7. LASER

7.1 Introduction The term LASER is originally comes from Light Amplification by Stimulated

Emission of Radiation. In 1917, Albert Einstein first theorized about the process which makes lasers possible called "Stimulated Emission." Before the Laser there was the Maser (microwave amplification by stimulated emission of radiation). In 1954, Charles Townes and Arthur Schawlow invented the maser, using ammonia gas and microwave radiation - the maser was invented before the (optical) laser. The technology is very close but does not use a visible light. On March 24, 1959, Charles Townes and Arthur Schawlow were granted a patent for the maser. The maser was used to amplify radio signals and as an ultrasensitive detector for space research. In 1958, Charles Townes and Arthur Schawlow theorized and published papers about a visible laser. In 1960, Theodore Maiman at the Hughes Research Laboratories in Malibu, (California), invented the ruby laser considered to be the first successful optical (or light) laser, consisted of a ruby crystal surrounded by a helicoidal flash tube enclosed within a polished aluminum cylindrical cavity cooled by forced air. Then, later that year in 1961, the Iranian physicist Ali Javan who was working with William R.Bennett and Donald Herriot made the first gas laser using helium and neon. In 1974, the first public laser was introduced in the way of barcode scanner at the supermarkets. The laserdisc player, introduced in 1978, was the first ever successful consumer product to include a laser. On the other hand a compact disc or CD player was the fist device that was equipped with laser which became a very common device in consumers' home. It was introduced in the year 1982.Let us now see the important frontiers of laser and the detailed study starting with the very fundamental entity that is atom.7.2 Interaction of Radiation with Mater

To understand the working principle of laser, one must consider the quantum processes that take place in a material when it is exposed to radiation. A material is made up of constituent atoms (or molecules) each having a set of discrete allowed energy states. An atom can move from one energy state to another when it receives or release an amount of energy equal to the energy difference between those two states. It is called a quantum jump or transition. Let E1 be the lower energy state and E2 be the excited state (the upper most state) and these states are identical to all the atoms in the medium as they are identical. The interaction between the radiation and atom (or molecule) takes place in three parts: (1) absorption, (2) spontaneous emission and (3) stimulated emission. Let us see each of them in detail.7.2.1 Absorption

If light (photons) of frequency ν pass through the group of atoms, there is a possibility of the light being absorbed by atoms which are in the ground state E 1, which will cause them to be excited to the higher energy state E2 as shown in the fig.7.1. This transition is known as stimulated absorption or induced absorption or simply absorption. Schematically, this may be represented as

(7.1)where A is an atom in the lower state and A* is an excited atom.

Fig. 7.1 Absorption of radiation by an atomThe probability of absorption is proportional to the radiation intensity of the light,

and also to the number of atoms currently in the ground state, N1.The more the number of atoms in the ground state, the more that can undergo absorption transition. In the same manner, the more the number of photons in the incident beam, the more the number of atoms that can be excited to the higher state. Therefore, the number of atoms participating in absorption or the absorption probability during time Δt is given by

(7.2)where, N1 is the number of atoms in the state E1, Q is the photon density per unit frequency range and is also known as spectral energy density, B12 is constant of proportionality. From above equation (7.2), it seems that as the number of ground state atoms (N1), spectral density Q and the interaction time duration (Δt) are higher, the probability of absorption is higher.7.2.2 Spontaneous emission

It is very natural fact that no any entity can remain exited for very longer time weather microscopic or macroscopic. This is also true for atoms. After being in the excited state, spontaneous decay events to the ground state will occur at a rate proportional to N2, the number of atoms in the excited state. The energy difference between the two states ΔE is emitted from the atom as a photon of frequency ν. In this transition, the excess energy is released as a photon of energy hυ=E2-E1.This type of transition is called spontaneous transition because this need not require any external impetus and represented in fig. 7.2. Schematically, it may be represented as

(7.3)

Fig. 2 Spontaneous emission of photon by an atom The probability of spontaneous emission Nsp is independent of photons in the

incident radiation and depends only on number of excited atoms N2. It is given by(7.4)

where, N2 is the number of atoms in excited state. The photons are emitted stochastically, and there is no fixed phase relationship between photons emitted from a group of excited atoms. Furthermore, the instant of transition, direction of emission and the polarization state of the photon are all random and this is the reason why spontaneous emission is incoherent. It contains the superposition of many waves of random phases. Such light is also not a monochromatic because of line broadening processes in the medium.7.2.3 Stimulated emission

If an atom is already in the excited state, it may be perturbed by the passage of a photon which has a frequency ν corresponding to the energy gap ΔE of the excited state to ground state transition. In this case, the excited atom relaxes to the ground state, and is induced to produce a second photon of frequency ν (See fig.7.3) The original photon is not absorbed by the atom, and so the result is two photons of the same frequency. This process is known as stimulated emission. Thus, the interaction of a photon with an excited atom triggers the excited atom to drop to the lower energy state giving up a photon. The rate at which stimulated emission occurs is proportional to the number of atoms N2 in the excited state, and the radiation density of the light. The base probability of a photon causing stimulated emission in a single excited atom was shown by Albert Einstein to be exactly equal to the probability of a photon being absorbed by an atom in the ground state. Therefore, when the numbers of atoms in the ground and excited states are equal, the rate of stimulated emission is equal to the rate of absorption for a given radiation density. Schematically, this may be represented as

(7.4)

Fig. 7.3 Stimulated Emission of photons by an atom.

The number of stimulated transitions Nst during the time Δt may be given as(7.5)

The critical detail of stimulated emission is that the induced photon has the same frequency and phase as the incident photon. In other words, the two photons are coherent. It is this property that allows optical amplification, and the production of a laser system. During the operation of a laser, all three light-matter interactions described above are taking place. Few important features of stimulated emission are as follows:

(1) The emitted photon is exactly identical to the incident photon in all respects. (2) The process is controllable from outside.(3) Photons are multiplied in this process. One photon induces an atom to emit a second photon, these two traveling along the same direction to de-excite two atoms in their path and producing four photons which in turn stimulate eight

photons and so on. The number of photons builds up an avalanche like manner as shown in the fig. 7.4.(Fig. 7.4_Fig.14.5_Avadhnulu)(4) The constructive interference of many waves traveling in the same direction with same frequency and phase produces an intense coherent light beam. The net amplitude of wave is proportional to the number of atoms that contributed to it and the net intensity is proportional to the square of the number of atoms.

7.3 Important Parameters of laser system Now let us discuss about some important parameters of laser system

without which it is not possible to generate the laser.7.3.1 Active Medium

A medium in which light gets amplified is called an active medium. The medium may be solid, liquid or gas. Out of the different atoms in the medium, only a small fraction of atoms are responsible for stimulated emission and light amplification. These atoms are known as active centres. The remaining part of the medium plays role of host and support active centres. 7.3.2 Population inversion

In physics, specifically statistical mechanics, a population inversion occurs when a system (such as a group of atoms or molecules) exists in state with more members in an excited state than in lower energy states. The concept is of fundamental importance in laser science because the production of a population inversion is a necessary step in the workings of a laser. Population inversion is required for laser operation, but cannot be achieved in our theoretical group of atoms with two energy-levels when they are in thermal equilibrium. In fact, any method by which the atoms are directly and continuously excited from the ground state to the excited state (such as optical absorption) will eventually reach equilibrium with the de-exciting processes of spontaneous and stimulated emission. At best, an equal population of the two states, N1 = N2 = N/2, can be achieved, resulting in optical transparency but no net optical gain.7.3.3 Metastable state

Normally, excited atoms have short life times and release their energy within nanoseconds (10-9 s) through spontaneous emission. Metastable state is the state at which the excited atoms can ‘wait’ for comparatively longer time with respect to highly excited state. In this regard, this state has good stability compared to highly excited state. The basic reason for the existence of metastable state is as follows: When the active centres are exposed to light (photons) of particular frequency, say υ, they are being excited by absorbing the photons and make transitions from ground state to higher excited state. This excitation has time limit of few nanoseconds. Meanwhile, they give up the part of their energy to the host neighbor atoms and try to achieve somewhat stable state. At this state, they remain stable for relatively longer time and this is called the metastable state. 7.3.4 Population and Thermal equilibrium

The population of any energy state is just the number of active atoms occupying that state. So, population of the lower energy state E1 is suppose N1 and that of upper E2 is suppose N2.Under thermal equilibrium condition, N1 and N2 are fixed by the Boltzmann factor. The population ratio is given by

(7.6)

The negative exponent indicates N2 <<N1 at equilibrium. This means that more number of atoms are in the lower energy level E1. This state is called normal state. When the external radiation incident to the N1 atoms having E1 energy sate then they will make transition to upper energy level E2 by absorbing the photons energy. Then they de excite to E1 once again. In order to maintain N1 and N2 constant, the number of upward and downward transitions must be same.

Thus, (7.7)

(7.8)This relation was predicted by Einstein and the coefficients A12,B21 and B12 are called Einstein coefficients. 7.3.5 Conditions for light amplifications

At thermal equilibrium, the ratio of the stimulated to spontaneous transitions is generally very small and the stimulated emission is negligible. The ratio is given by

(7.9)

The ratio of stimulated to absorption transitions is given by

(7.10)

Above eq.(7.9) indicates that in order to enhance the number of stimulated transitions, the radiation density Q is to be made larger. Here, B12=B21 as the probability of stimulated transition must be equal to the probability of absorption transition.7.4 Pumping

Pumping is the process by which the atoms are energized from the ground state to the excited state. Some of these atoms decay via spontaneous emission, releasing incoherent light as photons of frequency, ν. These photons are fed back into the laser medium, usually by an optical resonator. Some of these photons are absorbed by the atoms in the ground state, and the photons are lost to the laser process. However, some photons cause stimulated emission in excited-state atoms, releasing another coherent photon. In effect, this results in optical amplification. If the number of photons being amplified per unit time is greater than the number of photons being absorbed, then the net result is a continuously increasing number of photons being produced; the laser medium is said to have a gain of greater than unity. Recall from the descriptions of absorption and stimulated emission above that the rates of these two processes are proportional to the number of atoms in the ground and excited states, N1 and N2, respectively. If the ground state has a higher population than the excited state (N1 > N2), the process of absorption dominates and there is a net attenuation of photons. If the populations of the two states are the same (N1 = N2), the rate of absorption of light exactly balances the rate of emission; the medium is then said to be optically transparent. If the higher energy state has a greater population than the lower energy state (N1 < N2), then the emission process dominates, and light in the system undergoes a net increase in intensity. It is thus clear that to produce a faster rate of stimulated emissions than absorptions, it is required that the ratio of the populations of the two states is such that N2/N1 > 1; In other words, a population inversion is required for laser operation.

There are a number of techniques for pumping a collection of atoms to an excited state. Commonly used pumping techniques are optical pumping, electrical discharge and direct conversion. Optical pumping is used in solid state lasers in which light source such as flash discharge tube is used. In electric filed discharge, the electric field causes ionization of medium and raises it to an excited state. This technique is used in gas lasers. Direct conversion of electrical energy to light energy is adopted in semiconductor diode laser.7.5 Principal Pumping Schemes

Generally the atoms are characterized by large number of energy levels. Out of them only three or four levels will be pertinent to the pumping process. Therefore, only those levels are depicted in the pumping scheme diagrams. The upper most energy level is called pumping level and the terminal level is called the lower lasing level. The upward arrow in the diagram represents the pumping transition and the downward arrow represents the lasing transition. The slant arrows show the radiative or non-radiative spontaneous transition. Now let us see the important pumping schemes that are widely used. 7.5.1 Three-level pumping scheme

To achieve non-equilibrium conditions, an indirect method of populating the excited state must be used. To understand how this is done, we may use a slightly more realistic model, that of a three-level laser. Again consider a group of N atoms, this time with each atom able to exist in any of three energy states, levels 1, 2 and 3, with energies E1,E2 and E3, and populations N1, N2 and N3, respectively. Note that E1 < E2 < E3; that is, the energy of level 2 lies between that of the ground state and level 3.Initially, the system of atoms is at thermal equilibrium, and the majority of the atoms will be in the ground state; i.e., N1 ≈ N, N2 ≈ N3 ≈ 0. If we now subject the atoms to light of a frequency

, the process of optical absorption will excite the atoms from the ground state to level 3 as shown in the fig. 7.5. This process is called pumping, and does not necessarily always directly involve light absorption; other methods of exciting the laser medium, such as electrical discharge or chemical reactions may be used. The level 3 is sometimes referred to as the pump level or pump band, and the energy transition E1 → E3

as the pump transition, which is shown as the upward arrow. If we continue pumping the atoms, we can excite an appreciable number of them into level 3. In a medium suitable for laser operation, we require these excited atoms to quickly decay to level 2. The energy released in this transition may be emitted as a photon (spontaneous emission), however in practice the 3→2 transition is usually radiationless, with the energy being transferred to vibrational motion (heat) of the host material surrounding the atoms, without the generation of a photon.

An atom in level 2 may decay by spontaneous emission to the ground state, releasing a photon of frequency ν12 (given by E2 – E1 = hν12), which is called the laser transition. Because at least half the population of atoms must be excited from the ground state to obtain a population inversion, the laser medium must be very strongly pumped. This makes three-level lasers rather inefficient, despite being the first type of laser to be discovered (based on a ruby laser medium, by Theodore Maiman in 1960). A three-level system could also have a radiative transition between level 3 and 2, and a non-radiative transition between 2 and 1. In this case, the pumping requirements are weaker. In practice, most lasers are four-level lasers, described below.7.5.2 Four-level lasers

Here, there are four energy levels, energies E1, E2, E3, E4, and populations N1, N2, N3, N4, respectively as shown in the fig. 7.6. The energies of each level are such that E1 < E2 < E3 < E4. In this system, the pumping transition excites the atoms in the ground state (level 1) into the pump band (level 4). From level 4, the atoms again decay by a fast, non-radiative transition into the level 3. Since the lifetime of the laser transition is long compared to that of non-radiative transition, a population accumulates in level 3 (the upper laser level), which may relax by spontaneous or stimulated emission into level 2 (the lower laser level). This level likewise has a fast, non-radiative decay into the ground state. As before, the presence of a fast, radiationless decay transitions result in population of the pump band being quickly depleted (N4 ≈ 0). In a four-level system, any atom in the lower laser level E2 is also quickly de-excited, leading to a negligible population in that state (N2 ≈ 0). This is important, since any appreciable population accumulating in level 3, the upper laser level, will form a population inversion with respect to level 2. That is, as long as N3 > 0, then N3 > N2 and a population inversion is achieved. Thus optical amplification, and laser operation, can take place at a frequency of ν32 (E3-E2 = hν32).

Since only a few atoms must be excited into the upper laser level to form a population inversion, a four-level laser is much more efficient than a three-level one, and most practical lasers are of this type. In reality, many more than four energy levels may be involved in the laser process, with complex excitation and relaxation processes involved between these levels. In particular, the pump band may consist of several distinct energy levels, or a continuum of levels, which allow optical pumping of the medium over a wide range of wavelengths. Note that in both three- and four-level lasers, the energy of the pumping transition is greater than that of the laser transition. This means that, if the laser is optically pumped, the frequency of the pumping light must be greater than that of the resulting laser light. In other words, the pump wavelength is shorter than the laser wavelength. It is possible in some media to use multiple photon absorptions between multiple lower-energy transitions to reach the pump level; such lasers are called up-conversion lasers. While in many lasers the laser process involves the transition of atoms between different electronic energy states, as described in the model above, this is not the only mechanism that can result in laser action. For example, there are many common lasers (e.g., dye lasers, carbon dioxide lasers) where the laser medium consists of complete molecules, and energy states correspond to vibrational and rotational modes of oscillation of the molecules. This is the case with water masers, which occur in nature.7.6 Optical Resonator

A photon spontaneously emitted by an atom acts as a input and starts stimulated emission. To sustain stimulated emission acts and to increase the light intensity, a positive feedback of light in a medium is must. Moreover, spontaneous photons are emitted by atoms independently in various directions which produce stimulated emissions in different directions. This produces the incoherent light. Further, an intense beam of light can be available only when the optical gain of the system exceeds the optical losses. Moreover, a large radiation energy Q is required to keep the stimulated emissions dominating. The above requirements are met with the help of an optical cavity resonator, which converts the active medium in to oscillator and hence into a light generator.

An optical resonator generally consists of two opposing plane parallel mirrors, with an active material placed in between them. One of the mirrors is semitransparent while the other one is made 100% reflecting. The mirrors are set normal to the optic axis of the material. This structure is known as Fabry- Perot resonator which is shown in the fig.7.7.

Fig. 7.7 Fabry-Perot optical resonator

Working: Fig. 7.8 (FIG 7.8_FIG 14.11_AVADH) shows the successive stages of the action of optical resonator. Initially, the active centres in the medium are in ground state (fig. 7.8a). Through suitable pumping mechanism, the material is taken into a state of population inversion (fig 7.8b).Spontaneous photons are emitted in every direction (fig. 7.8c).To generate a coherent light, only those photons should be selected whose propagation direction is specific. This means that photons emitted in any direction other than parallel to the optic axis will escape from the sides of the resonator and are eliminated (fig.7.8c). Hence the number of photons having propagation direction parallel to the optic axis will increase rapidly. On reaching to the semitransparent mirror some of the photons are transmitted out and part of them will be reflected back.

While propagating in the opposite direction, they de excite more and more atoms and build up their strength (fig 7.8d). At the 100% reflecting mirror, some of the photons are absorbed but major portion will be reflected. The amplified beam will move along the same path as the starting photon (fig.7.8e) and undergoes multiple reflections at the mirrors and gain strength. At each reflection at the front mirror, the beam is partially transmitted through it and partially fed back into the medium. Thus the photon density in the medium will increase. Now the laser beam oscillations will begin when the amount of amplified light becomes equal to the total amount of amplified light becomes equal to the total amount of medium. As the oscillations builds up to enough intensity, light emerges through the front mirror as a highly collimated intense beam (fig. 7.8f).

The wave propagating within the cavity resonator should take on a standing wave pattern, so that the continuous increase in the wave amplitude occurs. The condition for forming the standing wave within the resonator is that the optical path length traveled by a wave between the consecutive reflections should be an integral multiple of the wavelength. Thus, (m=1,2,3……)

(7.11)Here, λm is the light wavelength within the material. The cavity resonates when there is an integer number of half wavelength spanning the region between the mirrors. Only wavelengths satisfying the above eq. (7.11) can exist inside the cavity ina steady state. Waves of other wavelengths interfere destructively with each other as they pass back and forth between the mirrors. Thus they attenuate very quickly. 7.7 Types of Laser

Lasers are mainly categorized in four groups: (1) solid state lasers, (2) gas lasers, (3) liquid lasers and (4) semiconductor lasers. Most lasers emit light in the red or IR region.7.7.1 Ruby Laser

The ruby laser is the first type of laser actually constructed, first demonstrated in 1960 by T. H. Maiman. The ruby mineral (corundum) is aluminum oxide with a small amount (about 0.05%) of chromium which gives it its characteristic pink or red color by absorbing green and blue light. After Schawlow and Townes published their paper on the possibility of laser action in the infrared and visible spectrum it wasn't long before many researchers began seriously considering practical devices. Most experts were speculating that gases would be the first to laser in the optical and infrared. It came as surprise that ruby was the first substance to produce laser action in the visible spectrum

Fig. 7.9 Ruby LaserConstruction: In ruby laser, ruby crystal surrounded by a helicoidal flash tube

enclosed within a polished aluminum cylindrical cavity cooled by forced air as shown in the fig 7.9. The ruby cylinder forms a Fabry-Perot cavity by optically polishing the ends to be parallel to within a third of a wavelength of light. Each end is coated with evaporated silver; one end is made less reflective to allow some radiation to escape as a beam. Ruby was the first substance to produce laser action in the visible spectrum. The ruby laser is used as a pulsed laser, producing red light at 694.3 nm. After receiving a pumping flash from the flash tube, the laser light emerges for as long as the excited atoms persist in the ruby rod, which is typically about a millisecond. A pulsed ruby laser was used for the famous laser ranging experiment which was conducted with a corner reflector placed on the Moon by the Apollo astronauts. This determined the distance to the Moon with an accuracy of about 15 cm.

Working: The energy levels of Cr+3 ions in the crystal lattice are shown in the fig 7.10. They form three level system. The Xenon discharge generates an intense burst of white light lasting for few milliseconds. The green component of the spectrum having wavelength around 5500Å is absorbed by Cr+3 ions raising them from ground state to excited state. The Cr+3 ions rapidly lose part of their energy (E3-E2) to the crystal lattice and perform non-radiative transitions to the state E2. Here, E2 is the metastable state and therefore Cr+3 ions accumulate there. Hence, the state of population inversion gets established between E2 and E1 levels. A spontaneous photon emitted by Cr+3 ions at E2

level initiates the stimulated emission by the other Cr+3 ions in the metastable state.

Photons traveling along the axial direction are repeatedly reflected and amplified, and emerge out of the semi transparent mirror in the form of a strong laser beam. The beam is red in the color having wavelength 6943 Å. Here, the green light plays the role f pumping

agent and it is a random red photon radiated spontaneously by one of the Cr+3 ions that acts as input and gets amplified.

Fig. 7.10The Xenon flash lasts for few milliseconds. However, the laser does not operate

throughout this period. Once, stimulated transition commences, the metastable state E2

gets depopulated very rapidly and lasing stops. The laser becomes inactive till the population inversion once again established. Therefore, the output of the laser is not continuous but occurs in the form of the pulses of microsecond duration. The efficiency of ruby laser is very less as only the green component of the pumping light is utilized while the rest of the components of the incident light are left unused.

7.7.2 He-Ne LaserThe bright, highly collimated, red (λ= 6328 Å) light beam from the helium-neon

(He-Ne) gas laser is a familiar sight in the scientific laboratory, the industrial workplace, and even at the checkout counter in most supermarkets. He-Ne lasers are manufactured in large quantities at low cost and with proper operation they can provide thousands of hours of useful service. Even though solid state diode lasers can now provide red laser light beams with intensities comparable to those obtained with He-Ne lasers, it is anticipated that the He-Ne laser will remain a common component in scientific and technical instrumentation in the foreseeable future

Construction: The schematic of a typical He-Ne laser is shown in the fig. 7.11. It consists of a long discharge tube of length about 50 cm and diameter 1 cm. The tube is filled with a mixture of helium and neon gases in the ratio of 10:1. Electrodes are provided to produce a discharge in the gas and they are connected to a high voltage power supply. The tube is hermetically sealed by inclined windows arranged at its two ends. On the axis of the tube, two reflectors are fixed which form the Fabry-Perot resonator. The distance between the mirrors is adjusted such that it equals mλ/2 and supports standing wave pattern.

Fig. 7.11 A schematic diagram of He Ne laser Working: A large potential difference (about 2000V) is placed across the cavity. 

This potential strips electrons off the conductor, giving the electrons a large amount of kinetic energy.  Some of these electrons collide with the Helium atoms in the laser.  In this collision, the kinetic energy of the electrons excites the Helium atoms to the metastable 2S state (fig.7.12). This newly excited atom then collides with a Neon atom in the ground state.  This collision causes the Neon atom to be excited into the 2s and 3s levels.  This occurrence is due to the striking similarity between the energy at the 2s and 3s state in the Neon atom and the 2S in the Helium atom. The Neon atom then decays down to lower levels (3p or 2p) releasing light with a wavelength 632.8nm or 115.23nm. This light is almost completely reflected by the mirror at the end of the cavity.  When this light is reflected back, it has the opportunity to cause the stimulated emission of more Neon atoms.  The small percentage of unreflected, transmitted light is emitted as a laser beam.

Fig. 7.12 Energy levels of He and Ne atoms and transition between the levels.

7.8 HolographyAlmost every field utilizes the laser nowadays. This is the reason why it is said

that next to computers it is the laser that is bringing revolutionary changes in this era of technology. Lasers are used in fundamental research, entertainment electronics, industrial electronics, communications, mechanical working, machining, welding, astronomy, information processes, and medical field and so on the list is unending and continues to grow. One of the most important applications of laser is in the production of three dimensional images of an object called holography. Let us see the different frontiers of holography starting with introduction.

In photography, one is concerned only with the brightness or irradiance distribution (square of the amplitude) of the image. The optical path to different parts of the object is not recorded because the photographic emulsion is a square law detector.It just records the amplitude. On the other hand, in holography, the aim is to record complete wave field (both amplitude and phase) as it is intercepted by a recording medium. The recording plane may not be even an image plane. The scattered or reflected light by the object is intercepted by the recording medium and recorded completely in spite of the fact that the detector is insensitive to the phase differences among the various parts of the optical field. In 1948 Denis Gabor gave an ingenious solution to the problem of recording phase information by means of a background wave, which converts phase differences into intensity differences. He introduced a two step lensless imaging process known as wavefront reconstruction technique or holography (Greek word holos means whole, complete), in which an interference between the object field and the background wave (known as reference wave) is formed and recorded on a photographic material as shown in the fig.7.13. (Fig. 7.13_fig. 14.25_AVADH). The record known as a hologram (whole record) captures the complete wave which can be viewed at a later time by illuminating the hologram with an appropriate light beam. Thus, in holography interference between the light reflected and scattered by the object, called the object beam and a reference beam is created and recorded on a photographic emulsion. If the amplitude of the object beam remains constant and the angle between the beams increases, the fringes will become finer. On the other hand, if the phase relation between the two interfering beams remains constant but the amplitude of the object beam changes, the contrast of the fringes will change. By this process the complex object information gets coded in the form of complicated fringe pattern. The object can be considered to be made up of a large number of point sources distributed in a three dimensional space. Each point of the object will interfere with the reference and produce fringes. The fringe patterns generated by different points will be varying in orientation, contrast and spacing. Gabor showed the applicability of this new process of wave front recording by using a mercury discharge lamp and taking collinear object and reference beams. The original in-line technique of Gabor produces both virtual and real images on the same axis, thus an observer focusing on one image, always sees it accompanied by the out-of-focus twin image.

Holography is thus a two stage process. In the first stage, a hologram is recorded in the form of interference pattern and in the second stage, the hologram acts as a diffraction grating for reconstruction beam and the image of the object is reconstructed from the hologram.Applications:

In routine life, holographic scanners are in use in post offices, larger shipping firms, and automated conveyor systems to determine the three-dimensional size of a package. They are often used in tandem with checkweighers to allow automated pre-packing of given volumes, such as a truck or pallet for bulk shipment of goods. Electron holography is the application of holography techniques to electron waves rather than light waves. Electron holography, invented by Dennis Gabor, is going to improve the resolution and avoid the aberrations of the transmission electron microscope.

Holographic data storage is a technique that can store information at high density inside crystals or photopolymers. Holographic storage has the potential to become the

next generation of popular storage media. The advantage of this type of data storage is that the volume of the recording media is used instead of just the surface. The use of holograms on credit cards and debit cards provide added security to minimize counterfeiting.

Holographic interferometry (HI) is a technique which enables static and dynamic displacements of objects with optically rough surfaces to be measured to optical interferometric precision (i.e. to fractions of a wavelength of light). It can also be used to detect optical path length variations in transparent media, which enables, for example, fluid flow to be visualized and analyzed. It can also be used to generate contours representing the form of the surface. The principle of holography can be used to study the image formed by sound waves. This is called the acoustic holography. The acoustic holography can be used to get a three dimensional image of our internal organs.

However, holograms suffer from aberrations caused by a change in the wavelength from construction to reconstruction and also by a mismatch in the reference and reconstruction beams. Both the chromatic and nonchromatic aberrations are quite important even when only small deviation from the recording geometry is present in the reconstruction geometry. The condition that will eliminate all the aberrations simultaneously is to duplicate exactly one construction beam in the reconstruction process.