OPTICAL TIME DOMAIN REFLECTOMETER

1. OTDR

2. • An optical time-domain reflectometer, or OTDR, is an
instrument used to test optical fibers.
• it is one of the most versatile and widely used fibre optic test
equipment, which is commonly used by technicians or
installers to certify the performance of new fibre optic links and
detects the issues of existing fiber links.
• It can be used to test for attenuation and overall fiber length,
as well as damage to splice and mating connectors.
• OTDR helps detect defects in optical fibers and determine
losses from optical return, which is light that is scattered or
reflected from various points in the fiber line.

5. • An OTDR contains a laser diode source, a photodiode
detector and a highly accurate timing circuit (or time base).
• The laser emits a pulse of light at a specific wavelength,
this pulse of light travels along the fiber being tested, as
the pulse moves down the fiber portions of the transmitted
light are reflected/refracted or scattered back down the
fiber to the photo detector in the OTDR.
• The intensity of this returning light and the time taken for
it to arrive back at the detector tells us the loss value
(insertion and reflection), type and location of an event in
the fiber link.

9. OTDR pulse width determines length of fiber that can be
measured before OTDR traces become noise.

10. • Setting the adjustable pulse width determines the duration of
the pulse being emitted into the fiber link.
• A shorter pulse width is usually selected for shorter cable
lengths,
• Longer pulse width settings may be called for when testing a
longer cable run, since more optical energy is required to
produce sufficient backscatter at great distances from the
OTDR.
• When the OTDR detector becomes saturated by a highly
reflective interface in the fiber link, the recovery period for the
OTDR translates to a distance from the event, known as a
dead zone, which is essentially a portion of the cable for which
no data will be available. Air gaps, bad splices, flat fiber end
faces (connectors or the fiber end) and other incidences
producing high Fresnel reflection are the usual causes of dead
zones.

12. • The distance range setting on an OTDR controls the display range
for the amount of cable to be presented on the screen. It also defines
the rate of pulse emission, since each pulse must be returned to the
detector before the next pulse is sent out.
For example, if the instrument has settings of 10, 100, 200 and 500
kilometers, and your actual fiber link is 150 kilometers, you would
select the 200 kilometer setting.
In general, more accurate measurements are usually produced by
averaging multiple repetitions of the same test. This same principle
holds true with OTDR measurements. Longer averaging times,
translating to more repetitions of the same test, will produce a
measurement with an improved signal-to-noise ratio, but take longer
to capture. For conditions where accuracy and noise are less critical,
a “real-time measurement”, with no averaging function, could be
sufficient. However, for circumstances where distance and loss data
must be as precise as possible, longer averaging times might be
justified.