Testing Fibre Optic Cabling

Fibre End Face Inspection

Fibre end face contamination is the number one source of issues in optical networks. Always “Inspect Before You Connect” to ensure fibre end faces are clean prior to mating connectors.

Fibre Inspection & Cleaning Flow Chart

All fibre connector end faces, including those of the Test Reference Cords (TRCs) and bulkhead adapters, should be inspected prior to testing to ensure that they are free from damage or contamination
                - damaged fibre end faces should be re-terminated
                - dust can be removed using a ‘One Click’ cleaner
                - grease or oil (finger prints) can be removed using a solvent dampened wipe or swab

Ideally the fibre end face should be inspected using a video microscope that can certify to industry standard IEC 61300-3-35
                - provides an automated Pass/Fail
                - has the ability to save an image of the fibre end face

Fibre End Face Images

Typical fibre end face inspection equipment

End Face Inspection Equipment

Fibre Testing Equipment

There are two tiers of fibre testing, tier 1 and tier 2. Tier 1 testing is suitable for certifying the performance of the fibre cabling and tier 2 testing can be used to compliment this. However, tier 2 testing is not considered suitable for certifying multimode fibre cabling by itself; it must always be used in conjunction with tier 1 testing.

Tier 1 Fibre Certification

The purpose of tier 1 testing is to:
         - 
Measure Length
         - 
Measure Loss
         - 
Check Polarity
         - 
Ensure Loss does not exceed a “limit” (loss budget)
         - 
Document results

Tier 1 testing can be performed using either an Optical Loss Test Set (OLTS) or a Light Source and Power Meter (LSPM).

Optical Loss Test Set (OLTS)
                - automated testing
                - tests a duplex fibre in both directions
                - provides a simple PASS/FAIL result
                - may be a dedicated device or a special ‘module’ that attaches to a copper cabling tester

Optical Loss Test Sets

Light Source & Power Meter (LSPM)
                - manual testing
                - tests a single fibre path
                - requires the use of a visual fault locator, fibre length meter and manual calculations

Light Sources & Power Meters

Tier 2 Fibre Certification

Optical Time Domain Reflectometer (OTDR)
                - not usually used for ‘short’ LAN fibre links
                - can be useful for locating faults
                - requires the use of special launch and tail leads to overcome OTDR dead zone limitations to
                  measure the loss and reflectance of the first and last connector in the link
                - testing from both ends of the link is required

Optical Time Domain Reflectometers
Tier 2 test equipment is generally more expensive than Tier 1 test equipment, requires a more skilled operator and requires more time for testing.

 

Encircled Flux (EF)

Different multimode light sources give different modal power distributions (commonly referred to as launch conditions). Launch conditions directly impact link loss measurement accuracy.
                LED overfills a multimode fibre, tending to overstate loss
                Laser under fills a multimode fibre, tending to understate loss
Encircled Flux provides controlled launch conditions for more repeatable and more accurate test results, which is important for high speed links with limited loss budgets.

Encircled Flux Launch Diagram

Encircled Flux requires the use of special launch adaptors or leads.

 

Test Reference Cords (TRCs)

TRCs use high performance connectors with optimal optical and geometrical characteristics
                - Numerical aperture (NA)
                - Core/ferrule concentricity
When mated with other TRCs they produce near zero loss and reduce test result uncertainty. TRCs are called for in various standards for loss measurements of installed fibre cabling. TRCs have typical attenuation losses between two reference connectors of ≤0.1dB for multimode and ≤0.2dB for singlemode.

 

Setting up for Testing (Referencing)

Correct Steps for Referencing

  • Allow test equipment warm up for 5 min
  • Select and configure the appropriate limit
  • Set the reference method on the test equipment
  • Connect the test equipment together according to reference method selected (Inspect Before You Connect (IBYC))
  • Perform reference
  • Verify reference (IBYC)
  • Test the installed fibre link (IBYC)

 

Tier 1 Fibre Certification Procedure (One jumper method)

1. Connect the light source to the power meter using a Test Reference Cord (TRC).

One Jumper Reference - Step 1

2. Reference the power meter (set to 0dB).
3. Disconnect the TRC from the power meter and connect a second TRC to the power meter port.
4. Verify the reference by connecting the two TRCs together using a precision bulkhead adaptor.

One Jumper Reference - Step 4

5. Measure the loss to ensure good quality TRCs (≤0.1dB for MM, ≤0.2dB for SM) and save the test result.
6. Disconnect the TRCs from the bulkhead adaptor and connect to the link under test to measure the loss.

One Jumper Reference - Step 6

The One Jumper Method calculates the link loss as the loss of the two adapters and the link under test. It is the preferred method for qualifying the link under test as outlined in ANSI/TIA-568-C. The power meter test lead must have the same connector type as the Link Under Test (LUT). This has been proven to be the most accurate and reproducible method of testing.

 

Calculating a Link Loss Budget

The following equation is used to estimate the attenuation, or link loss:
                Loss Budget (dB) = Cable Attenuation + Connector Attenuation + Splice Attenuation, where :
                Cable Attenuation. = Attenuation Coefficient x Length
                (See the table below for attenuation coefficients to apply to different fibre types at various
                 operating wavelengths)
                Connector Attenuation. = No. of Mated Pairs x Connector Loss
                                - Apply 0.75 dB loss per mated connector pair
                Splice Attenuation. = No. of Splices x Splice Loss
                                - Apply 0.3 dB loss per splice

Fibre Cable Attenuation Table

Example:

Link Under Test Diagram

132m multimode fibre at 850nm wavelength
2 connectors and 2 splices

Loss Budget (dB) = Cable Attenuation + Connector Attenuation + Splice Attenuation
Loss Budget = (3.5 x 0.132) + (2 x 0.75) + (2 x 0.3) = 2.56dB

If the reading at the Power Meter is less than 2.56dB then the Link Under Test has Passed.

 

Conclusions

  • Always inspect all of the fibre end faces before commencing testing

  • Use good quality Test Reference Leads

  • Use Encircled Flux launch conditions

  • Set the reference level using the one jumper method prior to commencing testing (at least once per day)

  • Save all of the results in the tester ‘native’ format and store on a secure drive or media