VDV Works Virtual Hands-On Training
Fiber Optics Testing: Using OTDRs
 
Part of the VDV Academy Fiber Optic Training Programs

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 Understanding OTDRs

"Virtual Hands-On" Termination

"Virtual Hands-On" Testing


OTDR Measurements

The OTDR can measure both distance and attenuation. Use the cursors and the measurement tool buttoms to determine the exact methods of both of these measurements.

The details in the information panel on the left side of the screen will change according to which measurement tool is active.

It is important to consider all of the tools when looking for an accurate measurement. 

This OTDR generally offers 4 modes of measurements.
2-pt loss: 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.
2-pt LSA: The OTDR measures distance and loss between the two markers but calculates the best fit line between the two points using the "least squares" method to reduce noise.
Conn/splice LSA: The OTDR measures distance and loss at an event - a connector or splice - between the two markers but calculates the best fit line between the two points using the "least squares" method to reduce noise.
ORL: The OTDR measures the back reflection or optical return loss for a connector or splice.
Remember the RED marker must be on the left and the BLUE marker on the right.


Fiber Attenuation by Two Point Method.


OTDR 2 pt loss

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 the two 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.




 

Notice in the trace shown the markers have been positioned between two events- the splice and the end of the fiber- and the information panel reports where the events are as well as the distance between the two markers.

L1 is located at 3.0991 km. L2 has been positioned at the end of the fiber segment before the noise begins (5.548km). The information panel also reports that the distance between the cursors is 2.4467 km. The dB loss between the two markers is 7.094 when the two point attenuation calculation is applied. With the noise at the end of the trace, this is not an accurate measurement, but we'll use a better method next.



Fiber Attenuation by Least Squares Method

The OTDR measures distance and loss between the two markers but calculates the best fit line between the two points using the "least squares" method to reduce noise. 

When the markers are selecting the noisy part of the fiber trace, the least squares attenuation (2-pt LSA) tool can be applied to calculate the dB loss between the cursors. Look closely and you will see a thick grey line between the markers, indicating the best fit to the trace, averaging all the noise.

 LSA Loss   

Notice when the LSA tool is activated the dB loss increases from 7.094 to 7.688. The LSA calculation aproximates a straight line amidst the many points between events to minimize the effect of the noisy trace.

Splice Loss by Two Point Method

OTDR 2 pt Splice loss

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 won’t 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.
M
easurements 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.

Using the 2-point method to measure the dB loss on this splice, the informtion panel reads 0.14 dB.

2-Point loss of a connector or splice requires careful placement of the markers. If the reflecrtance is high, the markers will have to be placed further apart due to the nonlinearities near the event.  Notice the distance between the markers is 0.045 km or 45 meters. The loss of the fiber 45 meters around the splice is included in this loss measurement, as is any nonlinearities caused by the OTDR pulse width or the location of the markers.

Splice Loss by Least Squares  (LSA)

The OTDR measures distance and loss at an event - a connector or splice - between the two markers but calculates the best fit line between the two points using the "least squares" method to reduce noise.

OTDR splice loss LSA

If you noticed, the markers are separated by some distance, which includes the loss of some fiber on either side of the actual connector or splice  Most OTDRs will calculate the loss for you by extrapolating the fiber traces on both sides of the event and calculating the loss without any influence from the fiber length. The mathematical method uses is called  "Least Squares Approximation", hence the term "LSA" used by many OTDRs in their display and setup menus.

Setting LSA requires setting seveeral markers - one on the peak, the two regular markers near the event and the two end markers which define the segments used for least-squares analysis. These segments should be long enough to allow good measurement but not so long as to approach other events.

Notice when the LSA method is used the information panel reveals the dB loss =0.23 dB.

The LSA method compensates for improper cursor placement and removes the effects of fiber between the markers. In most cases, the LSA method will be more accurate.

Reflectance
The OTDR  measures the amount of light that's returned from both backscatter in the fiber and reflected from a connector or splice.
OTDR ORL


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 the reflectance.

Reflectance

The OTDR can compare the amount of light in the reflected pulse to the backscatter level and calculate the backreflection or optical return loss directly. Plase the right marker on the peak of the pulse and the left marker on the fiber to the left of the peak and read the backreflection (-61 dB) on the information panel.








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