On measuring the loss of an optical cable after installation

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Oct 30, 2018

Once an optical cable has been installed, it is necessary to measure the loss of the newly installed section.
There are two types of equipment that can be used to measure loss.

1: Power Meter+Light Source
2: OTDR(Optical Time Domain Reflect meter)

Let’s take a look at the advantages and disadvantages of using each.

(1) Advantages/Disadvantages of measuring with a Power Meter+Light Source


Low cost of light source and power meter
Typically the light source and power meter are small and lightweight instruments
The results are a simple 'one number' evaluation of link loss that requires little or no understanding or interpretation of by the operator.
Easy to set a 'Go / No Go' threshold for the test that is easily understood by the operator


  1. When making a two-way measurement over a long segment, the measuring equipment must be carried from place to place.
  2. Only allows for the measurement of overall link loss.
    Does not locate the position of individual loss events in the measured link
    Uses the 'cut-back' method of measurement. If the reference measurements are made incorrectly, this will affect the test results giving incorrect values.
    In some situations, it may require use of special reference patch cords which might not be available when needed, or maybe in substandard condition, affecting the measured results.

(2) Advantages/Disadvantages of measuring through OTDR


Allows the operator to establish the position of high loss events in the overall link
Allows accurate measurement of the optical length of a link. (Providing the Group Index of the fibre under test is known).
Allows measurement of individual loss events: such as loss of an individual splice, or loss at a particular connector or splitter.
Creates very comprehensive characterisation of an optical link.
Measured data can be output to a PC for additional post processing and link characterisation.
Some OTDR platforms integrate other measurements such as PON splitter characterisation, Chromatic Dispersion measurement or Polarisation Mode Dispersion measurement into a single transportable unit.
Flexibility: the operator can trade off accuracy for speed to make either very fast measurements or slower more accurate ones.
No need to establish a prior cut-back reference measurement, removes a cause of possible errors


Requires skill and understanding to interpret the measurement data - although many current OTDRs include a 'Optical Link Mapper' simplified presentation of the results.
Accurate measurements must be made 'bi-directional'. So access to both ends of the link is required for ultimate accuracy.
Some fibres require special launch conditions if the operator is to achieve correct measurement results. User must have the correct launch fibres on hand to handle such testing.
Cost is higher than a simple light source and power meter combination
An OTDR is typically heavier and more bulky than a light source and power meter.

The OTDR is the ultimate fibre test 'toolbox'. It's more expensive and bulkier than a simple light source and power meter, but is far more versatile and provides a much more detailed characterisation of the link. So, OTDR is widely deployed in the inspection of optical fibre installation all over the world.

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