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LASERS

10.8 LASERS

The word ‘Laser’ stands for Light Amplification by Stimulated Emission of Radiations. Laser is biggest achieve of twentieth century in the field of research. The first laser was developed by Theodore Maiman in USA. It consist of a ruby rod around which a flash tube is bounded. When this rod was subjected to intense flashes of ordinary light it produced pulses of red laser light within a year of investigation-other laser using solids, liquids and gases were developed.

Principle of Laser: In material, atoms in excited state E2 can jump to lower energy state E1 by emitting photons of frequency

compelled to come down to lower energy state E1 such an emission is called stimulated emission. Further in this case energy (E2 - E1) is added to the incident photon in phase. As light wave or photon are all in phase so reinforce or, amplify each other. Such a process of amplifying light wave by stimulating atoms or molecules to emit identical waves is known as light amplification by stimulated emission of radiation Z Laser.

For stimulated emission the number of atoms in excited state must be more than number of atoms in lower energy state, this is known as population inversion. The population inversion can be achieved by pumping. -The materials in which population inversion can be achieved are called active medium. Under normal circumstance the number of atoms in the ground energy state are more than in excited energy state and electromagnetic radiations passing through it get attenuated instead of being amplified. Hence for amplification population inversion is must thus, a medium with population inversion is capable of amplification, but if the medium is to act as an oscillator, a part of the output energy must be fedback in to the system and it is brought about by placing by active medium between a pair of mirrors which are facing each other such a system is said to act as an oscillator. Hence there is light oscillator by stimulated emission of radiation in the early stage, there was a move to change the name of laser to loser but at that time it would have been difficult to get grant for research on loser so decided to retain the name laser.

10.8.1 Induced Absorption

In a material atoms are in ground state with energy (E1) when a photon of energy hv = E2 – E1 is incident on the material, the atoms in the ground state absorb energy and jump over to excited state E2. This process is called induced absorption as shown in Figure. 10.17. The frequency of photon absorbed is given as,

the atom in excited state can come to ground state by two process viz. spontaneous emission and stimulated emission.

10.8.2 Spontaneous Emission

The atoms in excited state E2 are less stable and have very small life time of the order of 10-8 second so they come to ground state after 10-8 second along with the emission of a photon of energy hv = E2 – E1. This phenomenon is called spontaneous emission. The frequency is given by

Stimulated Emission

In certain material the upper stage has longer life time of the order of 10-3 sec, known as metastable state and a material having metastable state is called as active material. If atom in excited state are compelled or forced to jump down to ground state by photon of energy hv = E2 – E1, then it jumps to ground state along with the emission of photon having energy hv = E2 – E1,this process is called stimulated emission both the photon in the same phase.

10.8.4 Einstein Coefficients

Let N1 and N2 represents the number in energy levels E1 and E2 respectively. Let density of radiation of particular frequency ν be represent by u(ν) then the absorption rate will be proportional to,

(i)                           the density of radiation u(v)

(ii)                        The number of atoms N1

Thus, absorption rate = B12 N1 u(v),                                   … (1)

where B12 is the coefficient of proportionality and is a characteristics of energy level.

Let us now consider the reverse process namely the emission of radiation at a particular frequency v when the atom de-excites from level E2 to E1. Einstein pointed out that an atom in the excited level can make a transition to a lower energy level either through spontaneous or through stimulated process. In spontaneous emission, the rate will be independent of energy density of radiation field u(v) and depends only on the number of atoms present in the upper energy level.

Rate of spontaneous transition = A21N2                                                                      … (2)

where A21 represents the probability of spontaneous transition.

But rate of stimulated emission will depend not only upon number of atoms in upper energy level but also the energy density.

Rate of stimulated transition = B21N2 u(v)                                             … (3)

where B21 u(v), given probability of stimulated transition, in steady state.

Rate of upward transition = Rate of downward transition using equation (I), (2) and (3) we get,

FIGURE 10.19 Stimulated, emission B21 N2 u(v)

Putting this value in equation (4) we get,

spontaneous transition and hence the emission from usual light source is incoherent.

10.8.5 Population Inversion

As we know earlier, the process of spontaneous emission is independent of external factor Einstein pointed out that two processes are equal probable under normal circumstances. Among two processes (spontaneous emission and stimulated emission) which predominate over other depend upon the number of atoms in two levels when N1 > N2, then absorption predominated over emission which is the case normally in normal materials. On the other hand when N1 < N2 then emission dominated over the absorption i.e., stimulated emission is possible under this condition the absorption coefficient of material becomes negative. Thus if the light traverses through such a medium it gets amplified instead of attenuation, this type of material in which N2> N1 is possible are called active material and this process is called population inversion thus for laser action population inversion is necessary.

We know that the population of a particular level is governed by Maxwell Boltzmann’s equation. According to if there is a level corresponding to energy E1 then number of atoms, N1 in this level will be given by

where N0 is the population is ground state, thus it is clear that the population of an energy level E2 such that E2 > E1 will be given by

So, N2 < N1 under normal condition. But as we have pointed out that for laser action population inversion is necessary.

So we have to supply energy from outside in order to achieve this population inversion (N2 > N1). The device which is used for this porpose is called pump and this process is called pumping. It is notable point that population inversion is possible only in the materials which has metastable stage i.e., Ruby, He-Ne mixture etc.

10.8.6 Laser Action : light Amplification

There are three main things which are required for laser action which are as follows:

  1.         i.            Active material.
  2.      ii.            An specially prepared cylindrical tube in which light intensity can build up by
    multiple reflection. The ends of the tube are silvered one end is completly silvered while the other end is partially silvered so that an intense beam can emerge out of it.
  3.    iii.            Pumping system in order to achieve population inversion.

In order to understand laser action, let us consider that three energy levels E1, E2 and E3 as shown in Figure 10.20 level, lifetime of which is = 10-3 sec.

 

FIGURE 10.20 laser action or light amplification

With the help of a pump some energy is given to the active material so that, they reach the level E2, As E2 is normal excited level, the lifetime of which is very small = 10-8 sec, so they immediately relax down to the intermediate energy level E3 which is metastable that is lifetime of E3 is large – 10-3 sec thus the atoms stay for a long time in this level. But on the other hand the atoms are lifted to E2. Only when these two condition are satisfied, the medium will act a amplifier this is the basic principle of laser when N2 larger than N1 i.e., the number of atom at higher energy is more than the number of atoms of lower energy the process is known population inversion, which is an artificial stimulation created in the medium. Anon-equilibrium state is to be produced in which the population of the upper energy level exceeds to a large extent the population of the lower energy level. When the situation occurs, the population distribution between levels E1 and E2 is said to be inverted and the medium is said to have gone into the state of population inversion.

10.8.7 Pumping

  1.         i.            Optical Pumping: In optical pumping light source is used to supply luminous energy most often this energy comes in the form of short flashes of light this method was first used by Maiman in the production of Ruby Laser and also widely used in solid state laser, the laser material is kept inside in a helical Xenon flash lamp of the type used in photography.
  2.      ii.            Electric Discharge: Another method of pumping is by direct electron excitation as occur in an electric discharge the method is preferred in gaseous ion laser for example Argon-ion. Laser the electric field of the order of several kV/m causes electrons emitted by the cathode, to be accelerated towards anode. Some of these electrons will collide with the atoms of the active medium, ionize the medium and raise it to the higher level. This produces the required population inversion.
  3.    iii.            Inelastic atom-atom collision: In an important class of lasers pumping by electrical discharge provides the initial excitation which raises one type of atoms. It is there laser atom which provide the population inversion required for the laser action. He-Ne laser is an example of this type.
  4.    iv.            Direct Conversion: A direct conversion of electrical energy into radiation occurs in light emitting diodes (LED’s) and the semiconductor laser drives from them.
  5.       v.            Chemical reaction: In a chemical laser, the energy comes from a chemical reaction without any need for other energy source for example hydrogen can combine with flourine.

H2 + F2 → 2HF

This reaction generates enough heat to pump a CO2 laser.

10.8.8 The Major Pumping Schemes

Atoms in general have a large number of energy levels among them, only three or four levels will be important for pumping process.

The transition between the two levels that generate stimulated emission is called the lasing transition. The top most level is called pumping level. Two important pumping schemes are widely used in the construction of lasers, these are known as three levels and four levels pumping schemes.

(a)             Three level pumping scheme: Consider an atomic system which has three energy levels as shown in Figure 10.21 the state E1 is the ground state and E2 and E3 are the excited states in the scheme, the energy states are such that atom could readily be excited to upper most state E3 (known as pumping level) this energy level is not a stable state. Hence atoms do not stay at this level and undergo downward transition

FIGURE 10.21 (a)

FIGURE 10.21 (b)

When the medium is exposed to radiation of frequency (vp) known as pumping
frequency, vp = (E3 - E1)/h, a large number of atoms will be excited to the higher energy level E3 some of there atoms make spontaneous transition to the lowest level E1 but many of them make spontaneous transition to the metastable level E2 through a non-radiative transition.

As spontaneous transition from E2 to E1 occur rarely the atoms get trapped in energy state E2. Because of pumping the process continue and after some time large number of atom have energy E2 when more than half of the ground state atoms accumulate at E2 the population inversion is achieved between the states E1 and E2. Now a photon of energy hv = E2 – E1 can trigger stimulated emission of atoms at E2. To achieve population inversion more than half of the ground state atoms must be pumped to the upper state. Therefore a very high pump power is required in this type of pumping scheme.

(a)             Four-level Pumping Scheme: A typical four level pumping scheme is shown in Figure 1O.22(a) and 1O.22(b), pump frequency take the active centres from ground state energy E1 to upper most energy E4 from the energy level E4, the atoms rapidly fall to the metastable state E3. The population at this energy grows rapidly while the energy level E2 is almost empty, therefore population inversion is achieved between the states E2 and E3 a photon of energy hv = E3 – E2 can start a chain stimulated emission, bringing the atoms in the energy state E2 from there, the atoms undergo nonradiative transitions subsequently to the ground state E1 and will again the available to participate in the process.

(a)             Two level Pumping Scheme: In case of two energy level E1 (lower energy) E2 (higher energy), the time span Δt for which the atom stay at upper level E2, must be large enough to achieve population inversion. However according to Heiserberg uncertainty principle

M will be longer if ΔE is small i.e., E2 is narrow if ΔE is smaller the pumping efficiency is smaller, as a consequence of this less number of atoms get excited through a sharp energy level supports the population inversion, enough population cannot be achieved at energy level E2, in view of all small ΔE. The result will be that the upward transition would be accompanied by premature downward stimulated transition and the population at energy levels E2 would not be possible to the required extent.

To obtain three or four level pumping scheme, the materials cannot be produced at
one’s will. One has to select the required materials from many available materials to suit a specific purpose.

10.8.9 The Ruby Laser

The first working laser was built in 1960 by Theodore Maiman, using a ruby crystal as the amplifying or active medium. Ruby belongs to the family of gems consisting of sapphire or alumina (Al2O3) with various types of impurities. For example pink ruby contains about 0.05% Cr atoms. Similarly, Al203 doped with Ti, Fe or Mn results in variously coloured sapphire. Most of these materials can be grown as single crystal.

Ruby crystals are available in rods several inches long, convenient for forming optical cavity (Figure 10.23). The crystal is cut and polished so that the ends are flat and parallel, with the end planes perpendicular to the axis of the rod. These ends are coated with a highly reflective material, such as Al or Ag, producing a resonant cavity in which light intensity can build up through multiple reflections. One of the end mirrors is construted to be partially transparent, so that a fraction of light will “leakout” of the resonant system. This transmitted light is the output of the laser. Of course, in designing such a laser one must choose the amount of transmission to be a small perturbation on the resonant system. The gain in photons per pass between the end plates must be larger than the transmission at the ends, as well as any other losses due to light scattering and absorption. The arrangement of parallel plates providing multiple interval reflections is similar to that used in Fabry-Perot interferometer; thus the silvered ends of the laser cavity are often referred as Fabry-Perot faces.

FIGURE 10.23 Schematic diagram of a ruby laser. A ruby crystal rod is cut and polished so that its ends form mirrors to create the resonant cavity. A flash lamp supplies the necessary energy in the form of photons to pump the rod

In case of ruby, chromium atoms in the crystal have their energy levels as shown in Figure 10.24, where only the energy levels which are important for stimulated emission are depicted.

FIGURE 10.24 Energy levels for chromium ions in ruby. The three level system include a ground level at E1 an excited level E3, and metastable state at E2 where the excited electrons rapidly go. The mean lifetime of the metastable state is long enough to ensure that population inversion can be achieved between the levels E1 and E2

This is basically a three-level system. Absorption occurs in the green part of spectrum exciting electrons from the ground state E1 to the band of levels designated E3 in Figure 10.24. Then electron decay rapidly to .the level E2. This transition is non-radiative. The level E2 is very important for the stimulated emission process since electrons in this level have a mean lifetime of about 5 ns before they fall-to the ground state. Because this lifetime is relatively long, E2 is called a metastable state. If electrons are excited from E1 to E3 at a rate faster than
the radioactive rate from E2 back to E1, the population of the metastable state E2 becomes larger than that of the ground state E1 (we assume that electrons fall from E3 to E2 in a negligibly short time).

In the experiment done by Maiman in 1960, population inversion is obtained by optical pumping of the ruby rod with a flash lamp such as the one shown in Figure 10.23. A common type of flash lamp is a glass tube wrapped around the ruby rod and filled with Xenon gas. A capacitor can be discharged through the Xenon-filled tube, creating a pulse of very intense light over a broad spectral range. If the light pulse from the flash tube is several milliseconds in duration, we might expect an output from the ruby laser over a large fraction of that time. However, the laser does not operate continuously during the light pulse but instead emits a series of very short spikes (Figure 10.25).

When the flash lamp intensity becomes large enough to create population inversion (the threshold pumping level), stimulated emission from the metastable level to ground level occurs with a resulting laser emission. Once the stimulated emission begins, the metastable level is depopulated very quickly. Thus the laser output consists of an intense spike lasting from a few nanoseconds to microseconds. After the stimulated emission spikes, population inversion build up again and a second spike results. This process continues as lone: as the flash lamp intensity is above the threshold pumping level.

In this situation, one can easily understand the metastable level never receives a highly inverted population of electrons. Whenever the population E1 reaches the minimum required for stimulated emission, these electrons depleted quickly in one of the laser emission spikes.

FIGURE 10.25 Laser spikes in the output of ruby laser: (a) typical variation of intensity of the flash lamp with time the intensity is above the threshold pumping level only during a certain period of time, not during the entire duration when the lamp is powered. (b) laser spikes occuring while the flash intensity is above the threshold pumping level. As soon as population inversion is achieved between the level E1, and E2 the laser emits a pulse of light. This process results in the series of
laser intensity spikes

To prevent this, we must some how keep the coherent photon field in the ruby rod from

Building up (and thus prevent stimulated emission) until after a larger population inversion obtained. This can be accomplished if we temporarily interrupt the resonant character of the optical cavity. This process is called Q-switching, where Q is the quality factor of the resonant structure. A straight forward method for doing this is illustrated in Figure 10.26. The front of the ruby rod is silvered to be partially reflecting but the back face is left unsilvered. The back reflector of the optical cavity is provided by an external mirror, which can be rotated at high speeds, when the mirror plane is aligned exactly

 

 

FIGURE 10.26. Schematic diagram of a Q-switched ruby laser in which one face of the resonant cavity is an external rotating mirror is to prevent stimulated emission by interrupting the resonant nature of the laser cavity, thus preventing the photon field building up. This allows a larger population inversion to be achieved, and consequently a higher laser light emission.

perpendicular to the laser axis, a resonant structure exists; but as the mirror rotates away from this position there is no build up of photons through multiple reflections and no laser action can occur-Thus during a flash from the Xenon lamp, a very large inverted population build up while the mirror rotates off axis. When the mirror finally returns to the position at which light reflects back into the rod, stimulated emission can occur, and the large population of the metastable level is given up in one intense laser pulse. This structure is called a giant pulse laser or a Q-switched laser. By saving the electron population for a single pulse, a large amount of energy can be given up in a very short time. For example of the total energy of the pulse is
1 Joule and the pulse width as 100 ns (10-7 s), the peak pulse power is 107 J s-1 = 10 MW.

10.8.10 Helium-Neon laser

A helium-neon laser, usually called a He-Ne laser, is a type of small gas laser. He-Ne lasers have many industrial and scientific applications, and are often used in laboratory demonstration of optics. Its usual operational wavelength is 633 nm, in the red portion of the visible spectrum.

The He-Ne laser was the first gas laser to be invented by Ali Javan, William Bennet Jr. and Donal Herriot at Bell Laboratory, USA, who in 1960 achieved continuous wave emissioin of the laser on the 1.15 urn wavelength line.

The active medium of the He-Ne laser is a mixture of helium and neon gas; in fact, He-Ne lasers perhaps could be labelled more properly as “neon lasers” since neon atoms are the active elements and helium only serves as a buffer gas (enhances lasing by causing the pumping efficiency to be increased).

The active/ gain medium of the laser is a mixture of helium and neon gases,
approximately in the ratio of 5 : 1., contained at low pressure (typically – 300 Pa) in a glass envelope. The energy of pump source of the laser is provided by an electrical discharge of around 1000 V through an anode and cathode at each end of the glass tube. The optical cavity of the laser typically consists of a plane, high reflecting mirror at one end of the laser tube, and a concave output coupler mirror of approximately 1% transmission at the other end.

 

FIGURE 10.27 Schematic diagram of a helium neon laser

He-Ne lasers are typically small, with cavity length of around 15 cm upto 0.5 m and optical output powers ranging from 1mW and 100 mW.

The red He-Ne laser wavelength is reported 632.816 nm. This is in fact the wavelength in air, and corresponds to a vacuum wavelength of 632.991 nm. The precise operating wavelength lies within about 0.002 nm of this value, and fluctuates within the range due to thermal expansion of the cavity. Frequency stabilized versions enables the wavelength to be maintained within about 2 parts in 1012 for months and years of continuous operations.

The laser process in a He-Ne laser starts with collision of electrons from the electrical discharge with helium atoms in the gas. He + e-) He+, where (*) represents an excited state. This excited helium from ground state to the 23s1 and 23s0 long-lived, metastable excited states. Collision of the excited helium atoms with the ground state neon atoms results in transfer of energy to the neon atoms, exciting neon electrons into the 5s level. This is due to coincidence of energy levels between the helium and neon atoms.

The process is given by the reaction equation:

He+ + Ne → He + Ne+ + ΔE

where  ΔE is the smaller energy difference between the energy states of the two atoms, of the order 0.05 eV.

The number of neon atoms entering the excited states builds up as further collision
between helium and neon atoms occur causing a population inversion between the neon 5s, 3p and other electronic levels. Spontaneous and stimulated emission between the 5s (2P1/2) and 3p (2P1/2) states results in emission of 632.991 nm wavelength light, the typical operating wavelength of a He-Ne laser.

FIGURE 10.28 Energy-level diagram for the He-Ne laser

After this, fast radiative decay occurs from the 3p to the 2p ground state via collisions of the neon with the container walls. Because of this last required step, the bore size of the laser cannot be made very large and the He-Ne laser is limited in both size and power.

With the correct selection of cavity mirrors, other wavelengths of laser emission of the He-Ne laser are possible. There are infrared transition at 3.39 µm and 1.15 µm wavelengths and a variety of visible transitions, including a green (543.5 nm), the so called Green Ne Laser), a yellow (594 nm) and an electron and gaseous atoms. Since a helium atom weighs only about one-fifth as much as a neon atom, electrons transfer energy much more readily to helium atoms via collisions. This process results in helium excitation to detectable levels. There after, excitation of neon atoms to level 5s and 4s occurs by means of collisons with excited helium atoms. The slight differences in energy between the helium and neon levels are accounted for a change in the kinetic energy of the atoms during collision.

The process of pumping the active laser gas indirectly by transfer of energy via collisions with another excited gas is called “resonant excitation” and has been utilized in several types of gas lasers. For example, although lasing action has been observed in CO2 alone, the pumping efficiency is greatly improved if a quantity of N2 is added as a buffer gas.

If resonant transfer of energy from the He metastable atoms to Ne atoms occurs at a rate faster than the decay rate from 5s and 4s to lower levels, then population inversion may be achieved. Stimulated emission of radiation in neon occurs at six different wavelengths as visualised in Figure 10.28. Until 1985 lasing was practiced at only three of these lines: the familiar line at 632.8 nm in the red portion of the spectrum and two additional lines at 1.152 µm and 3.39 µm in the infrared.

The 3.39 µn line and 632.8 nm line share the same upper lasing level and are in
competition for the same excited atoms that sustain the lasing process. When such a condition exists, the line having the greatest gain will loose, and the weaker line will not. In most cases the lower energy transition predominate. With neon the gain is so great for the 3.39 urn that lasing can be obtained at this wavelength without a pair of mirrors for feedback. The phenomenon of achieving an output from a laser with high gain without the use of a pair of mirrors is termed as “superradiant lasing” which can occur in large He-Ne lasers.

The 1.15 µm line and 632.8 nm line share the same lower lasing line. If 1.15 µm line laser, atoms from 4s are transferred to the level 3p, raising its population and reducing the population inversion for the 632.8 µm line.

All three of these wavelengths may be present in the laser output simultaneously. If one is interested in a He-Ne laser whose output is at the 632.8 run wavelength only, positive steps must be taken to suppress oscillations at the other two wavelengths. Both the 3.39 µm and 1.15 µm lines have greater gain than the 632.8 nm line and are capable of stealing power that otherwise would be available as output at the 632.8 nm wavelength.

Table 10.2 He-Ne Lasers

Wavelength 632.8 nm
Output power 0.5-50 mW
Beam diameter 0.5 – 2.0 nm
Beam divergence 0.5 – 3 m Radian
Coherence length 0.1 – 2 m
Power stability 5% /Hr
Lifetime >20000 Hrs.

10.8.11 Properties of Laser Beam

The ordinary light and laser light both are electromagnetic waves. But the laser beam is superior than ordinary light beam because of following characteristics:

(i)                           Highly directional

(ii)                        Highly intense

(iii)                      Highly monochromatic

(iv)                      Highly coherent.

(i)                           Directionality: Light from an ordinary source (sodium lamp, mercury lamp, sunlight etc.) spreads out in all possible directions. If light from an ordinary source is needed in a particular direction, an aperture is to be placed infront of the light source on the other hand, light from laser source travels only in one direction i.e., laser beam is highly directional. The directionally of laser beam is expressed in terms of full angle beam divergence and this angle is twice the angle made by outer edge with the axis of beam. The outer edge is the point where intensity of beam falls to 1/e of its value at the centre. A beam of light of wavelength (λ) radiated from an aperture of diameter (d) propagates as a parallel beam up to distance d2/λcalled Rayleigh range beyond this it begins to diverge due to diffraction. Angular spread on one side is given as θ =βλ/d where β is constant of proportionality having value nearly unity.

For typical laser beam divergence is about 0.01 milliradian i.e., 0.01 mm for every meter travelled by it from this one can imagine that spread of laser beam sent from earth to moon (384400 km away) will be just few kilometer.

(ii)                        Intensity: Intensity of light at any point is the amount of energy passing normally per unit area per second at that point, provided area being considered around that point as light from an ordinary source spreads out more or less uniformly in all direction so, if we look at 100 walt of ordinary lamp from a distance of 30 cm, the light entering the eye is hardly 1/1000 watt. On the other hand laser source gives out light as a narrow beam, so it’s whole of energy is concentrated in a small region and then accounts high intensity of laser. If we look at 1 watt laser directly it appears many thousand times more intence than 100 wall ordinary bulb.

The high intensity of laser beam can also be accounted by calculating and comparingthe number of photon output from a laser source and from an ordinary source photon energy (in negligible range) = hc/λ.

where as the no. of photon emitted per second from a hot source of light at 1000 K for same area is about 1012 thus laser beam is highly intense.

(i)                           Monochromaticity: The light from laser source is more or less strictly monochromatic whereas light from ordinary monochromatic source has frequency spread of the order of MHZ. Qualitatively the degree of monochromaticity is characterissed by spreads of frequency Δv of spectral line of frequency (v0). The degree of nonmonochromaticity is measured by the quantity Δv/v0. Thus lower is the value of (Δv/v0) higher will be the degree of monochromaticity and vice versa. For absolute monochromaticity I1v should be zero, which is not possible to attain.

For ordinary source of light,

(i)                           Coherence: Laser beam is highly coherent mass it has high degree of ordinary in the light field, which in turn is a measure of degree of phase correlation in the radiation field at different locations and at different times.

As we know that, the light emitted from source is the resultant of light radiations emitted due to transition of billions of atoms/molecules from higher energy state to lower energy state after initial exciation. As all the atoms/molecules operates independly and even does not radiates continuously, the wavefront so produced varies with space and time. Hence, there are two concept of coherence called spatial coherence and temporal coherence.

(a) Spatial Coherence: Spatial coherence is also known as lateral coherence it is measure of phase relationship between light field radiations at different points in space and describes how far apart two sources of two portion of the same source can be located along a transverse direction with respect to direction of observation. The concept of spatial coherence can be well explained by using Young’s double slit experiment.

Consider a cardboard having two pin holes S1 and S2 at variable separation (y), as shown in Figure 10.30 the card board is placed at distance D from point source S.

The interference pattern produced is observed on screen placed to the right of slit S1 and S2. The distance between the slit is varied and maximum separation i.e., S1S2 = ymax, for which sustained interference on the screen is observed, is calculated. This maximum distance is called coherence length of source.

If S is not source and has width ‘a’ as shown in Figure 10.30 then light from each point on the source will be spread out in angle = λ/a; Thus if angle S1SS2 =λ/a; than both the slits will receive light from each point on the source and this will be in coherence. From Figure 10.31 we get,  S1SS2 =y/D, therefore condition

y<λ/θ, where θ = a/D

Therefore the limiting value of y is ymax = a/D

where ymax known as spatial coherence length.

b)    Temporal Coherence: Temporal coherence refers to phase relation between light field at a point and the light field at the same point at later time a perfectly coherent source light radiations of constant amplitude as shown in Figure 10.32.

However in active practice no coherent source emits ideal sinusoidal field for all values of time.

This is because atoms in excited state emits pulse of short duration of the order 100 sec and returns to initial state. After this, phase changes abruptly as shown in Figure 10.32.

From the Figure 10.32 the interval for which field has definite phase relationship is known as coherence time and the length over which field remains sinusoidal is known as coherence length. Therefore,

Coherence length l = c.τ

where c is velocity of light and τ is coherence time.

Michelson Interferometer can be used for measuring coherence length and coherence time adjusts both the mirrors so as to obtain circular fringes. Let d be the difference in distance of mirrors from partially reflecting glass plate then path difference between interfering beam. So starting with equal distance of mirrors from partially reflecting glass plate i.e., (position of zero path difference) increase the path difference continuously, contrast between dark and bright fringes falls off gradually. The path diffence where frignes vanishes or disappear is an estimated coherence light.

Coherence length from sodium light is estimated to be 3 cm, so coherence time,

 [*For cadmium red light coherence length is 30 cm. ]

10.8.12 Applications of Laser

Laser have found wide applications in following fields:

(i)                           Material Processing: For mechanical and Metallurgical engineers, a knowledge of laser technology is of great use, laser can cut, drill, weld, remove metal from surfaces and perform their operations even at surface in accessible by mechanical methods.

(ii)                        Communication: Laser play the essential role in using thin strands of glass fibres to transmit light signal that can be received and translated in to communication format. With the help of laser beam, the distance between the earth and moon has been measured within accuracy of an inch. Laser can be used in computers to transmit an entire memory bank from one computer to another.

(iii)                      Medicine: Surgeous use laser to burnup brain tumors and remove tattoos. Blood vessels are reconnected by laser welding. He-Ne laser is used to stimulate nerves in the wrist and ankles for treatment of paralysis. Eye surgeons use laser to perform operation of common eye diseases e.g., glaucoma, cotaract and diabetic retinnopathy. Laser accupuncture is becoming popular for some disorders and relief pain.

(iv)                      Applications in physics and chemistry: Laser has initiated quite new fields of investigation in physics. An interesting example is of non-linear optics with special mension of harmonic generation and stimulated scattering. In the field of chemistry, laser used for both for diagnostic purposes and for producing inversible chemical change i.e., laser photochemistry. In diagnostic techniques particularly resonant
Raman scattering and coherent antistokes, Raman scattering gave considerable information on the structure and property of poly atomic molecules. The most interesting chemical application for laser is in the field of photochemistry.

(v)                         Military Applications: Due to great quality of energy which the laser can can concentrate, it has been mentioned as potential ‘death day’ type of incendiary weapon for use against energy missiles much research has been done in the field of laser weaponry e.g., development of higher energy output devices at great efficiency and active destructive effects of laser beam.