Laser is a process by which we can obtain a beam of light which is highly coherent, highly monochromatic and perfectly parallel.
A laser beam can be sent to a for all place and return back without any practical loss of intensity and it can be used for many purposes.
The first successful laser was constructed by T.H. Maiman in 1960. He used a small Ruby Rode (Al2O3 doped with .05% Cr oxide) known as Ruby Laser. May others have also been constructed e.g. Helium-Neon Laser, Carbon dioxide laser etc.
The production of laser is based on fact that when atoms are in excited state, they loose their energy and come back to ground state.
For this process a system is chosen in which a metastable state is there.
Hence E1 is ground state and E2 is metastable state. By some technique, the number of atoms in metastable state are kept more than in ground state at a time. When photon of energy E2- E1 is incident one of the atoms present in the metastable state, the atom will drop to lower energy state E1 emitting a photon of some energy in phase and in same direction as the incident one. Two photons interact with 2 more atoms present in metastable state and so on and thus the photons go on multiplying. All there photons possess the same energy in phase and in same direction. This process is called amplification of light. To proceed this process continuously the following requirements should be fulfilled.
1. The metastable state should all the time have larger number of atoms than in lower energy state.
2. The photons emitted due to stimulated emission should stimulate other atoms to multiply the photons inside the system.
However to kept the photons inside the systems, 2 mirrors are used to reflect back the photons to multiply their number. But one of the mirrors should be partially transmitting so as to allow the beam to emerge out of the system.
Induced absorption :- Consider that two energy states i. (lower) and j (higher) of an atom are associated with energy Ei and Ej respectively ratio of atoms in state a j and i at temp. 'T'
As Nj< Ni [Ej > Ei]
If we shine radiation of frequency.
n= Ej-Ei/h
On the atoms, more atoms will be raised to the higher energy state on absorbing photons of energy hn. This process is called induced or stimulated absorption.
Spontaneous Emission : If an atom is in higher state, it tends to return back to the lower energy state with in time=10-8sec. by emitting a photon of energy (Ej-Ei). This process is called spontaneous emission.
But in this process, atoms are emitted in a random manner. Due to which the beam produced is not coherent.
Stimulated Emission : In an atomic system, the higher energy states of the atoms are always populated to some extent. In the photons of energy hn are incident, the chief event that will take place is induced absorption. Atoms will be raised from lower to higher energy state. But higher energy state is little populated, the transmission from higher to lower energy state also takes place. As such the photons of energy hn will be emitted. This is called stimulated emission.
Populated inversion : When the higher energy state in an atomic system gets occupied due to induced absorption to a greater extent than the lower energy state, the phenomenon is called inverted population of energy levels or simply population inversion.
The process of induced absorption for producing he inverted population of energy levels is also known as pumping.
Metastable State : It is state of atom which has a mean life time of the order of 10-3 sec. or more. The atom in same system does not come down directly but it come Ist at metastable state and remain their for a time =10-3 sec.
Ruby Laser : Ruby is a crystal of aluminium oxide doped with .05% if chromium oxide. The adding of Chromium oxide impart red colour to ruby.
It consists of a ruby rode of length 5cm. and .5cm. in diameter. The 2 faces of the rode are mate plane and coated with silver. One face of the rode is made reflector by thick coating and one is kept some transmitting. A xenon flash lamp is wound around the ruby rode. The necessary pumping energy is provided to the ruby rode by the high intensity flash produced by the xenon lamp. It is kept cool by circulating liquid nitrogen gas.
Theory : It is a three energy level system. There are E0 (Ground state), E1 (Metastable state), E2(Higher energy state). When xenon lamp is flashed, it produces burts of very intense light (yellowish green) of wave length 5500 Ao. This light acts as the pumping radiation for chromium ions and atoms are raised from ground state to higher energy state. With in a time 10-8 sec. The electrons go rapidly to level E1. The energy lost to the rode and its temperature is kept low by liquid nitrogen. In a short time, there is population inversion. E1 level becomes more populated than E0. The electron go to E0 state by losing energy. The photon emitted get multiplied and a amplification process continues till a quit intense beam of laser emerge out from the system. Wavelength of the beam can be found from the relation.
A drawback with ruby laser is that, it requires more pumping energy. Because it is necessary to pump half of the electron of ground state to metastable state to achieve population inversion.
He - Ne have 4 energy level in contrast with the 3 levels of Ruby Laser.
The experimental arrangement consists of a cylindrical glass true containing a mixture of helium (90%) and neon (10%) at low pressure. Two parallel mirror M1 and M2 are fixed at the two ends of the glass tube. The mirror M1 is thickly silvered and M2 is kept partially transmitting. So as to emerge out of laser beam. A high voltage is applied across A and C electrodes.
Theory : It have 4 energy levels. In Neon. These are E0 (Ground energy level) E1 (18.70ev), E2(20.66ev), E3(high energy level). Helium has a metastable energy level 20.61ev, which is very close to the metastable level E2 of Neon. It allows helium to be used for pumping neon atoms to the metastable state E2 when atoms come down from metastable state to lower energy state E1 after population inversion, an intense laser beam emerge out of the mirror M2. The wavelength of beam is l =
hc/E2-E1=6328A0Lasers have proved a boon to the growth of modern scientific world. It is used in many modes.
1. In communication : There are used to send signals over long distances through optical fibres. They allows to send thousands of signals at the time due to narrow band width.
2. In measuring long distances : Being highly coherent these can travel long distances without any loss their intensity e.g. To measure distance of moon.
3. In surgery : These are used to perform practically blood less surgery. As the laser beam cuts through tissue, it heat seals blood vessels and this technique is used in removing tumour from the body.
4. It is used to ensure that roads, railway lines, pipes, oil tankers, buildings etc. are built accurately.
5. In Industry : Laser is used to drill holes and cuts of sheets of metals. They do this job accurately and quickly.
6. Nuclear Power Production : A day is not far when loses beams will be used in carrying out nuclear fusion reactions with much ease for power production.
7. For national defence : Three are used for controlling and guiding rockets, missiles, satellites and locating planes.
8. In Weather forecasting : Laser beams are used to obtain clear picture of clouds and wind movements for forecasting of weather.