Page 122 - Concise Encyclopedia of Robotics

Fiber-Optic Cable
Cladding
X
Step-index fiber Core Y
Cladding Core

Y

X

Graded fiber
Fiber-optic cable

down and bends light. An optical fiber has a core surrounded by a tubular
cladding, as shown in the illustrations. The cladding has a lower refractive
index than the core.
In a step-index optical fiber (top drawing), the core has a uniform index
of refraction and the cladding has a lower index, also uniform. The transi-
tion at the boundary is abrupt. In the graded-index optical fiber (lower
drawing), the core has a refractive index that is greatest along the central
axis and steadily decreases outward from the center. At the boundary,
there is an abrupt drop in the refractive index.

Operation
In the top illustration, showing a step-index fiber, ray X enters the core
parallel to the fiber axis and travels without striking the boundary unless
there is a bend in the fiber. If there is a bend, ray X veers off center and
behaves like Y. Ray Y strikes the boundary repeatedly. Each time ray Y
encounters the boundary, total internal reflection occurs, so ray Y stays
within the core.

