Basics for using an OTDR (launch coils)

In this fourth part, we continue the series of descriptive articles on the basic principles for using a OTDR. They are an excerpt from the book “Optical Fiber Communications. Guide of studies technicians ”provided by the author, Rodolfo Veloz Pérez, for publication in exclusive in our technical newspaper fiber

Read the Basics for using an OTDR (Part 1) and concepts Basic to use an OTDR (2nd part, pulse width) and Basics for using an OTDR (3rd part, dead zone).

Receiver linearity

Accuracy of attenuation or loss measurement
it is given by the "Linearity of the Receiver", and this depends on the following

  • Dynamic range
  • Dead zone of

The absolute calibration of the power levels of
Measured backscattering is not necessary in an OTDR. Measurements normally
they are displayed on a logarithmic scale with an arbitrary reference level that
this is usually the backscatter signal at the near end of the fiber,
after reflection of the front end. Therefore, the loss of a
fiber section, expressed in [dB], is simply equal to the difference between
the power values ​​at each end of the section. Of particular importance,
However, it is the linearity of the power measurements. The manufacturers of
instruments have invested a lot of time and effort in developing detectors and
High quality amplifiers that are very linear.

To quantify the linearity of an OTDR, you can use
a reference fiber, whose linearity has been calibrated by a laboratory
Independent. This is measured in the OTDR under test and the results are
They compare with the reference fiber. Another nonlinearity test consists of
a long length of uncalibrated fiber preceded by an attenuator
programmable. Here, the loss between two points given in the fiber, to certain
separation distance, is measured as the launch pulse is
attenuates progressively. Any deviation in the
loss represents a measure of the nonlinearity of the OTDR

Linearity may be affected by the nature of the
fiber bond and its components, so most manufacturers do not
They cite a linearity value. When used, the OTDR linearity is expressed
normally in units of [dB/dB].

Launch and Reception Coils

The launch and reception coil is a roll of
fiber optic cable connectorized at its ends that is used to
connect the OTDR to the fiber optic network under test. The roll,
it is usually a bare fiber optic filament, which given its extreme
Fineness can be wound between 100 meters to 2000 meters in a small box that
Contains the coil, adjusted to the size of the hand and fully transportable
for who performs the measurements. These must be of the necessary length, and with
connectors that match the optical port of the OTDR and fiber link
Optics under test.

One end of the OTDR launch coil
normally it is left connected to the OTDR and the other end moves
progressively along the connection panel as they are made
measurements in each fiber. Always, logically, performing the processes of
corresponding cleaning at each stage. At the other end of the network, you must
locate the receiving coil, in which no equipment is connected, since
The measurement with OTDR consists of a single instrument and an "open circuit". This
Reception coil is normally omitted due to ignorance, but it is
strictly necessary if we make measurements and we must characterize all the
network, to observe events including the first and last connector.

Basics for using an OTDR (4th part, launch coils)
Measures with an OTDR

For measurements with OTDR, the launch coils
and reception are essential and play a very important role in the
fiber optic measurements, since they allow the area to be removed from the measurement
dead at the beginning of the network, and Fresnel's reflection at the end of the network, of
such that it is possible to measure the total loss of the link, including the
First and last network connector. Therefore, care must be taken to
ensure that the correct coils are being used. The factors that
Affect the choice are as follows:

  • Fiber type
  • Kind of
  • Length of the
    launch / receive coil
  • Dead Zone of
  • Distribution
    modal required

The type of fiber is perhaps the most important factor for
Consider when choosing a launch / reception coil.

If the launch coil does not have the correct size,
the dynamic range is likely to be affected, and the measurements of
loss of fiber optic and connectors are not representative of the system
after commissioning

The type of connectors must be at one end
compatible with the OTDR connector. This could be, for example, a connector
SC or LC. The connector on the other end of the launch coil or
Reception, of course, is chosen to be compatible with the type of link
Fiber optic under test. Therefore, normally the professional who is dedicated
to measurements with OTDR has 2 coils of the same characteristics for
face a type of fiber optic and with specific connectors. So, you must
have pairs of coils to:

  1. Single Mode Fiber
  2. Multimode fiber
    1. OM1
    2. OM3
    3. OM4

And for different types of connectors too, those
You must connect to the network with your respective guide:

The combinations are not minor. Let's add now the
coil length, but this depends on the type of OTDR you have. This,
It depends on the ADZ and EDZ of the OTDR.

The length of the launch or reception coil is
choose so that the effects of receiver overload caused by the
Fresnel reflection on the OTDR end of the connection cable
Enough disappeared before measuring the fiber under test. Depending on
OTDR, this length can be a few meters or several meters today.

To define the length of the coil, you must set
Dynamic range and pulse width to see the full fiber and have
Enough backscatter. Then, consider that the propagation speed
of fiber optic light is approximately 0.2 [m/ns], which makes that
approximately 10 times the pulse width amplification is needed,
because enough backscatter is required to measure the loss by

Pulse width [ns] Launch Coil Length [m]



















If using a launch coil that is too long
It will reduce the resolution of the measurements.

View the table above, we can deduce that the length
it depends on the link being tested, since the pulse width is established
For the same effect. Usually used between 200 [m] to 500 [m] for
Multimode and 1000 network tests [m] to 2000 [m] for tests in single mode.

Another factor, which applies in the case of networks
multimode, is the amount of mixture of modes that is produced within it. In
In general, it is assumed that the modal volume of the fiber is quite full when
loss measurements are made using LSPM and that a mode filter is
effective to create a stable state distribution or EMD. The exact state of
filling so depends, in fact, on the length of the connection cable and the
way the connection cable is implemented. Conversely, a launch
with insufficient filling it can tend to approach EMD if there is any
mode coupling mechanism, such as curves. Here, we talk about the
modal distribution required.

Receiving Coil

The use of a receiving coil at the far end of
a fiber under test allows measuring the loss of a connector at the end
far. Usually, this is omitted, and the network measurement loses the value
of your actual measurement
. The length of the receiving coil must be more
long than the dead zone of attenuation of the OTDR. To not have differentiation
between launch and reception coil, two are usually acquired
coils in equal conditions, which allow to connect at both ends, and of
This way you can make bidirectional measurement without moving coils of your
physical location.

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