Lesson 4 Lab Experiment (s)

Radars and Light Sabers - How Fibers are Tested

These are the lab experiments for Lesson 4. Please conduct these experiments under direct supervision of an adult including a parent or teacher.

Experiment #1 - Create Reflected Wave

Experiment #2 - Observe Reflected Light Path

Experiment #1 - Create Reflected Wave
Objective: Observe Wave Motion to explain OTDR principles

Required Materials:
1. 20 foot length of string
2. Doorknob or equiv.

In this experiment, we will use a string to generate a wave, observing wave motion and properties. Tie one end of the string to the doorknob on a closed door as shown below.

Step back from the door as far as possible, bringing the string into a straight line, but not too tight. Use a quick up and down motion to cause a ripple or wave motion in the string while releiving tension just enough to allow the wave to travel at a good height. Observe how the wave travels along the string toward the door, then is reflected back toward you.

Creating Wave With String

Try generating the wave sideways as well. Adjust your technique to generate the highest wave that still travels the entire length of the string. Several observations on wave motion:

1. Short, quick motions generate smaller waves that are more closely spaced
2. Higher, longer motions generate larger waves that are farther apart
3. The higher the wave generated, the higher the reflection

This demostrates the way an OTDR is able to measure an optical fiber from the inside out. The OTDR generates a series of pulses, which travel along the fiber, sending back reflections when the waves encounter changes. The changes are called "events", and provide a very good picture of the ability of the fiber to pass light.

These same wave principles apply to police and airport radar systems in that they transmit pulses or radio energy and measure the time it takes for the reflections to be received back. Since there is a constant for travel time of radio signals in the air (nearly the speed of light), which is 300000 km/s (kilometers per second, or 186,000 miles per second). Since the travel speed is know it is a simple thing to measure the round-trip delay from transmit to receive and divide by two (2) to attain the one-way speed and calculate the distance to an event.

Experiment #2 - Observe Relected Light Path
Objective: Observe how light is reflected along the fiber Core

Required Materials:
1. Wall-mounted mirror or at least 4 feet wide (bathroom mirror)
2. Handheld mirror
3. Flashlight

To observe the way light travels along the Core of an optical fiber, position the handheld mirror along the wall mirror as shown below. Point the flashlight at the wall mirror at an angle that causes the light to reflect from the wall mirror to the handheld mirror and back again to the wall mirror as shown.

Observing Light Travel in Optics

Conclusions

As light travels along the Core of an optical fiber, it glances off the sides in a forward motion. As discovered in Lesson 4, if the bend radius of the fiber is too acute, most of the optical signal penetrates the cladding and is lost.

This concludes the experiments for Chapter 4- Radars and Light Sabers - How Fibers are Tested

Don't forget you can order a sample fiber from a real telephone cable for use in some experiments by clicking the ORDER FIBER button on the top right of the Home page. Prices $1.00 per fiber. Great way to earn extra credits in your next Technical Presentation, Science Fair or Merit Badge!