Elenco Fiber Optics Voice Data Kit - Manual (Page 10)

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A fiber's cladding will protect the core even underwater
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Earlier, when we immersed our U-shaped plastic rod, nearly all of the light which we sent into
the rod from the penlight escaped into the surrounding water. Now let's see what happens when we
use fiber with a permanent outer layer of cladding around the central light-carrying core.
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Penlight with batteries
Black rubber penlight boot
2 mm (.08 inch) diameter optical fiber, 50 cm (20 inches) long
Pan of water about 20 cm (8 inches) wide and 5 cm (2 inches) deep*
* Not contained in this kit.
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• Turn the penlight on. (Insert one end of the unjacketed 2 mm fiber into the rubber penlight boot
if you had removed it.)
• Dim the room lights. Place as much of the fiber's middle portion in the water as possible, but be
careful to keep the penlight and rubber boot out of the water.
• As you immerse the fiber, observe the amount of light coming out the end of the fiber.
• Bend the mid-section of 2 mm fiber into a "U-shape" similar to the acrylic rod and immerse it in
the pan of water.
• Again observe the fiber as light travels
down its length while immersed in water.
Can you see any light escaping from the
fiber? Where?
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This fiber does an excellent job of
transmitting light through the total length of the
fiber, even when under water. The light intensity
at the far end of the fiber does not change when
the midsection is immersed in water. (This
confirms the value of good cladding. It ensures
that light which enters one end reaches the other end virtually at the same intensity, even in unusual
circumstances.)
If you see a very small amount of light on the bottom of the pan it is due to light leaking out
between the fiber and rubber boot.
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Light does not escape from the central core because the boundary layer, or the cladding around
the optical core, does not change when the fiber is immersed in water. Similar to when you gripped
the fiber in your fist, the cladding provides an optical barrier to outside influences.
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Before fiber optics, when someone spoke into a telephone, their voice was converted to
electricity by a microphone. This converted electrical signal was sent via wires to another telephone.
There the electrical signal was converted by a speaker back into sound waves that the person on the
other end could hear. In all long-distance conversations over fiber optics, your voice is still converted
into electronics signals, but furthermore they are converted to optical (light) form for transmission. On
the receiving end, optical signals are converted to electrical signals and then to sound waves.
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