DENSE WAVELENGTH DIVISION MULTIPLEXED (DWDM) TESTING

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DENSE WAVELENGTH DIVISION MULTIPLEXED (DWDM)
TESTING

• BY
• TX-I FACULTY
• A.L.T.T.C.
• GHAZIABAD.

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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.

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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).

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Overview of a DWDM System
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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.

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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.

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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.

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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).

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DWDM Critical System Parameters
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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.

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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.

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(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.

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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.

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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.

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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.

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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)

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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-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)

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Resolution Bandwidth Calculation 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
  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)

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OSA Absolute Wavelength Accuracy
Power (dBm)
3dB
Resolution Bandwidth
A
B
Wavelength (nm)
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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.

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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.

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Traditional Wavelength Meter
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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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Wavelength and Power Accuracy
Power (dBm)
Wavelength meter measurement window
Power accuracy
Good wavelength accuracy
Wavelength (nm)
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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

• 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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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..)

• 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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Bending Loss Comparison at 1310nm, 1550nm and
1625nm
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(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.

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(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

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Optical Return Loss Spectral Measurement ORL)
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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.

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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.

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(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.

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(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

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PMD Effects on System Performance
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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.

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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.

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(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.

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DWDM Test Instrumentation
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Thank Youfor your patience listening