FAQs

What is a loop measurement?

Loop Measurements vs. Single Ended Measurements

With OTDR technology it is possible to perform a measurement in a uni-directional mode (also known as single ended) or in a loop configuration (double ended or combined measurements). For the loop measurement the sensing fibre is measured from both ends and then the geometric mean of the two traces can be combined to create a single measurement.

Advantages of Loop Measurements & when to use

Although, the single ended measurement configuration with OTDR technology is a very robust measurement in certain scenarios temperature anomalies can occur if the sensing fibre and related accessories are not configured within specifications. This can occur if splices are out of specification, if there are damaged fibre or if connectors are used in the line (not recommended). With the loop configuration, these anomalies are corrected for.

Additionally, you have the added security with a loop configuration that if the fibre breaks at one point you still have a continuous uni-directional measurement from either end and monitoring can continue undisturbed until the fibre is repaired.

Loop measurements are particularly useful when you have situations where

  • There are a lot of splices or connectors present (e.g. power cable when fibre is installed within the structure of the power cable)
  • There is potential for damaged fibre (e.g. in a hot oil well where there is high levels of hydrogen)
  • Where there is a need for high accuracy or temperature resolution

Cons of Double Ended Measurements

Although generally a higher quality measurement, there is 1 main drawback to double ended measurements. This drawback is that the measurement range is halved. For example if you are monitoring a 5km power cable, a system with a range of 10km measurement range will be required to provide this measurement. The range of DTS systems is generally quoted for uni-directional (single ended measurements).

Example of Loop Measurement

For example, if we take the example of a 2.2km power cable which is monitored with a sensing cable that contains 2 fibre optics. If you then splice these 2 optics together at the far end you will then have a loop of 4.4km which can be monitored using a DTS and a multiplexer (see diagram below)

If we assume that this cable has a rising temperature profile as shown in the graph below:

Then when we monitor the full loop (in single ended mode) we will have the following temperature profile.


Note the profile is now 4.4km in length and is symmetrical around the splice at 2.2 km. Notice that the quality of the measurement deteriorates with distance – as we would expect with a single ended measurement.

If we then take a second measurement from the other end we will obtain the following temperature profile.


As you can see the measurement looks like the mirror image, with the noisier signal at the opposite end to the fibre.
If we then take the geometric mean of these two single ended measurements we will obtain a double ended measurement, which will look like the graph below.

As you can see the noise levels are greatest towards the ends of the fibres and lowest in the middle of the fibre.
Below is a comparison on the same graph showing the different temperature resolutions of the single ended vs. the double ended measurement.

As you can see the resolution is very similar at the turnaround point of symmetry but quite different at the two ends.

Temperature Anomaly Correction Using Double Ended

The following experimental set up was installed at the Sensornet laboratory, which comprised 10km of regular fibre, calibration ovens and a 30m section of specialty fibre, which  had a differing differential loss profile.

As you can see in the temperature profile below, there is a very clear temperature offset at the section of differential fibre (around 4000m) which leads to a discrepancy along the length of the fibre e.g. it can be seen that the reference ovens at 10,000m are reading incorrect values.


However, with the double ended measurement, this temperature offset is removed as can be seen in the trace below.


 If we zoom in on the section of damaged fibre we are able to see more detail as to what occurs and it can clearly be seen that all temperature anomalies are corrected for.

 


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