Classification of Lasers
Depending upon state of active material lasers are classified into 3 types
i. Solid state lasers eg.: Ruby laser
ii. Gaseous lasers eg.: He-Ne Laser, CO2 laser
iii. Semiconductor laser eg.: Gallium Arsenide
Ruby Laser:
When solid substances are used in laser device the concentration of active material is less than 1%. The bulk of the remaining material is not participating in laser action, it is called host material.
For laser action to be possible, the atoms (or) ions of the active material will have to be excited to proper upper levels which is done by pumping process. The first successful solid state laser was operated to Maimann in 1960 using a crystal of Ruby mixed with chromium as active material.
Ruby is crystalline Al2O3 doped with 0.05% of some of the chromium ions replaces the Al ions, responsible for laser action.
The pumping of Cr ions is performed with flashed lamp to achieve population inversion. The Cr ions absorb radiation around the wave lengths 5500A (green light) & 4000 A (blue light) and are excited to the levels E1 & E2. The levels E1 & E2 are known as pump bands.
The Cr ions in turn, reach to the metastable state (M) during the process of stimulated emission. The level „M‟ contains a pair of energy levels which are known as upper energy level (2A) & lower energy level (E). The lifetime of Cr ions in metastable is 3m sec.
Laser emission occurs b/w the level „M‟ and ground state „G‟ at an o/p wavelength of 6943A0 . The laser action is obtained from both
transitions with corresponding wavelengths 6929A0 (R2 i.e., Rayleigh line 2) and 6943A0 (R1 i.e., Rayleigh line 1).
But the actual laser will be produced from
transition lying nearer to the wavelength of red light and travels less distance to get high intensity.
Experimental Arrangement:
The Ruby is taken in the form of rod and is enclosed in a discharge tube whose length is 4 cm and diameter is 1.5 cm. Both the ends of the ruby rod are polished and one end of the rod is completely coated with silver for getting perfect transmission.
A helical flash lamp filled with xenon (Xe) provides white light in the form of flash for every 1/1000 of a second to achieve population inversion. This type of pumping process is known as optical pumping. Because of pumping, population inversion takes place in the higher energy levels, than stimulated emission can able to produce laser beam.
An intense red laser beam comes out from the end of ruby rod with divergence of about ˝” and output power is 10 KW.
He-Le Laser:
The first gas laser to be operated successfully was the He-Ne laser in 1961 by the scientist A. Jawan.
In this method, two gases helium & Neon were mixed in the ratio 10:1 in a discharge tube made of quartz crystal. The dimensions of the discharge tube are nearly 80 cm length and 1.5 cm diameter, with its windows slanted at Brewster’s angle i.e.,
Where n = refractive index of the window substance.
The purpose of placing Brewster windows on either side of the discharge tube is to get plane polarized laser output. Two concave mirrors M1 & M2 are made of dielectric material arranged on both sides of the discharge tube so that their focci lines within the interior of discharge tube.
Note: There are 3 types of polarization for light
i) Plane Polarized – straight lines
ii) Circularly Polarized – arcs
iii) Elliptically Polarized – ellipse
One of the two concave mirrors M1 is thick so that all the incident photons are reflected back into lazing medium. The thin mirror M2 allows part of the incident radiation to be transmitted to get laser output.
Working:
The discharge tube is filled with Helium at a pressure of 1 mm of Hg & Neon at 0.1mm of Hg. The gas discharge is established by the application of large potential difference b/w the two electrodes of the discharge tube. Being large in number. He atoms are easily excited to higher energy levels by colliding with each other. Neon atoms present in less concentration, to excite the Ne atoms pumping process must be required.
The excited He atoms collide with Ne atoms and transfer the excitation energy to the Ne atoms. Because both the atoms having higher energy levels very close to each other i.e., 20.61 ev & 20.66 ev for He & Ne respectively. Here Ne is the lazing medium & He is the pumping medium.
Neon atoms undergo a radiative transfer through 18.7 ev energy level by emitting photons of wavelength 6328 A0 .
The released photons are transmitted through the concave mirror M2 there by producing lasers. A continuous laser beam of red colour at a wavelength of 6328A0 . By the application of large potential difference, Ne atoms are pumped into higher energy levels continuously. Laser beam of power 5 to 50 MW comes out from He-Ne laser.
Description of Energy Levels:
When electric discharge is set-up in the tube, collisions b/w the atoms raises the amount of He & Ne atoms to the higher energy levels F1, F2 and E4, E6 respectively. Ne has two energy levels E4, E5 at nearly same energy level of F2 to He.
The Ne atoms drop down to the levels E3, E5 a radiative transfer (St. emission) and reaches to the level E2 through non-radiative transfer (sp. Emission).
From level E2 Ne atoms
are brought back to the ground state through collisions with walls of the
discharge tube. The transitions from 
emit radiations
corresponding to the wavelengths 3390A0 , 1150A0 , & 6328A0 , respectively.
6328A0 is the wall known red light of He – Ne laser because other two
wavelengths are not in the region of electromagnetic radiation (visible
region).
Semi-Conductor Laser
Principle & Working:
1. After the invention of semiconductor leaser in 1961, laser have become at common use.
2. In conventional lasers, lasers are generated due to transition of q‟s from higher to lower energy level.
3. but in semi-conductor lasers the transition takes place from conduction band to valence band.
4. The basic mechanism responsible for light emission from a semiconductor laser is the recombination of e‟s and holes at Pn-junction when current is passed through the diode.
5. Stimulated emission can occur when the incident radiation stimulates an e in conduction band to make a transition into valence band in that process radiation will be emitted.
6. When current is passed through PN – junction under forward bias, the injected e‟s & holes will increase the density of e in CB & holes in VB. At some value of current the stimulated emission rate will exceed the absorption rate.
7. As the current is further increased at some threshold value of current the amplification will takes place and laser begin to emit coherent radiation.
8. The photon releasing zone is narrow and the method of use is simple.
9. The properties of semiconductor laser depends upon the energy gap i.e., the induced radiation must have a frequency greater than the value of the energy gap in order that transition will takes place.
10. The e‟s are pumped to conduction band by passing forward current through semiconductor and there is no role of metastable states.
11. Population inversion can be achieved by producing a large current to PN junction, causing stimulated emission.