3. Principle of optical fiber
Total internal reflection is the phenomenon which occurs when a propagated
wave strikes a medium boundary at an angle larger than a particular critical angle
with respect to the normal to the surface.
If the refractive index is lower on the other side of the boundary and the incident
angle is greater than the critical angle, the wave cannot pass through and is entirely
reflected.
The critical angle is the angle of incidence above which the total internal
reflection occurs. This is particularly common as an optical phenomenon, where
light waves are involved, but it occurs with many types of waves, such as
electromagnetic waves in general or sound waves.
When a wave reaches a boundary between different materials with different
refractive indices, the wave will in general be partially refracted at the boundary
surface, and partially reflected.
However, if the angle of incidence is greater than the critical angle – the angle of
incidence at which light is refracted such that it travels along the boundary – then
the wave will not cross the boundary, but will instead be totally reflected back
internally.
This can only occur when the wave in a medium with a higher refractive index
(n1) reaches a boundary with a medium of lower refractive index (n2).
For example, it will occur with light reaching air from glass, but not when
reaching glass from air.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17. Cladding in optical fibers is one or more layers of materials of lower
refractive index, in intimate contact with a core material of higher refractive
index.
The cladding causes light to be confined to the core of the fiber by
total internal reflection at the boundary between the two.
Light propagation in the cladding is suppressed in typical fiber.
Improving transmission through fibers by applying a cladding was
discovered in 1953 by Dutch scientist Bram van Heel.
Some fibers can support cladding modes in which light propagates in the
cladding as well as the core.
18. The core of a conventional optical fiber is a cylinder of glass or plastic that
runs along the fiber's length.
The core is surrounded by a medium with a lower index of refraction,
typically a cladding of a different glass, or plastic.
Light travelling in the core reflects from the core-cladding boundary due to
total internal reflection, as long as the angle between the light and the
boundary is less than the critical angle.
As a result, the fiber transmits all rays that enter the fiber with a sufficiently
small angle to the fiber's axis.
The limiting angle is called the acceptance angle, and the rays that are
confined by the core/cladding boundary are called guided rays.
19. The core is characterized by its diameter or cross-sectional area. In most cases the
core's cross-section should be circular, but the diameter is more rigorously defined as the
average of the diameters of the smallest circle that can be circumscribed about the core-
cladding boundary, and the largest circle that can be inscribed within the core-cladding
boundary.
This allows for deviations from circularity due to manufacturing variation.
Another commonly quoted statistic for core size is the mode field diameter. This is the
diameter at which the intensityof light in the fiber falls to some specified fraction of
maximum (usually 1/e2
≈ 13.5%).
For single-mode fiber, the mode field diameter is larger than the physical diameter of
the core, because the light penetrates slightly into the cladding as an evanescent wave.
The three most common core sizes are:
9 µm diameter (single-mode)
50 µm diameter (multi-mode)
62.5 µm diameter (multi-mode)[2]
