Lasers
Laser means light amplification by stimulated emission of radiation.
Specialty of Lasers:
Any light beam is composed of photons, two photons are said to be coherent if they have same energy, phase and direction. Laser beam is composed of such coherent photos whereas ordinary light contain incoherent photons. So the property of coherence adds special characteristics to LASER beam such as high directionality, high intensity, monochromacity and penetration.
Laser source is an electromagnetic radiation with absolute purity and high intensity which were never found in any other electromagnetic radiation.
Absorption: When at atom absorbs an amount of energy „hv‟ in the form of photon from the external agency and excited into the higher energy levels from ground state, then this process is known as absorption
Spontaneous Emission: When an atom in the excited state emits a photon of energy „hv‟ coming down to ground state by itself without any external agency, such an emission is called spontaneous emission.
Photons released in spontaneous emission are not coherent. Hence spontaneous emission is not useful for producing lasers.
Stimulated Emission:
When an atom in the excited state, emits two photons of same energy „hv‟ while coming to down to ground state with the influence of an external agency, such an emission is called stimulated emission.
In the two photons one photon induces the stimulated emission and the second one is released by the transition of atom from higher energy level to lower energy level. Both the photons are strictly coherent. Hence stimulated emission is responsible for laser production .
Laser Production Principle:
Two coherent photons produced in the stimulated emission, interacts with other two excited atoms, resulting in four coherent photons. Thus, coherent photons are multiplied in a lasing medium. The continuous successive emission of photons results for the production of laser beam.
Characteristics of Laser Beam:
Some of the special properties which distinguish lasers from ordinary light sources are characterized by:
1. Directionality
2. High Intensity
3. Monochromacity
4. Coherence
1. Directionality:
Laser emits radiation only in one direction. The directionality of laser beam is expressed in terms of angle of divergence i.e., it is twice the angle that the outer edge of the beam makes with the axis of the beam with reference to emitting source.
The outer edge of the beam is defined a point at which the intensity of beam falls to 1/37 times to that of value at the centre.
2. High Intensity:
Generally, light from conventional source spread uniformly in all directions. For example, take 100 watt bulb and look at a distance of 30 cm, the power enter into the eye is less than thousand of a watt. This is due to uniform distribution of light in all directions.
But in case of lasers, light is a narrow beam and its energy is concentrated within the small region. The concentration of energy accounts for greater intensity of lasers.
3. Monochromacity:
The light emitted by laser is highly monochromatic than any of the other conventional monochromatic light. A comparison b/w normal light and laser beam, ordinary sodium (Na) light emits radiation at wave length of 5893A0 with the line width of 1A^0 . But He-Ne laser of wave length 6328A0 with a narrow width of only 10^-7 A^ 0 i.e., monochromacity of laser is 10 million times better than normal light.
The degree of monochromacity of the light is estimated by line of width (spreading frequency of line).
4. Coherence:
If any wave appears as pure sine wave for long time and infinite space, then it is said to be perfectly coherent.
Practically, no wave is perfectly coherent including lasers. But compared to other light sources, lasers have high degree of coherence because all the energy is concentrated within the small region. There are two independent concepts of coherence.
i) Temporal coherence (criteria of time)
ii) Spatial coherence (criteria of space)
Population Inversion:
Generally, number of atoms in the ground state is greater than the number of atoms in higher energy states. But in order to produce a laser beam, the minimum requirement is stimulated emission. Stimulated emission takes place only if the number of atoms in the higher energy level is greater than the number of atoms in the lower energy level.
Simply population inversion is nothing but number of atoms in higher energy level is greater than the number of atom in lower energy level.
So, if there is a population inversion there by only stimulated emission will able to produce laser beam.
Population inversion is associated with three Phenomenon.
i) Stimulated emission
ii) Amplification
iii) Pumping Process
Stimulated Emission:
If majority of atoms are present in higher energy state than the process becomes very easy.
Amplification:
If „N‟, represents number of atoms in the ground state and „N2‟ represents number of atoms in the excited state than the amplification of light takes place only when N2 > N1.
If N2 > N1, there will be a population inversion so induced beam and induced emission are in the same directions and strictly coherent than the resultant laser is said to be amplified.
Boltzmann’s principle gives the information about the fraction of atom found on average in any particulars energy state at equilibrium temperature as
Pumping Process:
This process is required to achieve population inversion. pumping process is defined as: “The process which excites the atoms from ground state to excited state to achieve population inversion”.
Pumping can be done by number of ways
