removing the mystery from otdr measurements - bicsi · pdf fileremoving the mystery from otdr...

Removing the Mystery from OTDR Measurements

Keith Foord Product Manager Greenlee Communications


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


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


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


OTDR Standards:• CE, UL

• Safety• EMI susceptibility & emissions

• GR196; SR-4731• BelCore Standard• Defines specifications• Proves compliance to specification• Drop testing, dust ingress etc


The Optical Time Domain Reflectometer (OTDR) is an instrument that uses the inherent backscattering propertiesof an optical fiber to detect faults and categorize its condition. The OTDR sends high-power pulses of laser lightdown the fiber and captures the light that is reflected back (much like a radar system). By measuring the timingand power levels of the return pulses, the instrument correlates the reflected information with physical locationsalong the fiber and displays a “trace” that shows optical power versus distance. Attenuation of the fiber isdisplayed as the slope of the trace. Interruptions such as splices, connectors, bends, breaks or flaws in the fiberappear as transitions (“events”) that represent their nature and location.

Display Processor

Detector

Laser

Coupler

Fiber under test

Probe pulse

End of fiber

Rayleigh backscatter

Fresnel reflection

Theory:


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


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.


Larger pulse width(more energy into the fiber)

Results in lower resolution

Longer distance measured

Smaller pulse width(less energy into the

fiber)

Shorter distance measured

Results in higher resolution

OTDR Trade-offs - Pulse Width


Longer averaging time Best signal to noise

Shorter averaging time Lower signal to noise

OTDR Trade-offs - Averaging

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

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


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!


• 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

Fiber Scopes:

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



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

APC

UPCUPC

UPC

Big Reflection

Very Big ReflectionSmall Reflection

1m JumperCable


Typical Events• Fusion Splice or Macrobend

• Backscatter loss• No reflection

• Reflective Event• Reflective event (end of fiber, bad connector)• Return loss and Insertion loss

Insertion Loss

Insertion Loss

Reflectance

End of Fiber Bad Connector

Bad Splice or Macrobend


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!


Typical OTDR Trace using Trace Viewer Software:OTDR Bulkhead *Reflective Event (not a splice) Cursors End of Fiber

A ghost (not “real”) of the original reflection will be at 2X the original reflectionReflections will limit the bandwidth of fiber links due to ghosting effects!

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)


Mechanical connector with high loss (but passes mechanical connector specification) and high reflection

The insertion loss is <0.75dB but the return loss is > -40dB

Slope is Raleigh backscatter = 0.3dB/km


Fusion splice measurement Fusion Splice


Fusion splice measurement Fusion Splice = 0.048dB

The return loss was not measureable!


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

Launch Cable

Launch CableOTDR

1m Patchcord

Bulkhead & 5m Patchcord

Reflective Event with Loss

Fiber Under Test*Unable to resolve 1m patchcord


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

GainerMeasurement A to B

Measurement B to ALoss

After averaging

Actual insertion loss


Macrobend Analysis:Measurements at 1310nm and 1550nm are compared to identify macrobend losses at 1550nm that are not evident at 1310nm

1550nm

1310nm No loss at 1310nm

0.394dB loss at 1550nm

• Tight bends• Tie wraps


Documentation:• Trace viewers• Save as PDF• Proof of compliance to customer

design• Reference for future repairs


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


Greenlee Communications1390 Aspen WayVista, CA, 92081

www.greenleecommunications.com1- 800-642-2155

[email protected]

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