FTTH Handbook/FTTH Test Guidelines

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Chapter 11

Contents

[edit] Connector care

[edit] Why is it important to clean connectors?

One of the first tasks to perform when designing fibre-optic networks is to evaluate the acceptable budget loss in order to create an installation that will meet the design requirements. To adequately characterize the budget loss, the following key parameters are generally considered:

  • transmitter – launch power, temperature and aging
  • fibre connections – connectors and splices
  • cable – fibre loss and temperature effects
  • receiver – detector sensitivity
  • others – safety margin and repairs

When one of the above variables fails to meet specifications, network performance can be affected; in worst case scenario, degradation can lead to network failure. Unfortunately, not all variables can be controlled with ease during the deployment of the network or the maintenance stage; however, one component that is often overlooked is the connector, sometimes overused (test jumpers). This can be controlled using the proper procedure.

CONNECTOR CONTAMINATION IS THE #1 SOURCE 
OF TROUBLESHOOTING IN OPTICAL NETWORKS

A single particle mated into the core of a fibre can cause significant back reflection (also known as return loss), insertion loss, and equipment damage. Visual inspection is the only way to determine if fibre connectors are truly clean.

By following a simple practice of proactive visual inspection and cleaning, poor optical performance and potential equipment damage can be avoided.

Since many of the contaminants are too small to be seen with the naked eye, it is important that every fibre connector is inspected with a microscope before a connection is made. These fibre inspection scopes are designed to magnify and display the critical portion of the ferrule where the connection will occur.

[edit] What are the possible contaminants?

Connector design and production techniques have eliminated most of the difficulties in achieving core alignment and physical contact. However, maintaining a clean connector interface still remains a challenge.

Dirt is everywhere; a typical dust particle as small as 2–15μm in diameter can significantly affect signal performance and cause permanent damage to the fibre end face. Most field-test failures can be attributed to dirty connectors; the majority are not inspected until they fail, when permanent damage may have already occurred.

If dirt particles become attached to the core surface the light becomes blocked, creating unacceptable insertion loss and back reflection (return loss). Furthermore, those particles can permanently damage the glass interface, digging into the glass and leaving pits that create further back reflection if mated. Also, large particles of dirt on the cladding layer and/or the ferrule can introduce a physical barrier that prevents physical contact and creates an air gap between the fibres. To further complicate matters, loose particles have a tendency to migrate into the air gap.

Figure 99: Increased insertion loss and back reflection due to dirty fibre connection.

A 1μm dust particle on a singlemode fibre core can block up to 1% (0.05 dB loss) of the light; a dust particle the size of 9μm can incur considerable damage. An additional factor for maintaining endfaces contaminate free is the effect high-intensity light has on the connector end-face: some telecommunication components can produce optical signals with a power up to +30dBm (1W), which can have catastrophic results when combined with an unclean or damaged connector end face (e.g. fibre fuse).

Inspection zones are a series of concentric circles that identify areas of interest on the connector end face (see figure). The inner-most zones are more sensitive to contamination than the outer zones.

Figure 100: Connector end face inspection zones.

Dust, isopropyl alcohol, oil from hands, mineral oils, index matching gel, epoxy resin, oil-based black ink and gypsum are among the contaminants that can affect a connector end-face. These contaminants can occur on their own or in combinations. Note that each contaminant has a different appearance and, regardless of appearance, the most critical areas to inspect are the core and cladding regions where contamination in these regions can greatly affect the quality of the signal. Figure below illustrates the endface of different connectors that have been inspected with a video-inspection probe.

Figure 101: Appearance of various contaminants on a connector end-face.

[edit] Where do we need to inspect and clean?

12.1-where-inspect.jpg

The following network components should be inspected and cleaned:

  • all panels equipped with adaptors where connectors are inserted in one or both sides
  • test patch cords
  • all connectors mounted on patch cables or pigtails


[edit] When should a connector be inspected and cleaned?

Connectors should be checked as part of an inspection routine to prevent costly and time consuming fault finding later. These stages include:

  • on delivery
  • before installation
  • before testing

[edit] How to check connectors

To properly inspect the connector’s endface, a microscope designed for the fibre optic connector endface is recommended. The many types of inspection tools on the marke fall into two main categories: fibre inspection probes (also called video fibrescopes) and optical microscopes.

The table below lists the main characteristics of these inspection tools:

Inspection tool Main characteristics
Fibre inspection probes/ video fibrescopes Image display on an external video screen, PC or test instrument.

Eye protection from direct contact with a live signal. Image-capture capability for report documentation.
Ease of use in crowded patch panels.
Ideal for checking patchcords, patch panels, and multi-fibre connectors (e.g. MTP).
Different degrees of magnification available (100X/200X/400X).
Adapter tips for all connector types available.

Optical microscopes Safety filter* protects eyes from direct contact with a live fibre.

Two different types of microscopes needed: one to inspect patchcords and a different one to inspect connectors in bulkhead-patch panels.

* Never use a direct magnifying device (optical microscope) to inspect live optical fibre.


A fibre inspection probe comes with different tips to match the connector type: angle-polished connectors (APC) or flat-polished connectors (PC, SPC or UPC).

[edit] Inspection instructions

Visual fibre interconnect inspection is the only way to determine the cleanliness of the connectors prior to mating. A video microscope magnifies an image of a connector’s end face for viewing on a laptop or a portable display, depending on the product used.


12.6.1-inspect-clean-process-flow.png

INSPECT 1. Select the appropriate tip for the connector/adaptor you are inspecting.

2. Inspect both connector end faces (patchcord/bulkhead/ pluggable interface) using the microscope.

IS IT CLEAN?
CLEAN No – upon inspection, if defects are found on the end-face, clean the connector using a designed-for optics cleaning tool.
CONNECT Yes – if non-removable non-linear features and scratches are within acceptance criteria limits according to operator’s thresholds or standards, the fibre interfaces can be connected.

[edit] Tools needed for inspection

There are two methods for fibre end-face inspection. If the cable assembly is accessible, you can insert the connector ferrule into the microscope to do an inspection; this is generally known as patchcord inspection. If the connector is within a mating adaptor on the device or patch panel, you can insert a probe microscope into the open end of the adaptor and view the connector inside; this is known as bulkhead or through adaptor connector inspection.

Patchcord inspection

  1. Select the appropriate tip that corresponds to the connector type under inspection and fit it on to the microscope.
  2. Insert the connector into the tip and adjust focus to inspect.
12.1.7-patchcord-inspection.png

Bulkhead/through adaptor connector inspection

  1. Select the appropriate tip/probe that corresponds to the connector type under inspection and fit it to the probe microscope.
  2. Insert the probe into the bulkhead and adjust focus to inspect.
12.1.7-bulkhead-inspection.png

[edit] Cleaning wipes and tools

Dry Cleaning

Simple dry cleaning wipes including many types of lint-free wipes and other purpose-made wipes are available. This category also includes purpose-made fibre-optic connector cleaning cassettes and reels, e.g. Cletop cartridges.

WARNING! EXPOSED WIPES CAN EASILY
BECOME CONTAMINATED IN THE FIELD.
Figure 102: Examples of dry cleaning wipes and tools for fibre-optic connectors.

Cleaning materials must be protected from contamination. Do not open until just prior to use.

Wipes should be used by hand or attached to a soft surface or resilient pad. Applied using a hard surface can cause damage to the fibre. If applying by hand, do not use the surface held by the fingers as this can contain finger grease residue.

Damp cleaning

Figure 103: Examples of cleaning fluid and wipes.

Cleaning fluids or solvents are generally used in combination with wipes to provide a combination of chemical and mechanical action to clean the fibre end-face. Also available are pre-soaked wipes supplied in sealed sachets, e.g. IPA mediswabs. Caution: some cleaning fluids, particularly IPA, can leave a residue that is difficult to remove.

  • Cleaning fluid is only effective when used with the mechanical action provided by a wipe.
  • The solvent must be fast drying.
  • Do not saturate as this will over-wet the end-face. Lightly moisten the wipe.
  • The ferrule must be cleaned immediately with a clean dry wipe.
  • Do not leave solvent on the side walls of the ferrule as this will transfer onto the optical alignment sleeve during connection.
  • Wipes must be used in the hand or on a soft surface or resilient pad.
  • Use on a hard surface can cause damage to the fibre.

Bulkhead / through adaptor connector cleaning tools

Not all connectors can be readily removed from a bulkhead/through adaptor, and are, therefore, more difficult to access for cleaning. This category includes ferrule interface (or fibre stubs) and physical contact lenses within an optical transceiver; it does not however include non-contact lens elements within such devices.

Sticks and bulkhead cleaners are designed to reach into alignment sleeves and other cavities to reach the endface or lens, and aid in removal of debris. These tools make it possible to clean the endface or lens in-situ, within the adaptor or without removing the bulkhead connector. When cleaning transceiver or receptacles, care must be taken to identify the contents of the port prior to cleaning. Take care to avoid damage when cleaning transceiver flat lenses.

Figure 104: Examples of bulkhead/through adapter cleaning tools.

Recommendations when manipulating fibre-optic cables:

  • When testing in a patch panel, only the port corresponding to the fibre under test should be uncapped — protective caps should be replaced immediately after testing.
  • Unused caps should be kept in a small plastic bag.
  • The lifespan of a connector is typically rated at 500 matings.
  • Test jumpers used in conjunction with test instruments should be replaced after a maximum of 500 matings (refer to EIA-455-21A).
  • If using a launch cord for OTDR testing, do not use a test jumper in between the OTDR and launch cord or in between the launch cord and the patch panel. Launch cords should be replaced or sent back to manufacturers for re-polishing after 500 matings.
  • Do not allow unmated connectors to touch any surface. Connector ferrules should never be touched other than for cleaning.
  • Inspect each connector using a fiberscope or, preferably, a videoscope, after cleaning or prior to mating.
  • Test equipment connectors should be cleaned and inspected (preferably with a videoscope) every time the instrument is used.

[edit] Testing FTTH networks during construction

During network construction, some testing occurs in the outside plant. When fibre is laid down new splices have to be done and tested using an OTDR. For accurate measurements, bidirectional OTDR measurements should be performed.

For acceptance testing, it is important to test each segment of the construction. There are several testing methods, some of which are presented here. Each has specific advantages and disadvantages. Selecting the most appropriate method depends on the constraints faced: labour costs, budget loss, testing time combined with service activation time, maximum acceptable measurement uncertainty, etc.

An additional factor that must be considered when determining extent of testing is the skill levels of the technicians. Employing un-skilled fibre optic technicians during construction phase is very costly if mistakes need to be rectified ahead of and after service is added.

[edit] Method #1: Use of optical loss test sets

This first method involves using an optical loss test set (OLTS), comprising two test sets that share data to measure insertion loss (IL) and optical return loss (ORL). First, the two units should be referenced prior to measuring IL.

Figure 105: Test sets should be referenced prior to measurement.

Next, ORL sensitivity is set by calibrating the minimum ORL that the units can measure. The limitation comes from the weakest part of the test setup, which is most likely to be the connector between the units and reference test jumper. Follow the manufacturer’s instructions to set the ORL sensitivity on both units and to reference the source and the power meter.

Now you are ready to perform measurements on the end-to-end network or any individual installed segment, such as the fibres between the FCP and the drop terminal. The purpose of the test is to identify whether there are any transposed fibres, and measure the IL and ORL to make sure that the loss budget has been met.

Figure 106: Measuring distribution fibre IL and ORL using a pair of OLTS.
Results table for IL and ORL (Pr = premises, CO = central office)

The following table illustrates the expected ORL values for the network:

Length (metres) 1310nm (dB) 1490nm (dB) 1550nm (dB)
50 53 56 57
300 46 50 50
500 44 47 48
1000 41 45 46

These values only take into account two connections. FTTH networks often comprise of multiple connection points and, as reflectance values are very sensitive to dust and scratches, these values can easily be influenced by bad connections. For example, a single connector may generate an ORL of 40dB, which would exceed the expected value for the entire network. For point-to-multipoint network, the ORL contribution of each fibre is attenuated by 30 to 32 dB due to the splitter’s bidirectional loss.

Advantage of Method #1: OLTS Disadvantages of Method #1: OLTS
Accurate IL and ORL measurement Two technicians required (however with point-to-multipoint network, a single OLTS close to the OLT can be used for all customers within the same network)
Bidirectional IL and ORL values Communication required between technicians (when switching fibres)
Possibility to test every distribution fibre With a point-to-multipoint network, one technician needs to move from drop terminal to drop terminal
Macrobend identification during testing is performed at 1550 and 1310 nm or at another combination of wavelengths involving the 1625 nm wavelength In the event of a cut fibre or macrobend, an OTDR is required to locate the fault.
Transposed fibre identification on point-to point networks Impossible to detect transposed fibre on point-to-multipoint network
Fast testing  

[edit] Method #2: Use of an OTDR

This method uses an optical time-domain reflectometer (OTDR). Unlike an OLTS, the OTDR can identify and locate the position of each component in the network. The OTDR will reveal splice loss, connector loss and reflectance, and the total end to end loss and ORL.

Figure 107: Measurement with an OTDR.

All fibres between the OLT and before the first splitter (transport side) may be tested to characterize the loss of each splice and locate macrobends. The test can be conducted to cover both directions. Post-processing of the results will be required to calculate the real loss of each splice (averaged between each direction).

The engineer can measure the loss of the splitter and the cumulative link loss, as well as identifying whether any unexpected physical event has occurred before, or after, the splitter. Construction testing can significantly reduce the number of problems that occur after subscriber activation by certifying end-to-end link integrity.

Figure 108: Using a launch fibre makes it possible to characterize the first connector on any segment of your network. A pulse width of 300-500m will be sufficient for this test.
Figure 109: PON optimised OTDR test from the ONT to the OLT.


Advantages of Method #2: OTDR Disadvantages of Method #2: OTDR
Measures both IL and ORL values. When testing after the splitter on the ONT side, the ORL is not measured in the right direction (opposite from the video signal).
Possible to test every distribution fibre. The technician needs to move from drop terminal to drop terminal.
Macrobend identification during testing is performed at 1550 and 1310nm or at another combination of wavelengths involving the 1625nm wavelength. It requires a skilled technician to interpret the trace.
In case of a cut fibre or macrobend, the fault can be located.  
Only one technician required.  
Fast testing  


[edit] Service activation

The service activation phase may seem very straightforward initially, however this task should not be underestimated as this is the moment at which the subscriber experience begins. The service activation scheme can be different depending on topology of the fibre network. The trend is for pre-engineered plug-and-play components with multiple connection points, rather than an all-spliced approach, particularly for deployments in MDUs.

In terms of handling data relating to test and measurements in PON, the service activation brings two new dimensions:

  1. Results should be linked to customers or ONUs instead of fibres.
  2. More than one test location may be required, typically two or three.

Since the service-activation phase is often performed by subcontractors, reporting and data authenticity protection are important, especially in PON deployments where hundreds of results may be generated for a single PON activation. Following the right steps in daily activity ensures a smooth workflow and high productivity.

Figure 110: Activation testing using a PON power meter.

Multiple testing locations

Verifying optical levels at various locations along the same fibre path assists the test engineers in pinpointing problems and/or defective components before activating a subscriber’s service. Since FTTH network problems are often caused by dirty or damaged connectors, component inspection greatly reduces the need for troubleshooting, as power levels are verified for each network section. It is also strongly recommended that inspection of each connection point be conducted using a fibre inspection probe before each power measurement.

Testing points

Figure 111: Testing points in PON.
  1. By performing a power-level certification at the splitter, or more specifically at the output, enables users to verify if the splitter branch is working properly. This simple assessment makes it possible to confirm whether all network components from the CO (including the feeder fibre) to the splitter output are in good condition. Typically, the FDH includes SC/APC or LC/APC connectors but may also include fusion splices.
  2. Conducting a power-level certification at the drop terminal, engineers can characterize the distribution fibre and the drop terminal ports. Usually, a splice tray is included within the drop terminal which can cause macrobend problems.
  3. The fibre connecting the drop terminals and the subscriber’s premises is generally installed during service activation. To ensure reliable services to the subscriber, the network and the subscriber ONU must meet their specifications. The best method of guaranteeing this is to perform a pass-through connection to fully characterize all operating wavelengths (upstream and downstream) in the PON. This can only be achieved at the service-activation phase using a dual-port PON power meter with a pass-through connection; a normal power meter can only certify downstream signals from the CO.
Figure 112: Pass-through testing of all wavelengths.

[edit] Service activation reporting

Back at the office, engineers will have to generate reports to keep track of test results at the service activation phase. These results can be used later to pinpoint problems such as power degradation. Operators dealing with subcontractors may also use this information to keep track of activated subscribers.

A service activation report will typically include:

  1. customer name and/or phone number
  2. power level for each wavelength and each location
  3. time stamp for each measurement
  4. pass/warning/fail status compliant to standards such BPON, GPON or EPON
  5. thresholds used to perform the pass/warning/fail assessment
Figure 113: Service activation report.

Once the service activation report has been received from the installer, the operator can activate and validate the services.

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