 24

What tests are required to be conducted on “Backbone” cabling?

Answer 24

• Backbone cabling: 100% LS&PM & 100% Optical Time Domain Reflectometer (OTDR).