The attached narrated power point presentation attempts to explain the methods for measurement of length of Optical Fibers. The material will be useful for KTU final year students who prepare for the subject EC 405, Optical Communications.

1. 1
Measurement of Optical Fiber
Length
MEC

2. 2
Contents
• OTDR Based Measurement.
• OFDR Based Measurement.
• Fresnel Reflection Method.
• Time of Flight Technique.
• Other Methods of Measurement.

3. 3
OTDR
• Used in both laboratory and the field.
• Backscatter measurement method.
• Backscatter method first described by
Barnoski and Jensen.
• Nondestructive - no cutting back of the
fiber.
• Require access to one end of the optical
link only.

4. 4
Optical Time Domain Reflectometer
The Optical Radar
To measure:
Attenuation
Length
Connector Loss
Splice Loss
Reflectance
Level

5. 5
OTDR Building Blocks
• Optical Source and Receiver.
• Data Acquisition Module.
• Central Processing Unit.
• Information Storage Unit.
- Internal Memory.
- External Disk.
• Display.

6. 6
Backscatter Measurement

7. 7
Backscatter Measurement
• For location-dependent attenuation values.
• Provides overall picture of optical loss
down the link.
• Light pulse forwarded 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 contains noise.

8. 8
Backscatter Measurement
• Received optical signal power at a very low
level compared with the forward power at
that point.
• 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.

9. 9
OTDR
• Software to enable fast manipulation of the
measured data.
• Instant calculation of optical power link
budget.
• Generation of comprehensive reports.
• OTDR traces to determine ORL in an
optical fiber network.

10. 10
Backscatter Plot

11. 12
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.
↓
↓
↓

12. 13
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



13. 14
Locating Fiber Fault
• For a time difference of t, Fiber Length:
c- light velocity, n1- core refractive index.
Light travels a length L from source to the
break point and returns, hence a factor 2
included.
• Overall link length found from the time
difference between reflections from the
fiber input and output end faces.
12
ct
L
n


14. 15
Fiber Preparation
• Observe a proper fiber trace on the OTDR.
• Cut a short piece of fiber (1 to 2 cm) from
the far end of the cable.
• Cleave its end to produce a smooth,
perpendicular end on the fiber in the cable.
• Set OTDR to “real time” testing. The trace
should have a square peak at the far end
of the fiber.

15. 16
Fiber Preparation
• Place OTDR “cursor 2” at the beginning of
the peak at the end of the fiber, ensure that
it is on the linear portion of the trace.
• Connect the fiber under test to the lead-in
fiber using a mechanical connector or
fusion splicer.
• A trace will appear on the OTDR. Set
“cursor 1” to the splice between lead-in fiber
and fiber under test. It shall be positioned at
the upper point where the trace begins to
drop from the temporary connection loss.

16. 17
Fiber Preparation
• If the connection is reflective, position
“cursor 1” at the upper, left-hand portion of
the trace that rises from reflective
connection.
• Set “cursor 2” at the end of fiber under test.
• Since the fiber end is reflective “cursor 2”
shall be positioned at the last point on the
linear lead-in to the reflective spike at the
end of the fiber trace.
• Difference between two cursor positions is
the length of fiber in the cable under test.

17. 18
Fiber Length Measurement
Difference between the two cursor positions = length of
fiber in the cable under test.

18. 19
OTDR Measurement
• Measures fiber length using the principles
of Rayleigh backscattering.
• OTDR roughly show Fresnel reflections
due to large event dead zones (EDZ).
• Output signal pulse width and repetition to
be optimized to measure fiber length using
Rayleigh backscattering.

19. 20
Incoherent FMCW OFDR
• Modulating RF signal swept in frequency.
• Detected probe signal mixed with modulating
RF signal in the electrical domain.
• Resulting output contains mixing products,
observed using electrical spectrum analyzer.
• Frequency axis represents delay times
experienced by the probe signal.
• Knowing the speed of light within the fiber,
time axis converted into physical distance.

20. 21
Incoherent FMCW OFDR

21. 22
Length Measurement through
Fresnel Reflection
• Measures differential time between two
reflected pulses from the reference fiber
and the test fiber ends.
• Fresnel reflection intensities stronger than
Rayleigh backscattering.
• No event dead zone effect.
• Measurement system can be lightweight,
small and portable handheld.

22. 23
Length Measurement through
Fresnel Reflection
• Minimum and maximum measurement
lengths depend on source pulse width and
pulse repetition rate.
• Typical laser pulse width of about 50 ns,
repetition rate of 10 kHz at 1,550 nm
injected into the first port of the optical
circulator.
• Second port of the 3 port optical circulator
spliced with the reference fiber.

23. 24
Length Measurement through
Fresnel Reflection
• Reference and test fibers terminated with
FC/PC* connectors, strong Fresnel
reflections help measure test fiber length.
• Test fiber connected to the reference fiber
output with an FC/PC adapter.
• Test fiber length information from the
differential time delay for Fresnel reflected
pulses in both ends by a photodetector with
5 GHz bandwidth connected at the third
port of the optical circulator.

24. 25
Length Measurement through
Fresnel Reflection
• Reflected pulses measured in real-time
using a digital oscilloscope and processed
with a computer.
• Light slower in optical fiber than vacuum,
optical fiber refractive index n>1.
• Effective group index of refraction (neff)
- weighted average for all indices of
refraction, encountered by the light as it
travels within the fiber.

25. 26
Length Measurement through
Fresnel Reflection
50 ns
pulses,
1550 nm,
10 KHz
repetition
rate

26. 27
Length Measurement through
Fresnel Reflection
• Speed of light in a single mode fiber at
1,550 nm wavelength:
• Length of test fiber (L) by measuring the
time delay of reflected signals (Δt) from
the reference and the test fiber ends.
• 2L – light travels twice through the fiber.

27. 28
Time of Flight Technique
• To measure accurately fiber lengths of 1–
40 km at 1,550 and 1,310 nm.
• High-speed electro-optic modulator
(EOM), photodetector and high-resolution
time interval counter (TIC) used for TOF
measurement.
• TIC locked to Global Positioning System
(GPS)-disciplined quartz oscillator.

28. 29
Time of Flight Technique
FUT- fiber under test; EOM - electro-optic modulator;
C1 and C2 - connectors; TIC - time interval counter;
PD - photodetector; PG - pulse generator;
Q-GPS - GPS-disciplined quartz oscillator.

29. 30
Time of Flight System
• Consists of two distributed feedback lasers,
electro-optic intensity modulator (EOM),
oscilloscope, pulse generator, digital delay
generator, a photodetector and TIC.
• Pulse generator to send electrical pulses to
the EOM, which uses the pulses to
modulate laser light.
• Modulated laser pulses sent through fiber
under test (FUT) to a fast photodetector.

30. 31
Time of Flight System
• Photodetector converts fast laser pulses to
fast electrical pulses.
• Time delay between pulses sent through
the fiber and reference pulses from pulse
generator measured using TIC.
• FUT then removed, C1, C2 connected and
insertion delay of the setup measured.
• Time delay in the fiber calculated by
subtracting insertion delay from the total
delay, fiber length calculated.

31. 32
Time of Flight Technique
• Requires fast rise-time pulse generator,
fast photodetector and accurate time
interval counter.
• Time taken by a laser pulse to pass
through the fiber measured.
• Fiber Length
co - speed of light in vacuum, τ - TOF,
n -group refractive index of the fiber.
0
F
c
L
n



32. 33
Recirculating Loop Length
Measurement using TOF Technique
OTDR Trace produced by RDL
Recirculating Delay Line (RDL) Loop Length

33. 34
OTDR Trace Features
• First feature obtained from the optical pulse
travelling direct to the mirror through the
lead-in fiber.
• Second feature generated by optical pulse
travelling once through the loop, then to the
mirror, and then back direct to the OTDR.
• Third pulse travels through the loop twice,
etc.
• First pulse position indicates length of the
lead-in fiber.
• Difference between the first and the second
pulses indicates loop length.

34. 35
Setup to Measure Loop Length

35. 36
Recirculating Loop Length
Measurement
• Length of the lead-in fiber measured.
• Output of the photodiode connected with
equal cable lengths to both channels of
the TIC.
• TIC to directly give TOF of the pulses
inside the loop only.
• No need to measure insertion delay of the
setup separately.

36. 37
Other Methods of Fiber Length
Measurement
• Methods based on frequency-shifted
asymmetric Sagnac interferometer, mode
locking technique apart from TOF
technique.
• First two methods reported to have better
accuracy than the third method.
• With high-speed optoelectronics, third
method to reach similar accuracies with
the ease of use and ability to measure the
length of recirculating loop fibers.

37. 38
Thank You