Removing the Mystery from OTDR Measurements. Keith Foord Product Manager Greenlee Communications

Transcription

1 Removing the Mystery from OTDR Measurements Keith Foord Product Manager Greenlee Communications

2 Why an OTDR? Terminology Theory Standards Key specifications Trade-offs Cleaning and Inspection Measurements Reporting and documentation

3 Why an OTDR? Single ended measurement Locates individual insertion and return losses Full reporting for compliance and troubleshooting Auto shutdown feature so that live fibers are not interrupted Out of band testing is available to test live fibers at 1625nm Bi-directional measurements - gainers Macrobend analysis bending losses 930XC OTDR

4 OTDR Terminology: Range Set to capture the entire fiber link Pulse Width The width of the light pulse injected into the fiber Index of Refraction Speed of light in glass with respect to a vacuum (air) From data sheet Scattering Coefficient A property of the fiber that determines the amount of backscatter a given fiber will have From data sheet

5 OTDR Standards: CE, UL Safety EMI susceptibility & emissions GR196; SR-4731 BelCore Standard Defines specifications Proves compliance to specification Drop testing, dust ingress etc

6 Theory: Laser End of fiber Probe pulse Display Processor Coupler Detector Fiber under test Rayleigh backscatter Fresnel reflection The Optical Time Domain Reflectometer (OTDR) is an instrument that uses the inherent backscattering properties of an optical fiber to detect faults and categorize its condition. The OTDR sends high-power pulses of laser light down the fiber and captures the light that is reflected back (much like a radar system). By measuring the timing and power levels of the return pulses, the instrument correlates the reflected information with physical locations along the fiber and displays a trace that shows optical power versus distance. Attenuation of the fiber is displayed as the slope of the trace. Interruptions such as splices, connectors, bends, breaks or flaws in the fiber appear as transitions ( events ) that represent their nature and location.

7 Auto and Manual Modes: Auto mode allows for inexperienced users to make measurements OK for fast measurements More precise measurements require tweaking of the settings Manual mode allows for technicians to grow with additional features

8 Key Specifications: Dynamic range; typically 32-38dB for handheld; 42dB+ for tablet Higher dynamic range will allow the technician to probe longer fibers or links with high loss events Dynamic Range is quoted at the widest pulse width, this results in the poorest resolution. Event Deadzone; typically 1-2m The ability of the OTDR to resolve between two reflective events such as poor connectors and over-laid fiber events Resolution is quoted at the narrowest pulse width, this results in the poorest dynamic range. Attenuation Deadzone; typically 4m The ability of the OTDR to measure a backscatter event (fusion splice) after a reflective event.

9 OTDR Trade-offs - Pulse Width Larger pulse width (more energy into the fiber) Longer distance measured Results in lower resolution Smaller pulse width (less energy into the fiber) Shorter distance measured Results in higher resolution

10 OTDR Trade-offs - Averaging Longer averaging time Best signal to noise Shorter averaging time Lower signal to noise

11 Event Deadzone Considerations Two pulses closer together than the event deadzone Results in one pulse being displayed Using a shorter pulse width will result in the ability to measure two closely spaced events Deadzones are specified at -45dB reflectance Higher reflections will saturate the detector and cause longer deadzones

12 Resolution of Measurements Data points across the screen: More data points = Higher measurable resolution Longer distances: Wider pulse width required Each data point then represents larger distance Shorter distances have better resolution

13 Connectors must be CLEAN! Dirty connectors can cause: Complete failure Cross contamination Reduce bandwidth Permanent damage Reel type of cleaners work well Pens can clean both ferrule and bulkhead Photo: Courtesy Cletop Photo: Courtesy Greenlee Communications The technician will use the pens!

14 Fiber Scopes: Inspect Clean if Necessary Inspect Then Connect Replace if Necessary Validate that connector is clean Micron resolution Field of View most important specification must be able to view outside of the four zones Contamination can migrate with vibration

15 Single-Mode Criteria Table Zone Description Diameter Allowable Defects (Dia) Allowable Scratches (Width) A Critical Zone 25um None visible at 200X None visible at 200x B Cladding Zone 25 to 120um Any < 2um None >3um Total of five 2um - 5um None > 10um C Adhesive Zone 120 to 130um None > 10um Any scratch OK D Contact Zone 130 to 250um None > 10um Any Scratch OK

16 Protect the Bulkhead Use a 1m patch cord on the OTDR bulkhead Always keep connected to bulkhead Connect other end to the Outside Plant If the cable gets damaged it can be replaced Big Reflection 1m Jumper Cable UPC UPC APC Small Reflection UPC Very Big Reflection

17 Typical Events Fusion Splice or Macrobend Backscatter loss No reflection Bad Splice or Macrobend Insertion Loss Reflective Event Reflective event (end of fiber, bad connector) Return loss and Insertion loss Reflectance End of Fiber Bad Connector Insertion Loss

18 Length Measurements Measured length of the fiber inside the fiber jacket Actual fiber is typically 1 to 2% longer than the fiber jacket This is called the Helix specification for the fiber Watch out for spools of fiber and other slack storage Length of Fiber Cable Length of actual fiber is longer!

19 Typical OTDR Trace using Trace Viewer Software: OTDR Bulkhead *Reflective Event (not a splice) Cursors End of Fiber Some light is reflected back from the bad connection The reflected light hits the laser and is then transmitted again as signal but it is really noise (ghosting) A ghost (not real ) of the original reflection will be at 2X the original reflection Reflections will limit the bandwidth of fiber links due to ghosting effects!

20 Mechanical connector with high loss (but passes mechanical connector specification) and high reflection Slope is Raleigh backscatter = 0.3dB/km The insertion loss is <0.75dB but the return loss is > -40dB

21 Fusion splice measurement Fusion Splice

22 Fusion splice measurement Fusion Splice = 0.048dB The return loss was not measureable!

23 Launch Cables: Allow the OTDR to characterize input and output connectors Must be longer than the measurement pulse width At beginning and end of fiber link OTDR Launch Cable Launch Cable 1m Patchcord Fiber Under Test *Unable to resolve 1m patchcord Bulkhead & 5m Patchcord Reflective Event with Loss

24 Bidirectional Analysis: Eliminates gainers from improperly measured losses due to backscatter differences in dissimilar fibers Located at a splice or connector Two measurements averaged to null out gainers and overly stated losses Measurement A to B Gainer Loss Measurement B to A After averaging Actual insertion loss

25 Macrobend Analysis: Measurements at 1310nm and 1550nm are compared to identify macrobend losses at 1550nm that are not evident at 1310nm Tight bends Tie wraps 1550nm 1310nm No loss at 1310nm 0.394dB loss at 1550nm

26 Documentation: Trace viewers Save as PDF Proof of compliance to customer design Reference for future repairs

27 SUMMARY Proper training is imperative Buy an OTDR to suit technician ability Single ended measurement to locate failures & loss events Compare identical specifications Cleaning bulkheads and connectors is critical Pulse width and resolution require compromise for effective fault locating

28 Greenlee Communications 1390 Aspen Way Vista, CA,