OPTICAL TIME DOMAIN REFLECTOMETER(OTDR)

A Technical seminarBy

P.V.S.K.Bharadwaj11131A04C7

ECE-3GVPCOE(A)

Optical Time domain Reflectometer

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

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

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

Measuring Distance

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

Block diagram of OTDR

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

Display when there is no signal

Simple Measurement• 34q

34

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

DEAD ZONE

Length and attenuation

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=∆P/L=0.5/0.3=1.66dB/km

OTDR Display of a Typical System

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

Thank You