OPTICAL TIME DOMAIN
REFLECTOMETRY (OTDR)
Dr. BC Choudhary
Professor, NITTTR, Chandigarh
OTDR Models
WHAT IS AN OTDR?
 A measurement
technique which provides the loss
characteristics of an optical link down its entire length
giving information on the length dependence loss.
• Also allows splice and connector losses to be evaluated as
well as location of any faults on the link.
• Also called backscatter measurement method.
• It relies upon the measurement & analysis of the fraction of
light which is reflected back within the fiber’s numerical
aperture due to Rayleigh scattering within the fiber.
• A small proportion of the scattered power is collected by the
fiber in backward direction and returns to the transmitter,
where it is measured by a photodiode.
Back Scattering of Light
Incident light wavelength
intensity
Rayleigh
scattering
Brillouin scattering
Raman scattering
(Anti-stokes)
Raman scattering
(stokes)
Wavelength
Depending on temperature
Depending on strain and temperature
• In operation, an OTDR launches pulses of light into the line
fiber of an optical network and monitors the backscatter signal
as a function of time relative to the launch time.
• As the pulse propagates down the fiber it becomes weaker with
increasing distance due to power loss, and the measured
backscatter signal decreases accordingly.
• The rate of signal decrease for a continuous section of fiber
represents the fiber loss and any abrupt drops correspond to
losses from the presence of components, terminations or faults
which can be readily identified.

OTDR - the Industry standard for measuring the loss
characteristics of a link or network, monitoring the network
status and locating faults and degrading components.
Principle of OTDR
Incident light (Pulse)
Laser
Transmitted light
Detector
Fiber core
scattering light
Back scattering
light
z
Distance
z = t.V /2
t : two-way propagation delay time
V : velocity of light in the fiber

The received backscattered optical power as a function of
time ‘t’ down an uninterrupted fiber is given by:
1
PRa ( t )  Pi .S. R .w 0 .v g . exp(  v g t )
2
where;
Pi - optical power launched into the fiber.
S - fraction of captured optical power.
R - Rayleigh scattering coefficient.
wo - input optical pulse width.
vg - group velocity in fiber.
 - attenuation coefficient per unit length for the fiber.
 Generally, OTDR output is expressed in dB relative to the
launched power, and the directly measured loss is then halved
electronically before plotting the output trace.
OTDR Display
OTDR Testing to locate a fault point
Events in OTDR Traces
 In
addition to decaying signal associated with the
fiber losses;
• Abrupt drops in the backscatter signal on the trace:
Losses due to the presence of non reflective elements such as
fused coupler components, tight bends or splices.
• Presence of large return pulses - arise from Fresnel
reflections- followed by a drop in the background signal:
Fiber interruptions at connectors, non-fiber components,
termination or breaks.
 Such readily identified features on the OTDR signal  Events
Their location and loss associated with them may be
obtained directly from the trace.
Dead Zones and Ghosts
 Large Fresnel reflection signals can cause problems for the
detection system- they lead to transient but strong saturation of
the front end receiver which requires time to recover.
 Usually detection of strong Fresnel reflected pulses from the fiber
interruptions or termination drive the receiver into deep saturation.
The length of the fiber masked in terms of event detection by
this way is known as a Dead Zone
The length of which is determined by the pulse width and, for
reflection events, by the amplitude of the reflected pulse.
 Dead zones arising from large Fresnel reflection signals from
the fiber input and output(s) – near and far end dead zones
respectively.
Full OTDR trace of fiber reel at 1550 nm.
• Fiber attenuation = 0.185 dB/km
• Fiber length = 2.014 km
Expanded trace to show near end dead zone (NEDZ).
 Fluctuations in the trace after the NEDZ are due to the detection
electronics (receiver).

Strong Fresnel reflections can give rise to dead zones of the
order of hundreds of meters corresponding to detector recovery
periods of many tens of receiver time constant, whereas splices
result in dead zones of only a few tens of meters.
 Many OTDRs incorporate a dead zone masking feature, which
can be set up, to selectively attenuate large incoming reflected
signal pulses just for the period over which they arrive
 prevent deep saturation and hence minimum dead zone.
 Near end
dead zone and event dead zones present greater
problems in shorter networks.
 Large Fresnel reflected pulses from the terminations of shorter
branches of a network are reflected again from the fiber input face
to repeat the journey to the termination and back to the detector.
 If the signal strength is sufficiently high, these multiply reflected
pulses will be detected and will appear on the OTDR trace as what
are referred to as Ghosts.
 Ghosts are
much smaller than the detected pulses from primary Fresnel
reflections and appear at exact integer multiple distances relative to a
primary reflected pulse.
Trace with a ghost after Far end dead zone

Ghost signal is displayed at twice the fiber length.
 Can be confusing if appear within the maximum length of a
network, mostly appear in the noise region.
Measurement Resolution & Event Location

Spatial Resolution: One of the key performance features of an
OTDR – depends upon receiver BW and input pulse width
 Minimum separation at
which two events can be distinguished as
determined by the pulse width.
• For good resolution of 10m or less, require a 50ns pulse.
 Length of the fiber; L = vg.t ; vg = c/ne (ne- effective fiber index)
• Typical, ‘ne’ for fibers ranges from 1.45-1.47
• With this data, pulse travel approximately 1m in 5ns means that a pulse
width of 5ns in time has a spatial width of 1m.
• By definition, two events may be distinguished if they are
separated by ½ of the spatial pulse width – Spatial resolution of
the instrument- is defined as half the pulse width in time.
For pulse width of 50ns, spatial resolution is 5m.

Time Resolution: The precision to which a feature can be located
depends on the precision with which the OTDR can measure the
arrival time of an event and the accuracy to which the propagation
velocity is known.
 Uncertainty in
the measurement of arrival time can be very much less than
the pulse width and hence event location in principle may be achieved with
a precision which is much better than the spatial resolution.
• Timing resolution of the instrument is simply the sample period
used in the signal averaging scheme under a given set of
circumstances, and this corresponds to a spatial sample in the
fiber which can be taken as the precision to which event distances
can be measured.
• The sampling period and its corresponding sample length in space
(range resolution) vary enormously depending on the distance
range addressed and the number of samples used.
 Typically, however, time resolution is much less than the
spatial resolution.
• For example, if the spatial resolution is 10m (for a 50ns pulse)
on a range setting of 4km, the event distance (range) resolution
may be 2m corresponding to 2048 samples being used to record
the return signal from the 4km path.
• With the home-in feature on some instruments we can examine
a 1km section of the 4km range using 4096 samples to achieve a
range resolution of 0.25m for a spatial resolution of 5m.
Dynamic Range, Range & Range/Resolution
trade-off
 Obviously, the greater the energy of the launched pulses, the
better the SNR will be before and after averaging.
• The peak power from the laser is limited, but the pulse width
may be increased to deliver greater energy into the fiber at the
expense of resolution.
• With wide, high energy pulses and the levels of averaging, the
peak SNR at the beginning of a trace, referred to as the signal
dynamic range of the instrument, can be as high as 35dB.
 Range – maximum length of fiber which can be measured- is
determined by the signal dynamic range and is the fiber length
at which the signal has decayed to become equal to the noise.
Example:
In a system based on the fiber with an attenuation coefficient of 0.2
dB/km and in which the total loss of the components, splices and
connectors is 10dB,
• A good OTDR with a 35dB dynamic range can probe to range
of 125 km.
 Point to remember: As we decrease the pulse width (energy) to
improve the spatial resolution, the dynamic range will also be
reduced and the range of the instrument will be diminished;
 There is a range/resolution trade-off to be considered using
these instruments.
Technical Specifications of an OTDR
OTDR Trace Analysis of Networks

OTDR traces are analyzed to provide information about the loss
of a network & whether or not the loss is increasing with time
due to introduction of faults (sharp bend, breaks) or the
degradation of components (splices, couplers, WDMs etc.).
Schematic of a network, spices are shown as dots.
Testing port 1: Fibers with a single splice.
OTDR trace of fiber link with one splice
Testing of a Coupler spliced to fiber
OTDR measurement of coupler losses
OTDR trace of single splice and coupler
Testing Three component network
(WDM, 2 couplers)
Events
Near end
OTDR trace of 3 component network
Distance
0
WDM
265
Coupler 1
443
Short arm
end
587
Coupler 2
804
Short arm
end
941
Far end
1089
Fault Diagnosis with OTDR
 Link Loss Measurements : If loss is higher than its limit, then
OTDR testing is required to check the link health
 OTDR Testing : It will give graphical presentation with loss
table of each splice
 Bi-directional Testing : We have to take OTDR traces from
both ends
Distance Based Analysis
 Distance between A and B is 10 km.
 OTDR distance up to cut point C from A is 6
A
C
B
6.001 Km
OTDR distance from point B to check, if it is 4 km  a single
cut.
If OTDR distance from point B is less than 4 km  a possibility
of multi cut.
Distributed Sensing
Measurand field M(z,t)
OTDR
M(t)
M(z,t)
z
Fiber
Quasi-Distributed Sensing
Measurand field M(z,t)
OTDR
Fiber
M(t)
M(zj,t)
Sensitized regions
z
• FBG （Fiber Bragg Grating)
• Strain, Temperature
Optical Network Analysis with OTDR
If you have any query, feel free to contact :
Dr. BC Choudhary, Professor
NITTTR, Sector-26, Chandigarh-160019.
Phones: : 0172- 2791349, 2791351 Ext. 356 , Cell: 09417521382
E-mail: < bakhshish@yahoo.com>