Elenco Fiber Optics Voice Data Kit - Manual (Page 9)

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Commercial fiber is very pure and has a protective "cladding "
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The acrylic rod used in previous experiments carries light from one end to the other, but it
doesn't really do a very good job. To transmit light long
distances, commercial optical fibers must be composed of ultra-
pure transparent materials. For example, some commercial
optical fiber material is so pure that the light lost when traveling
through a one-kilometer (5/8 of a mile) length is more than 90
percent of the light which entered the fiber.
In the illustration to the right is a basic optical fiber, with
concentric layers of core and cladding. The fiber you will use in
this experiment contains a central "light carrying" core and a
very thin (10
µm/.0004 inches) cladding layer to trap the light
inside. (The cladding is also transparent. You probably won't be
able to distinguish it from the core.)
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Penlight with batteries
Black rubber penlight boot
2 mm (.08 inch) diameter optical fiber, 50 cm (20 inches) long
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• Insert one end of the 2 mm (.08 inch) diameter fiber into the rubber boot on the penlight, then
turn the penlight on.
• Take the penlight and fiber into a dark room and point the fiber end at a nearby wall.
• Grip the middle portion of the fiber in your fist so you enclose several inches of its length. Has
the light coming out of the fiber's tip decreased,
increased, or stayed the same in intensity? How
does this compare to what happened when you
gripped the 3 mm diameter plastic rod? Turn
the penlight off.
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Light should be visible exiting the end of the 2
mm fiber as soon as the penlight is turned on.
Gripping the fiber with your hand has no effect on the
light intensity emerging from the fiber end. (Moving
the fiber around in the rubber boot may vary the fiber
end's output intensity.)
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Light is transmitted from one end of the fiber to the other because light is being guided by the
central fiber core and trapped inside by the outer cladding layer. Light intensity doesn't change when
you grip the fiber in your fist because the refractive index (optical density) immediately surrounding the
central core doesn't change as it did in previous experiments. The cladding layer remains constant and
acts as an optical shield between the fiber core and the optical density of your hand.
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The fiber you just finished experimenting with is made of plastic. It is one of the two most
commonly used materials in commercial optical fibers. The other material is glass — commonly called
"silica" in the technical community.
1147.eps
Core
Cladding
1141