Physic Project
Refraction
Light travels at 300,000 km/s in a vacuum. Light travels slower in other media.
Different wavelengths travel at different speeds in the same medium. When light
passes from one medium to another it changes speed, which causes a deflection of
light called refraction. Because each wavelength changes speed differently, each is
refracted differently.
Total Internal Reflection
Total internal reflection is when light travelling in one material contacts a different
material and the light is reflected back into the original material.
Since the core and cladding consist of different compositions of glass, light entering
the core is confined to the boundaries of the core because it reflects down the core, so
long as the critical angle is not exceeded.
Critical Angle
In discussing refraction three terms are important: - The Normal: an imaginary line
perpendicular to the interface of the two materials. - The Angle of Incidence: the
angle between the incident ray and the normal.- The Angle of Reflection: the angle
between the normal and the refracted ray.
When light passes from one medium to another that has a higher refractive index, the
light is refracted toward the normal. When the index of the first material is higher than
that of the second, the light is refracted away from the normal (a small portion of the
light is reflected back into the first material by Fresnel reflection).
As the angle of incidence increases the angle of refraction approaches 90° with the
normal. The angle which yields a 90° angle of refraction is called the critical angle. If
the angle of incidence is increased past the critical angle the light is totally reflected
back into the first material. The angle of reflection is equal to the angle of incidence.
Numerical Aperture (NA)
Numerical aperture (NA) is the light gathering capability of a fibre. The calculated
material NA is related to the refractive indices of the core and cladding.
A fibre with a high NA gathers more light than a fibre with a low NA because the
critical angle is greater, and the fibre accepts light injected from larger angles.
Acceptance Cone
Since fibre is circular, a cone - the acceptance cone - defines the angles of incident
light that are totally internally reflected by the core.
A large acceptance cone, although allowing a fibre to accept and propagate light from
a larger field, coincides with greater dispersion. Conversely, a narrow acceptance cone
accompanies low dispersion but requires a narrow, more precise source of light
Dispersion
Dispersion relates to the spreading of the light pulse as it travels along the fibre. A
pulse seen at the output is wider than the input pulse.
Dispersion limits a fibres bandwidth.
Pulse rates must be slow enough that dispersion does not cause adjacent pulses to
overlap, the detector must be able to distinguish between each pulse
Fibre Types
Multimode Step Index, the simplest type has a core diameter in the 50um to more than
1000um range. The large core allows many modes of light propagation, some rays
taking longer paths than others. The lowest order mode, the axial ray travelling down
the centre of the fibre arrives at the end of the fibre before the higher order modes,
spreading a narrow pulse of light as it travels through the fibre. This is called modal
dispersion.
Single Mode Step Index limits this dispersion by having a core small enough to allow
only one mode of light to travel through the fibre.
A single mode fibre with a core of Sum to 10um is very efficient for high speed, long
distance applications.
Multimode Graded Index fibre also limits dispersion. Its core is a series of concentric
rings, each with a lower refractive index. Since light travels faster in a lower index
medium, light farther away from the fibre axis travels faster. Rays of light are not
reflected sharply by the core to cladding interface but are refracted successively by
the differing layers of the core. Since the high order modes have a faster average
velocity than the low order modes they arrive at any given point at nearly the same
time. The path of travel appears almost sinusoidal.
Intrinsic Attenuation Absorption
Absorption is caused by impurities in the glass. Although glass fibres are ultrapure
and have a purity exceeding that of semiconductors, some impurities remain as
residue after purification. Absorption is caused by transition-metal ions of copper, iron,
cobalt, and chromium and also by hydroxyl (OH-) ions associated with water
molecules in the glass. The amount of absorption by these impurities depends on their
concentration and the light wavelength.
Intrinsic Attenuation Scattering
Scattering results from imperfections in fibres and the basic structure of the fibre.
Rayleigh scattering comes from the atomic and molecular structure of the glass and
from density and composition variations that are natural by-products of
manufacturing.
Unintentional variations in density and fibre geometry occur during fibre manufacture
and cabling. Small variations in the core diameter, microbends and small incongruities
in the core diameter, microbends, and small incongruities in the core to cladding
interface cause loss. The angle of incidence of rays striking such variations at the core
to cladding interface change sufficiently that some rays are refracted onto new paths
not subject to total internal reflection.
Extrinsic Attenuation Macrobending
Macrobending is loss due to large scale bending.
Macrobending losses are reversible once bends are straightened.
Installers can minimise the effect during installation and testing by not exceeding the
installation and long term bend radii.
Extrinsic Attenuation Macrobending
Microbending is loss due to small scale bending or distortions.
Microbending may not be visibly apparent. Microbending losses are reversible once
cause is removed. Microbending may be: - temperature related. - tensile related. crush related.
Installers can minimise this effect during installation and testing.
Name : Au Ka Yiu
Class number : F.3C
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