The attached narrated power point presentation attempts to explain the working principle, applications and drawbacks of an optical time domain reflectometer. The material will be useful for KTU final year B Tech students who prepare for the subject EC 405, Optical Communications.

1. 1
Optical Time Domain
Reflectometry
MEC

2. 2
Contents
• Introduction.
• Backscatter Measurement.
• Optical Power and Power Ratio.
• Backscatter Plot.
• Dead Zone.
• Polarisation Noise.
• Polarisation OTDR.
• OTDR Drawbacks.

3. 3
Optical Fiber Fault Detection
Two Phases:
• Fault Detection:
- Locate the place where optical fiber cable
(OFC) has been cut.
- Optical Time Domain Reflectometer (OTDR)
used.
• Fault Rectification:
- Make an OFC joint with minimal spice loss.
- Fusion Splicer used.

4. 4
OTDR
• Wide application in both laboratory and the
field.
• Called backscatter measurement method.
• Backscattering method first described by
Barnoski and Jensen.
• Nondestructive - does not require the
cutting back of the fiber.
• Require access to one end of the optical
link only.

5. 5
Optical Time Domain Reflectometer
The Optical Radar
Measures:
Attenuation
Length
Connector Loss
Splice Loss
Reflectance
Level

6. 6
OTDR Building Blocks
• Optical Source and Receiver.
• Data Acquisition Module.
• Central Processing Unit.
• Information Storage Unit.
- Internal Memory.
- External Disk.
• Display.

7. 7
OTDR
• Measurement of attenuation on an optical
link down its entire length.
• Gives information on length dependence of
the link loss.
• When attenuation on the link varies with
length, averaged loss information is
inadequate.
• OTDR measures splice & connector losses
and the rotation of any faults on the link.

8. 8
OTDR
• Measurement and analysis of the fraction
of light reflected back within the numerical
aperture due to Rayleigh scattering.
• Possible to determine backscattered
optical power from a point along the link
length in relation to the forward optical
power at that point.

9. 9
Backscatter Measurement

10. 10
Backscatter Measurement
• For location-dependent attenuation values.
• Gives overall picture of optical loss down
the link.
• Light pulse launched in the forward
direction from an injection laser.
• Use of directional coupler or a system of
external lenses with a beam splitter.
• Backscattered light detected using an
avalanche photodiode receiver.
• Received signal swamped with noise.

11. 11
Backscatter Measurement
• Received optical signal power at a very low
level compared with the forward power at
that point by some 45 to 60 dB.
• Integrator averages over a number of
measurements, improves received SNR.
• Integrator output fed through a logarithmic
amplifier.
• Averaged measurements for successive
points within the fiber plotted on a chart
recorder.

12. 12
Backscattered Optical Power
• Backscattered optical power as a function
of time:
Pi - optical power launched into the fiber, S -
fraction of captured optical power, γR -
Rayleigh scattering coefficient (backscatter
loss per unit length), Wo - input optical pulse
width, vg - group velocity in the fiber, γ –
fiber attenuation coefficient per unit length.

13. 13
Captured Optical Power
• Fraction of captured optical power is the ratio
of the solid acceptance angle for the fiber to
the total solid angle.
• For step index fibers, fraction of captured
optical power:
• S for a graded index fiber a factor of 2/3
lower than for a step index fiber with the
same numerical aperture.

14. 14
Power Ratio
• Power Ratio
• Substitute for S
• Group velocity
• Hence,

15. 15
Backscatter Plot

16. 16
Backscatter Plot
• Initial pulse caused
by reflection and
backscatter from
the input coupler.
• Long tail due to
distributed
Rayleigh
scattering from the
input pulse as it
travels down the
link.
↓

17. 17
Backscatter Plot
• Pulse due to
discrete reflection
from a fiber joint.
• A discontinuity due
to excessive loss at
a fiber imperfection
or fault.
• Pulse due to
Fresnel reflection
incurred at the
output end face.
↓
↓
↓

18. 18
Back Scatter Plot
• Fresnel Reflection and Rayleigh Scattering
produce backscatter plot.
• Fresnel Reflection when light enters a
medium having a different refractive index.
• Reflected Power:
P0 – incident power, nfiber & nair – refractive
indices of fiber and air.
• Perfect fiber reflects about 4% of incident
power.
2
0P ( )
n nfiber air
n nfiber airref P



19. 19
Backscatter Plot
• Attenuation per unit length for the fiber by
computing the slope of the curve over the
length required.
• Location and insertion losses of joints
and/or faults obtained from the power drop
at the respective positions on the link.
• Overall link length determined from the
time difference between reflections from
the fiber input and output end faces.

20. 20
Backscatter Plot
• Measurement of splice or connector loss.
• Measurement of splice or connector return
loss.
• OTDR measurement based on operating
parameters (i.e. wavelength or band, data
transmission rate etc.).
• Dynamic range - total optical loss that
optical time domain reflectometer can
analyze.

21. 21
Locating Fiber Fault
• For a time difference of t, Fiber Length:
n1- core refractive index.
Light travels a length L from source to the
break point and then returns, hence a
factor 2 is included.
12
ct
L
n


22. 22
OTDR
• Software to enable fast manipulation of the
measured data.
• Instant calculation of optical power link
budget.
• Generation of comprehensive reports.
• OTDR traces used to determine ORL in an
optical fiber network.

23. 23
OTDR Performance Parameters
Dynamic Range:
• difference between initial backscattered power
level at the front end connector and the noise
level after around 3 minutes of measurement
time.
• expressed in dB of one way fiber loss.
• provides information on maximum fiber loss
that can be measured.
• denotes time required to measure a given fiber
loss.
• ranks OTDR capabilities.

24. 24
OTDR Performance Parameters
Measurement Range:
• maximum allowable attenuation between
OTDR and the event that still enables accurate
measurement.
• capability of identifying events such as splice
points, connection points or fiber breaks.
Tradeoff between dynamic range and
resolution.
• Small pulse width for high spatial resolution.
• Small pulse width reduces SNR, lowers dynamic
range.

25. 25
Dead Zone
• Strong optical reflection from a reflective
event reaches OTDR, saturates detection
circuit for specific time period (converted
to distance in the instrument) till recovery,
continues to measure backscattering
accurately again.
• Certain portion of fiber link following the
reflective event cannot be displayed by the
instrument.

26. 26
Polarisation Noise
• Exhibited by single-mode fiber optical time
domain reflectometers.
• State of polarization of backscattered light
dependent on distance of backscattering
fiber element from the input fiber end.
• Amplitude fluctuation in the backscattered
light.
• Employed to measure the evolution of
polarization (Polarisation OTDR).

27. 27
Polarisation Noise
• Reduction of polarization noise necessary
in single-mode OTDR instrument.
• Reduction using polarization-independent
acousto-optic deflector or a polarization
scrambler.
• POTDR for measurement of PMD on a
fiber link.
• POTDR to ensure uniform distribution of
PMD along the fiber during manufacturing.

28. 28
POTDR
• POTDR comprises a conventional OTDR
instrument, polarization controller,
polarization analyzer and pulsed laser.
• Employs a narrow band external cavity
laser or DFB laser (i.e. with pulse width ≤ 10
ns) for high spatial resolution with respect to
the frequency-dependent distortions due to
PMD.
• POTDR to monitor levels of PMD when
fabricating dispersion-compensating fibers.

29. 29
POTDR
Output signal power characteristic
Block schematic
POTDR can identify the levels of PMD on the fiber link
Measured on NZ-DSF
High PMD (solid circles)
Medium PMD (solid boxes)
Normal PMD (asterisks)

30. 30
POTDR
• Fiber under test connected to POTDR using
an optical circulator or fiber coupler.
• Pulses sent through optical circulator towards
fiber under test.
• Instrument performs measurements from the
state of polarization (SOP) of backscattered
field.
• Polarization controller & polarization analyzer
to determine SOP and degree of polarization
(DOP) for optical signals.
• Polarization analyzer provides data to plot
OTDR traces.

31. 31
Some OTDR Drawbacks
• Time Consuming.
- fault shows variation with increasing
distance.
- time to localise the place of actual fault.
• System Inefficiency.
- one fault may lead to escape of optical
signal.
- first fault to be rectified before other
faults can be detected.

32. 32
Some OTDR Drawbacks
• Labour Cost.
- trained manpower required.
• Fading of accuracy and precision.
- variation in measured distance with
increasing length.
- uncertainty in fault localisation.

33. 33
Thank You