Principle Of Light Emission And Laser Operation

The goal for this topics is:
To give the reader understanding of the basic principles of light emitting diode and the laser (both are optical source used in optical communication)

1. BASIC PRINCIPLE OF LIGHT EMISSION AND LASER OPERATION

1.1 Mechanism of light emission

internal energy : effective energy from outermost electron 
Plank's constant : 6.625 x 10-34 J-s

Energy levels and mechanism of transition are divided by three (see fig1.1):
1. Spontaneous emission 
atom is initially in the higher energy level 2 and the light of frequency is emitted spontaneously and randomly without any external light triggering
*both absorption and stimulated emission, the light frequency is pumped into the atom from an external source*
2. Absorption
atom in energy level 1 can absorb photon of the input light and make transition to energy level 2 
3. Stimulated emission
atom is initially in energy level 2 with a probability proportional to the intensity of the external light intensity  (forms of basis laser laser operation)

fig 1.1

1.2 Mechanism of Amplification and Light Oscillation

How light can be amplified?
"Laser" is an acronym for light amplification by stimulated emission of radiation, which describes very succinctly how a laser works.
An optical amplifier is a device which receives some input signal and generates an output signal with higher optical power. Typically, inputs and outputs are laser beams, either propagating as Gaussian beams in free space or in a fiber. The amplification occurs in a so-called gain medium, which has to be “pumped” (i.e., provided with energy) from an external source. Most optical amplifiers are either optically or electrically pumped.

Lets consider next about how light can be amplified. Now we consider Boltzmann distribution:

(k=1.38 x 10-23 J/K) T is temperature (K).

In equation above we can compre the ratio of N1 and N2. We can define it into two condition:
1. N1 < N2
The number of absorption exceeds the number of stimulated emissions and there is a net loss or attenuation, of the input light beam.
2. N1 > N2
This condition is achieved because of energy can be introduced from outside in appropriate way then sometimes it is possible to make some number of atoms in level 2 greater than the number in level 1. So the number of photons produced by stimulated emission is larger than the number removed by absorption. That's making the intensity of the input light is increased.