Differences between step index fibres and graded index fibres:

Step index fibre 

1. In step index fibres the refractive index of thecore medium is uniform through and undergoesan abrupt change at the interface of core and cladding.
 2. The diameter of core is about 50 to 200micrometers in case of multi mode and 10micrometers in single mode fibre
 3. The transmitted optical signal will cross thefibre axis during every reflection at the corecladding boundary.
 4. The shape of propagation of the optical signalis in zigzag manner.
5. Attenuation is more for multi mode step index fibres but Attenuation is less in single mode step index fibres
6. Numerical aperture is more for multi mode step index fibres but it is less in single mode step index fibres

  Graded index fibre
 1. In graded index fibres, the refractive index of the core medium is varying in the parabolic manner such that the maximum refractive indexis present at the center of the core.
 2. The diameter of the core is about 50 micro meters.
3. The transmitted optical signal will never cross the fibre axis at any time.
4. The shape of propagation of the optical signal appears in the helical or spiral manner
 5. Attenuation is very less in graded index fibres
 6. Numerical aperture is less in graded index fibres

Advantages of fibre optic communication

Advantages of fibre optic communication

The optical fibre communication has more advantages than convectional communication.
1. Enormous bandwidth
 2. low transmission loss
3. electric isolation
4. signal security
5. small size and less weight
6. low cost
7. immunity cross talk

  1. Enormous bandwidth
 The information carrying capacity of a transmission system is directly proportional to the frequency of the transmitted signals. In the coaxial cable (or convectional communication system) transmission the bandwidth range is up to around500MHZ. only. Where as in optical fibre communication, the bandwidth range is large as 105 GHZ.

2. Low transmission loss:- The transmission loss is very low in optical fibres (i.e. 1 2.0 − Km ) than compare with the convectional communication system. Hence for long distance communication fibres are preferred.

3. Electric isolation Since fibre optic materials are insulators, they do not exhibit earth and interface problems. Hence communicate through fibre even in electrically danger environment.

4. Signal security The transmitted signal through the fibre does not radiate, unlike the copper cables, a transmitted signal cannot be drawn from fibre with out tampering it. Thus the optical fibre communication provides 100% signal security.

 5. Small size and less weight The size of the fibre ranges from 10µm to50µm , which is very small. The space occupied by the fibre cable is negligibly small compared to convectional electrical cables. Optical fibres are light in weight.

  6. Low cost Since optical fibres made up of silica which is available in abundance, optical fibres are less expensive.

7. Immunity cross talk Since the optical fibres are dielectric wave guides, they are free from any electromagnetic interference and radio frequency interference. Since optical interference among different fibres is not possible, cross talk is negligible even many fibres are cabled together

Interference of light

The best evidence for the wave nature of light is interference phenomenon. This was experimentally demonstrated by Thomas Young in 180, through double slit experiment. Due to interference, we will observe many observations in our day today life, such as multiple colours on soap bubbles as well as on oil film when viewed under sun light. Interference concept is explained on the basis of superposition of wave’s concept. When two light waves superimpose, then the resultant amplitude or intensity in the region of superposition is different than the amplitude of individual waves

  Types of interference:- For the formation of interference pattern, two coherent light sources are required. To get two coherent sources form a single light source, two techniques are used. They are

 1. Division of wave fron
t
2. Division of amplitude

Division of wave front The wave front from a single light source is divided into two parts using the phenomenon of reflection, refraction, or diffraction. Young’s double slit experiment is belongs to this class of interference. Division of amplitude The amplitude of a single light beam is divided into two parts by parallel reflection or refraction. Newton’s ring experiment, Michelson’s interferometer is belongs to this class of interference.

  Conditions for interference

1) Two light sources of emitting light waves should be coherent.
 2) Two sources must emit continuous light waves of same wavelengths or frequency.
3) The separation between the two sources should be small.
4) The distance between the two sources and the screen should be large.
5) To view interference fringes, the background should be dark.
6) The amplitude of light waves should be equal or nearly equal.
7) The sources should be narrow.
 8) The sources should be monochromatic