Fiber Optic Testing, Lesson 4: Testing Insertion Loss
Objectives: From this self-study lesson you should learn:
- How does one test an installed cable plant and most components?
- What is insertion loss?
- How does insertion loss differ from OTDR testing?
- How do tests differ for cable plants, individual cables or patchcords and components?
- Who is installing FTTx?
There are two distinct ways to test a cable plant and most components, using insertion loss or OTDR testing.
Insertion loss refers to a test method that is similar to how a network actually transmits data through an optical fiber. A test source coupled to a launch cable injects a test signal into the fiber. The test signal will be attenuated by the connection into the cable plant under test, all the fiber in the link and every connection or splice. By using a power meter to measure the output of the test source and launch cable, then measure the signal at the far end, one calculates the loss as the difference between these two meter measurements. Insertion loss is a simple, direct way of measuring loss and if properly done accurately represents the loss of the cable plant that a typical transmission system will see. Insertion loss testing will work on any component or cable plant.The big issue for insertion loss testing is how to set the "0dB" reference - using one, two or three reference cables.
In the next lesson, #5, we will cover testing by the instrument called an OTDR - optical time domain reflectometer - which uses a very different indirect method of testing fiber that provides different kinds of information than insertion loss testing. The OTDR sends a pulse down the fiber and looks at the light scattered by the fiber back to the OTDR to create a "snapshot" of the fiber. That snapshot can be analyzed to provide information about the length and loss of the fiber, but as an indirect method has several ways it differs from a direct insertion loss test.
So this lesson will give you a good understanding of insertion loss testing of various components and discuss the uncertainty of measurements.
Testing gives you data - in this case the loss of the fiber in a cable plant. But does that test data tell you the cable plant loss is good (low enough) or bad (loss is too high)? You need to know that to pass or fail any fiber being tested. The way you estimate the loss of the cable plant is to calculate a "loss budget" for the fibers in a cable plant. Generally the loss budget is done during the design phase of a project and the data used to validate the design for use with communications equipment. But that same data should be used by the installer/tester to compare to test results and make the pass/fail judgement.
The course leads you to read online or printed materials (with an emphasis on the online using the FOA Guide), watch FOA YouTube Videos, complete some activities and take the quizzes. Each lesson plan will be self-contained. Lesson plans open in new pages so the course overview page stays open to lead you to the next lesson or you may use the link at the bottom of the page.
Read the references, watch the videos and take the quizzes (Test Your Knowledge)
FOA YouTube Videos
FOA Lecture 16: Insertion Loss Testing (How testing with a light source and power meter measures loss similar to the way a communications system uses the fiber.)
FOA Lecture 15: Five Ways To Test Fiber Optic Cable Plants (There are indeed 5 ways to test a cable plant, all covered in international standards, and this video will explain why. Be certain to think about this when you read "The Math of Insertion Loss Testing" on the Online Reference.)
FOA Lecture 26: Loss Budgets (Estimating the loss of a cable plant to compare to test results)
Online FOA Reference
FOA Reference Guide to Fiber Optics
Testing The Installed Fiber Optic Cable Plant, (The explanation of how to test a cable plant, including the options available.),
5 different Ways To Test Fiber Optic Cables According To International Standards (There are indeed 5 ways to test a cable plant, all covered in international standards, and this section will explain why.)
Calculating a loss budget, also What Loss To Expect When Testing Fiber Optic Cables (Estimating the loss of a cable plant to compare to test results)
Patchcord or Single Cable Testing (Different from cable plant testing; allows diagnosing bad connectors on one end.)
Accurately Testing Fiber Optic Cables and Metrology and Fiber Optics (What are the sources of measurement error in fiber optic measurements?)
The Math of Insertion Loss Testing - Reference Methods (This section explains how the test results from all the different measurement methods differ and why documenting the test method is important.)
Reference Cables (Chosing and using reference cables)
FOA Reference Guide to Fiber Optics, Chapter 8 (Testing) and loss budget section in Chapter 9 (Design)
FOA Reference Guide to Outside Plant Fiber Optics, Chapter 8 and loss budget section in Chapter 9 (Design)
Extra Credit Reading
While we call these "extra credit" you may find them very informative and useful for special testing issues.
You Tube: FOA Lecture 22 Mode Power Distribution in Multimode Fibers Demonstration and Modal Distribution Effects on Multimode Fiber and Cable Measurements (This is a major effect in measurement uncertainty when testing multimode fibers.)
Special Applications/Hybrid Cables (A extension of the explanations in the "5 Ways" article/video that gives more explicit directions.
Testing cables with different types of connectors. (Complements the section on Hybrid Cables)
Loss by Cable Substitution - when other methods will not work (Especially important with strange connectors or POF.)
Connector and Splice Loss Testing (Lab tests, not field, used for R&D or component qualification.)
Data Link or Network Testing (While many installers never test a system, end users and those troubleshooting systems need to know this.)
Test Your Knowledge
Online Quiz - Testing, Online Quiz - Loss Budgets
Next Lesson: #5 OTDR Testing