1. TEST
AND
MEASUREMENT

2. Loss- dB
Fundamental Of OTDR
Power, Laser Source Test
Link Loss Budget

3. Loss and it’s origin Loss in optical power due to…….. Scattering
Absorption
Bending
Micro bending
Macro bending

4. Scattering
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”.

5. Absorption
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).

6. Bending Loss
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.

7. Example of different types of Loss

8. Loss (dB) = 10*log10 (Pi / Po)
Input Power : Pi (w)
Output Power: Po (w)
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?

9. 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)

10. What is “dBm” and Why “dBm” ?

11. Optical Time Domain Reflectometer

14. Course Objectives Principles Of OTDR Block Diagram of OTDR
Specifications of OTDR
Using an OTDR(Operation of OTDR)

15. 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

16. Principles Of OTDR(Contd..)
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

17. 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

19. 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

20. OTDR Block Diagram

21. 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

23. 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”

24. 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

25. Typical OTDR Trace

26. Typical OTDR Trace

27. Apparent Signal Gain

28. OTDR Trace with Fiber Break

29. 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

30. OTDR Specifications Dynamic Range Dead Zone Resolution Accuracy
Wavelength

31. 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

32. 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

33. 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

34. Dead Zone

35. 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

36. 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

37. Attenuation vs Distance with increasing Resolution

38. 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)

39. 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

40. 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

41. 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)

42. Acquisition Parameters
Given below are various acquisition parameters and their
effect on the resulting trace:
Injection Level
Wavelength
Pulse Width
Range
Averaging

43. 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

44. 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

46. OTDR Wavelength
1550nm is more sensitive bends than 1310nm(as shown in the graph below)

48. 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

49. 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

51. 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

52. 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

53. Modes Of Operation Of OTDR
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

55. Noise in Free Run Mode

56. 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

58. TESTS PERFORMED USING OTDR

59. 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

60. 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

61. 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

63. 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

64. 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

66. 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

68. 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

69. Ghost Reflections
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”

70. 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

71. 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

72. Observations & Conclusion

73. Observations & Conclusion

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

75. EIA / TIA Standards defining standardized fiber optic test procedures

76. Power, Laser Source Test

77. 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.

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

79. 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

80. Link Loss Budget

81. 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

82. Losses…. General Losses: - Fiber Loss - Total connector loss
- Total Splice loss
Specific Losses:
- Total other component loss
- Manufacturer’s Specifications
- Total power penalties

83. 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, Optical
cross connects(OXC)
Conservative estimate is 0.5dB/connection
Splice Loss
Splices due to construction and repair
Conservative estimate is 0.1dB/splice

84. Span Loss Analysis
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.

85. 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

86. Link Budget

87. Attenuation/Span Loss Example...
Rx input needed -25 dBm Tx Rx
0.5 dB
.25dB / km
= 5.5dB
.25dB/km
=9.25 dB
Tx Output +0.5 dBm

88. Attenuation/Span Loss Example...
Total Attenuation:
Connector: dB Fiber1: dB Fiber2: dB Splices: dB
Total dB
Span Loss Analysis:
Tx Power : dBm
Rx Sensitivity : dBm
Available for span: dB
Available for span: dB
Attenuation on span: dB
Span Loss Margin: dB

89. 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