TEST

AND

MEASUREMENT

Loss- dB

Fundamental Of OTDR

Power, Laser Source Test

Link Loss Budget

Loss and it’s origin

Loss in optical power due to……..

Scattering

Absorption

Bending

Micro bending

Macro bending

Scattering, Primarily Rayleigh scattering, also contributes to attenuation. Scattering causes the light energy to be dispersed in all directions, with some of the light escaping the fiber core. A small portion of this light energy is returned down the core and is termed “backscattering”.

Scattering

Absorption may be defined as the conversion of light energy to heat, and is related to the resonance in the fiber material. There are intrinsic absorption (due to fiber material and molecular resonance) and extrinsic absorption (due to impurities such as OH- ions at around 1240 nm and 1390 nm).

Absorption

Bending losses which are caused by light escaping the core due to imperfections at the core/clad boundary (microbending), or the angle of incidence of the light energy at the core/cladding boundary exceeding the Numerical Aperture (internal angle of acceptance) of the fiber due to bending of the fiber (macrobending).Single mode fibers (for example) may be bent to a radius of 10 cm with no significant losses, however after the minimum bend radius is exceeded, losses increase exponentially with increasing radius. Minimum bend radius is dependent on fiber design and light wavelength.

Bending Loss

Example of different types of Loss

Loss = Pi -Po

Loss (dB) = 10*log10 (Pi / Po)

Loss per unit length (dB/Km) = (10/L)*log10 (Pi / Po)

What do u mean by 3dB loss?

Input Power : Pi (w)

Output Power: Po (w)

What is “dBm” and Why “dBm” ?

In Telecommunication transmitted power is very much low. ( in range of “mw” to “Microwatt” ).

dBm :

It is output power in decibel (dB) for unit milliwatt input power.

Remember : 5 dBm - 4 dBm = 1 dB ( not dBm)

What is “dBm” and Why “dBm” ?

Optical Time Domain ReflectometerOptical Time Domain Reflectometer

Course Objectives

Principles Of OTDR Block Diagram of OTDR Specifications of OTDR Using an OTDR(Operation of OTDR)

Principles Of OTDR

An OTDR is a fiber optic tester characterizing fibers and optical Networks

The aim of this instrument is to detect,locate and measure events at any location in the fiber optic link

An OTDR can test a fiber from only one end,that is it operates as a one dimensional Radar System

The OTDR technique produces geographic information with regard to localized loss and reflective events providing a pictorial and permanent record which may be used as a permanent baseline

The OTDR’s ability to characterize a fiber is based on detecting small signals returned to OTDR in response to the injection of a large signal

OTDR depends on two types of Optical Phenomena:

Rayleigh Backscattering Fresnel Reflections

Principles Of OTDR(Contd..)

Rayleigh Scattering

Rayleigh scattering is intrinsic to the fiber material itself and is present all along the length of fiber

If Rayleigh scattering is uniform along the length of fiber, then discontinuities in the back scatter can be used to identify anomalies in transmission along the length of fiber

Fresnel Reflections

Fresnel reflections are only point events

Fresnel reflections occur only where the fiber comes in contact with air or any other media such as at a mechanical connection/splice or joint

OTDR Block Diagram

OTDR

Light from the source is coupled to the fiber using a coupling device

If there are any non-linearities there will be a reflected ray from the fiber,which is coupled to the photodiode using a coupler

A pulse generator controls the LASER DIODE which sends powerful light pulses to the fiber

These pulses can have a width in the order of 2ns upto 20msec and a reoccurrence of some KHz

OTDR

The duration of the pulses can be selected by the operator for different measuring conditions(The repetition rate is limited to the rate at which the pulse return is completed, before any other pulse is launched

The OTDR measures the time difference between the outgoing pulse and the incoming backscattered pulses and hence the word “Time Domain”

The power level of the backscattered and reflected signal is sampled over time

Each measured sample is called an “Acquisition Point”

OTDR

These points can be plotted on an amplitude scale with respect to relative timing of launch pulse

It then converts this time domain information into distance based on the user entered index of fiber

The RI is inversely proportional to the velocity of propogation of light in the fiber

OTDR uses this data to convert time to distance on the OTDR display and divide this value by two to take round trip(or two way)into account

Typical OTDR Trace

Typical OTDR Trace

Apparent Signal Gain

OTDR Trace with Fiber Break

OTDR Time to Distance Conversion

V(Group Delay)=c/n

C: Velocity of light in Vacuum

n: Refractive Index

OTDR Time to Distance Conversion(Round Trip):

L(Distance) = v(Group Delay) * t/2

= (c/n) * t/2

OTDR Specifications

Dynamic Range

Dead Zone

Resolution

Accuracy

Wavelength

Dynamic Range

Dynamic Range determines maximum observable length of a fiber and therefore OTDR suitability for analyzing any particular network

The higher the signal to noise ratio,and the better the trace will be,with a better event detection

Dead Zone

OTDR is designed to detect the back scattering level all along the fiber link, it measures back scattered signals which are much smaller than the signal sent to the fiber

The device that receives these back scattered signals is an OTDR, which is designed to receive a given level range

When there is a strong reflection,then the power received by the photodiode can be more than 4000times higher than the back scattered power and can saturate the photodiode

Dead Zone

The photodiode requires time to recover from the saturated condition, during this time it will not detect any signal accurately

The length of the fiber which is not characterized during recovery is termed the dead zone

Dead Zone

Sampling Resolution

Sampling resolution is the minimum distance between two acquisition points

This data resolution can go down to centimeters depending on pulse width and range

The more data points an OTDR can acquire and process, the more the resolution

Distance Resolution

Distance resolution is very similar to sampling resolution, if OTDR samples acquisition points every 1meter,then only it can locate a fiber within +/- 1meter

The distance resolution is then like sampling resolution, a function of pulse width and range

Attenuation vs Distance with increasing Resolution

Accuracy

The accuracy of measurement is the capacity of measurement to be compared with a reference value

Linearity Accuracy: Determines how close an Optical level corresponds to an electrical level across the whole range

Distance Accuracy: Depends on the accuracy of group index(Index of refraction refers to a single ray in a fiber,while group index refers to propogation of all the light pulses in the fiber)

Wavelength OTDR’s measure according to wavelength

The major wavelengths are: 850nm, 1310nm and 1550nm A fourth wavelength is now appearing for monitoring live systems which is 1625nm

The wavelength is usually specified with central wavelength and spectral width

The attenuation of wavelength varies with wavelength, and any measurement should be corrected to transmission wavelength or to the central wavelength

Using an OTDR

We can broadly define the use of OTDR in two process:

Acquisition Step:where the unit acquires data and displays it graphically or numerically

Measurement Step:Where the operator analyzes the data and makes a decision based on the results to either store,print or go to the next acquisition

Acquisition

There are three major approaches to configure an OTDR:

A user may simply let the OTDR to auto configure and accept acquisition parameters selected by OTDR(Automatic)

A user may allow the OTDR unit to auto configure, analyze the results and change one or more parameters accordingly(Semi Automatic)

A more experienced user may choose not to use auto configuration feature altogether and enter the acquisition parameters based on his experience(Manual)

Acquisition Parameters

Given below are various acquisition parameters and theireffect on the resulting trace:

Injection Level

Wavelength

Pulse Width

Range

Averaging

Injection Level Injection level is defined an the power injected into the fiber under

test,the higher this level the higher the power level

The presence of dirt on connector faces and damaged or low quality pig tails or patch cords are the primary cause of low injection levels

Mating a dirty connector with a OTDR connector may scratch the OTDR connector,degrading the OTDR launch conditions

Some OTDRs will display the measured injection level during real time acquisition or just prior to averaging

OTDR Wavelength

A fiber must be tested with same wavelength as that used for transmission

For a given dynamic range 1550nm will see more distance than 1310nm

Single mode fiber has more mode field diameter at 1550nm that at 1310nm

OTDR Wavelength

•1550nm is more sensitive bends than 1310nm(as shown in the graph below)

Pulse Width

The OTDR pulse width controls the amount of light that will be injected into the fiber(It is the time for which the Laser is on and determines the resolution of waveform)

Longer the pulse width, more light is injected into the fiber

Longer pulse widths also produce longer dead zones in the OTDR trace waveform where the measurements are impossible

Short pulse widths inject lower levels of light but reduce dead zone

Pulse Width

By reducing the pulse width, there is a reduction in the dead zone of the fiber,compared to that of a larger pulse width and also an increase

But with the reduction in the pulse width, there is a reduction in the dynamic range, a reduction in the sensitivity of the receiver and also the distance

By proper selection of pulse width we can optimize the use of OTDR for making fiber measurements

Range

Range of an OTDR is the distance over which it can acquire data samples

The longer this parameter the more distance OTDR will shoot the pulses

This parameter is generally set to twice the distance of the end of fiber

Averaging

The OTDR detector works with extremely low optical power levels(as low as 100 photons per meter of fiber)

Averaging is the process by which each acquisition point is sampled repeatedly and the results averaged to improve signal to noise ratio

Averaging can be done by selecting the time of acquisition or the number of averages, the longer the time or higher the number of averages,the more signal the trace waveform will display in random noise conditions

Free Run Mode(Real Time):

It continually sends laser pulses down the fiber under test and obtains back scatter

This mode is useful for optimizing fiber alignment

The waveforms obtained in free run mode contain unacceptable amounts of noise making it impossible to determine small attenuation changes such as non-reflective splices

Modes Of Operation Of OTDR

Noise in Free Run Mode

Modes of Operation of OTDR Averaging Mode:

In the averaging mode each pulses are averaged from that of preceding pulses which makes the trace appear clear for each of the succeeding pulses

The number of samples that are to be averaged is predefined for an OTDR

The larger the number, the longer the OTDR takes for displaying the results

Recent OTDR specifies their averaging in terms of time taken for display, instead of number of samples

TESTS PERFORMED USING OTDR

Acceptance Test

Acceptance of fiber uses OTDR(TO measure loss per km):

This loss measurement is wavelength dependent, so the OTDR is set to the wavelength which matches with the fiber systems operating wavelength

When using an OTDR to make any measurement it is critical to correctly place reference markers so that the OTDR can display the loss & distance between them

Loss and Span Length This test has to be conducted in averaging mode, when ever we

choose averaging mode a trace will be displayed

To make any measurements it is critical to correctly place reference markers so that OTDR can display loss and distance between them

For making this measurement,a trace is obtained on OTDR in real time mode

Place the reference markers accurately, first reference marker is placed exactly where the back scatter starts,that is beyond dead zone(correct point is on the trailing edge of the

Span Loss and Span Length

Then move the cursor to end of the trace and place the second marker before the Refractive fiber end , the correct point is where the slope starts increasing faster than the normal slope of the trace

To exactly locate these reference markers use the horizontal and vertical zoom controls

Now choose the averaging mode and the display gives us the

loss per span and the span length

Attenuation of Splice or Connector

OTDR can be used to measure splice or connector loss, in order to do this a marker is placed on either of the aberration of the OTDR trace

OTDR will then display the attenuation between the two points

The vertical separation of the two marker points is the attenuation of the splice or the connector

Attenuation of Fusion Splice

Fusion splice has a loss value which is very negligible,so to accurately measure this value the OTDR is used in averaging mode

To measure the loss value,first amplify the slope the of the OTDR trace and then place the two reference points on either side of the aberration

To accurately place the markers use horizontal and vertical zoom controls

Automatic Operation

In two cursor method, sometimes the cursor might not have been placed properly and the OTDR also adds some losses and there by increasing the loss value

For short distance applications the effect is negligible,but becomes highly pronounced for long haul

Fortunately, most OTDRs have the provision to perform automatically

That is, in averaging mode the OTDR displays the splice loss as well as the connector loss systematically on the trace

Ghost Reflections

Sometimes there will be Fresnel reflection at points where it is not expected-usually after end of fiber,this usually happens when large reflection occurs in a short fiber

The reflected light actually bounces back and forth within a fiber,causing one or more false reflections to show up at multiple distances from the initial large reflection

Another type of ghosting happens when you set the range shorter than the actual length of the fiber

This allows OTDR to send additional pulses of light into the fiber before all the backscatter and reflections from the first pulse have cleared the whole fiber

When more than one pulse in the fiber at one time,a condition will be setup where returned light from different pulses arrive at the OTDR at

the same time producing “Unpredictable results”

Ghost Reflections

Ghost Reflections

Ghost Busting Techniques used to determine if ghosts are occurring and eliminate them:

Measure the distance of the suspect reflection,then place a cursor half this distance on the fiber if an expected reflection is at half way mark,then the suspect is probably Ghost

Suppress or reduce the known(true)reflection,by making the amount of

reflected power smaller, the ghost will also be reduced .To reduce the reflection, index matching gel at the reflection, or reduce the amount of

power going to the reflective point by selecting a shorter pulse width

Ghost Reflection

Ghost Busting Techniques used to determine if ghosts are occurring and eliminate them:

Change the distance Range(Display Range)of the OTDR.In some OTDRs,a ghost is caused when the Distance Range is too short

Increase the Range setting and ghost may disappear

If a ghost seems to occur in the fiber,then measure the loss across the suspected reflection.A ghost will show no loss across it when you do a

splice loss measurement

Observations & Conclusion

Observations & Conclusion

Various Instruments used for Fiber Testing(Power Meter,Laser Source,OTDR etc…)

EIA / TIA Standards defining standardized fiber optic test procedures

Power, Laser Source Test

OTDR can measure loss then why we measure

the loss with Power meter and Laser source again?

The most accurate way to measure overall attenuation in a fiber is to inject a known level of light in one end and measure the level when it comes out the other end.

Measurement of Loss in a Fiber using a LASER Source and a Power Meter

BER Test Using a VOA

To measure BER of a Optical Receiver,a VOA is used along with a BER Transmitter

As the attenuation increases, a technician can see the value of attenuation that causes a significant increase in the BER of the receiver

Link Loss Budget

What is Link Budget?

Computation of all the losses that comes into account from the source node to the destination node taking into account all the losses is called link budgeting for that particular link

Losses….

• General Losses: - Fiber Loss - Total connector loss - Total Splice loss

• Specific Losses: - Total other component loss - Manufacturer’s Specifications

- Total power penalties

General Losses(Typical Values)

•Fiber Loss Attenuation for 1310nm:0.3dB/km(G.652) Attenuation for 1550nm:0.25dB/km Largely due to impurities and imperfections in the glass of the fiber

•Connector Loss Connections at the termination points of fiber,patch panels in a site, Opticalcross connects(OXC)Conservative estimate is 0.5dB/connection

•Splice LossSplices due to construction and repairConservative estimate is 0.1dB/splice

Compares the allowable span loss for equipment against the total losses of the span.

The allowable span loss is the Transmit Power minus the Receive Power Level.

The total losses on the span is the sum of all attenuation due to fiber,connections,splices and other factors.

If the total span loss does not exceed the allowable span loss the system should work on this span.

Span Loss Analysis

Computation of Span Loss Margin

Total losses = (fiber length* loss/km) + (connector loss* No. of connectors) + (No. of Splices)*(loss/splice) + (loss due to components) + other losses

Span loss Allowed = Tx power - Receiver sensitivity

Span loss Margin = Total losses - Span loss

Link Budget

Attenuation/Span Loss Example...

Tx Output +0.5 dBm

Rx input needed -25

dBm

Tx Rx0.5 dB 0.5 dB 0.5 dB

22km @ .25dB / km

= 5.5dB

37km @ .25dB/km

=9.25 dB

Attenuation/Span Loss Example...

Total Attenuation:

Connector: 1.5dB Fiber1: 5.5dB

Fiber2: 9.25dB Splices: 0.9dB Total 21.25dB

Span Loss Analysis:

Tx Power : 0.5dBm Rx Sensitivity : -25dBm

Available for span: 25.5dB Available for span: 25.5dB

Attenuation on span: 21.25dB Span Loss Margin: 4.25dB

Signal/Noise Ratio

Signal is the information carrying optical pulse,Noise is the optical “static”created in the system Optical amplifiers amplify both signal and noise

If the signal travels long enough and through enough amplifiers,the noise will overwhelm the signal

This limits the number of consecutive amplifiers in an amplifier based system,before an optical-electrical-optical conversion is needed to restore the signal to clean low-noise pulse