Fiber optics is an exciting field, and the demonstration exercises that follow are sure to kindle one's interest in this fascinating area.

1. Demonstration of light guidance through optical fibers. Take a 2-meter length or so of optical fiber—preferably a large-diameter plastic optical fiber—and make perpendicular cuts at both the ends. Couple light from a laser pointer into one of the ends and see for yourself how the light is guided through the fiber. If the room is dark, you should be able to observe the entire fiber glowing all along its length due to scattering (Rayleigh scattering). Observe the light emerging from the other end of the fiber. Try tying the fibers in a knot and you will notice that light still is guided, in spite of the severe bend of the fiber. If you use a long reel of the optical fiber, you should still see the emerging light at the other end and the glow along the entire reel. Figure 7-1 shows a photograph of light propagating through a long optical fiber.
   
   

   Figure 7-1 A long, thin optical fiber transmitting a light beam (Photograph courtesy Dr. Chynoweth)

2. Differentiate between two kinds of fibers. Take short pieces (~ 2 m) of a multimode optical fiber and a single-mode optical fiber. Focus the light from a HeNe laser onto the input end of the fiber using a lens such as a microscope objective, and observe the output by projecting it onto a white screen. The output from the multimode fiber will show a speckle pattern while that from a single-mode fiber will show a very uniform beam much like the output from a good laser. In the former case, the light propagates via a large number of modes, and the interference between the various modes causes the speckle pattern.
   
   
3. Simple demonstration of sensing property of optical fibers. Perform the previous demonstration using a multimode fiber reel by fixing the entire setup on a table and observing the output speckle pattern as you bring your hand close to the optical fiber reel. The warmth of the hand is enough to change the phase difference between the interfering modes, which in turn changes the interference pattern. The speckle pattern will keep changing as you move your hand closer to and farther from the reel. If you try to press or twist the fiber, you will immediately notice the change in the speckle pattern. Such changes in the pattern due to external influence demonstrate in a very simple fashion the possibility of using optical fibers for sensing.
   
   
4. Demonstration of Rayleigh scattering in an optical fiber. Rayleigh scattering is responsible for the blue color of the sky and the red color of the setting sun. It is very interesting to demonstrate the wavelength dependence of Rayleigh scattering using a long optical fiber. Couple white light from a lamp, such as a tungsten halogen lamp emitting white light, into a 1-km length or so of multimode optical fiber and look into the output. Notice the color of the light. Cut the fiber, leaving about 1 m from the input end of the fiber, and repeat the experiment with this 1 m of fiber. You will see that, in the former case, the emerging light looks reddish while in the latter case it looks white. This difference is due to the decrease of loss with increase in wavelength due to Rayleigh scattering. Thus, shorter wavelengths toward the blue region have suffered greater scattering out of the fiber than longer wavelengths near the red region. Even though all wavelengths at the input end are coupled, there is more power in the red part at the output end, thereby giving the light a reddish color.