SUBJECT:- PHYSICS(US02CPHY21)�UNIT-IV LASER�

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DR. U.N.Patel

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1 INTRODUCTION�

• Laser is an acronym for Light Amplification by Stimulated Emission of Radiation.
• Two kind of emissions namely spontaneous and stimulated was first predicted by Albert Einstein in 1917, He made this prediction based on the thermodynamic equilibrium between atoms and the radiation field.
• He further proved that both spontaneous and stimulated emissions are necessary to obtain Planck’s quantum radiation law.

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Charles Towner demonstrated stimulated emission for the first time at microwave frequencies and Theodore Maiman demonstrated it at optical frequencies in a Ruby laser in 1960. Within a few months of operation of this device, Javan and his fellow workers constructed the first gas laser, i.e. He-Ne laser. The semiconductor laser was invented in1962. Since then lesser action has been obtained in a variety of materials like liquids, ionized gases, dyes, etc.

2 PROPERTIES OF LASERS:

• The important properties of lasers that are different from ordinary incoherent radiations may be stated as follows, i.e. a laser is characterized by
• (i) its directionality,
• (ii) its high intensity,
• (iii) its extraordinary monochromacity, and
• (iv) high degree of coherence.
• The above properties of the lasers are briefly discussed below one by one.

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Directionality

• Any conventional light source like a tube light emits radiations in all direction unlike a laser source which emits radiation only in one direction. The directionality of the laser beam is usually expressed in terms of full angle beam divergence which is twice the angle that the outer edge of the beam makes with the axis of the beam. The outer edge is defined as a point at which the strength of the beam drops to 1/e times its value at the centre.

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• A Gaussian shape of laser beam is shown in figure -1 and the full angle divergence in terms of minimum spot size is given by

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Where λ is the wavelength of the beam. For a typical planar wave front emerging from an aperture a diameter d, it propagates as a parallel beam for a distance of = d² / λ called the Rayleigh’s range, beyond which the beam due to diffraction diverges with an angular spread of Δθ = λ/d.

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For a typical laser the beam divergence is less than 0.01 milliradian, i.e. a laser beam spreads less than 0.01 mm for every meter. However, on the other hand, for ordinary light the spread is 1 m for every 1 m of travel.

If a₁ and a₂ are the diameters of laser radiation at distances d₁ and d₂ from a laser source respectively, then the angle of beam divergence in degree is given by

Intensity�

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A laser emits light radiation into a narrow beam, and its energy is concentrated in a small region. This concentration of energy both spatially and spectrally accounts for the great intensity of lasers. It can be shown that even a one-watt laser would appear many thousand times more intense than a 100 watt ordinary lamp. If we compare the number of photons emitted in one second from a square centimeter of a surface of a laser source with those from an ordinary source, the ratio is of the order of 10²⁸ to 10¹².

Monochromacity:

• The light from a laser source is highly monochromatic compared to light from a conventional incoherent monochromatic source. The monochromacity is related to the wavelength spread of radiation given by

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• The value of Δλ is in the order of 300 nm for white light, 0.01 nm for gas discharge lamp, while it is 0.0001 nm for laser.

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Coherence:

• Laser radiation is characterised by a high degree of ordering of the light field compared to radiation from other sources. In other words, laser light has a high degree of coherence, both spatial and temporal. The temporal coherence normally refers to the relative phase or the coherence of two waves at two separate locations along the propagation direction of the two beams. It is sometimes referred to as longitudinal coherence.

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If we assume that two waves are exactly in phase at the first location, then they will maintain the same phase at the second location up to a distance l_c where l_c is defined as the coherence length. For white light the coherence length is of the order of hundred nm while for monochromatic incoherent light its value is of hundred microns. For lasers the value of coherence length is of the order of several meters

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• Spatial coherence, on the other hand, also called transverse coherence, describes how far apart two sources or two portions of the same source can be located in a direction transverse to the direction of observation and still exhibit coherent properties over a range of observation points.
• The high degree of coherence of laser radiation makes it possible to realise a tremendous spatial concentration of light power such as 10¹³ watt in a space with linear dimensions of only 1 μm.
• The above remarkable properties of lasers paved the way for their unprecedented scientific and technological application. They have been used in telecommunications, meteorology, metrology, biology, cybernetics, etc.

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Example-1 : A laser beam having a wavelength of 16000 A° and aperture 1 mm is sent to moon. Calculate (i) the angular spread of the beam and (ii) the area of spread of the beam when it reaches the moon (distance between earth and moon is 4 × 10⁵ km).

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Given: λ =16000 A°, d = 1mm, D = 4 × 10⁵ km

Required : Angular spread and area of spread

Formulae : (i) φ =λ/d, (ii) area of spread = (D - φ)²

Solution:

(i)

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(ii)

Result : (i) Angular spread = 1.6 X 10^-4 rad

(ii) spread = 4.096 X 10⁹ m²

Example – 2 : For the He-Ne laser at 2 m and 4 m distances from the laser, the output beam spot diameters are 2 mm and 3 mm, Calculate the angle of divergence.

Given : a₁ = 2 mm, a₂ = 3 mm, d₂ = 4m, d₁ = 2m

Required : φ

Formula :

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Solution :

Result : Angle of divergence = 0.25 X 10^-3 rad

• Example – 3 : A laser beam has a power of 100 mW. It has an aperture of 10 mm and wavelength of 14400 A°. A beam is focused with a lens of focal length 0.1 m. Calculate the area and intensity of image.
• Given : λ = 14400 A°
• d = 10 x 10^-3 m

D = 0.1 m

Power =100 mW

Required : Area of spread of intensity of image

Formulae : (i) Angular spread = φ = λ/d

(ii) Area of spread = (D - φ)²

(iii) Intensity = power / area of spread

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• Solution :

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Result : (i) Area of Spread = 2.074 x 10^-10 m²

(ii) Intensity = 48.2 x 10⁷ W/m²

• STIMULATED ABSORPTION, SPONTANEOUS EMISSION AND STIMULATED EMISSION
• 1. Stimulated Absorption :
• An atom has a number of quantized energy states. Initially an atom is in the ground state, i.e. all of its electrons possess the lowest possible energy states. When energy is given in the form of electromagnetic radiation, the atom goes to the excited state, i.e. its electrons jump to a higher energy state by absorbing a quantum of radiation of photon. This process is called stimulated absorption or induced absorption and is shown in Figure

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• If E₁ and E₂ are the energies of an electron in the initial and final states respectively and ν the frequency of absorbed radiation, then
• E₂ - E₁ =h ν, h being the Planck’s constant

or

The probable rate of transition 1→2 depends on the properties of states 1 and 2 and is proportional to the energy density u(ν) of the radiation of frequency ν and to the number of electrons N₁ in the ground state.

Thus,

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Where B₁₂ is the proportionality constant called Einstein's coefficient for absorption of radiation.

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• 2. Spontaneous and stimulated Emission
• When the atom is in the excited state it can make a transition to a lower energy state through the emission of electromagnetic radiation; however in contrast to the absorption process, the emission can occur in two different ways.
• The first is referred to as spontaneous emission in which an atom in the excited state emits radiation even in the absence of any incident radiation. The atom is thus not stimulated by any incident signal but emission of radiation from it occurs spontaneously as shown in Figure

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• The rate of spontaneous emission is proportional to the number of electrons in the excited state, i.e.
• (P₁₂)_spont∝ N₂

=A₂₁N₂ ---------------(2)

Where A21 is called the Einstein’s coefficient of spontaneous emission of radiation. When an atom is in the excited state, then an incident photon of correct energy may cause the atom to jump to lower energy state, emitting an additional photon of the same frequency.

Thus now two photons of the same frequency are present. This phenomenon is called stimulated emission and is shown in figure

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• The rate of stimulated emission depends both on the intensity of the external radiation field and on the number of atoms in the upper level E2, i.e.

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Where B12 is called the Einstein’s coefficient of stimulated emission of radiation

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4. RELATION BETWEEN EINSTEIN’S A AND B COEFFICIENTS

Consider an assembly of atoms in thermal equilibrium at temperature T with radiation of frequency ν and energy density u(ν). Let N₁ and N₂ be the number of atoms in energy states 1 and 2 respectively at any instant.

The number of atoms in state 1 that can absorb a photon and give rise to absorption per unit time is given by Eq. (1). Conversely, the number of photons in state 2 that cause emission process is given by the sum of Eqs. (2) and (3).

At equilibrium the absorption and emission rates must be equal, i.e.

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Dividing throughout by N₂B₂₁, Eq. (1) becomes

According to Boltzmann distribution law, the number of atoms N₁ and N₂ in energy states E₁ and E₂ in thermal equilibrium at temperature T are given by

N₁ = N₀e^-E1/kT ------------(3)

And N₂ = N₀ e^-E2/kT ----------------(4)

Where N₀ is the total number of atoms present in the ground state and K is Boltzmann’s constant. From Eqs. (3) and (4)

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Where hν = (E₂ –E₁)

Putting Eq. (5) in Eq. (2), we have

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Comparing Eq. (6) with the Planck’s radiation formula given by

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We have

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Equations (8) and (9) are called Einstein’s relations. The ratio of spontaneous emission to stimulated emission is

Using Eq. (5), Eq. (9) can be written as

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Hence at thermal equilibrium at temperature T for ν<< kT/h, the number of stimulated emission far exceeds the number of spontaneous emissions while for ν >> kT/h, the number of spontaneous emissions far exceeds the number of stimulated emissions. It can be shown that for normal optical sources T ~10³ K and for optical range ν ~ 4 x 10¹⁵ S^-1, we find that the emission is predominately due to spontaneous transitions and hence the emission from usual light sources is incoherent.

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The ratio of spontaneous to stimulated emissions is proportional to ν³ (see Eq. (8)). It means that the probability of spontaneous emission dominates over the stimulated emission more and more as the energy difference between the two states increases.

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5. POPULATION INVERSION :

Consider an energy state E containing N atoms per unit volume. This number N is called population and is given by Boltzmann’s equation

N = N₀ e^-E/kT -------------------(1)

Where N₀ is the population of the ground state with E=0, k is the Boltzmann’s constant and T is the absolute temperature.

From Eq. (1), it is clear that the population is maximum in the ground state and decrease exponentially as we go to a higher energy state.

If N₁ is the population in energy state E₁, N₂ in E₂, then

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As N₁ > N₂ therefore, if an electromagnetic radiation is incident on the substance at the thermal equilibrium condition, then there is net absorption of radiation.

Usually the population of atoms decreases with the increase in energy of the state.

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If N₁, N₂, N₃ are the populations in energy states E₁, E₂, E₃ respectively such that E₁ < E₂ < E₃, then

N₁ > N₂ > N₃

This situation is shown in Figure (a).

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If the process of stimulated emission predominates over the process of spontaneous emission, it may then be possible that N₂ > N₁. If this happens then the state is called the population inversion. In the state of populated inversion the upper levels are more populated than the lower levels. Figure (b) represents a state in which N₂ > N₁, i.e. the state of population inversion. To achieve population inversion the external energy is supplied to excite the atoms of the substance. In some substances which contain metastable states, population inversion condition is achieved practically.

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6. PUMPING:

The population inversion can be achieved by exciting the medium with a suitable from the energy. This process is called pumping. There are several methods of pumping a laser and producing population inversion necessary for the occurrence of stimulated emission. Some of the commonly used methods are :

(i) Optical pumping

(ii) Electrical discharge

(iii) Inelastic atom-atom collision

(iv) Direct conversion

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Optical Pumping :

If luminous energy is supplied to a medium for causing population inversion, then the pumping is called the optical pumping. In optical pumping the luminous energy usually comes form a light source in the form of short flashes of light. The optical pumping mechanism is used for lasing material having broadband higher energy levels (width ~ 800 A° or more) as in Ruby laser or in Nd:YAG laser.

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Electric discharge :

The pumping by electric discharge is preferred in lasing materials whose higher energy levels have a narrow bandwidth, e.g. Argon-ion laser. When a potential difference is applied between cathode and anode in a discharge tube, the electrons emitted from cathode are accelerated towards anode. Some of these electrons collide with atoms of the active medium, ionise the medium and raise it to the higher level. This produces the required population inversion. This is also called direct-electron excitation.

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In-elastic atom-atom collision :

In electric discharge one type of atoms are raised to their excited state. These atom collide in-elastically with another type of atoms. The latter atoms provide the population inversion needed for laser emission. The example is He-Ne laser.

Direct Conversion :

A direct conversion of electrical energy into radiant energy occurs in light emitting diodes (LEDs). The example of population inversion by direct collision occurs in semiconductor lasers.

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Chemical conversion :

In a chemical laser, energy comes from a chemical reaction without any need for other energy sources. For example, hydrogen can combine with flourine to form hydrogen fluoride :

H₂ + F₂ → 2HF

This reaction is used to pump a CO₂ laser to achieve population inversion.

7. MAIN COMPONENTS OF A LASER :

The three main components of any laser device are :

(i) The active medium

(ii) The pumping source

(III) The optical resonator.

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The active medium consisting of a collection of atoms, molecules or ions acts as an amplifier for light waves. For amplification the medium has to be kept in a state of population inversion, i.e. in a state in which the number of atoms in the upper energy levels in greater than the number of atoms in the lower energy level. For many lasing materials the population inversion takes place in metastable levels where the lifetime charge carriers will be 10^-3 seconds. The pumping mechanism helps in obtaining such a state of population inversion between a pair of energy levels of the atomic systems. When the active medium is placed inside an optical resonator the system acts as

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an oscillator. Normally, a pair of mirrors (one having 100 per cent reflecting power and the other having a slightly lower reflecting power) acts as an optical resonator in most of the lasers.

8. ND:YAG LASER :

The Nd:YAG laser consists of yttrium aluminium garnet (Y₃ Al₅ O₁₂) crystal in which 1.5% neodymium ions (Nd³⁺ ions) are doped as impurities. These Nd³⁺ ions occupy yttrium ion sites and provide lasing transitions. Fig. 1 shows the schematic diagram of an Nd:YAG laser. This laser consists of three essential parts:

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1. Yttrium aluminium garnet in which 1.5% of Nd³⁺ is doped as impurities. Nd³⁺ is the lasing ion.
2. Two end mirrors M₁ and M₂ which will act as resonant cavity. While mirror M₁ is a totally reflecting one mirror M₂ is partially reflecting.
3. Krypton flash lamp which acts as a pump.

The Nd:YAG laser rod has a length of about 5 to 10

Mm with a diameter 6 to 9 mm. It is kept at one foci of an elliptical glass tube. A krypton flash lamp, the optical pump source, is placed at the other foci of the glass tube. As shown in Figure 1, on the left side there is a perfect reflecting mirror while on the right a partially reflecting mirror is kept.

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These two mirrors act as resonant cavity to produce a stimulated and amplification process. The krypton flash lamp is provided with the necessary power supply arrangement.

Fig. 2 shows the energy levels involved of Nd3+ ions (neodymium) in lasing action.

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When the krypton flash lamp is energised it gives out radiations and the Nd³⁺ ions by absorbing 0.73 μm and 0.80 μm from the input radiations get excited to the higher energy levels 2H_9/2 and 4F_5/2. These are energy levels where Nd³⁺ ions can stay for a duration of about 10^-8 s. Therefore these ions undergo a non-radiative decay process to reach the metastable level 4F_3/2.Therefore such a system will produce a population inversion process. If by chance a spontaneous transition takes place from the metastable level to still a lower energy level 4I_11/2 it will lead to a photon which on oscillating between the end mirrors will lead to a stimulated process to yield the laser output of wavelength 1.06 mm.

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some important features of Nd:YAG laser are :

1. It is a four-level laser.
2. Uses a rare earth ion like neodymium. Erbium and dysprosium may also be used instead of neodymium ions.
3. Energy levels of Nd³⁺ with the inner 4F shell is shown in Fig. 2.
4. Nd³⁺ ion concentration corresponds to ground state population of 6 x 10¹⁹ ions/cc.
5. The metastable level 4F_3/2 has a lifetime of 0.23 x 10^-3 s.
6. The Nd:YAG laser is a quasi-continuous wave laser.

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7. In some cases YAG is dopped with Cr³⁺ ions in addition of Nd³⁺ ions. In this case the xenon flash lamp can be used as a pumping source.
8. Gives output in the infrared region.

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9. CO₂ LASER :

Molecules like CO₂ are characterized not only by

electronic levels but also by vibrational and rotational

levels.

Each electronic level is split into various vibrational

levels due to vibrational motion and each vibrational

level is further subdivided into rotational sublevels.

The energy difference between various electronic

levels corresponds to visible and ultraviolet regions.

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The energy difference between various vibrational levels corresponds to infrared region while the energy difference between various rotational levels corresponds to far infrared region.

In CO₂ laser one makes use of transition from a rotational sublevel to a vibrational level to a lower rotational sublevel of a vibrational level in the ground electronic state. Thus CO₂ lasers are much more efficient compared to other gas lasers. This is because of the fact that in other gas lasers the de-excitation from the lower laser level to the ground level involves sufficient amount of energy that does not contribute to output laser beam.

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In contrast to this however in CO₂ laser the levels taking part in laser transitions are the vibrational rotational levels of the lowest electronic level. These levels are very close to the ground level and hence a large portion of the input energy is converted into output giving more efficiency.

A CO₂ laser consists of three essential parts. These are :

1. The lasing material is CO₂ which is mixed with nitrogen atoms to initiate the excitation process.
2. A resonant cavity of quartz tube about 5 m in length and 2.5 cm in diameter. This tube is provided with NaCl windows on either side and two mirrors one perfectly reflecting and the other partially reflecting.

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(iii) A suitable electrical discharge system which acts as a pumping mechanism.

The fundamental modes of vibration of the CO2 molecule are shown in Fig. 1

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In the symmetric mode of vibration the oxygen atoms oscillate along the axis of the molecule simultaneously departing or approaching the carbon atoms which is in stationary position. In the asymetric mode of vibration all the three atoms oscillate but while both oxygen atoms move in one direction, carbon atom moves in the opposite direction. In the bending mode of vibration, the molecule ceases to be exactly linear as the atoms more perpendicular to the molecular axis. In a CO₂ laser, transitions take place from a higher vibrational rotational level to a lower vibrational rotational level in the ground electronic state. The schematic diagram of a CO₂ laser is shown in Fig. 2.

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It consists of fused quartz tube of 5 m in length and 2.5 cm in diameter. CO₂, nitrogen and helium are mixed in the quartz tube with partial pressures of 1.5 T, 1.5 T and 12 T respectively. Nitrogen gas is mixed with CO₂ in order to help the CO₂ molecules to get lifted to the metastable level. Nitrogen gas plays a similar role in CO₂ laser as helium gas plays in He-Ne laser.

The purpose of helium mixed with CO₂ and nitrogen is to help CO₂ to get depopulate to lower energy levels. Further as helium has higher thermal conductivity is takes away the heat to the walls of the quartz tube thereby keeping the CO₂ molecules cold.

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When electrical discharge is given, the main excitation process involves the excitation of N₂ molecules into their first excitation vibrational level at 2360 cm^-1 above the ground level. This level is very close to the energy of the asymmetric stretching vibration frequency of CO₂ molecule at 2350 cm^-1, the resonance transfer of energy takes place and hence CO₂ molecules are excited due to collisions with the excited N₂ molecules. It may be noted that only 0.3 eV energy is required to excite a CO₂ molecule to its first vibrational and rotational levels whereas 20 eV energy is required to excite the helium atom in the He-Ne laser.

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The energy level diagram of N₂ – CO₂ system is shown in Fig. 3.

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The excited CO₂ molecule undergoes laser action by transition between the vibrational levels and rotational levels accompanying the vibrational levels and giving two wavelengths of 10.6 μm and 9.6 μm respectively as shown in Fig. 3.

Some important features of CO₂ laser are :

1. Partial pressures of CO₂ :N₂ :He :: 1.5 T : 1.5 T : 12 T
2. Water vapours can also be used instead of helium.
3. Lifetime of E₅ levels of CO₂ molecule is 0.4 millisecond.
4. CO₂ laser output-10 kW, with an efficiency of about 30%.
5. This high power laser is used for cutting metals, drilling, welding and etching processes.
6. CO₂ laser may be operated as a CW laser or pulsed laser.

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10 APPLICATIONS OF LASER IN MATERIAL PROCESSING:

10.1 Laser Cutting :

For high precision cutting, high power lasers such as a carbon dioxide laser is used. In the cutting process it is required that the materials along the cut must be removed. When pulsed lasers are used for the cutting process, the repetition frequency of the pulse and the motion of the laser across the material is adjusted so that a series of partially overlapping holes are produced. It must be taken care of that the width of the cut should be as small as possible and any rewelding must be avoided. The efficiency of laser cutting may be improved by using a gas jet co-axial with the laser as shown in Fig. 1.

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If a gas is projected at a very high velocity with the laser beam on the material, the gas will then remove the molten material away from the cutting point. This type of gas jet assisted laser cutting process is used to cut materials like low –carbon steel, stainless steel, titanium etc. Using a 190 W CO₂ laser with oxygen jet, it is possible

Fig. 1 LASER CUTTING SCHEME WITH A GAS-JET ASSISTED FOR REMOVING THE MOLTEN MATERIAL FROM THE CUTTING POINT.

to cut a stainless steel sheet of 0.13 cm thickness at a rate of 0.8 m/min. Nitrogen or argon may also be used as jet gases in the laser cutting process.

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Wood, paper, plastic etc. are also cut using such a method. When wood is cut with CO₂ lasers, carbonization occurs at the cut edges but it is usually limited to a small depth (about 10-20 μm) of the material. This causes a discoloration only and can be decreased by increasing the cutting speed. Cutting of nickel alloys, stainless steel and other materials by laser finds application in aircraft and automobile manufacturing units. It is estimated that laser cutting can save 60-70% power compared to conventional methods in aerospace industries. Laser cutting also finds a place in textile industries for cutting the cloth.

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10.2 Laser Welding :

High-power lasers such as CO2, Ruby laser, Nd: YAG lasers are important lasers used for accurate welding of materials. For example, a weld of ¼ inch thick stainless steel can be made using a CO2 laser with a power output of 3.5 kW. Laser welding finds an important place in the fields of electronics and microelectronics which require precise and accurate spot welding of very thin wires of 10 μm thickness or welding of two thin films together. Use of laser welding in electronics industries offers some unique advantages. As laser welding takes a shorter duration, welding can be done in regions adjacent to heat sensitive areas without affecting those elements.

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Further, welding in otherwise inaccessible areas like inside a glass envelope can also be done using a laser beam. In laser welding of two wires, we can achieve an effective welding joint even without the removal of the insulation.

Laser welding can be done easily between the two dissimilar metals. Thus a thermocouple may be easily welded to a substrate without much damage to the adjacent material. One can make a junction and attach the junction simultaneously to the substrate. This method has been used in attaching the measuring probes to transistors, turbine blades, etc. Laser welding not only achieves welding between dissimilar metals but also allows precise location ot

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the weld. In laser welding, material is added to join the two components. Thus the laser power used must not be that high which would evaporate the material. Hence the lasers used in welding processes must possess a high average power rather than a high peak power.

10.3 Hole Drilling :

Drilling of holes in different substances is another important applications of the laser. Using a laser with power of 0.05 J can produce a hole of 0.1 mm radius in a 1 mm thick steel plate in 0.001 second. Laser beams are also used for the drilling of diamond dies used for drawing wires. Conventional methods for drilling holes less than 250 μm diamter

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is very difficult.

At times it leads to the breakage of materials. However, using laser, even in the hardest materials drill holes of 10 μm diameter may be made easily. The Nd : YAG lasers are normally used for this purpose. Laser drilling has an advantage over conventional drilling. Not only the breakage of the material is avoided but also precise location of the hole is obtained.

10.4 Other Applications :

Lasers are also used to vaporize materials for subsequent deposition on a substrate. Some unique advantages offered by the laser in such a scheme include the fact that no contamination occurs, some

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preselected areas of the source material may be evaporated or the evaporant may be located very close to the substrate.

Brittle materials like rock, marble, etc. can be fractured using laser beams. This sort of technique is used in rock crushing and boring tunnels. Lasers are also being used in the removal of microscopic quantities of materials from balance wheels while in motion. They have also been used in trimming resistors to accuracies of 0.1%. Such micromaching processes find application in semiconductor circuit processing. The advantages offered by a system employing lasers for such purposes include the small size of the focused image with a precise control of

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energy, the absence of any contamination, accuracy of positioning and ease of automation.

10.5 CD-ROM (Compact Disk Read Only Memory) :

It is an optical ROM. Pre-recorded data can be read out from a CD-ROM. The manufacturer writes data on the CD-ROM. The disk is made up of a resin such as polycarbonate. It is coated with a material which changes when a high intensity laser beam is focused on it. The coating material is highly reflective, usually aluminium. The high intensity laser beam forms a tiny pit along a trace to represent a 1. For reading the data a laser beam of less intensity is employed. A laser system needs 25 mW for writing whereas only 5 mW for reading.

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In some cases separate laser beams are employed, one for writing and the other for reading. The reflected laser is sensed by a photodiode to read data. The intensity of the reflected light of laser changes as it encounters a pit. A pit spreads the light so that the photodiode receives very little reflected light. But the surface without the pit reflects sufficient light to the photodiode. Thus the change in reflected light is sensed and converted into electrical signals for data reading purposes.

If the coating of the special material on the optical disk is a thin film, a hole is formed when the laser beam falls on it. If the layer is a thick film, a pit is formed. In some process of thick film, a bubble is

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formed instead of a pit. The unaltered areas between the pits are known as lands.

Fig. 1 SPIRAL TRACK OF A CD-ROM.

The track is divided into blocks of the same size as shown in Fig.1. A CD-ROM disk rotates at a variable speed so that the pits are read by the laser at a constant linear speed. The speed of the disk is adjusted in such a manner that the track passes under the

read/write head at a constant linear velocity.

The advantages of CD-ROM are its high storing capacity, mass copy of information stored, removable from the computer etc. The main disadvantage is

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longer access time compared to that of a magnetic hard disk, i.e. as much as half a second. It cannot be updated because it is a read only memory. It is suitable for storing pieces of information which are not to be changed. Disks can be used for archival storage.

11 HOLOGRAPHY :

In the ordinary photography, the image of a three-dimensional object is recorded on a two-dimensional photographic film. The emulsion on the photographic plate is sensitive only to the intensity variations and hence when an ordinary photograph is recorded, the phase distribution of the scattered radiations from the object is lost. Since only the

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intensity pattern has been recorded, the three-dimensional character, e.g. parallax of the object environment is lost. Thus when we examine a photograph from various directions, we do not get new angles of approach and we cannot see what is happening on the other side of the object.

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