Page 58 - Optical Switching And Networking Handbook

03_200023_CH02/Batesx 1/17/01 8:17 AM Page 43

Basic Fiberoptics Technologies 43

Figure 2-11 8.3 Microns
Single-mode fiber is
Light
so pure and so thin
that light has only
one path from end
to end.

cladding
8.3 Micron

glass
125 Micron

Let’s try a different analogy this time. Think about a drain pipe. If
you have a 6-inch outer diameter drain pipe and you start to dump
water into the pipe, the water will swish back and forth on the bot-
tom of the pipe. As the water works its way to the end of the pipe, it
will have taken a circuitous route to get to there. However, if you
dump more water into the pipe, the water may start to experience
resistance and begin a higher motion up the sides of the pipe. The
more flow you put into the pipe (without totally filling it yet), the
higher up the sides the water moves. Now, the back-and-forth motion
may give way to a corkscrew effect swirling down the pipe toward
the opposite end. The path is lengthening to get to the other end as
the corkscrew effect gets more violent. This swirling motion changes
the path. Now suppose you decrease the size of the pipe to 1 inch and
put the same amount of water into it. The path is restricted to a
much smaller diameter opening and therefore decreases the size of
the swirl (making it much tighter). The shorter path means that the
water expels from the opposite end much more quickly. In fact, the
appearance is that the water speeds up. It gushes out the opposite
end in a jet stream instead of just flowing out.
We can achieve this same effect with single-mode fibers. Now, as
the light beams are moved into the glass, they move straighter and
quicker to the other end. Therefore, we can turn the spigot on faster