OPTICAL TIME DOMAIN

REFLECTOMETRY (OTDR)

Dr. BC Choudhary

Professor, NITTTR, Chandigarh

OTDR Models

WHAT IS AN OTDR?

• 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.

A measurement technique which provides the loss characteristics of an optical link down its entire length giving information on the length dependence loss.

Back Scattering of Light

Incident light wavelength

Rayleigh

scattering

Brillouin scattering

Raman scattering

(Anti-stokes) Raman scattering

(stokes)

Wavelength

intensity

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

Distance z = t.V /2 t : two-way propagation delay time

V : velocity of light in the fiber

Transmitted light

Incident light (Pulse)

scattering lightBack scattering

light

Laser

DetectorFiber core

z

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.

The received backscattered optical power as a function of

time ‘t’ down an uninterrupted fiber is given by:

)tvexp(.v.w..S.P2

1)t(P gg0RiRa

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.

Dead zones arising from large Fresnel reflection signals from

the fiber input and output(s) – near and far end dead zones

respectively.

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.

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.

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.

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.

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

• 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.

Obviously, the greater the energy of the launched pulses, the

better the SNR will be before and after averaging.

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.

OTDR trace of fiber link with one splice

Testing port 1: Fibers with a single splice.

OTDR measurement of coupler losses

Testing of a Coupler spliced to fiber

OTDR trace of single splice and coupler

Testing Three component network

(WDM, 2 couplers)

OTDR trace of 3 component network

Events Distance

Near end 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 BC

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

OTDR

Measurand field M(z,t)

M(z,t)

z

M(t)

Fiber

Quasi-Distributed Sensing

• FBG （Fiber Bragg Grating)

• Strain, Temperature

OTDR

Measurand field M(z,t)

M(zj,t)

z

M(t) Fiber

Sensitized regions

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>