OPTICAL TIME DOMAIN REFLECTOMETER(OTDR)

A Technical seminar

By

P.V.S.K.Bharadwaj

11131A04C7

ECE-3

GVPCOE(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• 3

4q34

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