OPTICS, Fiber optics Basics
OPTICS, Fiber optics Basics
Light or
visible light is
electromagnetic radiation that can be perceived by the human eye. Light energy is a form of
electromagnetic radiation. Light consists
of photons. Light travels
in waves and is the only form of energy visible
to the human eye.
There
are seven colors in white light:
Red, Orange, Yellow, green, Blue, Indigo and Violet. Wavelengths of light range from about 400 nm at the violet of the spectrum to 700 nm at the red.
The speed of light C =
3x108 m/s.
Reflection :
Whenever
we look into a mirror, or see at sunlight glinting from a lake, we are seeing a
reflection.
Laws of
Reflection :
Fig A1. The incident ray,
the reflected ray and
the normal to the surface of the mirror all lie in the same plane.
2. The angle of incidence
is equal to the angle of reflection.
i.e. < i = < r .
3. The incident ray
and the reflected ray are on the opposite side of normal.
Refraction :
Refraction is the bending of the path of a light wave when
passes across the boundary separating two media. Refraction is caused
by change in speed a wave when it changes
medium. Fig B
Laws of refraction :
Fig B1. The incident ray refracted ray, and the
normal are in the same plane.
2. The
ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. This is
known as Snell's law of refraction.
The constant is known as Refractive Index n.
n = sin i / sin r
When
light travels from air to glass then refractive index is written as ang and read as refractive index of glass with
respect to air.
Also ang = Speed of light in air / Speed of light in glass = C / V
The amount of change in speed of light in a
medium depends upon the property of
medium(refractive index). Refractive index is a measure of
how much speed of light changes when it enter the medium from air.
Total Internal Reflection (TIR ) :
Communication is very important aspect of living organisms. Hence human tried to communicate each other
by using different means. Today the
important thing about communication is fiber optics communication. Let us consider the principle of fiber
optics. This is based on total internal reflection ( TIR ).
We know that the refractive index of a medium is ratio of speed of light in the air
to the speed of light in medium. n = C / V
The refractive index of
glass is about 1.5 and refractive index of air is 1.
Consider the
light ray travelling from glass to air.
The light ray, when traveling from denser medium to rarer medium, the
refracted ray bend away from the normal.
It means the angle of incidence is less than the angle of refraction.
Consider different cases
of angle of incidence at the boundary of glass and air.
Fig C 1. If the ray incident normal to the boundary, it will passes without bending, as seen at point A in Fig C .
2. Consider angle of
incidence at B, the refracting angle is greater than the angle of incidence. (
Point B in Fig C )
3. If the angle of incidence increases then the angle
of refraction also increases as seen at a point C . If the angle of incidence
goes on increasing the angle of refraction also goes on increases. At a particular angle of incidence the angle of
reflection becomes 900 .
The angle of incidence corresponds to refracting angle 900 , is called
Critical angle ic .
4. Now if the angle of
incidence i is greater than critical angle ic (i > ic ), then ray will not
refract , but ray will reflect in the glass medium only. This phenomena is
called total internal reflection ( TIR ).
When the ray of light travelling from denser medium to rarer medium,
if angle of incidence is greater than critical angle then ray will reflect in
the denser medium only ,then this is called Total Internal Reflection.
The fiber optics works on principle of total internal
reflection.
Optical Fiber ( or Fibre ) or Fiber optics :
Fiber optics is the device used to transmit information as
pulses of light through fiber made of glass or plastic over long distances.
Optical fibers are about the diameter of
human hair and when bundled into a fiber-optic cable, they are capable of
transmitting more data over longer distances and faster than other media. It is
this technology that provides homes and offices with fiber-optic internet,
phone and TV services etc.
Structure
of Fiber optics :
All optical fiber contain a core and a cladding. These two components make propagation of light with minimal loss possible. Other layers of a fiber optics called sheath are used for fiber protection and manufacturing purposes.
Fig D Optical fiber is
composed of several elements. The construction of a fiber optics consists of a core, cladding,
coating buffer, strength member and outer jacket. The optic core is the light-carrying element at
the center (Fig D ). The core is having
refractive index n1 , usually made up of a
combination of Silica and Germania. The cladding having refractive index n2 surrounding the core
is made of pure silica. The cladding has a slightly lower index of refraction
than the core ( n1 > n2 ). The lower refractive index causes the light in the core to
reflect off the cladding and stay within the core. The diameter of core is
about 50 μm , diameter of cladding is 125 μm and that of sheath is 150 μm
Path of light through Fiber optics :
Fig EThe light in a fiber-optic cable travels through the core by constantly bouncing from the
cladding by the principle of total
internal reflection. The cladding does not absorb any light from the core, hence, the light wave can travel through great distances.
The ray AB of light made to enter in cable of
fiber optics in core ( refractive index n1 ) at an
angle less than acceptance angle as shown in Fig E. the ray BC strikes at C,
the boundary between core and cladding ( refractive index n2 and n1 > n2 ) , such that the angle of incidence at C is
greater than critical angle. Due to this the ray BC experiences total internal
reflection along CD. As the wall is parallel, at D also the ray has total
internal reflection and the ray travels along DE. Thus every time the ray
experiences same effect, and follow the
zigzag path throughout the cable without any loss of energy.
In one
fiber number of rays can be send with different angles, without disturbing
signals.
Applications of Fiber optics :
1. Internet:
Fiber optics transmit large amounts of
data at very high speeds. Thus this is widely used in internet cables.
2. Computer Networking: Networking between computers is made
easier and faster with the use of fiber optics.
3. Surgery and Dentistry:
Fiber optics is widely used in the
fields of medical field. Optical communication is an important part of
non-intrusive surgical methods, widely known as endoscopy.
4. Telephone: Calling telephones within or outside the
country is now so easy due to use of fiber optics.
5. Cable Television : The use of fiber optics in the transmission of cable signals of television has made this simple.
6. Automotive Industry : Fiber optics play an important role in
the lighting and safety features of automobiles.
7. Military and Space Applications:
With the high level of data
security required in military applications, fiber optics gives the ideal
solution for data transmission with high level security.
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