Biomedical Sensors - Introduction to Biomedical Engineering - page 652

FIGURE 10.36 Principle of optical fibers.

Optical fibers are typically made from two concentric and transparent glass or plastic
materials, as shown in Figure 10.36. The center piece is known as the core, and the outer
layer, which serves as a coating material, is called the cladding.
The core and cladding of an optical fiber have a different index of refraction, . The index

of refraction is a number that expresses the ratio of the light velocity in free space to its
velocity in a specific material. For instance, the refractive index for air is equal to 1.0,
whereas the refractive index for water is equal to 1.33. Assuming that the refractive index
of the core material is

n

n 1 and the refractive index of the cladding is n 2 (where n 1 > n 2 ),

according to Snell's law,

n sin f 1 ¼ n sin f 2 ð 10 : 24 Þ

1 2

where is the angle of incidence, as shown in Figure 10.37.

Accordingly, any light passing from a lower refractive index to a higher refractive index
is bent toward the line perpendicular to the interface of the two materials. For small inci-
dent angles,

f

f 1 , the light ray enters the fiber core and bends inward at the first core/clad-
ding interface. For larger incident angles,

f 2 , the ray exceeds a minimum angle required
to bend it back into the core when it reaches the core/cladding boundary. Consequently,
the light escapes into the cladding. By setting sin f 2 ¼ 1.0, the critical angle, f cr , is given by

f cr ¼ n

2
n 1

sin ð 10 : 25 Þ

f cr are inter-
nally reflected inside the core of the fiber by the surrounding cladding. Conversely, any
entering light rays with incidence angles smaller than

Any light rays entering the optical fiber with incidence angles greater than

f cr escape through the cladding

and are therefore not transmitted by the core.

FIGURE 10.37 Optical fiber illustrating the incident and refracted light rays. The solid line shows the light ray
escaping from the core into the cladding. The dashed line shows the ray undergoing total internal reflection inside
the core.