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UNIT IV

Optics with Laser and Optical Fibre

Chapter-15

LASERS AND HOLOGRAPHY

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LASER:

Laser stands for “light amplification by stimulated emission of radiation”.

Laser produces a highly directional and high-intensity light with a narrow frequency range.

ATOMIC EXCITATION AND ENERGY STATES :

In an atom, an electron in a ground state is stable and moves continuously in its orbit without radiating energy.

When the electron receives an amount of energy equal to the difference of the energy of the ground state and one of the excited states, it absorbs energy and jumps to the excited state.

The electron can stay for a very short lifetime (10^–8s) in the excited state, but sometime may stay in the metastable state having relatively longer lifetime of10^{– 3}s.

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Let us consider in an atom, an electron absorbs an energy equal to the difference of the ground state (E₁) and the first excited state (E₂), i.e., ΔE = (E₂ – E₁).

When this excited electron comes to the ground state by the process of de-excitation, then it releases a radiation of energy ΔE = (E₂ – E₁). The process of excitation and de-excitation of atom is shown in Fig.1

Fig.1 (a) Presentation of excited and de-excited atom with the basic structure of atom

(b) demonstration of excitation and de-excitation with energy diagram

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INTERACTION OF EXTERNAL ENERGY WITH THE ATOMIC ENERGY STATES:

There are three kinds of interactions of the external energy with the atomic energy states.

Absorption: In absorption suitable amount of energy is absorbed by the atoms of the ground state to get excited to the higher energy states. This is shown in fig 2.

Fig.2 Absorption process: (a) before absorption and (b) after absorption

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The rate of absorption transition is defined as the number of atoms per unit volume per second which are raised from the lower energy level to the excited energy level. It is expressed as

Where, −dN₁/dt is the rate of decrease of population at the lower energy level E₁.

If dN₂/dt is the rate of increase of population at the higher energy level (E₂), then the absorption transition rate can also be expressed as

From the above expression, we may conclude that

(1)

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It is clear that the rate of absorption transition is proportional to the population of atoms in the lower energy state and the energy density of incident light [(ρ(ν)]. Hence, the rate of absorption transition can be given as

(2)

Where B₁₂ is known as the Einstein coefficient for induced absorption transition, which represents the probability of induced transition from level 1 → 2.

Spontaneous emission: In spontaneous emission the excited atoms emit photon to come back in the lower energy state without any external impetus.

Fig.3 Spontaneous emission process

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The rate of spontaneous transitions R_spis given as

(3)

Where T_spis the spontaneous transition lifetime.

Number of photons generated during the process of spontaneous transition will be

proportional to the number of excited atoms only. It may be expressed as

(4)

Where A₂₁ is known as Einstein coefficient for spontaneous emission. It gives the probability of spontaneous transition, from level 2 → 1.

Comparing Eqs. (3) and (4), we get

(5)

Thus, the reciprocal of the coefficient A₂₁represents the lifetime of the spontaneous emission.

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Some important features of spontaneous emission are as follows:

It is very difficult to control the process of spontaneous emission from outside.

(ii) It is essentially probabilistic in nature.

(iii) The emitted photons (radiation) during the process of spontaneous emission have different direction of propagation, initial phase, and plane of polarisation.

(iv) The light emitted in the spontaneous emission is incoherent and non-monochromatic.

(v) The intensity of light decreases rapidly with distance from the source.

(vi) The emitted light during the process of spontaneous emission is incoherent.

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Stimulated emission: In stimulated emission, with the influence of suitable energy impetus, excited atom is triggered to the lower energy state, with the release of appropriate energy.

A photon with suitable energy (hν=E₂ – E₁) interacts with an excited atom. This photon c triggers the excited atom for the transition from higher energy level (E₂) to lower energy level (E₁) by emitting another photon as shown in Fig.4

Fig. 4 Stimulated emission process

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The rate of stimulated emission of radiation can be given as

Where B₂₁ is known as Einstein coefficient for stimulated emission, which indicates the stimulated emission from level 2 → 1.

Some important features of stimulated emission are as follows:

The process of stimulated emission can be controlled from outside.

(ii) The emitted photon and incident photon have same direction, phase, frequency, and

plane of polarisation.

(iii) The light produced during stimulated emission is coherent and monochromatic.

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EINSTEIN COEFFICIENTS:

Einstein’s A and B coefficients give the idea about the relation between spontaneous and stimulated emission probabilities.

Let us consider that under thermal equilibrium, N₁ and N₂ are the mean population of atoms in the lower energy level E₁ and the upper energy level E₂, respectively. For the condition of equilibrium, the number of transitions from E₂ to E₁must be equal to the transitions from E₁ and E₂ .

If ρ(ν) is the photon density, then the number of atoms absorbing photons per unit time per unit volume is given as

The number of atoms emitting photons per unit volume per unit time can be given as the sum of spontaneous emission and stimulated emission as

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At the condition of equilibrium, transitions from E₁ to E₂ must be equal to the transitions from E₂ to E₁. Thus, we can write

The ratio of the populations in these two states, N₁/N₂, is called the relative population, which is expressed as

By putting the value of N₁/N₂ ,we get

(6)

From the Planck’s radiation law, we know that

(7)

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Energy density ρ(ν) given by Eq. (6) will be consistent with Planck’s law given by Eq. (7) only if

(8)

And (9)

and hence (10)

Equations (8) and (9) are known as Einstein relations and Eq. (10) gives the relationship between Einstein's A and B coefficients.

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POPULATION INVERSION:

At the state of thermal equilibrium, the number (population) of atoms in lower energy level (E₁) is more than the population of atoms at the higher energy level (E₂).

But for emission processes and for laser action, it is essential that the number of atoms in higher energy level (E2) must be greater than the number of atoms in lower energy level (E1).

The process by which this condition is achieved is known as the process of population inversion. At the condition of population inversion, stimulated emission can produce a cascade of light. Population inversion process is shown in fig (5).

Fig.5 Population inversion process

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PUMPING MECHANISM:

The process by which atoms are raised from the lower energy level to the upper energy level is called pumping. In this process, it is necessary that atoms must be continuously promoted from the lower level to the excited level. Different methods of pumping are given below:

Optical Pumping:

In optical pumping, a light source (suitable photons) is used to supply luminous energy. Most often this energy is given in the form of short flashes of light.

(2 )Electric Discharge:

In this method of pumping direct electron excitation occurs through an electric discharge.

This method is preferred in gaseous ion lasers. An electric current flowing through the gas excites the atoms to the excited level from where they drop to the metastable upper laser level leading to population inversion.

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(3) Inelastic Collisions Between Atoms:

In an important class of lasers, pumping by electrical discharge provides the initial excitation which raises one type of atoms to their excited states.

These atoms collide inelastically with another atoms and provide them enough energy to excite them to the higher energy level and thus help in population inversion. This type of pumping occurs in helium–neon laser.

(4) Direct Conversion:

In light-emitting diodes (LEDs) and semiconductors, the electrons recombine with holes producing laser light. Thus, the direct conversion of electrical energy into radiation takes place.

(5) Chemical Reaction:

In chemical lasers, radiations come out of a chemical reaction, without any need of other energy source.

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COMPONENTS OF LASER SYSTEM:

In laser, the amplification of light is achieved by stimulated emission of radiations.

The stimulated emission produces completely coherent and intense radiations.

There are three essential components of laser action, which are shown in Fig.(6).

Fig. 6 Essentials of laser

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(1) Active Medium:

It is the medium in which the laser action is made to take place.

It may be in solid, liquid, or gaseous state where atoms/ions are lying in excited state (metastable state) to facilitate stimulated emission.

The most important characteristic of laser medium is that it should be capable to obtain the population inversion in it.

(2) Population Inversion:

Under the normal conditions at thermal equilibrium, the lower state of energy is more populated than the higher states.

In order to facilitate the stimulated emission (laser action), it is must that the number of excited atoms should be greater than the number of atoms in the ground state.

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This condition can be achieved by pumping mechanism in which energy is supplied to the atoms by external impetus through different processes such as optical pumping, electric discharge, direct conversion, and chemical reactions.

(3) The Optical Resonator:

It consists of a pair of plane or spherical mirrors in which one is perfect and the other is a partial reflector having common principal axis.

The reflection coefficient of one of the mirrors is very near to 1 and that of the other is kept somewhat less than one.

Due to the stimulated emission, there are waves between the two mirrors propagating along both the directions which undergo interference to form a standing wave.

For a stable standing wave, the wavelength must satisfy the following condition:

Where, n= 1,2,3 ………..

l = length of the cavity

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THREE-LEVEL LASER:

Laser action is highly dependent on the pumping scheme and the number of excited atoms ready for stimulated emission.

The simplest method of pumping is two-level pumping scheme, but a two-level pumping scheme is not suitable for population inversion because in this scheme, the lifetime of spontaneous emission is very fast.

In order to achieve suitable population inversion for laser action, three-level pumping scheme is used. In this scheme, the excited atoms ready for stimulated emission have more lifetime than the atoms excited under two-level pumping scheme.

In this scheme, the lower laser level is either the ground state or the level whose separation from the ground state is small compared to kT.

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A model of a three-level pumping scheme is shown in Fig. 7.

Fig. 7 Three-level pumping mechanism

The major disadvantage of three-level scheme is that it requires very high pump powers.

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FOUR-LEVEL LASER:

A simple sketch of four-level pumping scheme is shown in Fig.8. In this scheme, the terminal energy level E2 is above the ground level by the energy more than kT.

The pump energy elevates atoms to a short lived uppermost energy level E4 from where the atoms drop spontaneously to the metastable energy level E3.

Fig. 8 Pumping mechanism and laser action in four-level laser

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An impetus of energy hν= E₃ – E₂ can initiate a chain of stimulated emissions, which give the laser transitions from E₃ to E₂.

The atoms reaching at the energy level E₂ lose the rest of their excess energy by radiative or non-radiative transitions and finally reach the ground state E₁. Thus, the atoms are once again available for excitation.

In comparison to the three-level scheme, the lower laser transition level in the four-level scheme is not the ground state, but it is above the ground level, which is virtually vacant.

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RUBY LASER:

Historically, ruby laser is the first laser developed in 1960. It is a solid state, three-level laser.

Principle:

Ruby laser rod consists of a synthetic ruby crystal, Al₂O₃, doped with chromium ions with the concentration of about 0.05% by weight.

With this concentration of doping, there are about 1.6 × 10²⁵ Cr³⁺ions per cubic metre. These ions have a set of three energy levels suitable for the laser action.

Structure:

The schematic diagram of a ruby laser is shown in Fig.9. A cylindrical ruby rod of length 4 cm and width 0.5 cm is used.

The end faces of ruby laser are polished in such a way that one face is partially reflecting and the other is fully reflecting.

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Fig. 9 Schematic diagram of ruby laser

The rod is surrounded by a helical photographic flash lamp filled with xenon.

This xenon flash tube produces white light whenever activated by the power supply.

For the cooling of cavity, there is a provision of circulating coolant around the ruby rod.

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Working:

At the thermal equilibrium condition, most of the chromium ions are in the ground state E1.

When flash light falls upon the ruby rod, radiations of 5500 Å are absorbed by chromium ions and they are pumped to the excited state E3. Energy level diagram and different transitions of ruby laser are shown in Fig.10.

Fig.10 Energy level diagram and transitions in ruby laser

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The population inversion is achieved between E2 and E1.

Due to the transition of excited atom from E2 to E1 (transition 3), a photon of wavelength 6943 Å is emitted.

This photon travels through the ruby rod back and forth to get the suitable condition to stimulate another excited atom to emit a photon of same wavelength.

This process remains continued until the emitted photons becomes sufficiently intense.

Now, a part of this intense photons beam emerges through the partially reflecting end of the system to give the laser light.

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HELIUM–NEON LASER:

Helium–neon laser is a four-level gas laser. In this laser, population inversion is achieved by electric discharge.

A mixture of about 7 : 1 of He and Ne at the pressure of about 1 mm of mercury is used as active material in this laser. A schematic diagram of He–Ne laser is shown in Fig.11.

Fig. 11 Schematic diagram of He–Ne laser

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The mixture of these gases in suitable ratio is filled in a glass tube at the pressure of 1 mm of mercury.

At both ends of the tube, there are optically plane and parallel mirrors, out of which one is fully silvered mirror and other is partially silvered mirror.

An electric discharge is produced in the gas mixture by electrodes connected to a high-frequency electric source.

The excitation process of helium and neon atoms with their different transitions is shown in Fig. 12.

Fig. 12 Excitation of neon atom and different transitions

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The excited neon atom passes spontaneously from 20.66 eV to 18.70 eV which emits a photon of 6328 Å. This photon stimulates another excited atom which leads to laser transition.

CARBON DIOXIDE LASER:

CO₂ is a four-level laser operating on a set of vibrational–rotational transitions. This laser produces a light of middle infrared (IR) range of 10.6 mm and 9.4 mm wavelength.

Structure:

A schematic diagram of carbon dioxide gas laser is shown in Fig.13.

Fig. 13 Schematic diagram of CO2 laser

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In this laser, there is a discharge tube filled with a mixture of carbon dioxide, nitrogen, and helium gases in 1 : 4 : 5 proportions, respectively.

A high value of dc voltage is used for electric discharge in the tube due to which CO₂breaks into CO and O.

In order to maintain the equilibrium of CO2 molecules, a small amount of water vapour is added to the gaseous mixture which regenerates the CO2 molecule.

Working:

Different states of excitation and transition levels of CO2 laser are shown in Fig. 14.

Fig. 14 Different energy levels and transitions in CO2 laser

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The lowest vibrational level of N₂ has nearly as much energy as the asymmetric stretching mode of CO₂ molecule.

Hence, nitrogen can easily transfer energy to CO₂ molecules. Thus, the CO₂ molecules are excited to the energy level E_5.

Corresponding to the different states of vibrations, transitions are allowed between E₅ to E₄and E₅ to E₃to produce the photons of wavelength 10.6 mm and 9.6 mm, respectively. These transitions are known as laser transitions.

Helium is also used to keep CO₂ cooled.

SEMICONDUCTOR LASERS:

A semiconductor laser is a specially fabricated p–n junction device that emits coherent light when it is forward-biased.

Difference between solid-state lasers and semiconductor lasers is that in the solid-state lasers only 1% of the active material participates in the process of laser action while in semiconductor lasers, the whole material is active.

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Following conditions are essential for the semiconductor laser:

(i) The semiconductor must have a very high transition probability between the conduction and the valance bands.

(ii) The excess population can be maintained across the laser transition.

The first semiconductor laser was made in 1962 by R.N. Hall and Coworkers in USA. In this laser, gallium arsenide (GaAs) is used, which is usually operated at low temperature and emits laser light in the near IR.

Room temperature semiconductor lasers were made in 1970.

Now, the p–n junction (diode) lasers are made to emit light almost anywhere in the spectrum from UV to the IR. Diode lasers are remarkable due to their small size (0.1 mm long) and high efficiency of the order of 40%.

The main advantage of a semiconductor laser is that it is a portable and easily controlled source of coherent radiation.

Semiconductor lasers have a variety of applications in optical fiber communications, in CD audio players, CD-ROM drives, optical reading, high-speed laser printing, etc.

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APPLICATIONS OF LASER:

Some important applications of lasers are as follows:

Laser is used in optical fiber communication.

(ii) It is frequently used in the cutting and welding of metallic rods.

(iii) It is used to vaporize unwanted material during manufacturing of electronic circuits and chips.

(iv) These are used for different important purposes in medical, defense, industries, research and development organizations.

(v) Laser is frequently used in CD players, laser printers, laser copiers, etc.

(vi) It is used for specific task in thermonuclear reactions.

(vii) Laser is used for separating various isotopes.

(vii) Laser is used in holography.

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LASER BEAM WEILDING :

It is a welding technique used to join multiple pieces of metal through the use of a laser.

The beam provides a concentrated heat source, allowing for narrow, deep welds and high welding rates.

The process is frequently used in high-volume applications such as in the automotive industry.

Operation:

Laser beam welding has high power density (of the order of 1 MW/cm²) resulting in small heat-affected zones, and high heating and cooling rates.

The spot size of the laser can vary between 0.2 mm and 13 mm.

The speed of welding is proportional to the amount of power supplied but also depends on the type and thickness of the work pieces.

LBW is a versatile process, capable of welding carbon steels, HSLA steels, stainless steel, aluminum, and titanium.

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LBW is particularly dominant in the automotive industry.

A derivative of LBW, laser-hybrid welding, combines the laser of LBW with an arc welding method such as gas metal arc welding (GMAW).

This combination allows for greater positioning flexibility, since GMAW supplies molten metal to fill the joint, and due to the use of a laser, increases the welding speed over what is normally possible with GMAW.

(2) LASER CUTTING:

It is a technology that uses a laser to cut materials and is typically used for industrial manufacturing applications.

Laser cutting works by directing the output of a high-power laser, by computer, at the material to be cut.

The material then either melts, burns, vaporizes away, or is blown away by a jet of gas, leaving an edge with a high-quality surface finish.

Industrial laser cutters are used to cut flat sheet material & structural and piping materials.

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Advantages of laser cutting over mechanical cutting include easier work holding and reduced contamination of work-piece and high precision.

There is also a reduced chance of warping the material that is being cut, as laser systems have a small heat-affected zone. Some materials are also very difficult or impossible to cut by more traditional means.

A disadvantage of laser cutting is the high energy required.

(3)LASER DRILLING WITH YTTERBIUM LASERS:

Laser drilling is a successful manufacturing solution in many industries. Advantages include non-contact processing, low-heat input into the material, flexibility to drill a wide range of materials, accuracy, and consistency.

The other benefits associated are drilling submicron holes and small holes with large aspect ratios, and drilling at angles.

The common techniques used in drilling are percussion hole drilling and trepanning.

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Percussion drilling is a process where multiple pulses are applied per hole to achieve the desired results. High-speed on-the-fly drilling is a percussion type drilling process often used in drilling filter and guide vanes.

Trepanning is a process cutting large holes or contouring shaped holes. The advantages of trepanning include large holes, consistency, and ability to drill shaped holes. Trepanning also reduces the hole taper.

Fiber lasers can be focused to spot sizes as small as 10–20 mm. The Q-switch fiber laser offer very good drilling capabilities in thin sheets, ceramics, and silicon.

High-power fiber lasers are used for rock drilling applications and for oil and gas exploration industries. The high peak and energy/pulse are also used for drilling thick metals.

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Application Examples :

● Drilling of flow filters and strainers

● Submicron drilling in flexography ceramic rolls

● High-speed drilling of guide vanes

● Hole drilling of silicon

● Drilling diamonds for removing imperfections

● On-the-fly-cooling holes

● Rock drilling

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HOLOGRAPHY:

In the photographic methods for images of objects the information about the phase of the wave (reflected from the object) is not recorded.

During 1948, Dennis Gobor invented a two-step lens less imaging process. It is the new technique of photography of objects which is known as wave front reconstruction, or holography.

The word holography is the combination of two Greek words: holos and graphein.

Holos stands for whole and graphein stands for to write. Hence, holography means writing the complete image.

Holography is actually a recording of interference pattern formed between two beams of coherent light coming from the same source. In holography, intense coherent light is required.

In this process, both the amplitude and the phase components of the light wave are recorded on a light-sensitive medium such as a photographic plate.

This recording is known as a hologram.

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BASIC PRINCIPLE OF HOLOGRAPHY:

Holography is a method in which one not only records the amplitude of the light wave (reflected from the object), but also the phase of it.

Holography is a two-step process. First step is the recording of hologram where the object is transformed into a photographic record .

The second step is the reconstruction in which the hologram is transformed into the image.

Holography is the lens less photography in which hologram is a result of the interference occurring between the coherent light (from laser), reflected from the object and the light from a coherent reference beam, obtained by splitting the light from the same laser source.

Hologram actually contains information not only about the amplitude but also about the phase of the object beam, which produces a three dimensional image of an object.

The image will change its appearance if you look at it from a different angle.

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CONSTRUCTION OF THE HOLOGRAM:

To construct the hologram, a broad laser beam is divided into two beams, namely, a reference beam and an object beam, by the beam splitter in the form of mirror as shown in Fig.15.

Fig. 15 Construction of hologram

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Object beam directly illuminates the object, while the reference beam, after being reflected from the mirror, is collected on the photographic plate. Thus, the film is exposed simultaneously by both reference beam and object beam.

These two beams are coherent because they are from the same laser source.

These coherent sources interfere and give a complicated inference pattern on the photographic plate.

The developed negative of these interference fringe patterns is a hologram.

Thus, the hologram does not contain a distinct image of the object, but carries a record of both the intensity and the relative phase of the light waves at each point.

Thus, the resulting hologram contains all the information needed to reproduce the exact replica of the object.

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RECONSTRUCTION OF THE IMAGE:

The reconstruction of the object is schematically shown in Fig.16.

In this process, the hologram is illuminated by a parallel beam of light, called the reconstruction beam, from the laser source. Most of the light passes straight through, but the complex of fine fringes acts as an elaborate diffraction grating.

Fig. 16 Reconstruction of image

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Light is diffracted from this grating and two images: a virtual image and a real image are produced.

Virtual image is obtained at the original position of the object where the hologram was constructed. The real image can be photographed directly without using a lens. The virtual image observed through the hologram appears in full three-dimensional form.

This type of hologram is known as a transmission hologram since the image is seen by looking through it.

The basic difference between a hologram and an ordinary photograph is given below:

In a photograph, the information is stored in an orderly fashion, i.e., each point in the object relates to a conjugate point in the image. Whereas in a hologram, there is no such relationship, i.e., the light from every point on the object goes to the entire hologram.

Hologram has two main advantages:

(i) According to the viewing style of the observer, the image is seen in three dimensions.

(ii) Each part of the hologram would reconstruct the whole object, and not just a part of it.

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IMPORTANT PROPERTIES OF A HOLOGRAM:

Holograms have many unique properties given as follows:

(i) Each part of a hologram contains information about the entire object.

(ii) Hologram is a reliable medium for data storage, because a small part of hologram can reconstruct the entire image.

(iii) Information holding capacity of a hologram is extremely high. For example the hologram of size 6 × 9 mm can hold the information of one printed page.

(iv) On the hologram, information is recorded in the form of interference pattern. The type of the pattern obtained depends on the reference beam which is used to record the hologram.

(v) Any beam which is coherent and identical to the original reference beam can be used for the reconstruction of the image of the hologram.

(vi) If the wavelength of the reconstructing beam is greater than that of the original reference beam, the reconstructed image will be a magnified image. This magnification is proportional to the ratio of the two wavelengths.

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TYPES OF HOLOGRAMS:

According to the selection of the source, photographic plate, and viewing angle, there are many types of holograms. Some of them are given below:

Reflection holograms:

Reflection holograms are viewed with white light by choosing a suitable white-light source such as spot light, sun light, and flash light.

They are made with two beams approaching the holographic plate from opposite sides.

(ii) Volume holograms:

In this method, the object wave is reflected from the object and propagates backward, and overlaps the incoming reference wave. The two waves form a standing wave pattern.

The fringes are recorded by the photo-emulsion throughout its entire thickness to form a volume hologram.

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(iii) Multiple-channel holograms:

In the multiple-channel hologram, two or more images are visible from different angles.

These multiple channels can be classified as simple one and multiplex.
In the case of simple one, there are few images, and each is viewed from different angles.

In the case of multiplex, a large number of flat pictures of a subject, viewed from different angles, are combined into a single three-dimensional image of the object.

(iv) Rainbow holograms:

Rainbow hologram is also known as white-light transmission hologram.

When this hologram is viewed with a white-light point source, a very bright colour image can be reconstructed. A true-colour hologram image can be observed when the hologram is viewed in the correct plane.

(v) Polymer hologram:

These are made from light-sensitive plastics.

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(vi) Dichromate holograms:

In this type of holograms, glass-sandwiched dichromated gelatin is used as a holographic recording medium.

APPLICATIONS OF HOLOGRAPHY:

Some important applications of Holography are listed below:

Holograms, made with X-rays or ultraviolet rays, are able to record images of

particles smaller than visible light such as atoms or molecules.

(ii) A holographic lens is used in an aircraft “head-up-display” to allow a fighter pilot to see critical cockpit instruments while looking straight ahead through the wind screen.

(iii) Holography is widely used in non-destructive testing to study distortions resulting from stresses, strain, heat, and vibrations.

(iv) Three-dimensional acoustical hologram of an opaque object is used to see the internal structure of an object.

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(v) Holograms are used for security in many industries and are commonly found on a host of products and packaging.

(vi) Holographic lenses are used in supermarket scanners to read bar codes on merchandise for the store’s computers.

(vii) Artists use holography to express their creativity.

(viii) Holography is used for point-of-purchase advertising, taking the place of a photography of a product.

(ix) Holograms are also used for data storage such as holographic hard devices.

(x) Dichromate holograms are used as jewellery pendants, watches, etc.

(xi) Holographic techniques, such as holographic endoscopies, X-ray holography, and laser holograms are frequently used to diagnose dangerous diseases.

(xii) Advanced holographic techniques have a variety of applications in in ophthalmology, urology, otology, pathology, and orthopedics.

(xiii) Holographic three-dimensional images of eyes and interferometric testing of human teeth and chest motion during respiration were carried out quite early.