How to Certify Optical Fiber Cabling with an OTDR

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There are different types of OTDRs available. TIA 568C.0 and ISO 14763-3 recommend OTDR testing as a complementary test to ensure the quality of fiber installations meets component specifications. The standards don’t designate Pass/Fail limits for this test. You should consider generic cabling requirements for components and design criteria for the specific job. You can use an OTDR as a single ended tester, bidirectionally (if desired) and optionally with a receive fiber for certification testing.

What you need to know about OTDRs. OTDRs used to be hard-to-operate laboratory equipment – impractical for field use. They were big, heavy and complicated for inexperienced technicians to set up and operate. Test results were difficult to understand. This created fear and confusion. Today, however, many new OTDRs are small, light and easy to use. An ordinary technician can troubleshoot like an expert – but a basic understanding of ow an OTDR works is still helpful.

• Basic operation – An OTDR infers loss, reflectance and location of events. It sends pulses of light into a fiber and uses a sensitive photo detector to see the reflections and plot them graphically over time. In order to accurately test, the optical characteristics of the fiber must be determined and set prior to testing.

• OTDR trace – The OTDR plots the reflectance and loss over time in a graphical “trace” of the fiber. Experienced technicians can “read a trace” and explain it. For example, in figure 11, an experienced eye can spot that one side of a cross connect shows excessive loss.

• Event analysis software – The latest OTDRs run sophisticated software that automates trace analysis and set up of test parameters. Fluke Networks’ OTDRs can automatically choose setup parameters, not only telling you where events (instances of reflectance and loss) are on the trace, but also indicating what the events are and qualifying each of them.

• Dead zone – The length after a connector, splice, break or macro-bend along the fiber cabling where the OTDR can make an attenuation measurement or differentiate between closely spaced events such as connectors. Event dead zone is the minimum distance between two consecutive reflective events that the OTDR can make a measurement. Attenuation dead zone is the minimum distance after a reflective event that the OTDR can make a loss measurement.

• Dynamic range – Determines the length of fiber that can be tested. The higher the dynamic range, the longer the fiber-under-test can be. However, as the dynamic range increases, the wider the OTDR pulse becomes and as a result, the dead zone increases.

• Ghosts – Not as scary as they might seem, ghosting is caused by an echo due to highly reflective events in the link under test. Fluke Networks’ OTDRs identify ghosts on the trace and tell you where the source of the ghost is so you can eliminate it.

• Gainers – Another misunderstood phenomenon on an OTDR trace is a gainer. Simply put, a gainer is an apparent negative loss at an event where there is a change in the optical performance. This is usually due to a mismatch between the index of refraction of two spliced fibers or connection of a 50 µm multimode fiber into a 62.5 µm fiber. This type of event will often exhibit excessive loss in the other direction.

OTDR built specifically for the enterprise.

The OptiFiber Pro OTDR is an Optical Time Domain Reflectometer that locates, identifies and measures reflective and loss events in multimode and singlemode fibers. Typical maximum test ranges are less than 35 km at 850nm and/or 1300 nm wavelengths for multimode fiber and well below the instrument’s range of 130 km for singlemode fiber which should be typically tested at 1310 and/or 1550 nm for singlemode fiber.

The tester can show the OTDR results in three formats (Figure 12):

• Table shows a table of the events on the fiber. Use this screen to quickly see measurements for all events and see the types of events on the fiber. The table includes the distance to the event, the loss of the event, the size of the reflection from the event and the type of the event. To see details for an event, tap the event in the table.

• EventMap™ shows a diagram of the events on the fiber, the fiber length and the overall loss of the fiber. Use this screen to quickly locate connectors and faults on the fiber. To see details for an event, tap the event in the map, then tap the information window for the event.

• Trace shows the OTDR trace. Use this screen to see the dead zones of reflective events and examine the characteristics of unexpected events such as ghosts and gainers.

– The characteristics of the link will show in the EventMap

– Note: Test result (Pass/Fail) and events shown depend on the characteristics of the test link

– Tap “Summary Bubble” to look at the details

EventMap™ View – In figure 13 is an example of a PASSING test using launch and tail fiber

• Different icons distinguish events such as:

– Passing reflective event

– Failing reflective event

– Hidden reflective event

– Passing loss event

– Failing loss event

– Hidden event’s loss is added to previous event’s loss

• Details are provided for the event’s loss, reflectance and segment attenuation.

Trace View – When testing a fiber link or channel, change the wavelength easily to view the trace. Zoom in to see detail loss info and event characteristics.

• Jump to next/previous event

• Pinch to zoom out

• Reverse pinch to zoom in

• Drag to move the trace

• Slide to adjust x or y zoom

• Double tap to zoom to 100%

As mentioned at the beginning of this section, there are different types of OTDRs available. The OptiFiber Pro OTDR, with its advanced feature set and capabilities, would be an excellent choice for certifying enterprise fiber  cabling. But, since many contractors and network owners use the DTX Compact OTDR for “closet to closet” certification, we will demonstrate how to perform this type of testing with the DTX Cable Analyzer view.

OTDR certification set-up

Setting up for OTDR Certification Testing Setup: Turn the rotary switch to “Setup” and choose “Settings” from menus in five setup screens.

1. First, select which port you want to test from (multimode or singlemode), which test limit you want to use, the fiber type and desired wavelength.

• It is possible to create multiple sets of OTDR test limits and select one for a particular job. Each OTDR test passes (Figure 16) or fails (Figure 17) based on a comparison against the selected set of test limits.

2. On the second setup screen, you can set launch fiber compensation, designate which end you are testing from and note what you want to call each end of the fiber.

Using launch fiber compensation (LFC)

Launch fiber compensation is used to simplify testing and remove the launch and receive fibers’ losses and lengths from measurements (See figure 15).

• It shows where your launch (and/or receive) fiber is on the trace, and eliminates it from the certification test results. If you are a contractor, your customers want to know where an event is in their fiber plant, not where it is on your test setup. When you enable “LFC”, a connector 50m from the patch panel will show up at 50m, not 150m on the trace. Just turn the rotary switch to “Setup”, go to the second tab and enable “Launch Fiber Compensation”. Then turn it again to “Special Functions”, and choose “Set Launch Fiber Compensation”. Choose “Launch” only if you are just using a launch fiber, or “Other Options” if you are also using a receive fiber.

3. Third, designate the fiber characteristics, allow default to the selected fiber in the first step or choose “User Defined” and select “Numerical Aperture” and “Back-scatter coefficient” for the fiber-under-test.

4. Now choose from the menu to set “Distance Range”, “Averaging Time”.

5. Finally, choose from the menu to set “Pulse Widths” and “Loss Threshold”. With the DTX Compact OTDR, many settings such as “Distance Range”, “Averaging Time”, “Pulse Widths” and “Loss Threshold” can be automatically set. Just turn the rotary switch to “Autotest” and when you push the “Test” button, the OTDR will choose the most appropriate setting for the fiber you are testing.

Running an autotest. Now that you’re all set up for testing, turn the dial to “Autotest”, plug in your launch fiber and press “Test”. If it passes, press “Save”, name the test and test the next fiber. If you want to see a trace, just press the f1 softkey. The event table and limits are also accessible via softkeys on the main screen.

Summary of extended certification

• OTDR traces characterize the individual components of a fiber link: connectors, splices and other loss events. Extended certification compares the data to specifications for these events to determine if they are acceptable

• Critical because it identifies faults that may be invisible to basic certification

• Evidence that every component in a fiber optic cabling system was properly installed.

See Pass result at 1550nm in figure 18 and at 850nm in figure 19 Figure 19 – “Pass” trace screenshot on the DTX Compact OTDR

Setting up for OTDR Certification Testing

Setup: Turn the rotary switch to ‘Setup’ and choose ‘Settings’ from menus in five setup screens.

1. First, select which port you want to test from (multimode or singlemode), what test limit you want to use, the fiber type, and desired wavelength.

• It is possible to create multiple sets of OTDR test limits and select one for a particular job. Each OTDR test passes (Figure 6) or fails (Figure 7) based on a comparison against the selected set of test limits.

2. On the second setup screen, you may then set launch fiber compensation, designate which end you are testing from, and notate what you want to call each end of the fiber.

Using launch fiber compensation (LFC) Launch fiber compensation is used to simplify testing and remove the launch and receive fibers’ losses and lengths from measurements.

• It shows you where your launch (and/or receive) fiber is on the trace, and eliminates it from the certification test results. If you are a contractor, your customers want to know where an event is in their fiber plant, not where it is on your test setup. When you enable ‘LFC’, a connector that is 50m from the patch panel will show up at 50m, not 150m on the trace. Just turn the rotary switch to ‘Setup’, go to the 2nd tab, and enable ‘Launch Fiber Compensation’. Then turn it again to ‘Special Functions’, and choose ‘Set Launch Fiber Compensation’. Choose ‘Launch’ only if you are just using a launch fiber, or ‘Other Options’ if you are also using a receive fiber.

3. Third, designate the fiber characteristics or allow default to the selected fiber in the first step or choose ‘User Defined’ and select ‘Numerical Aperture’ and ‘Back-scatter coefficient’ for the fiber-under-test.

4. Now choose from the menu to set ‘Distance Range’, ‘Averaging Time’.

5. Finally, choose from the menu to set ‘Pulse Widths’ and ‘Loss Threshold’. With the DTX Compact OTDR, many settings such as ‘Distance Range’, ‘Averaging Time’, ‘Pulse Widths’, and ‘Loss Threshold’ can be automatically set. Just turn the rotary switch to ‘Autotest’, and when you push the test button, the OTDR will choose the most appropriate setting for the fiber that you are testing.

Running an autotest. Now that you are all set up for testing, turn the dial to ‘Autotest’, plug in your launch fiber and press ‘Test’. If it passes, press ’Save’, name the test, and test the next fiber. If you want to see a trace just press the f1 softkey. The event table and limits are also accessible via softkeys on the main screen.

Summary of extended certification

• OTDR traces characterize the individual components of a fiber link: connectors, splices and other loss events. Extended certification compares the data to specifications for these events to determine if they are acceptable

• Critical because it identifies faults that may be invisible to basic certification

• Evidence that every component in a fiber optic cabling system was properly installed