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DENSE WAVELENGTH DIVISION MULTIPLEXED DWDM TESTING

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Title: DENSE WAVELENGTH DIVISION MULTIPLEXED DWDM TESTING


1
DENSE WAVELENGTH DIVISION MULTIPLEXED (DWDM)
TESTING
  • BY
  • TX-I FACULTY
  • A.L.T.T.C.
  • GHAZIABAD.

2
Topics To Be Covered
  • 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.

3
Overview of a DWDM System
  • 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
    re-cabling 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).

4
Overview of a DWDM System
5
Topics To Be Covered
  • 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.

6
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 dimensionWavelengthmu
    st be considered.
  • Field test equipment suitable for troubleshooting
    in Single-Wavelength Systems cannot cope with
    these needs.

7
TOPICS TO BE COVERED
  • 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.

8
Spectral Measurements
  • Need to characterize Components and Link
    accurately as a function of Wavelength.
  • To do so already existsthe 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).

9
DWDM Critical System Parameters
10
Topics To Be Covered
  • 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.

11
Parameters to Be Measured in the Field
  • Channel Powerto measure the Optical Power in
    each channel to verify the equal distribution of
    power over the Bandwidth of the Optical
    Amplifiers (EDFAs) that are used in the Link.
  • Channel Center Wavelength and SpacingThe 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 RatioThis 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.

12
(Contd..)
  • Total Optical PowerBecause 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 talkThis 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.

13
TOPICS TO BE COVERED
  • 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.

14
Optical Spectrum Analyser
  • The application of OSA is to measure the
    following-
  • 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.

15
TOPICS TO BE COVERED
  • 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.

16
OSA Characteristics.
  • 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)

17
Dynamic Range Calculation for an OSA
Power (dBm)
1 nm
Dynamic Range
A
B
Wavelength (nm)
18
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-to-noise 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)

19
Resolution Bandwidth Calculation for an OSA
Power (dBm)
3dB
Resolution Bandwidth
A
B
Wavelength (nm)
20
OSA Characteristics (Contd.)
  • Wavelength- There are three terms used in
    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/1570-1620nm)
  • Tuning Repeatability-It refers to the wavelength
    accuracy of returning to a wavelength after
    having tuned to a different wavelength.
    (0.002nm)
  • 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 with in or less than 1 minute)

21
OSA Absolute Wavelength Accuracy
Power (dBm)
3dB
Resolution Bandwidth
A
B
Wavelength (nm)
22
TOPICS TO BE COVERED
  • 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.

23
Wavelength Meter
  • 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.

24
Traditional Wavelength Meter
25
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.

26
Wavelength and Power Accuracy
Power (dBm)
Wavelength meter measurement window
Power accuracy
Good wavelength accuracy
Wavelength (nm)
27
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.

28
TOPICS TO BE COVERED
  • 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.

29
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.

30
(Contd..)
  • 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)


31
Bending Loss Comparison at 1310nm, 1550nm and
1625nm
32
(Contd..)
  • 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.

33
(Contd..)
  • 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
    DWDM Channel

34
Optical Return Loss Spectral Measurement ORL)
35
TOPICS TO BE COVERED
  • 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.

36
Characterizing Fiber for DWDM Applications
  • Theory predictsand field experience
    confirmsthat 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.

37
(Contd..)
  • 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.

38
(Contd..)
  • 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

39
PMD Effects on System Performance
40
TOPICS TO BE COVERED
  • 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.

41
Field Testing DWDM Systems
  • The Optical Spectrum Analyzer is eminently
    suited to almost all of the Field Testing needed
    in DWDM systems. Measurements of Signal Levels,
    Signal-to-Noise Ratio and Cross Talk as well as
    Channel Spacing and Stability. However the
    Wavelength Meter will be normally used in the
    field.

42
(Contd..)
  • However to complement OSA testing in the field,
    Center Wavelengths must be accurately measured.
  • Other instrumentation offering more accurate
    Wavelength Calibrationa Wavelength Meter, most
    likely to be used for such operations as the
    measurement of DFB characteristics.

43
DWDM Test Instrumentation
44
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