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May 19, 2011

May 19, 2011. Bill Reynolds Technical Support Engineer. Agenda Fiber Optic Theory Connector Cleaning OTDR Testing OLTS Testing CD Testing PMD Testing CWDM/DWDM. Fiber Optic Theory. Spectrum. Optical fiber domain 850 nm  353 000 GHz 1650 nm  182 000 GHz λ (nm)=c (m/s) / f (Hz).

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May 19, 2011

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  1. May 19, 2011

  2. Bill Reynolds Technical Support Engineer

  3. Agenda Fiber Optic Theory Connector Cleaning OTDR Testing OLTS Testing CD Testing PMD Testing CWDM/DWDM

  4. Fiber Optic Theory

  5. Spectrum Optical fiber domain • 850 nm  353 000 GHz • 1650 nm  182 000 GHz λ (nm)=c (m/s) / f (Hz) Units Micrometers (mm) - 10-6 m Nanometers (nm) - 10-9 m

  6. Index of Refraction • The speed of light = c = 299793 km/s under vacuum • Any material that can transmit light has it’s own index of refraction represented by n Example n = c vac / c mat • In a given index of refraction, the speed of light gets slower • Speed of light in water ≈ 225 000 km/s • Speed of light in fiber optic ≈ 215 300 km/s

  7. Fiber Optic Coating Acrylate, Teflon, polyimide Cladding Glass index n2 Core Glass index n1

  8. Fiber Optic Coating diameter = 250 µm Cladding diameter = 125 µm Core diameter = 9, 50, 62.5 µm

  9. Reflection When a light beam I hits a material with a different index of refraction, a portion of the beam is reflected R The angle of this reflected beam is the same as the incident beam Reflection n1 θi θR Core I R n2 Cladding θi = θR

  10. Refraction For the same light beam hitting a different material, another portion is refracted This occurs when a the light goes through a material with a different index of refraction The angle of this beam changes because the speed of propagation changes Refraction n1 θi θR Core I R n2 θr T Cladding n1 sin(θi) = n2 sin(θr)

  11. Critical angle (Total internal reflection) There is a certain angle where 100% of the light is reflected and no light is refracted, we call this angle, the critical angle Fiber optics use this concept to propagate light Total Internal Reflection Cladding Core I R1 θC θR R2

  12. There are 2 fiber optics types in telecommunications Fiber Types Singlemode Multimode Core Core Core Cladding Cladding Cladding 62.5/125 (µm) 50/125 (µm) 9/125 (µm) ITU-T G.652D For telecommunications applications

  13. Agenda Fiber Optic Theory Connector Cleaning OTDR Testing OLTS Testing CD Testing PMD Testing CWDM/DWDM

  14. Inspecting & Cleaning

  15. Inspection • Inspection techniques: • A microscope or fiber probe can be used to inspect connectors • A microscope acts as a magnifying glass. If you inspect a connector on a live fiber, permanent damage can be done to your eyes! • Using a fiber probe is the safest was to inspect a connector:

  16. Cleaning • Cleaning Techniques: • The best way to clean connectors can be done by following these easy steps: • Clean the outside of the ferrule with a wet pad • Clean the ferrule using a dry pad • Inspect the connector using a fiber probe • If the connector is still dirty, repeat the 2 previous steps with a wet pad Dirty ferrule Clean ferrule

  17. Why Clean? Permanent damage can occur on dirty connectors on high power systems RF video may reach +23 dBm @ 1550nm MDUs now includes MPO connectors MPO are tricky to clean 80% of network problems are related to dirty connectors! Clean Permanently burnt – combined high power and dirt View of an multi-fiber angle-polished connector

  18. Cleaning • Bad cleaning results Broken surface

  19. Damaged Dirty Clean ICIC: Inspect – Clean – Inspect - Connect

  20. Agenda Fiber Optic Theory Connector Cleaning OTDR Testing OLTS Testing CD Testing PMD Testing CWDM/DWDM

  21. OTDR Testing

  22. Water peak Attenuation • Attenuation in fiber is wavelength-dependent C: 1310nm = 0.34 dB/km D: 1383nm = 0.50 dB/km E: 1550nm = 0.19 dB/km • Optical fiber is normally tested at the same wavelength that the fiber system will be operated. Water peak Corning SMF-28 SM Fiber

  23. OTDR Principle • The OTDR can be compared to a submarine radar. • Instead of sending RF or audio (submarine) signal to detect distant objects, it sends short pulses of light to detect events in fiber. • The OTDR locates and identifies events along the fiber.

  24. Setting Up the OTDR

  25. Dopant particules Source Ray of light Rayleigh Backscattering • Rayleigh Backscattering • Comes from the “natural” reflection of the fiber • The OTDR will use the Rayleigh back reflections to measure the fiber’s attenuation (dB/Km). • Back reflection level is around -75 dB (depends on pulse length) • Higher wavelength will be less attenuated by the Rayleigh Backscatter

  26. Fresnel Reflections • Fresnel back reflections • Will come from abrupt changes in the IOR, ex: (glass/air) • Fiber break, mechanical splice, bulkheads, connectors • Will show as a “spike” on the OTDR trace • Reflection are typically UPC –45 dB and APC –65 dB (Typical OTDR results) • Fresnel reflections will be approximately 20,000 times higher than the fiber’s backscattering level • Will create a « Dead Zone » after the reflection

  27. Pulse versus resolution and dynamic range Short Pulse : more resolution but less energy Long Pulse width: more energy but less resolution

  28. Pulse Width • Short pulses will give a better resolution but less dynamic range: Two connectors 3 meters apart End of link (patch panel) Connectors are measured for distance and marked as separate events 5ns pulse End of fiber is not reached due to low power of short pulses Long pulses will give a better dynamic range but less resolution Connectors are « merged » and identified as one event 30ns pulse End of fiber is reached and located when using a larger pulse

  29. 10 us pulse

  30. 20 us pulse

  31. OTDR Trace • Single ended measurement Fusion splice OTDR Connector Connector (P.P.) Connector (P.P.) End of link UPC Reflection Power (dB) Loss APC Slope shows fiber attenuation Distance (km)

  32. Acquisition at 1310 nm Acquisition at 1550 nm Macrobend • Loss in fiber is wavelength-dependent • Shorter wavelengths are more attenuated by fiber’s scattering • Longer wavelengths tend to leak out of the fiber more easily due to bending

  33. Agenda Fiber Optic Theory Connector Cleaning OTDR Testing OLTS Testing CD Testing PMD Testing CWDM/DWDM

  34. OLTS Testing Insertion Loss

  35. Insertion testing is done in pairs Dual ended measurement

  36. dBm The dBm is use to measure the output power of a light source Measurement Units Instrument reading - 3.50 dBm Fiber optic Detector Laser source • The laser output is -3.50 dBm

  37. Measurement Unit mW How to convert the dBm in mW • dBm = 10*log(mW) The laser seen on previous page was emitting -3.50dB • -3.50 dBm so 0.45 mW

  38. dB (relative power) dB is the difference between 2 power measurements Take the -3.50 dBm laser shown previously Measurement Units Laser output = -3.50 dBm There is an event on the fiber and the detector reads -4.25 dBm -3.50 dBm To calculate this difference: (-3.50 dBm) – (-4.25 dBm) = 0.75 dB We have lost 0.75 dB So the insertion loss is -0.75dB -3.50 dBm -4.25 dBm

  39. Connector Mec. Splice Fiber section Fiber section Fiber section Patch Panel Patch Panel Reflectance • Will come from abrupt changes in the IOR: Fiber break, mechanical splice, bulkheads, connectors, etc. • We use the term « reflectance » when speaking of the amount of energy returned by specific points within the network • Expressed as a negative value Connector 1 = -40dB Connector 2 = -50dB Q: Which one has the best reflection value? Reflectance [dB] = Preflected [dBm] - Pincident [dBm] Connector reflectance: -55dB Connector reflectance: -45dB Mechanical splice reflectance: -45dB

  40. Connector Mec. Splice Fiber section Fiber section Fiber section Patch Panel Patch Panel Optical Return Loss (ORL) • Comes from the amount of energy lost within components and fiber due to back reflections • We use the term « ORL » when speaking of the amount of energy returned by a section or an entire link • Expressed as a positive value Link ORL = 35dB Section 2 & 3 ORL = 45dB

  41. Rayleigh Backscatter + Reflectance = ORL Optical Return Loss (ORL) represents the sum of all of the light returned to the source from the fiber link under test.

  42. Consequences of ORL • Less light is transmitted • Causes interference with light source signals • Creates higher bit error rate (BER) in digital systems • Reduces signal-to-noise ratio (SNR) in analog systems • Causes fluctuations in the light source’s central wavelength • Causes fluctuations in its output power • Damages the light source permanently

  43. UPC Connectors SC/UPC Ultra Physical Contact FC/UPC LC/UPC UPC Connector UPC Connector

  44. APC Connector SC/APC FC/APC Angle Physical Contact LC/APC APC Connector APC Connector

  45. Mitigating ORL • Backscatter • Is an inherent property of fiber and therefore there is nothing you can do about it. • However backscatter will have minimal affect on ORL • Reflectance • Dirty connectors!!! • Damaged endfaces • Improper mating • APC-UPC etc. • Clean connectors, clean connectors, clean connectors!!

  46. Agenda Fiber Optic Theory Connector Cleaning OTDR Testing OLTS Testing CD Testing PMD Testing CWDM/DWDM

  47. Dispersion

  48. Data rate on long fibers! • Limits length on high data rate fibers! Dispersion is the Fundamental limiting factor in transmission links and determines the:

  49. Dispersions Multimode Fiber Optical Paths Optical 1 Frequencies 2 Difference in Arrival Times Chromatic Dispersion Polarization Modes Polarization Mode Dispersion Input Pulse Output Pulse

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