How to Test Fiber Optic Cable Using an OTDR for Incoming Inspection

Written by Don Schultz, Senior Technical Marketing Specialist, Fluke Networks Certified Copper/Fiber CCTT, BICSI TECH, INSTC, INSTF Certified

You've just received a spool of fiber optic cable and you are excited to get it run and working! Many installers skip incoming inspection and regret it later. However, professional installers know something; they know to inspect it upon receipt. How is this accomplished? Well, you could opt for a simple VFL (Visual Fault Locator) or you can use the right tool for the job -- an Optical Time Domain Reflectometer (OTDR). If you wish to save a great deal of time and money, this is a step you should never skip, especially when time is money.

Why Fiber Inspection Matters — Don't Skip This Step

Fiber optic cable is more fragile than traditional metallic (copper) communications cable. Breaks and micro bends can occur during shipping and handling without any visible external damage to the spool or jacket. Manufacturer test results are performed at the factory before shipping — they do not reflect what happened to the cable in transit. A dropped pallet or tight bend during loading is enough to crack a fiber. To be sure, some fiber types are more susceptible to this than other varieties but it is still a good idea to inspect prior to installation. Read more in Top 10 Things You Should Never Do With Fiber Optic Cable.

Inspecting incoming cable before installation takes minutes, especially if the manufacturer already attached a connector (or connectors) at one end of the cable.

Discovering a break after the cable is pulled through conduit, terminated, and spliced takes hours — and may require completely replacing the run. It's important to inspect your fiber cable before you install it, since you can at least confirm the cable is defect free prior to installation and can pin down post-installation damage to a particular installer! Retraining, anyone?

What Is an OTDR?

• OTDR stands for Optical Time Domain Reflectometry
• An OTDR works by sending short pulses of powerful laser light down the fiber and measuring the light that is reflected back — similar in principle to how radar works, but with light instead of radio waves
• By analyzing the time it takes for reflections to return and the intensity of those reflections, the OTDR can calculate the location and severity of any anomaly in the fiber — splices, connectors, bends, breaks, and contamination
• The OTDR will use the normal “backscatter” light reflection naturally found in glass as the reference baseline to measure events in the fiber and distance
• OTDR testing is classified as Tier 1 testing in fiber qualification — the comprehensive baseline inspection standard
• An OTDR produces a visual output called a trace (or reflectogram) — a graph that maps the fiber's loss characteristics across its entire length, with events shown as distinct features on the trace

 

Example real OTDR trace. The smooth downward slope is called “attenuation” or signal degradation over distance and this is normal.

OTDRs are not only useful for simple incoming fiber inspection; they can be used for detailed troubleshooting post installation too (their primary use case is post installation)

OTDR vs. VFL — Why OTDR Wins for Inspection

• A VFL (Visual Fault Locator) is a simple, low-cost tool that shines visible red laser light down the fiber — breaks and severe bends glow red, making them visible to the naked eye
• VFLs are useful for confirming fiber continuity on short runs and fiber patch cords, quickly checking connectors, and finding obvious breaks near the ends of a cable
• VFLs cannot locate faults beyond a few hundred meters, detect microbends or gradual loss events, provide quantified loss measurements, or generate a documented test record
• An OTDR can do all of the above — it locates faults precisely (to within centimeters in some cases), measures loss at every event, and generates a trace that can be saved and shared as a test record
• An OTDR also can help experienced technicians diagnose specific faults depending on how they present themselves on the trace record

Example VFL. Notice the visible laser light emitting from the end connector.

Example VFL test with a high loss event in the cable. Light does not reach the end.

OTDRs will provide a graphical representation of any events in the fiber, where VFLs will require visually walking around to find any.

Considering a VFL requires your ability to literally see a red light as a disruption, you are not likely to see a less severe crack/bend if the cable is on a spool and the defective spot is buried. An OTDR has no such issue.

For incoming inspection of bulk fiber spools and post-installation troubleshooting, the OTDR is the right tool. The VFL is a supplement, not a substitute. Please see more on VFLs in Visual Fault Locators - A Guide to Fiber Optic VFL’s.
In fact, if you are familiar with copper communications cable (specifically Cat5e and above) you might have used a simple wiremap tester to check for continuity. A VFL is essentially the same thing for fiber optical cable. These devices are low cost and can provide meaningful (albeit extremely limited) diagnostic data.

How to Perform an Incoming Fiber OTDR Test — Step by Step Guide

Step 1 — Set Up the OTDR

Each brand/type of OTDR may have different setup procedures. Please follow the manufacturer instructions first and foremost! Many OTDR’s have a simple setup feature where you can run an autotest using minimal user input.

• Select the correct wavelength for your fiber type — typically 850nm and 1300nm for multimode; 1310nm and 1550nm for singlemode.
• Set the range to slightly longer than the expected fiber length — this ensures the full trace is captured.
• Set the pulse width — shorter pulses give better resolution for nearby events; longer pulses give better range for long cables. For incoming inspection of a spool, use a shorter pulse width.
• Connect a launch cable (also called a launch lead or dead-zone fiber) between the OTDR and the cable under test — this is a short length of factory certified and laboratory grade fiber that moves the OTDR's dead zone away from the connector being tested, allowing the near-end connector to be evaluated. This is a critical step many beginners skip!
• For single ended incoming fiber inspection, you will need to set a “1 jumper reference” that will reference out the launch fiber.

Note that most OTDR’s have a “warm up” time of 5 to 10 minutes before the light source stabilizes and will give accurate readings.

Step 2 — Connect the Fiber Optic Cable Under Test

Before you begin this step, you will need to terminate a connector onto the beginning of the fiber spool. Sometimes manufacturers will leave that connector on for your convenience after they factory test, or you can order the fiber with a connector on it. In other cases, the fiber will be raw and unterminated which means going through fiber termination. Please see Preparing your Fiber Optic Cable for Connectors or Splices.

For more on fiber cleaning, please see Optimal Fiber Performance Starts with Clean Fiber Connections.

If your bulk fiber spool arrives with a connector on it, you are in good shape! No need to terminate the fiber just to test the spool. HOWEVER, don’t assume that the connector is clean. Inspect and clean it!

You are now ready to connect up!

• Connect the launch cable to the fiber spool or cable being tested (the one that was used to set the “1 jumper reference”.
• For a spool of bulk fiber, you do not need to terminate the far end — the OTDR will detect the end of the fiber as a reflection event.
• Ensure the connector interface(s) is clean — a dirty connector is one of the most common sources of measurement error in OTDR testing. Use a fiber cleaning tool before connecting.
• Additionally, it is a good idea to clean any end faces or couplers used for the test to be sure you get accurate results.

Step 3 — Acquire the Trace

• Initiate the OTDR test — the device will send pulses and build the trace over several seconds to a few minutes depending on settings.
• Most modern OTDRs (Fluke, VIAVI, EXFO) have an auto-test mode that sets parameters automatically — this is a good starting point for incoming inspection. You will also get a pass or fail result (such as with Fluke)
• Once the trace is acquired, review it on the OTDR screen or export it for analysis.

Step 4 — Read the OTDR Trace

The trace shows optical power (dB) on the Y axis and distance on the X axis
A healthy fiber appears as a gradually sloping straight line — this slope represents the fiber's natural attenuation (signal loss per unit length).

Events to look for:

• Reflective events — sharp upward spikes followed by a drop. These indicate connectors, mechanical splices, or the end of the fiber. Expected at known connector locations; unexpected spikes elsewhere indicate damage.
• Non-reflective events (loss events) — a sudden step down in the trace without a spike. These indicate fusion splices, bends, or areas of localized loss. A small step at a fusion splice is normal; a large step indicates a poor splice or physical damage.
• End of fiber — the trace drops steeply at the far end of the cable. The OTDR calculates the distance to this point, confirming the cable length.
• Breaks — the trace ends abruptly before the expected cable length. The OTDR shows the distance to the break, allowing precise fault location.

For incoming inspection look for any unexpected loss events or reflective spikes along the cable length. A clean spool should show a smooth, consistent slope from end to end with only the expected start and end reflections.

What to Do If You Find a Fault in Your Fiber Optic Cable

• When inspecting incoming fiber, large contractors will test incoming spools before the delivery driver leaves. If you find faults, then reject shipment in part or full. Don’t take delivery! If that does not apply to you, or the driver dropped the spools and inspection could not be done immediately, then return per your supplier guidelines.
• In all cases, contact your supplier immediately. Document the fault location on the trace — the distance reading from the OTDR tells you exactly where in the spool the damage is. Most suppliers will work with you on a replacement if damage is documented before installation.

Conclusion

So there you have it! OTDR testing is not optional for professional fiber installations — it's the standard. Incoming inspection before pulling cable is the single highest-ROI testing step you can take. Nobody wants to pull thousands of feet of fiber and then discover the fiber was defective prior to installation. Buy an OTDR and then take a look at trueCABLE’s collection of fiber products too! We offer a growing selection of multimode bulk fiber, single mode bulk fiber, fiber patch cords, pigtails, and fiber tools.

HAPPY NETWORKING!

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