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Singlemode
fiber was developed after the “pulse spreading”
problems were realized during the initial installations
of multimode fiber by the telephone companies in
the 1980’s. While the outside diameter of the cladding
is the same as multimode, that being 125 microns,
the core of a singlemode fiber is much smaller,
approximately 1/6th of the 62.5 micron core of multimode.
The core of singlemode is so small that, even with
the rigid controls used in the manufacturing process,
the size of the core cannot be completely controlled.
Even within the same roll the core of a singlemode
fiber will vary along the length between 7 and 9
microns.
The “mode field diameter”, which includes a
portion of the cladding immediately surrounding
the core, is where the light signal travels in singlemode
fiber. Because the diameter of the core varies, so will
the mode field diameter. |
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