Wel – Come

1. Wel – Come

2. Presentation ON Types of Lasers B.Sc. III, Paper XIX ( VI Semester) By Dr. ShitreA.R. Head, Department of Physics, YashwantraoChavanMahavidyalaya, Tuljapur.

3. THE RUBY LASER In the first laser fabricated by Maiman in 1960, the population inversion was achieved in the following manner. It was made from a single cylindrical crystal of ruby whose ends were flat, with one of the ends completely silvered and the other partially silvered (see Figs. 26.16 and 26.17). Ruby consists of Al2O3 with some of the aluminum atoms replaced by chromium. The energy states of the chromium ion are shown in Fig. 26.18. The chief characteristic of the energy levels of a chromium ion is the fact that the bands labeled E1 and E2 have a lifetime of ~10 –8 s whereas the state marked M has a lifetime of ~3 x 10 –3 s—the lifetime represents the average time an atom spends in an excited state before making a transition to a lower energy state. A state characterized by such a long lifetime is termed a metastable state. Fig. 26.17 The first ruby laser

4. The chromium ion in its ground state can absorb a photon (whose wavelength is around 6600 Å) and make a transition to one of the states in the band E1. It could also absorb a photon of  ~ 4000 Å and make a transition to one of the states in the band E2 —this is known as optical pumping, and the photons which are absorbed by the chromium ions are produced by the flash lamp (see Fig. 26.16). In either case, it immediately makes a nonradiative transition (in a time ~ 10–8 s) to the metastable state M—in a nonradiative transition, the excess energy is absorbed by the lattice and does not appear in the form of electromagnetic radiation. Also since state M has a very long life, the number of atoms in this state keeps increasing and one may achieve population inversion between states M and G. Thus we may have a larger number of atoms in states M and G. Once population inversion is achieved, light amplification can take place, with two reflecting ends of the ruby rod forming a cavity. The ruby laser is an example of a three-level laser.

5. In the original setup of Maiman, the flash lamp (filled with xenon gas) was connected to a capacitor (see Fig. 26.16) which was charged to a few kilovolts. The energy stored in the capacitor (~ a few thousand Joules) was discharged through the xenon lamp in a few milliseconds. This results in a power which is about a few megawatts. Some of this energy is absorbed by the chromium ions, resulting in their excitation and subsequent lasing action.

6. Spiking in Ruby Laser The flash operation of the lamp leads to a pulsed output of the laser. Even in the short period of a few tens of microseconds in which the ruby is lasing, one finds that the emission is made up of spikes of high-intensity emissions as shown in Fig. 26.19. This phenomenon is known as spiking and can be understood as follows. When the pump is suddenly switched on to a value much above the threshold, the population inversion builds up and crosses the threshold value, as a consequence of which the photon number builds up rapidly to a value much higher than the steady-state value. Since the photon number is higher than the steady-state value, the rate at which the upper level depletes (because of stimulated transitions) is much higher than the pump rate. Consequently, the inversion becomes below threshold, and the laser action ceases. Thus the emission stops for a few microseconds, within which time the flash lamp again pumps the ground-state atoms to the upper level, and laser oscillations begin again. This process repeats itself till the flash lamp power falls below the threshold value and the lasing action stops.

7. THE He–Ne LASER The He-Ne laser was first fabricated by Ali Javan and co-workers at Bell Telephone Laboratories in the United States. This was also the first gas laser to be operated successfully. The He-Ne laser consists of a mixture of He and Ne in a ratio of about 10:1, placed inside a long, narrow discharge tube (see Figs. 26.20 and 26.21). The pressure inside the rube is about 1 torr. The gas system is enclosed between a pair of plane mirrors or a pair of concave mirrors so that a resonator system is formed. One of the mirrors is of very high reflectivity while the other is partially transparent so that energy may be coupled out of the system. Fig. 26.21 A helium-neon laser demonstration at the Kastler-Brossel Laboratory at Univ. Paris 6. The glowing ray in the middle is an electric discharge producing light in much the same way as a neon light. It is the gain medium through which the laser passes, not the laser beam itself, which is visible there. The laser beam crosses the air and marks a red point on the screen to the right.

8. The first few energy levels of He and Ne atoms are shown in Fig. 26.22. When an electric discharge is passed through the gas, the electrons traveling down the tube collide with the He atoms and excite them (from the ground state F1 ) to the levels marked F2 and F3 . These levels are metastable; i.e., He atoms excited to these states stay in these levels for a sufficiently long time before losing energy through collisions. Through these collisions, the Ne atoms are excited to the levels marked E4 and E6 which have nearly the same energy as the levels F2 and F3 of He. Thus when the atoms in levels F2 and F3 collide with unexcited Ne atoms, they raise them to the levels E4 and E6 , respectively.

9. Thus, we have the following two-step process: Helium atom in the ground state F1 + collision with electron  Helium atom in the excited state (F2 or F3 ) + electron with lesser kinetic energy. The excited states of He (F2 or F3 ) are metastable —they would not readily lose energy through spontaneous emissions (the radioactive lifetime of these excited states would be about 1 h). However, they can readily lose energy through collisions with Ne atoms: He atom in excited state F3 + Ne atom in ground state  He atom in ground state + Ne atom in excited state E6 Similarly, He atom in excited state F2 + Ne atom in ground state  He atom in ground state + Ne atom in excited state E4

10. This results in a sizeable population of the levels E4 and E6 . The population in these levels happens to be much more than those in the lower levels E3 and E5. Thus a state of population inversion is achieved, and any spontaneously emitted photon can trigger laser action in any of the three transitions shown in Fig. 26.22. The Ne atoms then drop down from the lower laser levels to the level E2 through spontaneous emission. From the level E2 the Ne atoms are brought back to the ground state through collision with the walls. The transitions from E6 to E5, E4 to E3, and E6 to E3 result in the emission of radiation having wavelengths of 3.39 m, 1.15 m, and 6328 Å, respectively. Note that the laser transitions corresponding to 3.39 and 1.15 m are not in the visible region. The 6328 Å transition corresponds to the well-known red light of the He-Ne laser. A proper selection of different frequencies may be made by choosing end mirrors having high reflectivity over only the required wavelength range. The pressures of the two gases must be chosen so that the condition of population inversion is not quenched. Thus the conditions must be such that there is an efficient transfer of energy from He to Ne atoms.

11. Also, since the level marked E2 is metastable, electrons colliding with atoms in level E2 may excite them to level E3 , thus decreasing the population inversion. The tube containing the gaseous mixture is also made narrow so that Ne atoms in level E2 can get de-excited by collision with the walls of the tube. Actually there are a large number of levels grouped around E2 , E3 , E4 , E5 , and E6 (see Fig. 26.22). Only those levels are shown in the figure which correspond to the important laser transitions. Gas lasers are, in general, found to emit light, which is more directional and more monochromatic. This is so because of the absence of such effects as crystalline imperfection, thermal distortion, and scattering, which are present in solid-state lasers. Gas lasers are capable of operating continuously without need for cooling.

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