unit – v

UNIT – V

LASER

5.1. INTRODUCTION:

Laser is an outstanding achievement of science and technology in the twentiethcentury.

Today Lasers find wide applications in communication systems, computers,navigation equipments, measuring instruments and in medicine. The term LASERstands for Light Amplification by Stimulated Emission of Radiation.

Einstein predicts the existence of stimulated emission of radiation by an atom.The theoretical prediction for laser oscillations was given by A.L. Schawlow and C.H.Townes in the year 1958 using the process of stimulated emission.

The first laser was demonstrated by T.H.Maiman in the year 1960 using a rubycrystal as the active material.

Basic requirements of laser operation:

To achieve laser operation, the following basic requirements are required.

H Active medium − To create population inversion.

H Active centers − atoms that actually take part in population inversion.

H Population inversion.

H Resonance cavity − For optical feedback.

H Threshold inversion density − for emission of inphase photons satisfyingcharacteristics of Laser beam.

Stimulated absorption:

In order to understand the working principle of laser, let us study the quantumprocesses that take place in a material matter when it is exposed to light radiation.Let us consider an assembly of atoms exposed to light radiation. According toquantum theory, light radiation consists of photons with energy hν. Let us assumetwo energy levels E1 and E2 for the atom.

An atom residing in the lower energy state E1 can absorb a photon and go to

the higher energy level E2 (excited state) provided the photon energy hν equals to

energy difference (E2 − E1). This process is known as stimulated absorption or simply

absorption.

Fig. 5.1: Stimulated absorption

Spontaneous emission:

After absorption of photon the atom return to ground state by emitting a photonof energy hν. The emission process can take one of the following forms.

(i) Spontaneous emission

(ii) Stimulated emission

In the case of spontaneous emission, the atom passes from higher energystate E2 (excited state) to lower energy state E1 (ground state) spontaneously by

emission of a photon of energy hν (Figure 5.2).

Fig. 5.2: Spontaneous emission

The spontaneous emission is random in character. The radiation emittedspontaneously by each atom has a random direction and a random phase. Thusradiation in this case is a random mixture of quanta having various wavelengths.

Moreover, they are not in phase. Thus the radiation is incoherent and has abroad spectrum. It is the process of spontaneous emission that dominates inconventional light sources.

5.2 ENGINEERING PHYSICS – I

Stimulated emission:

The transition from a higher to a lower energy state with the emission ofradiation depends upon the presence of incidence radiation of the same frequency inthe surrounding.

According to Einstein, an interaction between the excited atom and a photoncan trigger the excited atom to make a transition to the ground state. The transitionproduces a second photon which would be identical to the triggering photon in respectof frequency, phase and propagation direction. This process of induced emission ofphotons caused by the incident photon is called stimulated emission (Figure 5.3).

Fig. 5.3: Stimulated emission

Now the emission contains both incident photon and photon due to stimulatedemission. Both the photons are in same phase. The remarkable feature of thestimulated emission of radiation is that it is coherent with the incident radiation.

The photon emitted due to stimulated emission has the same frequency andphase as the photon in the incident radiation. This is the basic concept of laser.It is the process of stimulated emission that dominates in the laser light source.

Differences between spontaneous and stimulated emissions:

Spontaneous and stimulated emissions are distinguished from each other due tofollowing reasons.

S.No. Spontaneous emission Stimulated emission

1. It is random in character. That is,this type of emission is astatistical phenomenon.

This type of emission takes placedue to inducement of incidentphoton.

2. It is incoherent. It is highly coherent.

3. This type of emission dominates inconventional light sources.

This type of emission dominates inlaser sources.

LASER 5.3

S.No. Spontaneous emission Stimulated emission

4. It is less intense. It is more intense.

5. The radiation of this type containsmany wavelengths.

The radiation of this type containsmonochromatic wavelength.

6. Less directionality with moreangular spread.

High directionality with lessangular spread.

5.2. AMPLIFICATION OF LIGHT BY POPULATION INVERSION:

The ratio of stimulated emission to spontaneous emission gives

Nst

Nsp =

B21 N2 Q

A21 N2 =

B21

A21 Q .... (5.1)

Equation (5.1) indicates that the stimulated emission will dominate thespontaneous emission if density (Q) of the incident photons is very large. Thus, thepresence of a large number of photons in the active medium is required.

However, it will lead to more absorption transitions. Hence large photon densityalone will not be sufficient for more stimulated emissions.

Population inversion:

To achieve stimulated emission, population of excited state N2 should be made

larger than population of lower state (N1) and this condition is called population

inversion.

The ratio of stimulated emissions to stimulated absorptions is

Nst

Nab =

B21 N2 Q

B12 N1 Q.... (5.2)

But B21 = B12

Nst

Nab =

N2

N1.... (5.3)

The equation (5.3) indicates that the stimulated emissions will dominate theabsorption process if N2 is greater than N1. It means that there should be more

atoms present in the higher energy level than in the lower energy level for stimulatedemission to dominate over spontaneous emissions.


The establishment of situation in which the number of atoms in higher energystate is larger than the number of atoms in the lower energy state is called populationinversion.

Fig. 5.4: Population inversion

Different methods of achieving population inversion:

There are five different methods by which the population inversion can beachieved, viz.,

(a) Optical pumping.

(b) Direct electron excitation.

(c) Inelastic atom-atom collision.

(d) Direct conversion.

(e) Chemical process.

Pumping action:

The process of raising more number of atoms to excited state by artificial stateis known as pumping process.

Active Medium:

This is the basic material in which atomic or molecular transitions take placeleading to the laser action.

Pumping System:

This is a device by itself or simply a process by which population inversion canbe achieved in the medium.

LASER 5.5

5.3. PROPERTIES OF LASER:

The wide use of laser in science and technology is due to specific properties oflaser radiation. The most striking features of laser are:

(i) High degree of coherence

(ii) Extraordinary

(iii) Directionality and

(iv) High intensity

Coherence

Laser radiation has a high degree of coherence. That is, all its constituentphotons have the same energy, same direction of momentum and same direction ofpolarization. Coherence is characterized by two parameters, namely coherence

length and coherence time.

The length of the wave train over which it may be assumed to have a fairlysinusoidal character and predictable phase is known as coherence length. The timeinterval during which the phase of the wave train can be predicted reliably is calledcoherence time.

The coherence length is about 600 km for laser beam and 3 cm only for sodiumlamp. The high coherence of laser beam results in extremely high power radiation.

Fig. 5.5: Coherence

Monochromaticity

Laser beam is more monochromatic (single wavelength or frequency) to any otherconventional monochromatic light sources. Laser beam spreads over a very smallfrequency range whereas ordinary light beam spreads over a very large frequencyrange.


Fig. 5.6: Monochromaticity of laser beam and ordinary light beam

The degree of non-monochromaticity is defined as ξ = ∆ νν0

Where, ∆ν is the band width and νo is the average frequency.

For laser beam, ∆ν = 500 Hz and νo = 5 × 104 Hz.

Thus, the degree of monochromaticity of laser beam ξ = ∆ν

ν0 =

500

5 × 1014 = 10− 22

For conventional monochromatic light source ξ = ∆ν

ν0 =

109

1014 = 10− 5.

Hence the monochromaticity of the conventional monochromatic light source ispoorer than the laser source. It is concluded that laser beam is highly monochromatic.

Directionality

Laser beam travels as a parallel beam over very long distances but conventionallight source emits in all directions. The directionality of laser beam is usuallyexpressed in terms of full angle beam divergence. The angular spread is related withthe aperture diameter ‘d’ by. For a typical laser, the beam divergence is less than0.01 milliradian. That is, the beam spread is less than 0.01 mm for every metre.But in the case of ordinary light beam, the spread is about 1 metre for every onemetre that light traverses.

LASER 5.7

Intensity

Laser beam gives out light into a narrow beam of light and its energy isconcentrated in a small region. Even a 1 watt laser light is more intense than100 watt ordinary incandescent lamp.

Ordinary light and laser light:

Laser light is distinguished from ordinary light due to the following reasons.

S.No. Ordinary light Laser light

1. It is not coherent It is highly coherent

2. It is not directional It is more directional

3. It is less intense It is highly intense

4. The angular spread is more The angular spread is less

5. Examples are sunlight, mercuryvapour lamp etc.

Examples are helium-neon laser,carbon dioxide laser etc.

5.4. DIFFERENT KINDS OF LASERS:

Solid state lasers, Gas lasers, Liquid lasers, Dye Lasers and Semiconductorlasers are the important kinds of lasers.

Solid state lasers : Ruby laser, Nd : YAG laser

Gas lasers : He-Ne laser, CO2 laser, Argon-ion laser

Liquid lasers : Se OCl2 laser, Europium chelate laser

Dye lasers : Rhodamine 6G laser, Coumarin dye laser

Semiconductor lasers:Ga As laser, In P laser.

5.5. RUBY LASER:

Ruby laser is a three level solid state laser and was constructed by Maiman.It is a pulsed laser having very high power of hundreds of Mega watt in a singlepulse with 10 nanosecond duration. It is used for various industrial applications likesurface hardening, hard facing, cladding of various industrial products, etc. Recently

erbium (Er3+) doped ruby lasers are available and have higher merits than ordinary

chromium (Cr3+) doped ruby laser.


Fig. 5.7: Ruby laser

Construction

In ruby laser the active element is pure aluminium oxide (Al2 O3) with 0.05%

of chromium and it is shaped into a cylinder with parallel transparent sides andreflectors at both ends. Among the two end reflectors, one is a perfect reflector andother is a partial reflector.

The diameter of the active element is about 0.5 cm. and its length is about fewcentimetres. Pumping source is a flash lamp. Chromium atoms are particularlyresponsive to light having a wavelength 5600 Å. Most flash lamps like Xenon flashlamps are able to supply energy in this wavelength range.

Flash lamp tube is spirally wound over the curved surface of the ruby rod andis connected to a power supply as shown in figure 5.7.

Working:

(a) Once chromium atoms have been excited to an upper energy level ‘H’ byabsorbing light photons of wavelength 5600 Å from the flash lamp theyrequire two steps to return to their ground state ‘G’. First step is from ‘H’state to metastable state ‘M’ which is a shorter jump and energy emitted inthis transition is passed to the crystal lattice as heat. The energy is notradiated in the form of photons and this transition is called “radiation lesstransition” (Figure 5.8).

LASER 5.9

Fig. 5.8: Three levels in Ruby Laser “Transition”

(b) The chromium atoms returned to M level can remain in this state for severalmilliseconds. The accumulation of excited atoms at M level increases thepopulation at M level and then transition occurs from M to G level emittingout some photons by spontaneous emission initially in a random manner.

(c) Due to continuous working of flash lamp, the chromium atoms arecontinuously raised to higher energy state ‘H’ and then to M level.

(d) At a particular stage population of excited chromium atoms are more at Mthan at G. Hence there is population inversion. The emitted photons ofwavelenegth 6943 Å stimulate or induce the chromium atoms at level M toundergo transition. This results in stimulated emission of other identicalphotons and a cascade begins.

(e) The photons travelling parallel to the axis of the ruby rod are used forstimulation while the photons travelling in other directions will pass outfrom the ruby rod. In the mean time, the photons undergo multiple reflectionsfrom the mirrors placed at the ends of the ruby rod and the intensity of thelaser radiation grows to a higher value and some of its bursts is coming outthrough the partial reflector and it serves as output laser beam.

(f) The emitted photon and stimulating photon are in phase and have samefrequency and are travelling in the same direction. Thus the laser beam hasdirectionality along with spatial and temporal coherence.

(g) The output beam has red wavelength 6943 Å and frequency

4.32 × 1014 Hertz. Its power is more than few hundred Mega watt.


[Note: Metastable state ‘M’ consists of two closed sublevels. There is another lasertransition between one of these closed sub-levels and ground state giving an emissionline at a wavelength 6927 Å. But this transition probability is very small].

Applications of Ruby laser:

H Since it is a pulsed laser, it is used in pulsed holography.

H It is used in LIDAR.

H It is used in Remote sensing.

H It is used in Ophthalmology.

H Because of its coherence it is used in drilling small areas.

5.6. ND-YAG (NEODYMIUM – YTTRIUM ALUMINUM GARNET) LASER:

Characteristics of Nd-YAG laser:

Type − Doped insulator laser (solid state laser)

Active medium − Yttrium Aluminum Garnet (Y3 Al5 O12)

Active centre − Neodymium (Nd3 + ions)

Pumping method − Optical pumping

Pumping source − Xenon flash lamp

Optical resonator − Ends of the rod polished with silver and twomirrors, one of them is totally reflecting and theother is partially reflecting.

Power output − 2 × 104 Watts

Nature of output − Pulsed

Wavelength emitted − 1.064 µm

Introduction:

Nd-YAG laser is a doped insulator laser. It is a four level system in which theactive medium is taken in the form of a crystal. Here, the crystal is intensitionallydoped during its growth. Those type of lasers have number of energy levels with thesame energy. The laser is used to generate high power intensity.

Principle:

The term “doped insulator laser” refers to the active medium, yttrium aluminium

garnet doped with neodymium Nd3 + . The neodymium ion has many energy levels.Due to optical pumping, these ions are raised to excited levels. During the transitionfrom metastable state to E1state, the laser beam of wavelength 1.064 µm is emitted.

LASER 5.11

Construction:

The active medium is made as a rod which has yttrium aluminum garnet

(Y3 Al5 O12) doped with a rare earth metal ion neodymium Nd3 + ions normally

occupies the yttrium ions and provides the energy levels for both the lasingtransitions and pumping. This rod is placed inside a highly reflecting elliptical cavityas shown in fig. 5.9.

Fig. 5.9

A close optical coupling is made by placing the xenon flash lamp near by thelaser rod, in such a way that most of the radiation from the flash type passed throughthe laser rod due to the elliptical cavity. The flash tube may be switched ON andcontrolled with the help of a capacitor. The discharge of capacitor is initiated usinga high voltage source.

The optical resonator is formed by grinding the ends of the rods and coatedwith silver accompanied by two mirrors, one is 100% reflecting and the other ispartially reflecting which is included to increase the efficiency of the output beam.

Working:

1. The xenon flash lamp is switched ON and the light is allowed to fall on thelaser rod.

2. The intense white light excites the neodymium (Nd3 + ) ions from the groundstate to various energy levels above E2 hence the atoms are raised to group

of higher levels in E3 as illustrated in the energy level diagram.


3. From these energy levels the ions make non − radiative decay and is gatheredin a state called metastable state, until the population inversion is achieved.

4. Once the population inversion is achieved, the stimulated emission builds uprapidly.

5. Hence, pulsed form of laser beam of wavelength 1.064 µm is emitted duringthe transition from E4 to E1 (lower).

6. A large amount of heat is produced by the flash tube during the working.Hence cooling arrangement is made either by blowing air or circulating waterover the crystal.

Applications of Nd-YAG laser:

1. It is used in transmitting signals to a longer distance.

2. It is used in ling haul communication system.

3. It is also used in the endoscopic applications.

4. It plays a vital role in remote sensing applications.

5.7. HELIUM-NEON LASER:

For the continuous laser beam, He-Ne laser is used. Light radiations with highercoherence, higher directionality and higher monochromaticity can be obtained fromit. But the output power is generally few milliwatts. Helium-Neon lasers are veryuseful in making holograms and for interferometric experiments. In medicine, thisacts as an aiming laser which is normally used to identify the spot where the lasersurgery has to be performed.

Construction

The Helium-Neon laser system consists of a gas discharge tube which is theactive medium. The tube is made up of quartz and is filled with the mixture of Neonunder a pressure of 0.1 mm of mercury and Helium under a pressure of 1 mm ofmercury. The ratio of the Helium-Neon mixture is 10 : 1 i.e., the number of Heliumatoms is greater than the number of Neon atoms. The power output from these lasersdepends upon the length of the discharge tube and pressure of gas mixture. Furtherelectrodes at the ends of the discharge tube are connected to a radio frequencyoscillator to produce electrical discharge in the Helium-Neon mixture so as to pumpthe Helium atoms to higher energy states.

LASER 5.13

1. Perfect concave reflector 2. Active medium 3. Electrodes 4. R.F. Oscillator5. Brewster angle windows 6. Partially reflecting concave mirror.

Fig. 5.10: Construction of Helium-Neon Laser

The end faces of the gas discharge tube are tilted at the Brewster angle asshown in figure 5.10. These are called Brewster angle windows. An emittedunpolarised light wave consists of two components called perpendicularly polarisedwave and parallel polarised wave. The perpendicularly polarised wave is completelyattenuated by the window plate while the parallel polarised wave is transmitted bythe window in the same direction. The parallel polarised wave is repeatedly reflectedby the concave mirrors situated behind the Brewster angle windows andcorrespondingly it passes repeatedly through the active medium. Among these concavemirrors, one is partially reflecting and the other is perfectly reflecting. Through thepartially reflecting concave mirror the laser output is coming out as parallel polarisedbeam of light.

Working:

Fig. 5.11: Helium-Neon energy levels and laser transitions


1. By the electrical discharge in the gas tube, the ground level (E0) helium atomsare excited to higher levels E1 and E2 of helium. This process of excitationis called electron excitation which is taking place by the transfer of fractionof kinetic energy of electrons to helium atoms (Figure 5.11).

2. By reasonance collisional transfer method, the helium atoms at E2 give up

their excitation energy to the ground state neon atoms. Thus the neon atomsare excited to their higher energy level E5. Meanwhile these helium atoms

are de-excited and returned to their ground state.

3. Similarly the helium atoms at E1 give up their excitation energy to the ground

state neon atoms and the neon atoms are excited to another higher energylevel E3 as shown in figure 5.11. The helium atoms are de-excited and

returned to their ground state.

4. Since E5 and E3 of Neon atoms are metastable states, population inversion

takes place at these levels. Any one of the spontaneously emitted photonswill trigger the laser action.

5. Thus the stimulated emission takes place between E5 (3s) and E2 (2p) giving

a laser light wavelength of 6328 Å.

6. Similarly the stimulated emission between E5 (3s) and E4 (3p) gives a laser

light wavelength of 3.39 µm.

7. Another stimulated emission between E3 (2s) and E2 (2p) gives a laser light

wavelength of 1.15 µm.

8. The neon atoms undergo transition from E2 to E1 and from E4 to E1 in the

form of fast decay giving photons by spontaneous emission. These photonsare absorbed by optical elements placed inside the laser system.

9. The neon atoms are returned to the ground state from E1 by non-radiative

diffusion and collision proceses. Therefore there is no emission of radiation.

10. After arriving the ground state, once again the neon atoms are raised toE5 and E3 by excited helium atoms. Thus one can get continuous output form

the He-Ne laser.

11. Some optical elements placed inside the laser system are used to absorb theinfrared laser wavelengths 3.39 µm and 1.15 µm.

12. Hence the output of He-Ne laser contains only a single wavelength of 6328Å and the output power is about few milliwatts.

Applications of He-Ne Laser:

H Because of its high power it is used in open air communications.

H It is used to produce holograms.

H It is used in determining the size of tiny particles.

LASER 5.15

5.8. CARBON DI-OXIDE LASER:

In the Nd YAG laser or in He-Ne laser, the transitions are taking place amongthe various excited electronic states of an atom or an ion.

In CO2 laser, the laser transitions are occurring between different vibrational

states of the carbon di oxide molecule. An Indian Engineer, Patel was the first personwho designed CO2 laser.

Special features of CO2 lasers

1. High power CO2 lasers find applications in materials processing, welding, hole

drilling, cutting, etc.

2. Since the atmospheric attenuation is low at 10.6 µm, CO2 lasers are also

used in open air communication.

3. It is used in surgery as a direct substitute for a scalpel. In cutting tissueswith CO2 laser beam, the laser light is absorbed in a small volume, causing

a little damage to the surrounding tissues.

4. Laser radar or Lidar (Light detection and ranging) has proved to be apowerful tool for investigation of a variety of atmospheric features. Mostlypulsed laser type is used.

(a) Modes of vibration and vibrational energy levels:

The CO2 molecule consists of a central carbon atom with two oxygen atoms

attached one on either side. We know that every molecule has electronic, vibrationaland rotational energy levels.

Fig. 5.12: Three independent modes of vibration of the carbon di-oxide molecule


In CO2 laser, laser transition is taking place between vibrational-energy levels.

According to figure 5.10, CO2 molecule can vibrate in symmetric stretching, bending

and asymmetric stretching modes. Each of these modes is characterised by a definitefrequency of vibration.

The molecule’s vibrational energies in symmetric stretching mode is given by

Ex = (m + 1 ⁄ 2) hνi where m = 0, 1, 2

The degree of excitation is characterized by the integer ‘m’. Since the CO2

molecule is vibrating with a combination of three modes, the state of vibration canbe described by three integers (m n q) where m, n, and q are the integerscorresponding to the degree of excitation in the symmetric stretching, bending andasymmetric stretching modes respectively.

Construction:

Fig. 5.13: CO2 Laser

Symmetric stretching mode of a CO2 molecule:

Here the carbon atom is stationary and the oxygen atoms oscillate or vibratealong the axis of the molecule.

Asymmetric stretching mode of a CO2 molecule:

Here all the three atoms will vibrate. Here the oxygen atom vibrates in theopposite direction to the vibration direction of carbon atom.

Bending mode of a CO2 molecule:

Here the atoms will not be linear; rather the atoms will vibrate perpendicularto the molecular axis.

LASER 5.17

Figure 5.13 shows the schematic diagram of a CO2 laser. Along with CO2,

there are also nitrogen and helium gases in the apparatus. Nitrogen helps to increasethe population of the upper level of CO2, while helium helps to depopulate the lower

level. This is achieved due to high thermal conductivity of helium. Further heliumhelps to conduct heat away to the walls of the discharge tube keeping CO2 cold.

The discharge tube has 2.5 cm in diameter and 5 m in length and discharge isproduced by d.c. excitation. Sodium chloride Brewster windows are used at the end.Near confocal silicon mirrors coated with aluminium, form the resonant cavity. Thepartial pressures of CO2, N2, and He are around 0.33 torr, 1.2 torr and 7 torr

respectively. The partial pressure values depend on the diameter of the tube. Toremove the dissociation products which may contaminate the laser, the continuousflow of the gas mixture is maintained in the tube. [1 torr = 1 mm height of mercurycolumn].

Working:

Figure 5.14 shows the various vibrational energy levels taking part in the lasertransition. In the figure E5 refers to (001) level, E4 refers to (100) level and E3 refers

to (021) level. E2 refers to (010) level and E1 refers to ground state energy level.

Fig. 5.14: Vibrational Energy levels of CO2 molecule and important laser

transitions having 9.6 µm and 10.6 µm wavelength

The laser transition at 10.6 ?m occurs between the (001) and (100) levels ofCO2. The excitation of the CO2 molecules to the long lived level (001) occurs both

through collisional transfer from nearly resonant excited nitrogen molecules and alsofrom the cascading down of CO2 molecules from higher energy levels.


Laser action in CO2 Laser:

1. When a discharge is passed through the tube, the nitrogen molecules areexcited and are raised to higher excited state.

2. The excited energy of nitrogen molecules is transferred to carbon-di-oxidemolecules through collisions and carbon-di-oxide molecules are raised to theirexcited vibrational energy level E5 (001) from their ground state.

3. The energy level ‘E5’ is a metastable state energy level. Hence there is

population inversion.

4. Stimulating photons of wavelength 10.6 µm and 9.6 µm induce the CO2

molecules to undergo stimulated emission by laser transitions from E5 to

E4 giving laser wavelength of 10.6 µm and from E5 to E3 giving laser

wavelength of 9.6 µm.

5. Since the laser transition from E5 to E4 has higher gain than from E5 to

E3, the laser usually oscillates at 10.6 µm.

6. The CO2 molecules from E4 and E3 are returned to their ground through fast

decay and diffusion.

7. When there is longitudinal flow of gases, the maximum power obtained isabout 50-60 W/m.

8. If the gas flow is perpendicular to the discharge the output power can beraised to about 10 kilowatt/m. This type of CO2 laser is known as

Transversely Excited Atmospheric pressure laser or TEA laser. Thus the gasflow is maintained along the axis of the tube at normal atmospheric pressureand the current in the arc flows at right angles to the axis of the laser.

5.9. GALLIUM ARSENIDE (GA AS) DIODE LASER (SEMICONDUCTOR LASER)

Principle:

Among the semiconductors there are direct bandgap semiconductors and indirectbandgap semiconductors. In the case of direct bandgap semiconductors, there is alarge possibility for direct recombination of hole and electron emitting a photon.

But in indirect bandgap semiconductors, like Germanium and Silicon, directrecombination of hole and electron is not possible and hence there is no photonemission.

LASER 5.19

Fig. 5.15: GaAs diode laser

GaAs is a direct bandgap semiconductor and hence it is used to make lightemitting diodes and lasers. The wavelength of the emitted light depends on thebandgap of the material.

Construction (Homo junction GaAs semiconductor diode laser). Referfigure 5.15.

The active medium is a p-n junction diode made from crystalline GalliumArsenide. The p-region and n-region in the diode are obtained by heavily dopingGermanium and Tellurium respectively in GaAs.

At the junction the sides through which emitted light is coming out are wellpolished and parallel to each other. Since the refractive index of GaAs is high, thereflectance at the material-air interface is sufficiently large. So that the externalmirrors are not necessary to produce multiple reflections.

Electric current is applied to the crystal platelet through a strip electrode fixedto its upper surface. The threshold pumping current density to start the stimulated

emission is so high about 400 A ⁄ mm2. Homojunction laser can give the laser outputin the form of pulsed mode at room temperature.

Working:

1. A population inversion is obtained by injecting electrons across the junctionfrom the n doped region to the p region by means of a forward bias voltage.

2. The excess minority electrons in the p region and excess minority holes inthe n region produce population inversion of minority charge carriers.


3. Particularly when a relatively large current of the order of 104 ampere/cm2

is passed through the junction to provide excitation, the direct recombinationprocess is taking place efficiently.

4. Further the emitted photons increase the rate of recombination of injectedelectrons from the n region and holes in p region by inducing morerecombinations. Thus more number of photons are produced.

5. Hence the emitted photons from induced recombinations are having the samephase and frequency as that of original inducing photons; otherwise we havestimulated emission of radiation along the P-N junction as shown in figure.

6. The wavelength of the emitted radiation depends upon the concentration ofdonor and acceptor atoms in Ga As. Particularly when the donor and acceptorconcentrations are about 1024 atoms/m3 the emitted wavelengths are 0.9020,0.8425 and 0.8370 microns.

7. The efficiency of laser emission increases when we cool the Ga As diode. Sincethere is no optical confinement, the emitted laser radiation has largedivergence and power coherence.

8. In the reverse bias, no carrier injection takes place and consequently no lightis emitted.

5.10. APPLICATIONS OF LASERS:

Due to high intensity, high monochromaticity, high coherence and highdirectionality of lasers, they are widely used in many fields of Science, Engineeringand Medicine.

Lasers in microelectronics:

Micro electronics is a sub-field of electronics. It is related to the study andmanufacture of electronic components which are very small. These devices are madefrom semiconductors with the help of lasers using a process called photolithography.

The proliferation of consumer microelectronic devices such as cell phones,computers, CD’s, DVD’s, PDA, flat panel TV’s and miniaturised audio equipment, hashad a huge impact on our daily lives. Laser-based processes are used at many pointswithin the manufacture and test of the microscopic circuits that make up thesedevices.

Laser’s application in semiconductor and microelectronics fabrication process isgrowing exponentially due to its unique characteristics.

LASER 5.21

Material Processing using Lasers:

We know that lasers are the light sources having very high energy density andtheir radiation can be focused well. This fact is utilized for material processing likesurface modification, welding, cutting, drilling, etc. The material processing usinglasers can be done in open air. But the electron beam processing requires vacuum.Today we have industrial lasers having very high beam power (> 1 MW) and very

narrow pulse width (< 10− 9 s). Particularly by mode locking and Q switchingprinciples, the power of the laser beam is further increased to a very large value.

Mode locking means the different modes of laser oscillation developed in theresonant cavity of laser source, enter the output in a periodic manner as well as inphase condition. The laser output has very high power density due constructiveinterference among the modes. The period of occurring of giant pulse having highenergy density is about 2L/C second where L is the length of the resonant cavityand C is the velocity of light. In other times, the output from the laser beam isalmost zero due to destructive interference among the different modes.

Similarly Q switching means the increase of population inversion in the excitedstate having long life time above the threshold condition for oscillation and makingall the atoms to undergo laser transition simultaneously. This will bring a verynarrow beam with high power.

Industrial lasers and their uses:

Laser type Wavelength Power (µ m)Applications

(kW)

CO2 10.6 ≤ 0.1 Ceramic scribing, cutting, drilling

0.1 to 1 Welding and cutting

1 to 10 Heat treating and heavy sectionwelding

> 10 Large scale heat treating, claddingand alloying

Nd YAG(pulsed)

1.06 0.05 Spot welding

0.1 to 0.4 Seam welding, high speed spotwelding, cutting and large holedrilling

Nd YAG(continuouswave)

1.06 ≤ 0.1 Ceramic scribing, resistor trimmingand sawing

0.2 to 0.8 Welding and heat treating


Table shows the different lasers, their wavelength and power and thecorresponding applications. Generally when we use high power lasers, thickerspecimens can be processed well.

The carbon di-oxide laser is a gaseous laser and Nd-YAG laser is a solid statelaser.

Fig. 5.16: Time dependence of laser output for different laser types

Figure 5.16 shows the time dependence of laser output in continuous wavelaser (figure 5.16(a)), pulsed laser (figure 5.16(b)), and Q switched laser(figure 5.16(c)). Depending upon the application different lasers are used. Forexample, in the case of welding of thicker specimens, Q switched lasers are used.

Fig. 5.17: Operational regimes in laser material processing

LASER 5.23

Figure 5.17 shows the various power densities required for different operations.

Laser instrumentation for material processing:

Fig. 5.18: Laser instrumentation for material processing

Figure 5.18 shows the laser setup used for surface modification, welding, cuttingand drilling. The output of the laser beam is incident on the plane mirror. Afterreflection, it passes through a shutter to control its intensity. Then there is focusingassembly to get a fine beam. Further there are shielding gas jet and powder feeder.The shielding gas is used (i) to remove the molten material and to favourvaporisation, (ii) to provide cooling effect, (iii) to protect the focusing opticalarrangement against smoke and fumes and to increase the absorption of energy bythe sample. For different materials, different gases are used. Air is the assisting gasfor Ti, Nb, Ta, Zr and glass. Using high power lasers, Copper and Aluminium canbe welded, drilled, alloyed or cut without any assisting gas. During the cutting ofreadily inflammable glass materials like ceramics, wood and paper, the nitrogen gasis used. This will increase the cutting rate by blowing the molten material out ofthe hole produced by laser energy. Further it reduces the firing and fire accidents.Oxygen gas is mainly used for brittle materials and metals. It will blow out themolten material and contribute some exothermal chemical reaction. So that metalcan absorbs maximum energy from the laser beam. Argon gas is used for cuttingand drilling of wood and ceramics. Powder feeder is used to spray the metal powderon the substrate for alloying or cladding. For example the satellite powder (cobaltbased carbide consisting of various elements C, Cr, Mo, W and Fe) is used during


cladding to improve the hardness of the surface. There are various arrangements tomonitor the flow of feeder gas or powder and shielding gas.

Laser surface treatments

Laser surface treatments are used for altering the compositions andmicrostructure of surface layers and there by improving the surface hardness, wearresistance, corrosion resistance and fatigue strength. Laser surface modificationprocesses for changing the surface composition can be divided into (i) laser surfacealloying and (ii) laser cladding.

(i) Laser surface alloying:

Alloying elements in the laser melt pool may be introduced by means ofpre-deposition methods like thermal spraying, boronizing, nitrating, carburizing,laying of loose metal powders, thin sheets or rods on the surface of substrate, etc(all of which are done prior to laser melting) and co-deposition methods like injectingpowder, wire or rod forms of alloys into the laser melt pool. By this method one canhave the control over the supply of alloying elements.

Fig. 5.19: Different laser surface treatment processes

Figure 5.19 shows the different laser surface treatment processes. In annealing,there is no heat affected zone and melting also takes place over few picometresthickness. In hardening process, there is heat affected zone in the form of

LASER 5.25

hemispherical manner. In the case of shock hardening there is melting over athickness of few micrometres. In surface alloying and surface cladding, there is acoating of alloying or cladding material on the substrate surface. There are localmelting and heat affected zone also. Laser alloying means that there is controlledmelting of a work piece surface to a desired depth using laser beam withsimultaneous addition of powdered alloying elements. Since the alloying elementsdiffuse in a thin liquid layer at the surface, it is feasible to obtain the required depthof alloying in small time intervals (~0.1s to 10s). Alloy composition will govern themicrostructure and the degree of microstructural grain reinfinment. Laser alloyingwill increase the resistance to wear, erosion, corrosion and high temperatureoxidation.

(ii) Laser cladding (or) hardfacing:

Here a laser beam melts a very thin surface layer of the work piece. This thinliquid layer mixes with the liquid cladding alloy and subsequently freezes to form ametallurgical bonding between the cladding and substrates. Cladding alloys areusually cobalt, nickel or iron based and are used for the applications involving theincrease of resistance of metal-to-metal wear, impact, erosion, corrosion and abrasion.Laser cladding is more superior than the conventional cladding techniques for highquality surface modification. That is both mechanically and metallurgically superior.

Laser welding, cutting and drilling:

For laser welding, cutting and drilling high power lasers are generally used.The quality of welding or cutting is affected by beam power, beam diameter, welding(or) cutting speed, focussing conditions and TEM mode of laser. When the beampower is more, the penetration depth is also more. The diameter of the beam shouldbe very small to get highly focussed radiation. For a given power, if the welding orcutting speed increases, the penetration depth decreases. So to increase the weldingor cutting speed as well as the penetration depth it is advisable to use key holemode of melting and high power lasers.

Advantages of laser welding, cutting and drilling:

1. Here the heat affected zone is very narrow. Therefore the microstructure ofsurrounding layers are not affected.

2. Laser welding, cutting and drilling can be done at room temperature andpressure without preheating and vacuum condition.

3. Difficult materials such as titanium, quartz and ceramics can be welded, orcut or drilled.

4. The work piece is need not be clambed rigidly.


5. No electrode or filler or material is required.

6. Higher welding speed or cutting speed can be achieved.

7. There is minimum residual stress and distortion.

8. Heating and cooling are so rapid. Hence there is fine grain structure in theweld or cut or drilled region so that it increases the strength of that region.

9. The drilled hole has uniform diameter throughout the specimen and the wallsof the hole are perfectly uniform.

10. The weld region is correctly bonded together and there is no dry welding.

11. The laser cutting has improved edge quality. That is the surface quality ismaintained in the original condition since there is no bead formation due toscattering of molten material.

Laser welding, cutting and drilling can be done by the following manner.

1. By movement of laser source keeping the material stationary.

2. By movement of material alone keeping the laser source stationary.

3. Keeping the material and laser source stationary and moving the laser beamby means of mirrors and lenses.

4. Keeping the laser source stationary and moving the material and the laserbeam.

Medical Applications:

Surgery:

The three lasers most often used in medical treatment are

1. Carbon dioxide (CO2) laser. Primarily a surgical tool, this device converts

light energy to heat strong enough to minimize bleeding, while cutting throughor vaporizes tissue.

2. Neodymium: yttrium-aluminum-garnet (Nd: YAG) laser.

3. Argon laser.

LASER 5.27

Purpose:

Laser surgery is used to:

1. Cut or destroy tissue that is abnormal or diseased without harming healthy,normal tissue.

2. Shrink or destroy tumors and lesions.

3. Close off nerve endings to reduce postoperative pain.

4. Cauterize (seal) blood vessels to reduce blood loss.

5. Seal lymph vessels to minimize swelling and decrease spread of tumor cells.

6. Remove moles, warts, and tattoos.

7. Decrease the appearance of skin wrinkles.

Description:

Lasers can be used to perform almost any surgical procedure. In fact, generalsurgeons employ the various laser wavelengths and laser delivery systems to cut,coagulate, vaporize, and remove tissue. In most “laser surgeries,” they actually usegenuine laser devices in place of conventional surgical tools-scalpels, cryosurgeryprobes, electrosurgical units, or microwave devices − to carry out standard procedures,like mastectomy (breast surgery). With the use of lasers, the skilled and trainedsurgeon can accomplish tasks that are more complex, all the while reducing bloodloss, decreasing postoperative patient discomfort, decreasing the chances of infectionto the wound, reducing the spread of some cancers, minimizing the extent of surgery(in some cases), and achieving better outcomes in wound healing. Also, because lasersare more precise, the laser can penetrate tissue by adjusting the intensity of thelight. Lasers are also extremely useful in both open and laparoscopic procedures.Common surgical uses include breast surgery, removal of the gallbladder, herniarepair and solid organ surgery. The first working laser was introduced in 1960.Initially used to treat diseases and disorders of the eye, the device was first used totreat diseases and disorders of the eye, whose transparent tissues gave ophthalmicsurgeons a clear view of how the narrow, concentrated beam was being directed.Dermatologic surgeons also helped to pioneer laser surgery, and developed andimproved upon many early techniques and more refined surgical procedures.


SOLVED PROBLEMS:

1. Calculate the relative population in the laser transition levels in a ruby laserin thermal equilibrium (without pumping of atoms). The wavelength of theruby laser light in 6943 Å at 300K.

Solution:

HereN1 = Number of atoms per unit volume in the ground state E1

N2 = Number of atoms per unit volume in the meta stable

ν = Cλ

= 3 × 108

6943 × 10− 10 = 4.32 × 1014 Hz

Now

∴ N1

N2 = e(6626 × 10− 34 × 4.32 × 1014

) ⁄ (1.38 × 10− 23 × 300)

= e69.14 = 1.064 × 1030

2. HeNe laser emits light at a wavelength 632.8 nm and has an output powerof 2.3 mW. How many photons are emitted in each second by this laser when

operating? If the spot size of the laser beam is 1 mm2 what is its powerdensity?

Solution:

Frequency = Emitted energy

Time =

n h νt

∴ n = Power × time

Energy of each photon

= 23 × 10− 3 × 1

6.626 × 10− 34 × 4.74 × 1014

= 7.323 × 1015 photons ⁄ second

Power density = Power

spot area =

2.3 × 10− 3

1 × 10− 6 = 2.3 k Wm− 2

LASER 5.29

3. Calculate the wavelength of emission from Ga As whose bandgap is 1.44 eV.

Solution:

λ = hCEg

= 6.626 × 10− 34 × 3 × 108

1.44 × 1.6 × 10− 19 = 8628 Å

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unit – v

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