TYPES OF LASER - PowerPoint Presentation

Slide1

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

laserSlide2

NEODYMIUM LASER

Rare earth LASER system

Two types of Nd lasers

Nd:YAG LASER

Nd Glass LASERSlide3

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 LASERSlide4

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

ApplicationsWidely used in industries for cutting, welding, drilling and surface hardening of the industrial productsUsed to treat gastrointestinal bleeding and to do intraocular eye surgeriesSlide5

Nd: YAG Laser (Doped insulator laser) :Lasing medium

:

The host medium for this laser is Yttrium Aluminium Garnet (YAG = Y

3

Al5 O12) with 1.5% trivalent neodymium ions (Nd3+) present as impurities. The (Nd3+) ions occupy the lattice sites of yttrium ions as substitutional impurities and provide the energy levels for both pumping and lasing transitions.Nd:YAG LASERSlide6

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.Slide7

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 Slide8

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 Nd3+ ions from the ground state E1 to the multiple energy states at E4 Excited Nd3+ ions quickly decay to the metastable upper laser level E3 & excess energy is released to the crystal lattice (radiationless transition)Slide9

Nd:YAG LASER

Population inversion takes place at E

3

with respect to E

2 (E2 is virtually empty at room temperature) Stimulated emission takes place between E3 and E2 (laser radiation of wavelength 1.06 μm) E2 is about 0.25 eV above the ground state E1 (Transition of Nd3+ ions from E2 to E1 is radiationless and quite fast)Slide10

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 pumpingSlide11

Schematic arrangement of a gas laser

Gas LASERSlide12

CO2

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 CO2 molecule C.K.N. Patel designed CO2 laser in the year 1964Carbon di-oxide (CO2) LASERSlide13

Vibrational modes of CO2

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 stretchingEnergy 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 vibrationSlide14

Molecular Vibrations of CO

2

Slide15

CO2

LASER (Construction)

CO

2

laser system consists of CO2, N2 and He gases N2 → Increase the population of upper level of CO2 He → Decrease the population of lower level of CO2 Slide16

CO2

LASER (Construction)

Excited N

2

molecules collides with ground state CO2 molecules and exchange energy so that N2 goes to the ground state and CO2 goes to excited state (This process is effective because N2 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 CO2 molecules falls to the lower level Slide17

CO2

LASER (Construction)

Active medium :

It consists of a mixture of CO2, N2 and helium The active centres are CO2 molecules Optical resonators :A pair of concave mirrors placed on either side of the discharge tube, one completely polished and the other partially polishedA high dc voltage causes an electric discharge in the tubeSlide18

CO2

LASER (Construction)

Population inversion is created by electric discharge of the mixtureSlide19

CO2

LASER (Working)

CO

2

molecules are raised to their excited vibrational energy level E5 from their ground stateEnergy level E5 is metastable state energy level (so population inversion)When a electric discharge is passed through the tube, N2 molecules are excited to higher energy state Excited energy of N2 molecules is transferred to CO2 molecules through collisionsSlide20

CO2

LASER (Working)

Since the laser transition from E

5

to E4 has higher gain, the CO2 laser transition wavelength is normally 10.6 μmCO2 molecules from E4 and E3 return to the ground state through fast decay and diffusionStimulating photons of wavelength 10.6 μm and 9.6 μm induce the CO2 molecules to undergo stimulated emission by laser transition from E5 to E4 (

10.6

μ

m wavelength) and E

5

to E

3

(9.6

μ

m wavelength)Slide21

Absorption

Spontaneous

Emission

Stimulated

EmissionSlide22

Multiplication of Stimulated PhotonsSlide23

Active centers in the laser medium

are in the ground state

Population inversion through

pumping

Spontaneous emission of photonsSpontaneous photons trigger many stimulated transitionsPhotons along the axis of mirrors stimulate more and more atoms and build up their strengthPhotons 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