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Optical Loss Budget (Example 2)

1000BASE-ZX GBIC Pt max = 5 dBm Pt min = 0 dBm Pr max = -3 dBm Pr min = -23 dBm. Questions: Can you connect one GBIC to another with only a patchcord? How can you ensure that the fiber system does not exceed the maximum loss?. Optical Loss Budget (Example 2). Pt max 5 dBm.

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Optical Loss Budget (Example 2)

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  1. 1000BASE-ZX GBIC Ptmax = 5 dBm Ptmin = 0 dBm Prmax = -3 dBm Prmin = -23 dBm Questions: Can you connect one GBIC to another with only a patchcord? How can you ensure that the fiber system does not exceed the maximum loss? Optical Loss Budget (Example 2) Ptmax 5 dBm Minimum Loss (dB) = 8 dB Optical Loss Budget: Bmax = Ptmin – Prmin Bmin = Ptmax -Prmax Prmax -3 dBm Ptmin 0 dBm Optical Power Level (dBm) Maximum Loss (dB) = 23 dB Prmin -23 dBm

  2. Test! Basic Tests Visual Fault Locator (VFL) Optical Insertion Loss Optical Power Levels Advanced Tests Optical Return Loss (ORL) Optical Time Domain Reflectometer (OTDR) Chromatic Dispersion (CD) Polarization Mode Dispersion (PMD) Optical Spectral Analysis (OSA)

  3. Visual Fault Locator VFLs provide a visible red light source useful for identifying fiber locations, detecting faults due to bending or poor connectorization, and to confirming continuity. VFL sources can be modulated in a number of formats to help identify the correct VFL (where a number of VFL tests may be performed). FFL-100 FFL-050

  4. Advanced Tests • Optical Return Loss (ORL) • Optical Time Domain Reflectometer (OTDR) • Detect, locate, and measure events at any location on the fiber link • Fiber Characterization • Determines the services that the fiber can be carry • Basic tests plus: • Chromatic Dispersion (CD) • Polarization Mode Dispersion (PMD) • Optical Spectrum Analysis (OSA) • Spectral analysis for Wavelength Division Multiplexing (WDM) systems

  5. Introduction to OTDR It’s the single most important tester used in the installation, maintenance & troubleshooting of fiber plant • Most versatile of Fiber Test Tools • Detect, locate and measure events at any location on the fiber link • Identifies events & impairments (splices, bends, connectors, breaks) • Provides physical distance to each event/ impairment • Measures fiber attenuation loss of each event or impairment • Provides reflectance / return loss values for each reflective event or impairment • Manages the data collected and supports data reporting. T-BERD 4000 FTTx / Access OTDR

  6. Background on Fiber Phenomena OTDR depends on two types of phenomena: • Rayleigh scattering • Fresnel reflections. Light reflection phenomenon = Fresnel reflection Rayleigh scattering and backscattering effect in a fiber

  7. How does it work ? • The OTDR injects a short pulse of light into one end of the fiber and analyzes the backscatter and reflected signal coming back • The received signal is then plotted into a backscatter X/Y display in dB vs. distance • Event analysis is then performed in order to populate the table of results. OTDR Block Diagram Example of an OTDR trace

  8. Dynamic Range & Injection Level • Dynamic Rangedetermines the observable length of the fiber & depends on the OTDR design and settings • Injection levelis the power level in which the OTDR injects light into the fiber under test • Poor launch conditions, resulting in low injection levels, are the primary reason for reductions in dynamic range, and therefore accuracy of the measurements • Effect of pulse width: the bigger the pulse, the more backscatter we receive

  9. What does an OTDR Measure ? • Distance • The OTDR measurement is based on “Time”: The round trip time travel of each pulse sent down the fiber is measured. Knowing the speed of light in a vacuum and the index of refraction of the fiber glass, distance can then be calculated. • Fiber distance = Speed of light (vacuum) X time • 2 x IOR

  10. What does an OTDR Measure ? • Attenuation (also called fiber loss)Expressed in dB or dB/km, this represents the loss, or rate of loss between two events along a fiber span

  11. What does an OTDR Measure ? • Event LossDifference in optical power level before and after an event, expressed in dB Connector orMechanical Splice Fusion Splice or Macrobend

  12. What does an OTDR Measure ? • ReflectanceRatio of reflected power to incident power of an event, expressed as a negative dB value The higher the reflectance, the more light reflected back, the worse the connection A -50dB reflectance is better than -20dB value • Typical reflectance values • Polished Connector ~ -45dB • Ultra-Polished Connector ~ -55dB • Angled Polished Connector ~ -65dB

  13. What does an OTDR Measure ? • Optical Return Loss (ORL)Measure of the amount of light that is reflected back from a feature: forward power to the reflected power. The bigger the number in dBs the less light is being reflected. The OTDR is able to measure not only the total ORL of the link but also section ORL Attenuation (dB) ORL of the defined section Distance (km)

  14. PPC PAPC PAPC PAPC Light Source Photo-diode PF PF PF PT ORL (dB) = 10Log > 0 Optical Return Loss (ORL) • Light reflected back to the source PT: Output power of the light source PAPC: Back-reflected power of APC connector PPC: Back-reflected power of PC connector PF: Backscattered power of fiber PB: Total amount of back-reflected power

  15. Effects of High ORL Values All laser sources, especially distributed feedback lasers, are sensitive to optical reflection, which causes spectral fluctuation and, subsequently, power jitter. Return loss is a measure of the amount of reflection accruing in an optical system. A -45dB reflection is equivalent to 45dB return loss (ORL). A minimum of 45-50dB return loss is the industry standard for passive components to ensure normal system operation in singlemode fiber systems. • Increase in transmitter noise • Reducing the OSNR in analog video transmission • Increasing the BER in digital transmission systems • Increase in light source interference • Changes central wavelength and output power • Higher incidence of transmitter damage SC - PC SC - APC • The angle reduces the back-reflection of the connection.

  16. OTDR Events How to interpret a trace

  17. How to interpret an OTDR Trace

  18. Front End Reflection Connection between the OTDR and the patchcord or launch cable Located at the extreme left edge of the trace Reflectance: Polished Connector ~ -45dB Ultra-Polished Connector ~ -55dB Angled Polished Connector up to ~ -65dB Insertion Loss: Unable to measure

  19. Dead Zones Attenuation Dead Zone (ADZ) is the minimum distance after a reflective event that a non-reflective event can be measured (0.5dB) • In this case the two events are more closely spaced than the ADZ, and shown as one event • ADZ can be reduced using shorter pulse widths Event Dead Zone (EDZ) is the minimum distance where 2 consecutive unsaturated reflective events can be distinguished • In this case the two events are more closely spaced than the EDZ, and shown as one event • EDZ can be reduced using shorter pulse widths

  20. Connector A connector mechanically mates 2 fibers together and creates a reflective event • Reflectance: • Polished Connector ~ -45dB • Ultra-Polished Connector ~ -55dB • Angled Polished Connector up to ~ -65dBInsertion Loss: ~ 0.5dB (loss of ~0.2dB w/ very good connector)

  21. Fusion Splices A Fusion Splice thermally fuses two fibers together using a splicing machine Reflectance: None Insertion Loss: < 0.1dB A “Gainer” is a splice gain that appears when two fibers of different backscatter coefficients are spliced together (the higher coefficient being downstream) Reflectance: None Insertion Loss: Small gain

  22. Fusion Splices Direction A-B Direction B-A

  23. Macrobend • Macrobending results from physical bending of the fiber. • Bending Losses are higher as wavelength increases. • Therefore to distinguish a bend from a splice, two wavelengths are used (typically 1310 & 1550nm) Reflectance: None Insertion Loss: Varies w/ degree of bend & wavelength

  24. Mechanical Splice A Mechanical Splice mechanically aligns two fibers together using a self-contained assembly. Reflectance: ~ -35dB Insertion Loss: ~ 0.5dB

  25. Fiber End or Break A Fiber End or Break occurs when the fiber terminates. The end reflection depends on the fiber end cleavage and its environment. Reflectance: PC open to air ~ -14dB APC open to air ~ - 35dB Insertion Loss: High (generally)

  26. Ghosts A Ghost is an unexpected event resulting from a strong reflection causing “echos” on the trace When it appears it often occurs after the fiber end. It is always an exact duplicate distance from the incident reflection. Reflectance: Lower than echo source Insertion Loss: None

  27. Typical Attenuation Values • 0.2 dB/km for singlemode fiber at 1550 nm • 0.35 dB/km for singlemode fiber at 1310 nm • 1 dB/km for multimode fiber at 1300 nm • 3 dB/km for multimode fiber at 850 nm • 0.05 dB for a fusion splice • 0.3 dB for a mechanical splice • 0.5 dB for a connector pair (FOTP-34) • Splitters/monitor points (varys with component)

  28. Best Practices with OTDRs

  29. Performing an OTDR Test • Inspect & Clean connector end faces (patch cords & bulkheads (including test instrument) • Set up instrument for test environment • Test • View trace/table of results • Store / Report Results • Further analysis optional (for advanced users)

  30. Key OTDR Setup Parameters for Manual Operation • Pulse Width • Controls the amount of light injected into the fiber • A short pulse width enables high resolution and short dead zones, but limited dynamic range • A long pulse width enables high dynamic range but less resolution and longer dead zones 5ns 1µs • Short Pulse: • More Resolution • Shorter Dead Zones • Less Dynamic Range • More Noise • Long Pulse: • Less Resolution • Wider Dead Zones • More Dynamic Range • Less Noise 100ns

  31. Key OTDR Setup Parameters for Manual Operation • Acquisition Time (Averaging) • Length of time the OTDR takes to acquire and average the data points • Increasing acquisition time improves the dynamic range w/o affecting the resolution or dead zones. 5s 30s 20s

  32. Key OTDR Setup Parameters for Manual Operation • Index of Refraction (IOR) • The IOR converts time, measured by the OTDR, to distance, which is displayed on the trace • Entering the appropriate value into the OTDR will ensure accurate length measurements for the fiber.

  33. How to select the right OTDR Test Module OTDR modules are primarily specified in terms of dynamic range Select the optimum test module as follows: • Determine the longest span you will be testing w/ this module • Determine the expected link loss budget this will translate to • Select the module by subtracting 6 dB from the rated dynamic range of the module (this is the range of the unit to view backscatter signal or measure a splice loss)

  34. Example: Link Loss / OTDR Module selection calculation Dynamic Range requirement for Module

  35. Tools to Optimize OTDR testing • Launch Cable • Using a launch cable allows the characterization of the connector at the origin of the link. • This shifts the first connector outside the dead zone of the OTDR connector • The last connector can also be measured by using a receive cable About Launch Cables • Launch cables are typically 100 – 1,000 meters in length. • The length required depends upon the dead zone performance of the OTDR. A minimum 2x the attenuation dead zone length is recommended, although in practice, most are much longer

  36. TB6000/8000 OTDR Distance Chart

  37. Fiber Characterization Step-by-step review

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