otdr -optical time domain reflectrometer prepared & presented by rahat azim chowdhury for...
DESCRIPTION
Junior ResearcherResearch & Development Genesis Technologies [email protected]TRANSCRIPT
Introduction to OTDRPrinciples, Operation and Uses
Prepared By : Rahat Azim Chowdhury
Junior Researcher
Genesis Technologies Limited
www.gtl.com.bd
PTICAL
IME
OMAIN
EFLCTOMETER
What is OTDR?
OTDR is Optical Time Domain Reflectometer
It is basically a Fiber Optic Tester
It is used to monitor loss characteristics of a link
It determines network status
It locates the fault
It shows the degrading components of a link
Principles of OTDR
An OTDR is a fiber optic tester characterizing fibers and optical Networks
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.
An OTDR detects, locates and measures events at any location in the fiber optic link
An OTDR can test a fiber from only one end. So it may be called as a one dimensional Testing Equipment.
OTDR produces results from reflective events providing a pictorial and permanent record by saving it. The saved data may be used as a permanent baseline.
OTDR tests fibers by the help of two types of Optical Phenomena:
Rayleigh Backscattering
Fresnel Reflections
Rayleigh Backscattering
Rayleigh scattering occurs for the intrinsic impurities present in the fiber
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 Reflection
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
Incident Light Passed Light
Reflected Light
Connector Joint Splice
OTDR Block Diagram
Operation of OTDR Light from the source is coupled to the fiber using a coupling device If there are any impurities 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. Laser diodes are selected according to the wavelength i.e. 850 &
1300 for multimode and 1310, 1550 for single mode. These pulses can have a width in the order of 2ns up to 20msec and
a reoccurrence of some KHz 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”
Operation of OTDR (Cont.) The control unit is the BRAIN of the OTDR. It takes all the acquisition
points, performs the averaging, plots them as a log. function of time and then displays the resulting trace on the OTDR screen.
The time base controls the pulse width, spacing between pulses and the signal sampling.
The RI is inversely proportional to the velocity of propagation 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
OTDR Time to Distance Conversion (Round Trip):
L (Distance) = v (Group Delay) * t/2 = (c/n) * t/2
Where, V (Group Delay)=c/n
C: Velocity of light in Vacuum
n: Refractive Index
OTDR Specifications
Dynamic Range Dynamic Range is the maximum observable length of a fiber. So
we have choose suitable OTDR to analyze any particular network
Resolution Resolution is of two types:
Sampling resolution is the minimum distance between two acquisition points. It is dependent on pulse width and range. The more data points an OTDR can acquire and process, the more the resolution
Distance resolution is very similar to sampling resolution, if OTDR samples acquisition points every 1meter,then only it can locate a fiber within (+/-) 1 meter. The distance resolution is then like sampling resolution, a function of pulse width and range
OTDR Specifications (Cont.)
Dead zone
OTDR receiver is very sensitive. Big reflections(1% percent of the outgoing signal) will saturate the receiver or overloads it.
When there is a strong reflection, then the power received by the photodiode can be more than 4000times higher than the normal back scattered power and can saturate the photodiode.
Once saturated, the receiver requires some time to recover, which is equivalent to 50 m to 1 km depending on the OTDR design, wavelength and magnitude of the reflection
During this time it will not detect any signal accurately
This length of the fiber is termed as “Dead Zone”
Spool (500 to 1000m) can be used. It allows the OTDR to settle down properly and to see the condition of the initial connector on the cable plant.
OTDR Specifications (Cont.)
Accuracy The accuracy is the measurement of capacity to be compared
with a reference value
Most OTDR have attenuation accuracy of 0.05dB/dBm & can go up to 0.02dB/dBm
Wavelength OTDR measures according to wavelength
The major wavelengths are: 850nm, 1310nm and 1550nm. A fourth wavelength is now appearing for monitoring live systems which is 1625nm
Attenuation varies with wavelength and any measurement should be corrected to transmission wavelength or to the central wavelength
Using an OTDR
The use of an OTDR can be defined in two processes Acquisition Step: In this step the unit acquires data and displays
it graphically or numerically Measurement Step: In this step the operator analyzes the data
and makes a decision based on the results to either store, print or go to the next acquisition
There are three approaches to configure an OTDR Automatic: A user may simply let the OTDR to auto configure and
accept acquisition parameters selected by OTDR Semi Automatic: A user may allow the OTDR to auto configure,
analyze the results and then change one or more parameters accordingly
Manual: A more experienced user may choose not to use auto configuration feature and enter the parameters based on his experience
Parameters of an OTDR
Injection Level Injection level is defined as 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 pigtails or patch cords are the primary cause of low injection levels
Connecting a dirty connector to an OTDR may scratch the OTDR connector which may lead to degrading OTDR Injection Level
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
Parameters of an OTDR (Cont.)
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 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
Modes of operation of OTDR
Free Run Mode (Real Time): In this mode OTDR continuously 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
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 can be predefined for an OTDR. The larger the number, the longer time the OTDR needs for displaying the results
Recent OTDR specifies their averaging in terms of time taken for display, instead of number of samples
Uses of OTDR Acceptance of Fiber (By measuring loss)
This loss measurement is wavelength dependent, so the OTDR must be set to the wavelength which matches with the fiber systems operating wavelength
When using an OTDR to make any measurement reference markers should be placed correctly so that the OTDR can display the loss & distance between them
Measuring Span Loss and Length This test has to be conducted in averaging mode, then a trace
will be displayed in real time mode
Then the first reference marker should be placed exactly where the back scatter starts, that is beyond dead zone
Then the second marker should be placed before the Refractive fiber end, the correct point is where the slope starts increasing faster than the normal slope of the trace
Then choosing the averaging mode will give the desired result
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 Fusion splice has a loss value which is very negligible, so to
measure this value accurately 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
For accurate result horizontal and vertical zoom control should be used
Uses of OTDR (Cont.)
Uses of OTDR (Cont.)
For fault finding OTDR can be used for finding the place of fiber cut
From the trace we can find the fiber cut point
In the trace there is a sharp loss in the place of fiber cut
We can also measure the distance of fiber cut directly from the OTDR in the analysis mode
OTDR gives the result by trace, direct distance and by giving data in tabular manner
It gives all the data in tabular form of any incident (Connector loss, Splice loss, Bending loss, Fiber loss, Cut point etc.) inside fiber in the fiber route
OTDR Trace
Observations from Trace
Non reflective event (Splice)
Reflective event (Connector)
Reflective ghost
Slope of Fiber
End of Fiber
Problem when two types of fibers are spliced(Extra loss is added shown in dotted line)
Link Loss 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) This loss is typically 0.5dB/connection
Splice Loss Splices due to construction and repair Typical value of this type of loss is 0.1dB/splice
Total losses = (fiber length* loss/km) + (connector loss* No. of connectors) + (No. of
Splices)*(loss/splice) + (loss due to components) + other losses
Feature of OTDR
Introduction The Corning 450 OTDR is a PC based equipment designed for
field applications. It is a dedicated OTDR which offers best in class OTDR
performance and user-friendly operation. It offers different wavelength and dynamic range options. The OTDR is also available with a combination of power meter
and light source with the wavelengths of the OTDR. An optional Video Inspection Probe (VIP) for connector end face
inspection is available. A battery, power supply and a hard shell transport case are
included with all OTDR kits.
Feature of OTDR
Features / Benefits Extremely high resolution: 0.5 m at 125 km and 1 m at 250 km
One of the fastest OTDR s in the industry: 60 to 80% of range is acquired in 15 to 30 seconds
Up to 256,000 data points.
Superior event analysis software provides accuracy and detection consistency
Multiple test modes simplify and automate tests for several applications including fiber reel validation measurements of long-haul, metro or PON applications
Automated reporting
Large, color display with touch screen
Feature of OTDR
Features / Benefits Hard buttons and application-specific soft keys
20 GB hard drive
Multiple I/O ports for easy connection of accessories, including
two USB ports, infrared interface and 10/100 MB Ethernet
network interface port
CD-RW and floppy drive options available
Powering options: Li-ion battery or AC mains supply
Built-in operator’s manual with help functions and explanation
screens
General Specification
Parameter Specification Display Type Touch screen
10.4 in (26.4 cm)
Active Matrix Color (TFT) Units of Measure Meters, Feet (selectable) Operating Temp. 0° to +45°C (32° to 122°F) Storage Temp. -25° to +60°C (-13° to 140°F) Humidity 95% RH maximum, non-condensing Altitude No limitation Power Supply Battery: 4 hrs typical battery life
Recharge time: less than 3 hrs; Mains Supply 100 to 250 V AC, 50 to 60 Hz
Auto ranging
General Specification
Parameter Specification Weight 5.4 kg Dimensions 24.1 x 34.3 x 9.5 cm (9.5 x 13.5 x 3.75 in) Processor Ultra-low power 300 MHz Intel Celeron System Memory 256 MB Operating System Windows® XP embedded Control Interface Touch screen, cursor control
Dedicated hard buttons and status LEDs
I/O Ports USB (2), Ethernet 10/100 (1), IrDA (1), Parallel (1)
Data I/O (module)CD-RW (optional)
3.5-in 1.44 MB floppy drive (optional) Data I/O (fixed) 20 GB hard drive Loss Resolution 0.001 dB
New Feature of OTDR
In the new version of OTDR called Corning 500 OTDR has the following extra features It has Chromatic Dispersion Compensating capabilities.
Corning 500 OTDR does this by phase shift method
It can be used in WDM network
Maintenance of OTDR
The connector face of OTDR should be cleaned every 7 days
Clean patch cord should be used for measuring purpose
The OTDR should be charged in every week
It should be calibrated every year
The screen should be cleaned regularly
Prepared and Presented By :Prepared By : Rahat Azim Chowdhury
Junior Researcher
Genesis Technologies Limited
www.gtl.com.bd