Transcript Document

BSNL
DENSE WAVELENGTH
DIVISION MULTIPLEXED
(DWDM) TESTING
BY
TX-I FACULTY
A.L.T.T.C.
GHAZIABAD.
7/7/2015
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Topics To Be Covered
BSNL
•
•
•
•
•
•
•
•
Overview of a DWDM System.
Need for New Testing Tools.
Spectral Measurements.
Parameters to be Measured in the Field.
Optical Spectrum Analyzer.
OSA Characteristics.
Wavelength Meter.
New Requirements for Traditional Fiber-Optic Test
Instruments.
• Characterizing Fiber for DWDM Applications.
• Field Testing DWDM Systems.
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Overview of a DWDM System
BSNL
 DWDM system can be viewed as a parallel set of
Optical Channels, each using a slightly different
light Wavelength, but all sharing a single
Transmission Medium.
 DWDM can increase the capacity of existing
networks without the need for expensive recabling and can significantly reduce the cost of
network upgrades.
 The planning, installation & maintenance of DWDM
networks demand to discuss performance
parameters as well as other factors involved in
field of Testing DWDM Systems(see Figure).
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BSNL
Overview of a DWDM System
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Topics To Be Covered
BSNL
•
•
•
•
•
•
•
•
Overview of a DWDM System.
Need for New Testing Tools.
Spectral Measurements.
Parameters to be Measured in the Field.
Optical Spectrum Analyzer.
OSA Characteristics.
Wavelength Meter.
New Requirements for Traditional Fiber-Optic Test
Instruments.
• Characterizing Fiber for DWDM Applications.
• Field Testing DWDM Systems.
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BSNL
Need For New Testing Tools
• DWDM systems call for considerably more care than
has been needed for conventional systems.
• New parameters must be measured and Component
Characteristics once of interest only before
installation must now be verified regularly.
• Accuracy and Stability requirements reach new
levels and an entirely new dimension—Wavelength—
must be considered.
• Field test equipment suitable for troubleshooting in
Single-Wavelength Systems cannot cope with these
needs.
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TOPICS TO BE COVERED
BSNL
•
•
•
•
•
•
•
•
Overview of a DWDM System.
Need for New Testing Tools.
Spectral Measurements.
Parameters to be Measured in the Field.
Optical Spectrum Analyzer.
OSA Characteristics.
Wavelength Meter.
New Requirements for Traditional Fiber-Optic Test
Instruments.
• Characterizing Fiber for DWDM Applications.
• Field Testing DWDM Systems.
7/7/2015
DWDM/ALT/TX-I/Opti.Test/06
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Spectral Measurements
BSNL
• Need to characterize Components and Link
accurately as a function of Wavelength.
• To do so already exists—the Optical Spectrum
Analyzer (OSA) has long been a fixture in Network
Development and Test Laboratories.
• Now,however,similar capabilities must be provided
in the field.
• Capabilities must be usable for maintenance
personnel working in conditions that are very
different from those in the stable, controlled
laboratory environment(see Figure).
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DWDM Critical System Parameters
BSNL
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Topics To Be Covered
BSNL
•
•
•
•
•
•
•
•
Overview of a DWDM System.
Need for New Testing Tools.
Spectral Measurements.
Parameters to be Measured in the Field.
Optical Spectrum Analyzer.
OSA Characteristics.
Wavelength Meter.
New Requirements for Traditional Fiber-Optic Test
Instruments.
• Characterizing Fiber for DWDM Applications.
• Field Testing DWDM Systems.
7/7/2015
DWDM/ALT/TX-I/Opti.Test/06
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Parameters to Be Measured in the
BSNL
Field
• Channel Power—to measure the Optical Power in
each channel to verify the equal distribution of power
over the Bandwidth of the Optical Amplifiers (EDFA’s)
that are used in the Link.
• Channel Center Wavelength and Spacing—The
precise value of the Center Wavelength of each channel
must be measured in order to detect unacceptable drifts
in DFB Laser Sources.
• Signal-to-Noise Ratio—This is one of the most
important parameters to be measured for each channel
in a DWDM System, as it is the best indicator of the
overall performance of the channel. The Noise
measurement it incorporates must be based on
measurements of the Noise Floor between Channels.
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(Contd..)
BSNL
• Total Optical Power—Because adverse
effects of nonlinear phenomena in the Optical
Fiber depend on the total power carried, the
parameter must be measured or calculated by
summing the individual Channel Powers.
• Cross talk—This parameter reveals the level
of unwanted signal (Noise plus contribution
from other channels) in the Pass Band of the
tested channel. It is awkward to incorporate
its measurement into Field Tests because it is
a two-step operation, but it can be critical.
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TOPICS TO BE COVERED
BSNL
•
•
•
•
•
•
•
•
Overview of a DWDM System.
Need for New Testing Tools.
Spectral Measurements.
Parameters to be Measured in the Field.
Optical Spectrum Analyzer.
OSA Characteristics.
Wavelength Meter.
New Requirements for Traditional Fiber-Optic
Test Instruments.
• Characterizing Fiber for DWDM Applications.
• Field Testing DWDM Systems.
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Optical Spectrum Analyser
BSNL
 The application of OSA is to measure the
following:1.
2.
3.
4.
5.
6.
7.
Channel Wavelengths.
Source Power.
Gain & Noise Figure of an Amplifier.
Source Mean Wavelength.
Sum of Source signal Power.
Amplifier Mean Wavelength.
Sum of Amplifier signal Power.
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TOPICS TO BE COVERED
BSNL
•
•
•
•
•
•
•
•
Overview of a DWDM System.
Need for New Testing Tools.
Spectral Measurements.
Parameters to be Measured in the Field.
Optical Spectrum Analyzer.
OSA Characteristics.
Wavelength Meter.
New Requirements for Traditional Fiber-Optic Test
Instruments.
• Characterizing Fiber for DWDM Applications.
• Field Testing DWDM Systems.
7/7/2015
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OSA Characteristics.
BSNL
These are essential to an OSA, while
measuring the following core parameters:• Dynamic Range:- It is a measure of the ability to
see low-level signals that are located very close to
a stronger signal. For example, in a system with
100GHz (1550 nm at ± 0.8nm) channel spacing,an
OSA must be able to measure an Optical signal at
a given wavelength and just 0.4 nm away an ASE
noise level that may be 30 dB to 35 dB weaker.It
may be –60dB. (see Figure)
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BSNL
Dynamic Range Calculation
for an OSA
Power (dBm)
1 nm
Dynamic
Range
A B
Wavelength (nm)
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OSA Characteristics (Contd.)
• Optical Sensitivity:- It is defined as the signal level
that is equal to six times the RMS value of the noise. It
must be low enough to permit the measurement of
component insertion loss and assess the signal-tonoise ratio in all parts of a network. It is generally
determined by electronic considerations viz. the dark
current of detectors, noise in detector preamplifiers
etc.
• Resolution Bandwidth:- The resolution bandwidth
of an OSA determines its ability to deal with close
optical channel spacing. It is measured as the width
of the response curve at half peak power (i.e; 3 dB
down) of the instrument to a monochromatic test
signal. This specification is often called full-width
half-maximum (FWHM) (see Figure)
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Resolution Bandwidth Calculation
BSNL
for an OSA
Power (dBm)
3dB
Resolution
Bandwidth
A B
Wavelength (nm)
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OSA Characteristics (Contd.)
•
Wavelength:- There are three terms used in
1.
measuring the wavelength with OSA.
Absolute Accuracy:- It refers to the wavelength
accuracy after the user has performed the internal
wavelength calibration using a source of known
wavelength. (±0.01 nm/1480-1570nm; ±0.025nm/15701620nm)
2. Tuning Repeatability:-It refers to the wavelength
accuracy of returning to a wavelength after having
tuned to a different wavelength. (±0.002nm)
3. Reproducibility:- It refers to the amount of
wavelength drift (in DFB laser sources) which can
occur over the specified time while the OSA is swept
across a source of known wavelength. (±0.002nm
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with in or less than
1 minute)
BSNL
OSA Absolute Wavelength
Accuracy
Power (dBm)
3dB
Resolution
Bandwidth
A B
Wavelength (nm)
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TOPICS TO BE COVERED
BSNL
•
•
•
•
•
•
•
•
Overview of a DWDM System.
Need for New Testing Tools.
Spectral Measurements.
Parameters to be Measured in the Field.
Optical Spectrum Analyzer.
OSA Characteristics.
Wavelength Meter.
New Requirements for Traditional Fiber-Optic Test
Instruments.
• Characterizing Fiber for DWDM Applications.
• Field Testing DWDM Systems.
7/7/2015
DWDM/ALT/TX-I/Opti.Test/06
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Wavelength Meter
BSNL
• It is an EXCELLENT complementary instrument
for the complete characterization of DWDM
systems.
• The Wavelength Meter is expected to be the
instrument of choice for DWDM systems to
measure the Centre Wavelengths of DFB Lasers.
•
It will also monitor how they change with Time
(both short-term & long-term),Temperature and
other environmental conditions.
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Traditional Wavelength Meter
BSNL
Fixed mirror
Beam splitter
Fibre
Movable
mirror
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Detector
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BSNL
Characteristics of Wavelength
Meter
• Absolute Wavelength Accuracy:- The ability to
accurately measure the absolute wavelength of a
channel. The absolute accuracy of the wavelength
meter should be better than about 0.005nm. It is
adequate to locate individual DWDM channel
wavelength.
• Absolute Power Accuracy:- The ability to
measure the exact power in each DWDM channel is
important to verify the power flatness throughout the
link. It is usually little lower than that of the OSA,
which is ± 0.2 dB.
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BSNL
Wavelength and Power Accuracy
Power (dBm)
Wavelength meter measurement window
Power accuracy
Good wavelength accuracy
Wavelength (nm)
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BSNL
Characteristics of Wavelength
Meter
• Dynamic Range:- The ability to measure weak
signals in the presence of strong ones-to measure
the noise floor in a multi channel transmission
system. It can attain a dynamic range of 20dB to
25dB.
• Number of Channels:- It varies from vendor to
vendor. But Forty to a Hundred Channels should be
attainable. These are enough to characterize DWDM
System properly.
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TOPICS TO BE COVERED
BSNL
•
•
•
•
•
•
•
•
Overview of a DWDM System.
Need for New Testing Tools.
Spectral Measurements.
Parameters to be Measured in the Field.
Optical Spectrum Analyzer.
OSA Characteristics.
Wavelength Meter.
New Requirements for Traditional Fiber-Optic Test
Instruments.
• Characterizing Fiber for DWDM Applications.
• Field Testing DWDM Systems.
7/7/2015
DWDM/ALT/TX-I/Opti.Test/06
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BSNL
New Requirements for Traditional
Fibre-Optic Test Instruments
• Optical Loss Test Sets (OLTS)
Optical Loss Test Sets will also be used at the
Wavelengths used for Optical Supervisory
Channels (OSC)—1480 nm, 1510 nm and 1625 nm- depending on the System Design. Dedicated
DFB Light Sources will be needed to verify the
Loss Budget when the Fiber is installed.
The Longest OSC wavelength, 1625 nm, requires
particular attention, as this wavelength lies
outside the range in which the fiber or cable
manufacturer guarantees the performance of its
product.
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(Contd..)
BSNL
• Optical Time Domain Reflecto meter(OTDR)
• Ability to test & troubleshoot the important 1625nm
Optical Supervisory Channel, live fibers may be tested at
the 1625nm wavelength while normal DWDM transmission
continues in the EDFA spectral region.
Since Optical Losses due to fiber bending are more
pronounced at 1625nm than at the shorter DWDM
operational wavelengths, OTDR testing can reveal critical
points in the installed fiber which could degrade over
time(see Figure)
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BSNL
Bending Loss Comparison at
1310nm, 1550nm and 1625nm
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(Contd..)
BSNL
• Back Reflection Meter
In a non- WDM Network, the Optical Return Loss (ORL)
can be determined with a single measurement using a
Back Reflection Meter at the operating wavelength. This
ORL variation with wavelength may be caused from the
bad connectors at the output port of a Multiplexer or
Demultiplexer. Excessive ORL can cause instability in
DFB source lasers which affects the overall system
performance.
In DWDM Systems, there are two possibilities:1. An aggregate measure covering the entire
wavelength band in use.
2. A detailed one, giving results for each channel
wavelength.
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(Contd..)
BSNL
• Mostly the first one is carried out with a Broadband
Source & an independent Power Meter .
• If the simple aggregate test fails on a particular link,
then it is measured using a high-power broadband
source,usually an erbium-amplified spontaneous
emission (ASE) source.
• High power is needed to provide enough power in
each measurement band (0.1nm wide) to give an
adequate S/N Ratio at the detector for the lowest ORL.
• The detector is nothing but an Optical spectrum
Analyser(OSA) of adequate resolution & sensitivity.
• The result is an individual ORL reading for each
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BSNL
Optical Return Loss Spectral
Measurement ORL)
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TOPICS TO BE COVERED
BSNL
•
•
•
•
•
•
•
•
Overview of a DWDM System.
Need for New Testing Tools.
Spectral Measurements.
Parameters to be Measured in the Field.
Optical Spectrum Analyzer.
OSA Characteristics.
Wavelength Meter.
New Requirements for Traditional Fiber-Optic Test
Instruments.
• Characterizing Fiber for DWDM Applications.
• Field Testing DWDM Systems.
7/7/2015
DWDM/ALT/TX-I/Opti.Test/06
35
BSNL
Characterizing Fiber for DWDM
Applications
• Theory predicts—and field experience
confirms—that the characteristics of the
Fiber itself can have significant impact on
the performance of DWDM Networks.
• The particular characteristics which are
most important are not necessarily those
of greatest concern in conventional
Single-Wavelength Links.
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(Contd..)
BSNL
1. Chromatic Dispersion “Variation of the
Index Of Refraction(IOR) of the Fiber with
Wavelength”
•
Chromatic Dispersion is determined during Fiber
Manufacturing. However, few situations have arisen
in which it is necessary to verify this value in the
Field also.
•
As DWDM systems are operated ever closer to their
limits, however, a need is likely to emerge to verify
that this Parameter is adequately controlled at
every point in the Optical Path.
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(Contd..)
BSNL
2.Polarization Mode Dispersion “Various
polarization states of the optical signal propagate
at different velocities”
It effects the following:•
Prevent many present day Optical Systems from
using High- Bandwidth.
•
The transmission quality by spreading signal
pulses which in turn raise the BER.
Post-installation testing may be needed to ensure
that a network does not overly suffer from PMD
and that the installed facilities can be upgraded
to support tomorrow's Higher Bit Rates
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PMD Effects on System
Performance
BSNL
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TOPICS TO BE COVERED
BSNL
•
•
•
•
•
•
•
•
Overview of a DWDM System.
Need for New Testing Tools.
Spectral Measurements.
Parameters to be Measured in the Field.
Optical Spectrum Analyzer.
OSA Characteristics.
Wavelength Meter.
New Requirements for Traditional Fiber-Optic Test
Instruments.
• Characterizing Fiber for DWDM Applications.
• Field Testing DWDM Systems.
7/7/2015
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Field Testing DWDM Systems
BSNL
•
The Optical Spectrum Analyzer is
eminently suited to almost all of the Field
Testing needed in DWDM systems.
Measurements of Signal Levels, Signal-toNoise Ratio and Cross Talk as well as
Channel Spacing and Stability. However the
Wavelength Meter will be normally used in
the field.
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(Contd..)
BSNL
•However to complement OSA testing in
the field, Center Wavelengths must be
accurately measured.
•Other instrumentation offering more
accurate Wavelength Calibration—a
Wavelength Meter, most likely to be used
for such operations as the measurement of
DFB characteristics.
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DWDM Test Instrumentation
BSNL
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BSNL
Thank You
for your patience listening
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