Three types of Laser diode structures :

1. Gain indexed guide
2. Positive indexed guide
3. Negative indexed guide

Three requirements of Laser action :

1.Absorption

2. Spontaneous emission

3. Stimulated emission.

Two types of LED configurations :

1. Homo junction
2. Single and double hetero junction.

Two types of confinement used in LED's :

1. Optical confinement.
2. Carrier confinement.

Advantages of LED :

1. LED's are less complex circuits than Laser diodes.

2. Fabrication is easier.

3. They have long life.

Direct band gap materials and indirect band gap materials :

• In direct band gap materials direct transition is possible from valence band to conduction band.

                            e.g. GaAs, InP, InGaAs

Advantages of optical communication :

1. Low transmission loss.

2. Small size and weight.

3. No electromagnetic interference.

4. Electrical isolation.

Extrinsic absorption :

• Absorption phenomena due to impurity atoms present in the fiber.

Losses responsible for attenuation in optical fibers :

1. Absorption losses, 
2. Scattering losses and  
3. Bending losses

Measure of information capacity in optical wave guide :

• It is usually specified by bandwidth distance product in MHz. 
• For a step index fiber the various distortion effects tend to limit the bandwidth distance product to 20MHz.

Intra-modal delay :

• The other factor giving rise to pulse spreading is intra-modal delay which is a result of each mode having a different value of Group velocity at a single frequency.

Inter-modal dispersion :

• Inter-modal dispersion is a pulse spreading that occurs within a single mode. 
• The spreading arises from finite spectral emission width of an optical source. 
• It is called group velocity dispersion or intermodal dispersion.

Rayleigh scattering :

• The index variation causes a Rayleigh type of scattering of light. 
• Rayleigh scattering in glass in the  same phenomenon that scatters light from sun in the atmosphere, giving rise to blue sky.
• The expression for Rayleigh scattering loss is given by

αscat =(8π3/3λ2)(n2-1)2kBTfβT

n = refractive index

kB = boltzman constant

βT = isothermal compressibility

Tf = fictive temperature

λ = operative wavelength

Scattering losses :

• Scattering losses in glass arise from microscopic variation in the material density from compositional fluctuation and from structural in homogeneities or defects occurring during fiber manufacture.

Intra-modal dispersion :

• The intra-modal dispersion depends on wavelength and its effect on signal distortion increases with the spectral width of the optical source. ( It is a band of wavelength over which the source emits light )