The document discusses the optical time domain reflectometer (OTDR), a fiber optic tester used to characterize optical fibers. It works by sending light pulses into the fiber and measuring the backscattered light. Changes in the refractive index cause Rayleigh scattering and Fresnel reflections, which the OTDR detects to measure losses and locate faults. It can determine distances along the fiber based on the time delay of reflected pulses. The OTDR is useful for measuring fiber length, attenuation coefficient, and locating faults like cuts.

1. OPTICAL TIME DOMAIN
REFLECTOMETER(OTDR)
A Technical seminar
By
P.V.S.K.Bharadwaj
11131A04C7
ECE-3
GVPCOE(A)

2. Optical Time domain Reflectometer

3. What is an OTDR?
• It is a fiber optic tester
• It allows complete characterization of the fiber
• It provides information regarding local losses and reflective
elements
• It is mainly used to find the place of fault in an optical fiber

4. Principle of Operation
• OTDR uses the effects of Rayleigh scattering and Fresnel
reflection
• RAYLEIGH SCATTERING occurs when the light travelling down
the fiber encounters small material variations and
discontinuities in the refractive index
• The light is scattered in all directions
• FRESNEL REFLECTIONS occur when the light encounters
abrupt variations in the material properties that are caused by
a break or air gaps or Connectors
• Fresnel reflection is 10,000 times greater than Rayleigh
scattering in terms of reflected power

5. Back Scatter
• As the light is scattered in all directions some of it happens to
return back along the fiber to the light source
• The returned light is known as BLACK SCATTER
• The OTDR sends short light pulses and continuously measure
the returned power level and hence deduces the losses
involved

6. Measuring Distance

7. Measuring Distance
(Contd..)
• Consider the Refractive index of the core n1=1.5
• Then the speed of light in the core=V=c/n1=2x10^8 m/s
• If the Reflected Light reaches the OTDR 1.4us later
• Since the Light has travelled back and forth along the length
of the fiber(L)
• 2L=Vx delay time
• 2L=2x10^8 x1.4us=280m
• Hence L=140m
• Hence the OTDR uses the principle of RADAR .It sends a
optical pulse and then listens to the ECHO

8. Block diagram of OTDR

9. Display
• It can be either a CRT or an LCD
• It displays the returned signal in a XY plot where the Range
across X axis and power level in dB along Y axis

10. Display when there is no signal

11. Simple Measurement
• 34
q3
4

12. Ringing at Amplifier
• The Fresnel reflection at the launch connecter causes a high
energy at the receiver amplifier which swings above and
below real levels
• This phenomenon is known as Ringing
• The Receiver takes a few microseconds to return back to the
normal state
• During which the OTDR cannot determine any reflections .
Hence it is known as Dead Zone
• We can avoid the dead zone by adding a patch chord of
100m which finishes the problem of dead zone before the
fiber under the test is reached

13. DEAD ZONE

14. Length and attenuation

15. Attenuation coefficient Measurement
• Length appears to be 400m
• 100 meters for patch chord to deal with dead zone
• Hence length=300m=0.3km
• Power level at 100m (at the end of patch chord)=-10.8dB
• Power level at 400m (at the end)=-11.3dB
• Attenuation coefficient=ΔP/L=0.5/0.3=1.66dB/km

16. OTDR Display of a Typical System

17. Fault localization
• In case of fiber cut due to various reasons like rodents,
road repairs etc the position of the cut should be
identified
• Since optical fibers are sent underground it is difficult
with manually digging all through and finding the cut
• So we proceed with OTDR and find if the length of the
fiber obtained from test is same as that in the records
and to check if there is a cut and its location

18. Thank You