otdr.ppt

OPTICAL TIME DOMAIN REFLECTOMETER

Lt Cdr Harshavardhan

2011 CRF 3631

CONTENT

• Basic Introduction

• Reflection is the key

• Block diagram

• Evaluation of few traces

• Factors affecting OTDR trace

• Cost analysis

April 17, 2023 Optical Time Domain Reflectometer 2

Optical Time Domain Reflectometer (OTDR)

• A device for characterizing a fiber optic cable • An optical pulse is transmitted through the fiber and the resulting

backscatter and reflections to the input are measured as a function of time

• Useful in estimating attenuation as a function of distance

• Identifying defects and other localized losses

• Check the quality of fiber optic links by measuring backscatter

April 17, 2023 3Optical Time Domain Reflectometer

FRESNEL’S REFLECTION• Light moves from a medium of a refractive index (n1 )into a second medium with

refractive index (n2)

• Both reflection and refraction of the light may occur. • The Fresnel equations describe what fraction of the light is reflected and what

fraction is refracted

n1 sin1 n2 sin2April 17, 2023 4Optical Time Domain Reflectometer

Source-wikipedia

RAYLEIGH BACKSCATTERING

• Result of the elastic collisions between the light wave and the silica molecules in the fiber.

• Accounts for about 96 percent of attenuation in optical fiber

Light scattered in all directions after interaction with silica

some portion of the light propagates forward

deviates out of the propagation path and escapes from the fiber core

reflected back toward the light source


REFLECTION IS THE KEY• Rayleigh backscattering

– used to calculate the level of attenuation in the fiber as a function of distance – Expressed in dB/km– shown by a straight slope in an OTDR trace– Higher wavelengths are less attenuated than shorter ones and thus require less power

to travel over the same distance in a standard fiber

• Fresnel’s reflection– detects physical events along the link. – When the light hits an abrupt change in index of refraction (e.g., from glass to air) a

higher amount of light is reflected back, creating Fresnel reflection– thousands of times bigger than the Rayleigh backscattering. – Fresnel reflection is identifiable by the spikes in an OTDR trace – Reflections from connectors, mechanical splices, fiber breaks


HOW OTDR WORKS

• An OTDR calculates the distances to Events by measuring the time elapsed between transmission of the light and reception of the reflection

• The rising edge of the reflection of the front panel connector and the reflection from a connector/splice/cracks

• The distance and the time measured depends upon the the refractive index of the fiber


BLOCK DIAGRAM


Adapted from http://www.EXFO.com/application note 194

EVENTS• Anything that causes loss or reflections other than normal scattering of

the fiber material

• Applies to all kind of connectors as well as damages such as bendings, cracks or breaks.

The vertical axis is the power axis and the horizontal one is the distance axis


Source- OTDR pocket book by Agilent Technologies

BEGINNING OF A FIBER

• For a normal straight connector, the beginning of a fiber always shows a strong reflection at the front connector



FIBER END

• A strong reflection at the end of the fiber before the trace drops down to noise level



FIBER BREAK

• If the fiber is interrupted or broken, this is called a break

• Breaks are non-reflective Events. The trace drops down to noise level



CONNECTOR OR MECHANICAL SPLICE• Connectors within a link cause both reflection and loss

• A mechanical splice has a similar signature to a connector. Usually it has lower loss and reflection values



FUSION SPLICE• A fusion splice is a non-reflective Event, only loss can be detected.

• Modern fusion splices are so good, they may be nearly invisible

• In the case of a bad splice, we may see some reflection



CRACKS

• A crack refers to as a partially damaged fiber that causes reflection and loss



PATCHCORDS

• Patch cords are used to connect the OTDR to the fiber under test

• The initial reflection is not covering the beginning of the fiber

• This allows better examination of the first connector

April 17, 2023 16Optical Time Domain ReflectometerSource- OTDR pocket book by Agilent Technologies

FACTORS AFFECTING OTDR TRACE

• Dead zone

• Pulse width

• Dynamic range


DEAD ZONE

• The time during which the detector is temporary blinded by a high amount of reflected light, until it recovers and can read light again

– Assume yourself driving on a highway during night

• More reflection causes the detector to take more time to recover, resulting in a longer dead zone

• Determines how close together two events can be measured

• Dead zone increases as the pulse width increases


PULSE WIDTH• The time during which the laser is ON

• The shorter the pulse, the less energy it carries and the shorter the distance it travels due to the loss along the link (i.e., attenuation, connectors, splices, etc.).

• A long pulse carries much more energy for use in extremely long fibers


NOISY TRACE

• Increased averaging time– Results in a considerable improvement in SNR, while maintaining the good

resolution of the short pulse. – does not improve SNR indefinitely.

• Longer Pulse width– Use the next available higher pulse (more energy). – Dead zones extend along with the pulse width


DYNAMIC RANGE• Maximum optical loss an OTDR can be analyzed from the backscattering level

down to a specific noise level.

• It is the maximum length of fiber that the longest pulse can reach

• Bigger the dynamic range (in dB), the longer the distance reached. Typical values 20-50 dB

• Directly related to pulse width

• Specified for the longest pulse width at a three-minute averaging time with signal-to-noise ratio (SNR) = 1

• Improved by using longer pulse width and decreasing noise through averaging


COST ANALYSIS – M/s Fluke OTDRs

Measurement Type Model Cost

Multimode OTDR OF-500-M $9,265.00

Singlemode OTDR OF-500-S $10,295.00

Multimode plus Single Mode OTDR

OF-500-MS $14,415.00


THANK YOU


REFERENCES

• http://www.EXFO.com/application note 194 by Jimmy Gagnon, Product Specialist, Optical Business Unit

• OTDR pocket guide – Agilent Technologies• Wikipedia


TYPES OF FO CABLES

• Single-mode fibers – used to transmit one signal per fiber. They have small cores(9 microns in diameter) and transmit infra-red light from laser.

• Multi-mode fibers – used to transmit many signals per fiber (used in computer networks). They have larger cores(62.5 microns in diameter) and transmit infra-red light from LED.


FO CABLE

• Core – thin glass center of the fiber where light travels.• Cladding – outer optical material surrounding the core• Buffer Coating – plastic coating that protects the fiber.


TYPES OF CONNECTOR

TYPE FULL NAME COUPLING TYPE

SIZE USE

MT-RJ

Mechanical Transfer Registered Jack /Media Termination - recommended jack

Snap (duplex) 2.45×4.4 mm Duplex multimode

connections

SCSubscriber Connector /Square connector /Standard Connector

Snap (push-pull coupling) 2.5 mm

Datacom and telcom; extremely common

ST / BFOC

Straight Tip/Bayonet Fiber Optic Connector Bayonet 2.5 mm Multimode, rarely

single-mode;


INDEX OF REFRACTION (IOR)

• The Index of Refraction is a way of measuring the speed of light in a material.

• Light travels fastest in a vacuum.

• Index of Refraction is calculated by dividing the speed of light in a vacuum by the speed of light in some other medium (such as glass in the case of fiber optics!).


OTDR SETUP – IOR• Each different optical glass fiber has a different refractive index profile

consistent with it’s type and manufacture process.

• Typical G.652.B singlemode fiber from Draka has an index number of 1.467 @ 1310nm and 1.468 @ 1550nm

• The longer the wavelength, the faster the light travels through the core.

• The user must set the OTDR to the proper GIR (Group Index of Refraction).

• If the GIR is not set to the proper number, the OTDR may overestimate or underestimate linear cable footage.


otdr.ppt

Documents

optical pulse

fiber breaks

fiber materialapplies

fiber optic cable

otdr trace reflections

quality of fiber optic

distance axis

strong reflection