{"id":630,"date":"2018-11-26T05:48:11","date_gmt":"2018-11-26T05:48:11","guid":{"rendered":"http:\/\/phyp10.epgpbooks.inflibnet.ac.in\/?post_type=chapter&p=630"},"modified":"2019-04-30T09:03:04","modified_gmt":"2019-04-30T09:03:04","slug":"laser-system-li","status":"publish","type":"chapter","link":"https:\/\/ebooks.inflibnet.ac.in\/phyp10\/chapter\/laser-system-li\/","title":{"rendered":"laser system-ll"},"content":{"raw":"
\"epgp<\/a>\r\n<\/span><\/div>\r\n
\r\n\r\n1. CO2 laser<\/strong>\r\n\r\n \r\n\r\n \r\n

CO2 is a linear and symmetric molecule and its ground electronic state is a 1\u2211. As CO2 is a linear molecule, therefore it has 3N \u2013 5 =4 internal vibrational degree of freedom where N = 3 is the number of nuclei in the molecule.<\/p>\r\n \r\n

These four vibrations correspond to (i) symmetric stretching (ii) doubly degenerate bending mode and (iii) asymmetric stretching mode and have been shown in the figure.<\/p>\r\n \r\n

In the symmetric stretching mode, the oxygen atoms oscillate along the axis of the molecule simultaneously departing or approaching the carbon atom that is stationary.<\/p>\r\n \r\n

In the bending mode, that is doubly degenerate, the molecule ceases to be exactly linear. It occurs both in the plane of the figure and the plane perpendicular to it. In this mode all the three atoms in the molecule undergo vibrational motion perpendicular to the molecular axis.<\/p>\r\n \r\n

In antisymmetric stretching all the three atoms oscillate but both the oxygen atoms move in one direction and carbon atom moves in the opposite direction.<\/p>\r\n\"\"\r\n\r\n(a) symmetric stretching (b) bending mode (c) antisymmetric stretching\r\n\r\n \r\n

In the first approximation these three modes are described as independent harmonic oscillators, and hence the excited vibrational states are completely described by the number of excited quanta in each mode. These states are represented by a set of three vibrational quantum number v1, v21 , v3 where subscript 1, 2 and 3 refer to the symmetric bending and antisymmetric modes, respectively. The superscript 1 gives the angular momentum of this vibration about the axis of molecule in units of h\/2\u03c0. For example, 020<\/sup>0 indicates that the two vibrations combine to give an angular momentum <\/span>l<\/em>= 0.<\/span><\/p>\r\n \r\n

The Fig depicts the energy level diagram for the lowest levels. Each of the vibrational levels has superimposed upon it a set of rotational levels. The rotational levels are spaced by energies that are more than three orders of magnitude smaller than the vibrational energies.<\/span><\/p>\r\n\r\n<\/div>\r\n

<\/div>\r\n
\r\n\r\n\"\"\r\n

The CO2<\/sub> laser uses a mixture of CO2<\/sub>, N2<\/sub> and He or water vapor. The active centres are the CO2<\/sub> molecules lasing on the transition between the rotational levels of the vibrational bands of the electronic ground state. The figure also shows the vibrational energy level for the ground state of N2 molecule.<\/p>\r\n \r\n

Nitrogen being a homonuclear diatomic molecule does not posses a permanent electric dipole moment and hence radiation decay from v\u2019\u2019 = 1 to v\u2019\u2019=\u00a0\u00a0\u00a0 0 is forbidden. The lifetime of the level v\u2019\u2019 = 1is therefore quite long and ~ 0.1 s at one torr. The vibrational level 000<\/sup>1 of CO2<\/sub> is in near coincidence (\u2206E = 18 cm-1<\/sup>) with v\u2019\u2019= 1 level of nitrogen.<\/p>\r\n \r\n\r\nThe broken lines in the figure show non \u2013 radiative transitions.\r\n\r\n \r\n

When a discharge is passed in a tube containing mixture of CO2<\/sub>, N2<\/sub> and He, electron collisions excite the molecules to higher electronic vibrational rotational states. The electronic collision cross section for the excitation to the level 000<\/sup>1 is very large. The electron collision excite the N2<\/sub> molecules by<\/p>\r\n\"\"\r\n

The cross section for this process is quite large and produce significant amount of metastable N2 in the discharge. These nitrogen molecules transfer their internal energy to CO2 anymmetric stretching modes through near resonance collision.<\/p>\r\n\r\n<\/div>\r\n

\r\n\r\n\"\"\r\n\r\n \r\n

The cross section for this process is also large therefore, the excitation of CO2<\/sub>000<\/sup>1 level is unusually efficient. Further, the discharge containing the CO2<\/sub> produces dissociation of CO2<\/sub> into CO and oxygen. As Co vibrational frequency is not grossly different from that of the CO2<\/sub> asymmetric mode, it will rapidly transfer its internal energy to the CO2<\/sub>system through near resonant collision in the same manner as N2<\/sub>.<\/p>\r\n \r\n\r\nThe high density of population arises principally from three factors\r\n\r\n \r\n\r\n(i) the long vibrational lifetime of the asymmetric mode of CO2<\/sub>\r\n\r\n(ii) the near resonant N2<\/sub>\u2013 CO2<\/sub> energy transfer,\r\n\r\n(iii)\u00a0 the large cross section for the production of the vibrationally excited nitrogen through inelastic\r\n

collision with electrons<\/p>\r\n(iv)\u00a0 population gain from other levels of the asymmetric mode.\r\n\r\n \r\n

The 100<\/sup>0 and 020<\/sup>0 levels are in resonance. The coupling between these levels is sufficiently strong so that 100<\/sup>0 and 020<\/sup>0 level system behaves like a single level. The separation between these levels ~100cm-1that is significantly less than the thermal energy of motion kBT ~ 210 cm-1<\/sup> at room temperature. The similarity of vibrational motion in combination with the near degeneracy of the levels causes the rate of transfer between them by collision process, corresponding to<\/p>\r\n \r\n\r\n100<\/sup>0 + 000<\/sup>0 \u2192020<\/sup>0 + 000<\/sup>0 + \u2206E\r\n\r\n \r\n\r\nto be extremely rapid. Thus 100<\/sup>0 and 020<\/sup>0 levels of CO2<\/sub> reachs thermal equilibrium in a very short time. For population inversion, between upper 000<\/sup>1 level and lower 100<\/sup>0 and 020<\/sup>0 levels, it is necessary that population of lower levels should decay very fast. The presence of He has a considerable influence on the population decrease of lower levels. The He atoms collide with CO2<\/sub>and increase the rate of relaxation of 010<\/sup>0 levels.\r\n\r\n \r\n\r\nThe rate is approximately twenty times in collision of the type<\/span>\r\n\r\n010<\/sup>0 + He \u2192 000<\/sup>0 + He + \u2206E that for the corresponding<\/span>\r\n\r\n010<\/sup>0 + 000<\/sup>0 \u2192 000<\/sup>0 +000<\/sup>0+ \u2206E<\/span>\r\n\r\n \r\n

process where two CO2<\/sub> molecules collide. This reduces the bending mode population, that in turn reflects itself in reduction of lower level population, and hence increases the gain.<\/span><\/p>\r\n \r\n

Another function of He is to keep CO2<\/sub> cold. The excited CO2<\/sub> can decay spontaneously and directly to the ground state, liberating energy as heat. It is necessary to keep temperature of CO2<\/sub> as low to avoid population of lower level by thermal excitation. Helium has high thermal conductivity and hence helps to conduct heat away to the walls keeping CO2<\/sub> cold. Thus while N2<\/sub> helps to increase the population of the upper level, helium helps to depopulate the lower level.<\/span><\/p>\r\n \r\n

The laser oscillation is at a wavelength corresponding to the transition from the upper vibrational level 000<\/sup>1 to the lower vibrational level 100<\/sup>0. With each vibrational level, rotational levels are also associated. Thus a number of laser lines are possible. The population of J\u2019= 21 of 000<\/sup>1 has the maximum population and transition corresponding to J\u2019= 21 to J\u2019\u2019 = 22 or P(22) is strongest and this corresponds to the wavelength of 10.6\u00b5m. Another laser line of = 9.6 \u00b5m from transition between 000<\/sup>1 and 020<\/sup>0 is also prominent.<\/span><\/p>\r\n \r\n

A Schematic of CO<\/span>2<\/sub> laser is shown in the Figure. The discharge is produced in tube having diameter of 2.5 cm and length 5m. Brewster windows are used at the end. These windows are made of NaC1, CaC1, BaFC1, ZnSe or KC1 as these materials are transparent to 10.6\u00b5m radiation. As glass strongly\u00a0<\/span>absorbs most of the frequencies in infrared region therefore, near confocal Si mirrors coated with A1 are used.<\/span><\/p>\r\n \r\n

He, N2 and CO2are filled in the ratio 8:1:1 with pressure of He, N2 and CO2 as 0.7 torr, 1.2 torr and 0.33 torr, respectively. To remove the dissociation products such as Co and O, that would contaminate the laser, the continuous flow of the gas mixture is maintained in the tube.<\/span><\/p>\r\n \r\n

Grating can be used to separate out different laser lines. CO2 laser can be made to operate in pulsed or CW mode. Pulse energy ~ 2KJ and pulse width is of few \u00b5s. For CW operation, power > 50 W is easily available.<\/span><\/p>\r\n\r\n<\/div>\r\n

\r\n\r\n \r\n\r\nApplications<\/strong>\r\n\r\n \r\n\r\nThe advantage of using this laser is based on the fact that it provides very intense heating source over a very small area. It has applications in\r\n\r\n \r\n
    \r\n \t
  1. Material processing: cutting, drilling, material removal, welding, cladding, alloying, hardening, melting etc.<\/li>\r\n \t
  2. In medicine it is used for cutting and cauterizing.<\/li>\r\n<\/ol>\r\n \r\n\r\n2.<\/strong>\u00a0\u00a0\u00a0 Nitrogen Laser<\/strong>\r\n\r\n<\/div>\r\n
    \r\n\r\n\"\"\r\n\r\n\"\"\r\n\r\n\"\"\r\n\r\n \r\n

    The values of re<\/sub> indicate that the minimum of potential for C state lies almost vertically above that of the X state. On the other hand the minima of A and B shifted to high r. The electron collides with the molecules and excites them to various levels, when the electric discharge is passed through the gas, According to Franck Condon principle the probability of transition from X to C is much larger than that of X to A or X to B. Thus, population inversion occurs at level C (v = 0 level) relative to level B (v = 0 level). A lasing action is observed between C and B levels (corresponding to 0-0 and 0-1 vibrational levels). As the lifetime of state C is about 40 ns and that of B is about 100\u00b5s, therefore, the molecule remains in C state for a short duration and after transition accumulates in level B. Hence the population difference between B and C decreases very rapidly and this renders laser action impossible. The population inversion can therefore be achieved only for a short time and after this the laser action is self-terminating. Due to high gain of this self-terminating transition, oscillations take place in the form of amplified spontaneous\u00a0emissions. Thus the laser can be operated without mirror. The mirror placed at one end of the tube reduces the threshold power and also provides unidirectional output. This also reduces the beam divergence. The pulse width for nitrogen laser is normally 10ns and emission wavelength is at 337.1 nm. Peak power up to 1MW can be achieve.<\/span><\/p>\r\n \r\n

    A pulsed high voltage of about 20kV, triggered by a thyratron or spark gap is applied across the tube through which N2 at a pressure of about 100 torr, is flowing. On one end of the tube a mirror is used while a window used on the other end has a very high transmission. The length of the tube is between 0.1 \u2013 1.0m.<\/span><\/p>\r\n\r\n<\/div>\r\n

    \r\n\r\n \r\n\r\n3. Titanium Sapphire Laser<\/strong>\r\n\r\n \r\n

    Introduction: <\/em><\/strong>In 1960 T. H. Maiman demonstrated the operation of the first optical maser, or laser, a crystal of synthetically grown ruby, or sapphire (Al2<\/sub>O3<\/sub>) doped with a small amount of chromium was used in the experiments. P. F. Moulton demonstrated a widely tunable laser at Lincoln Laboratory, twenty-two years later, where titanium instead of chromium was incorporated as an impurity into sapphire. Titanium-doped sapphire or Ti:Al2<\/sub>O3<\/sub>, has the largest tuning range of any laser that is, from 660 to 1180 nm.<\/p>\r\n \r\n

    The Ti:Al2<\/sub>O3<\/sub> crystals used in the initial experiments exhibited significant scattering and an absorption at the laser wavelength and these losses affected the efficiency of the laser, and only pulsed operation was possible. Further with the advances in the development of higher-quality laser crystals of Ti:Al2<\/sub>O3<\/sub>, (with significantly smaller losses).room-temperature continuous-wave operation became areality that was first reported in 1986.<\/p>\r\n \r\n

    Sapphire is an ideal host crystal in both the ruby and Ti:Al2<\/sub>O3<\/sub> laser. It is transparent from ultraviolet to infrared and also, it is nonhygroscopic and very hard, that is required for producing good optical-quality surfaces..<\/span><\/p>\r\n\r\n<\/div>\r\n

    \r\n\r\n \r\n

    The most common tunable lasers are the organic dye lasers. A dye laser consists of an organic dye (such as polymethine, xanthenes, or coumarin dye) in a liquid solvent or host. A typical dye laser such as Rhodamine 6G can be tuned from 570 to 610 nm; other dyes have laser bandwidths that cover portions of the spectrum from the ultraviolet to the infrared.<\/p>\r\n \r\n

    In a Ti:Al2<\/sub>O3<\/sub> laser the laser transition is between two electronic levels of a single Ti3+<\/sup> ion, and in dye lasers the laser transition is from one molecular electronic level to another. The broadening of the absorption and luminescence bands is caused by the multitude of rotational levels associated with each molecular electronic level. However there are undesirable characteristics of dye lasers as toxicity of the dyes and solvents, degradation of the dyes with time, and amplitude noise in the laser output (because the dyes are flowed to reduce thermal loads).<\/p>\r\n \r\n

    Another class of tunable solid state lasers is the alkali-halide color-center lasers. A color center is a crystal defect in which an electron becomes trapped. For example, an F color center consists of an electron trapped at an anion vacancy of the crystal lattice. The excitations of the trapped electron strongly couple to the phonons of the lattice and give rise to broad absorption and emission bands. Color-center lasers can span the wavelength range from 800 nm to 4 \u03bcm where organic dye lasers are of limited usefulness. Color-center lasers have many drawbacks;<\/p>\r\n\r\n