Optical sources: Lasers

 The main three optical sources that we deal with n day to day life are incandescent light, LED, and laser.

Noise should be very less.

1610 nm is the usual wavelength of a light source to be used.

LED has a larger spectrum range.

Basic concept -

E= E2-E1 = hf

Absorption-> When the electron gains energy hf and moves to a higher state or energy level.

Spontaneous emission is when the electron moves from a higher state to a lower state.

Stimulated emission is when the transition of electron takes place from a higher energy level to a lower energy level on the supply of external energy hf, at the cost of two coherent energies released by the electron.

The Einstein relation ---

Two energy level E2 E1

Population density-   number of atoms per unit volume

Conditions for LASER-

1. metastable state -

2. population inversion- 

3. stimulated emulation- 

Stimulated emission-

1. it is controlled from outside
2. energy transition takes place between definite selected energy levels
3.  emitted photons are same in direction. phase and state of polarisation 
4. are monochromatic
5.  are all coherent
6.  output narrow and highly intense
7.  in the output, photons are multiplied

spontaneous emission

1. cannot be controlled from outside
2. a probabilistic or random process
3. emitted photons are in a random direction.