Optical Time Domain Reflectometer Flashcards
What does OTDR stand for?
Optical Time Domain Reflectometer (OTDR)
What Pre-Use check are required on an OTDR prior to use?
- 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.
State the benefits of an OTDR?
- 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.
State the drawbacks of an OTDR?
- 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.
What aspects/events should an OTDR be able to measure?
- Length of the fibre.
- Loss of the fibre per Km.
- Splice loss.
- Insertion loss of connectors.
- Return loss of connectors.
Explain how a Fresnel Reflection occurs?
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.
State what events would cause a Fresnel Reflections?
Ends of fibres, at fibre breaks, and sometimes at splice points.
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?
Rayleigh Scattering
Explain how would a Fresnel Reflection appear on an OTDR trace?
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.
Explain how Rayleigh Scattering is depicted on an OTDR trace?
The Light Source does not travel as far as it should and you would show attenuation even with nothing attached to the Fibre.
After what event would you expect to find a “Dead Zone” on an OTDR trace?
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.
How can Dead Zones be reduced on an OTDR?
Since the dead zone is directly related to the pulse width, it can be reduced by decreasing the pulse width.
Explain the difference between Loss Resolution and Spatial Resolution on an OTDR?
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).
Explain what Dynamic Range is?
The dynamic range of an OTDR determines how long of a fibre can be measured.
State why Saturation would occur on an OTDR trace?
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.
