TYPES OF LASER Solid State lasers:Ruby laser, Nd:YAG laser, Nd:Glass laser Gas lasers:He-Ne laser, CO 2 laser, Argon laser Liquid/Dye lasers:Polymethene dye, Courmarine dye, Rhodamine laser Semiconductor laser:GaAs laser, InP laser

NEODYMIUM LASER Rare earth LASER system Two types of Nd lasers (1)Nd:YAG LASER (2)Nd Glass LASER

Expansion – neodymium (Nd) : yttrium aluminum garnet (YAG) First demonstrated by (1964) 1. J. E. Geutic 2. H. M. Marcos 3. L. G. Van Uitert Nd:YAG LASER

Nd:YAG LASER is a four level LASER Emit high power pulses having short pulse width & high repetition rate Can give continuous power output also Applications Widely used in industries for cutting, welding, drilling and surface hardening of the industrial products Used to treat gastrointestinal bleeding and to do intraocular eye surgeries

Nd: YAG Laser (Doped insulator laser) : Lasing medium : The host medium for this laser is Yttrium Aluminium Garnet (YAG = Y 3 Al 5 O 12 ) with 1.5% trivalent neodymium ions (Nd 3+ ) present as impurities. The (Nd 3+ ) ions occupy the lattice sites of yttrium ions as substitutional impurities and provide the energy levels for both pumping and lasing transitions. Nd:YAG LASER

Nd: YAG laser rods are typically 5–10 cm length and 6–10 diameter The YAG laser rod and a linear flash lamp are housed in a elliptical reflector cavity Since the laser rod and the lamp are located at the foci of the ellipse, the light emitted by the lamp is effectively coupled to the rod The ends of the rod are polished and made optically flat and parallel.

Nd:YAG LASER The optical cavity is formed either by silvering the two ends of the rod or by using two external reflecting mirrors The system is cooled by either air or water circulation One mirror is made hundred percent reflecting while the other mirror 90% reflecting (slightly transmitting) to draw the output

Nd:YAG LASER The Nd 3+ ions are pumped by a xenon flash lamp Absorption bands are in the range 700–800 nm (0.7–0.8  m) Flash lamp excites the Nd 3+ ions from the ground state E 1 to the multiple energy states at E 4 Excited Nd 3+ ions quickly decay to the metastable upper laser level E 3 & excess energy is released to the crystal lattice (radiationless transition)

Nd:YAG LASER Population inversion takes place at E 3 with respect to E 2 (E 2 is virtually empty at room temperature) Stimulated emission takes place between E 3 and E 2 (laser radiation of wavelength 1.06 μm) E 2 is about 0.25 eV above the ground state E 1 (Transition of Nd 3+ ions from E 2 to E 1 is radiationless and quite fast)

Nd:YAG LASER The laser output is in the form of pulses with higher repetition rate Xenon flash lamps are used for pulsed output Nd: YAG laser can be operated in CW mode also using tungsten-halide incandescent lamp for optical pumping

Schematic arrangement of a gas laser Gas LASER

CO 2 lasers belong to the class of molecular gas lasers In Nd:YAG LASER, the transitions takes place among the various excited electronic states (electron energy levels) of an atom or an ion Each electron level is associated with vibrational and rotational energy levels The laser transitions occur between different vibrational states of the CO 2 molecule C.K.N. Patel designed CO 2 laser in the year 1964 Carbon di-oxide (CO 2 ) LASER

Vibrational modes of CO 2 The principle of CO 2 laser is transition between vibrational states of the same electronic state by achieving population inversion between these states Three modes of Vibration Symmetric stretching Bending Asymmetric stretching Energy states of a molecule are represented by three quantum numbers (m n q) – These numbers denote the amount of energy associated with each mode of vibration

Molecular Vibrations of CO 2

CO 2 LASER (Construction) CO 2 laser system consists of CO 2, N 2 and He gases N 2 → Increase the population of upper level of CO 2 He → Decrease the population of lower level of CO 2

CO 2 LASER (Construction) Excited N 2 molecules collides with ground state CO 2 molecules and exchange energy so that N 2 goes to the ground state and CO 2 goes to excited state (This process is effective because N 2 remains in excited metastable state for long time) He gas depopulates the lower vibrational energy levels by different energy transfer process (vibrational energy is converted into translational KE of the He atoms) He takes greater amount of energy during collision due to its small mass and CO 2 molecules falls to the lower level

CO 2 LASER (Construction) Active medium : It consists of a mixture of CO 2, N 2 and helium The active centres are CO 2 molecules Optical resonators : A pair of concave mirrors placed on either side of the discharge tube, one completely polished and the other partially polished A high dc voltage causes an electric discharge in the tube

CO 2 LASER (Construction) Population inversion is created by electric discharge of the mixture

CO 2 LASER (Working) CO 2 molecules are raised to their excited vibrational energy level E 5 from their ground state Energy level E 5 is metastable state energy level (so population inversion) When a electric discharge is passed through the tube, N 2 molecules are excited to higher energy state Excited energy of N 2 molecules is transferred to CO 2 molecules through collisions

CO 2 LASER (Working) Since the laser transition from E 5 to E 4 has higher gain, the CO 2 laser transition wavelength is normally 10.6 μm CO 2 molecules from E 4 and E 3 return to the ground state through fast decay and diffusion Stimulating photons of wavelength 10.6 μm and 9.6 μm induce the CO 2 molecules to undergo stimulated emission by laser transition from E 5 to E 4 (10.6 μm wavelength) and E 5 to E 3 (9.6 μm wavelength)

Absorption Spontaneous Emission Stimulated Emission

Multiplication of Stimulated Photons

Active centers in the laser medium are in the ground state Population inversion through pumping Spontaneous emission of photons Spontaneous photons trigger many stimulated transitions Photons along the axis of mirrors stimulate more and more atoms and build up their strength Photons are fed into the medium by the mirrors & photon amplification Laser output will become bright when the photons have enough power and coherent laser light