FOA Basic Skills

Lesson: How Fiber Works And How To Find Faults

Objectives: From this lesson you should learn:
How fiber transmits light
How a visual fault locator (VFL) can be used to troubleshoot fiber optic cable
How fiber transmits signals as pulses of light

Safety Glasses
Visual Fault Locator (VFL)

Tools for stripping fiber optic cable

Fiber optic cable



Lennie works safely

Always wear safety glasses when doing any of these exercises and dispose of all fiber scraps properly.

Safety Rules - Read before beginning any exercises.

Do not look directly into the light from the visual fault locator - it's bright!


Attach your cable to the Visual Fault Locator (VFL)

2. Turn the VFL on and note how the light travels through the fiber. You can see how bright the glow is at the end of the fiber, diffused through the fiber protective cap. This shows how fiber transmits light by total internal reflection as you learned in the lesson on optical fiber.

3. Gently put bends in the cable and see what happens. The glow you see is the light lost in the bend you caused. Stress on a fiber can cause loss and the lost light shines through the jacket of the fiber.

4. Remove the bend on the cable and see how the light disappears when you remove the stress on the fiber. Look closely and you can probably see how the light at the end of the fiber changes as you cause light loss at the bend you put in the fiber.

5. In later lessons you will see how we use visual light from the VFL to help in splicing and terminating.

Watch this to see how it's done:


6. The VFL has both continuous and pulsed modes to help locate fibers and find faults. That helps us understand how fiber optics sends communications with light.

7. Change the VFL mode from continuous light to pulsed using the switch on the unit. Now you can see how the light is pulsed - the VFL is sending pulses of laser light down the fiber which you can see at the far end. That's the same way fiber optic communications systems work - sending pulses of light - digital 1s and 0s - that are encoded digital data. The big difference in that a
fiber optic communications system is transmitting pulses at millions or billions of times faster than the VFL! And it transmits infrared laser light invisible to the human eye.

Watch this to see how it's done:


You have successfully completed this exercise when you have observed the use of the VFL to show how fiber transmits light and communications signals as pulses of light.

Complete the exercise and fill in your

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This information is provided by The Fiber Optic Association, Inc. as a benefit to those interested in teaching, designing, manufacturing, selling, installing or using fiber optic communications systems or networks. It is intended to be used as an overview and/or basic guidelines and in no way should be considered to be complete or comprehensive. These guidelines are strictly the opinion of the FOA and the reader is expected to use them as a basis for learning, as a reference and for creating their own documentation, project specifications, etc. Those working with fiber optics in the classroom, laboratory or field should follow all safety rules carefully. The FOA assumes no liability for the use of any of this material.


Table of Contents: The FOA Reference Guide To Fiber Optics

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