SEMINAR

HOLOGRAPHY AND ITS USAGE

Avtorica: Eva Grum Mentor: prof. Igor Poberaj

Ljubljana, May 2009

Abstract The seminar sums up the basic principles of holography. A few main types of holograms and their practical applications are described. Then it orientates on the different fields of application and tries to embrace those already established and some new ones.

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1. Preface ....................................................................................................................... 2

2. Physics behind holography ...................................................................................... 3

3. Most common types of holograms .......................................................................... 5

3.1 Transmission and reflection hologram ................................................................. 5

3.2 Rainbow hologram ............................................................................................... 7

3.3 Stereogram ........................................................................................................... 8

5. Fields of application ................................................................................................. 8

5.1 Holographic interferometry .................................................................................. 9

5. 2 Data storage ....................................................................................................... 10

5.3 Security holograms ............................................................................................. 12

5.4 Smart holograms ................................................................................................ 13

5.5 Dynamic holography .......................................................................................... 15

6. Conclusion ............................................................................................................... 16

Reference ..................................................................................................................... 17

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1. Preface Holography is a technique that allows the light scattered from an object to be recorded on photographic emulsion and later reconstructed so that it appears as if the object is in the same position relative to the recording medium as it was during the recording. As the position or orientation of the viewer changes, the image changes in exactly the same way as if the object was still present, thus making the hologram to appear three dimensional. Holography was discovered in 1947 by Hungarian physicist Dennis Gabor, for which he received the Nobel Prize in Physics in 1971 [1]. The discovery was an unexpected result of research into improving electron microscopes. He carried out his experiments using visible light from a filtered mercury arc. Because of the limited coherence of that source his holographic images were restricted to transparencies little larger than a pinhead. The field did not really advance until the development of the laser in 1962. The laser provides a powerful source of coherent light which enabled recording holograms of diffusely reflecting objects with appreciable depth. A common misunderstanding is that a hologram is simply some sort of a three-dimensional photograph [2]. Certainly, both photography and holography make use of photographic film, but that is about all they have in common. The difference is in the way the image is produced. A photographic image produced by a camera lens can be described using a simple geometric or ray model for the behaviour of light, whereas the holographic image cannot be described by wave propagation in geometrical optics. Its existence depends on diffraction and interference, which are wave phenomena. With photography we only store information about the amplitude of light. In a hologram we store information about the phase and amplitude. Information about the phase is stored in a form of fringe pattern that is created by two interfering beams falling on a photographic plate. The relative phase (difference between the wave vectors) between the two beams varies across the photographic plate and is encoded as the maxima and minima of the fringe pattern.

Figure 1: Picture of Denis Gabor with his holographic portrait [1].

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2. Physics behind holography At first let us have a look at the optical system presented in figure 2, which was used by Gabor where the light source, the object and the photographic plate are located along an axis [1]. This system is also called an in-line hologram. When the object is illuminated with a collimated beam, we consider the incident light consisting of two parts. These parts are expressed as exponential functions )(

00Φ= ieEO and )( ri

r eER Φ= where O describes the object wave (or scattered wave as seen in figure 2) and R the reference wave (directly transmitted light). I stands for intensity, E for amplitude, Φ for phase angle. We presume RO ⟨⟨ .

Figure 2: Holography setup for in-line hologram [3].

Both waves are coherent in time and space. We will compare the reference wave trough interference with the object wave. The reference picture is written as a hologram. We can write the formula for the intensity in the hologram as

)1(

)()(

)()(

)(0

)(0

220

2)(0

)(0

20

00

00

0000

00

rr

rrrr

rr

ir

irr

iir

ir

ir

ii

ir

iii

eEEeEEEE

eEeEEeEEeE

eEeEEreeE

ROROI

Φ−Φ−Φ−Φ

Φ−ΦΦ−Φ−Φ−ΦΦ−Φ

Φ−Φ−ΦΦ

∗∗

+++=

+++=

+⋅+=

+⋅+=

The phase information is contained in the two last terms of equation 1. While in ordinary photography only the intensity distribution in the object is recorded, the hologram contains also information about the phase. We can describe a transmission distribution on the photographic plate as ( )20 ItTT β+= . Where T0 is transmission

constant for the photographic plate, β equals photographic parameter, I is exposure intensity and t is exposure time. We put the equation for the intensity (1) into equation (2), and we get for the transmission distribution

)3()()(

)()(

0)(

022

00

)(0

)(0

220

00

00

rr

rr

ir

irr

ir

irrO

eEEeEEtEEtTT

teEEeEEEETTΦ−Φ−Φ−Φ

Φ−Φ−Φ−Φ

++++=

++++=

ββ

β

4

This equation describes the transmission of a holographic photographic plate. This means that the equation describes a hologram, which is a blackened area on the plate. For the reconstruction of the hologram, we have to illuminate the holographic plate with a reference beam as seen in figure 3. The reconstruction can then be expressed as

TeEE Rirr

Φ= .

Figure 3: Setup for hologram reconstruction [3].

The use of R instead of r is justified as the reconstruction beam is not necessarily the same as the reference beam.

)4()()( )(0

)(0

2200

00 rrRRR ir

ir

irr

ir

irr eEEeEEteEEEteETeEE Φ−Φ−Φ−ΦΦΦΦ ++++= ββ

This equation can also be written as

)5()( )(20

)(20

20

20

00 RrRrRR ir

ir

irr

irr eEEteEEteEtEEtTeEE Φ+Φ+Φ−Φ+Φ−ΦΦΦ ++++= ββββ

The first term in the equation 5 represents a wave that is directly transmitted beam. The second term is extremely small in comparison to the others because of our assumption that RO ⟨⟨ and can be neglected. The third term differs for a constant factor from the complex amplitude of the scattered wave from the object that was originally incident on the plate. This wave reconstructs an image of the object in its original position and is located behind the transparency. We call it a virtual image. Similarly, the fourth term corresponds to a wavefront that resembles the original object wavefront, except it has the opposite curvature. This wave forms a real image in front of the hologram. The problem with this approach is that the waves overlap which results in twin images [1]. An observer viewing one image sees it superposed on the out of-focus twin image and strong coherent background. The problem is solved with off-axis hologram technique, presented in figure 4 on the next page. There the reference beam is diverted by a beam splitter and is incident on the photographic plate under an offset angle θ (figure 4). The procedure is very similar to the one described. The difference is that the real and virtual image are formed at different angles and there are no more twin images and wave overlapping.

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Figure 4: Symbolic setup for off-axis hologram, where reference and object beam split up [4].

We should add that if we break the photographic plate in half, we can still recreate the holographic image, but we pay the price in decreasing resolution quality.

3. Most common types of holograms There are various types of holograms, classified differently concerning their way of recording, thickness etc. In this paragraph some of the most common types are presented with their characteristics and fields of application.

3. 1 Transmission and reflection hologram The transmission hologram was one the first holograms made [5]. A transmission hologram is formed when the light from the object and the reference beam enters the recording material from the same side. Like a photographic transparency, transmission holograms are lit from the back and bend light as it passes through the hologram to our eyes to form the holographic image. In order to reconstruct the holographic image, the hologram is placed in its original position in the reference beam as during its recording. We see an image of the object if we look along the reconstructed object beam. We see object from different perspectives as we shift viewpoints. Thus the object gives a three-dimensional effect. In reconstructing transmission hologram, the light does not actually pass through the image, but it creates a wave front that makes it appear as though the light had been generated in the position of the object. This image is called virtual image.

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Figure 5 and 6: Set-ups for recording transmission and reflection hologram. The difference is the direction of the reference beam [6].

Reflection holograms are viewed with the light source on the same side as the viewer [5]. In comparison to a transmission hologram, the recording of a reflection hologram requires 10 to 100 times more power. This leads to longer exposure time. During the process of hologram recording, the two beams-the reference beam and the object beam-illuminates the film plate from opposite sides and interfere on it. The fringes form flat layers more or less parallel to the emulsion's surface. The reflection hologram selects the appropriate band of wavelengths to reconstruct the image if it is illuminated by a highly directed beam of white light like a spotlight or sun light. This is because a reflection hologram reflects light within a narrow band while the rest of the light passes straight through.

Figure 7: Difference between reflection and transmission hologram[6]. Those two holograms are commonly used on packaging, for security ID and labelling, for identification purpose, for labeling CDs and DVDs etc [6].

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3.2 Rainbow hologram The rainbow holograms are very popular holograms because the images are reconstructed by white light [1]. Rainbow holograms are made by a double holographic process. Here an ordinary hologram or a transmission hologram is used as the object as seen in figure 8.

Figure 8: Scheme of rainbow hologram recording process. First we make a transmission hologram of an object. Than we record this hologram again trough the slit [6].

The second hologram is made through a slit. This is a horizontal slit which limits the vertical perspective of the first image and there is no vertical parallax in the resultant rainbow hologram. This limits the range of angle in vertical plane from which the real image can be viewed, without restricting the range of viewing angles in horizontal plane. This means that if the viewer eye is moved vertically, no parallax is seen and the image colour moves through the rainbow spectrum from blue to red. It is seen that when a rainbow hologram is viewed at an average height, the image appears yellow-green. Viewed higher the colour changes from orange or red and viewed lower it becomes blue or violet. That is why it is referred to as a rainbow hologram. The colour changes when we view it at different angle, but the hologram is still monochromatic.

Figure 9: An example of rainbow hologram from MIT [7].

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3.3 Stereogram A stereogram is a type of hologram that reconstructs a three-dimensional image from a series of two-dimensional views of an object recorded from different angles [1]. A series of photographs of the object is taken from equally spaced positions along a horizontal line. Contiguous, vertical strip holograms are then recorded of these photographs o a photographic plate. When we illuminate the stereogram with a point source of monochromatic light, the viewer sees a three-dimensional image. The image lacks vertical parallax, but it exhibits horizontal parallax over the range of angles covered by the original photographs.

Figure 10: Stereogram of planet Earth [8].

The method is widely used for simple presentations of objects. In medicine we can use computer tomography shots to create a stereogram and so we can present the internal organ more clearly.

5. Fields of application Holography is present in our lives on every step although we don’t notice it. Everybody has already come across with security hologram on bank notes and credit cards. Many people see a holography to be a new form of art and is now and then used in movies for special effects. Some of the more technical and scientific areas are less known.

Figure 11: Usage of holography in the movie Star Wars.[9].

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5.1 Holographic interferometry The industry takes advantage of holograms ability to record two successive holograms of deformed object and display the deformation [10]. This technique is called holographic interferometry and is used as aid in design, testing, quality control, and analysis. Major advantage of holographic technique it is that is nondestructive and allows us high resolution identification of vibration modes, displacements, and motion geometries. If the object we examine is changed or disturbed in some way during the hologram, then a pattern of “fringes” will appear on the image itself. These fringes represent maps of the surface displacement caused by the force applied to the object. Such a displacement map represents an extremely sensitive picture of the actual motion the object has experienced. An example is presented in figure 12, where we see that condensation of fringes marks a flaw in a pneumatic. A powerful feature of holographic interferometry is that information is obtained over the entire illuminated surface of the object being studied as a full and continuous field, which is important in understanding what is happening to the object as a whole.

Figure 12: With interferometry they have discovered a flaw in a tire [11].

With a technique known as double-exposure interferometry we measure defects by taking two successive holograms of the object [12]. The first hologram is taken when the object is under stress. Because of it a point on the surface of the object dislocates for distance ∆y. When the object is relieved of the stress, the small change in distance vanishes and the second hologram is taken. Therefore, two slightly different holograms are recorded onto the same photographic plate and the difference between their recorded images creates an interference pattern of light and dark fringes. We will look at a simple case, where we assume the object is locally flat as shown in Fig. 13.

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Figure 13: Objects flat surface and directions of travelling light rays [12].

As seen on figure 13, we have a light ray, z1, deflecting off the unstressed surface to z3, followed by a light ray, z2, deflecting of the stressed surface to z4. Both rays deflect at an angle θ to the normal. The optical path difference between the stressed and unstressed path is given by ∆P = (z1+ z3) − (z2 + z4) (6) and is represented by the green line segments in Fig. 13. If the difference between the surfaces is ∆y, we can geometrically determine the optical path difference to be

∆P = 2 ∆ycos θ. (7)

To achieve interference the optical path difference must fulfill the requirement 2λnP =∆ (8) where λ is the wavelength of the light ray, n is the total number of

fringes. By combining eq. 7 and 8, we can derive an equation that measures the total effect of the applied stress, or

θλ cos4ny =∆ (9). Technique is used in different areas of industry- for testing and observation of tires, manufacture of optical elements, vibration modes of motors, turbines, music instruments etc.

Figure 14: Hologram of a pop can stressed with a rubber band. The course of the fringes in the middle shows us its position [11].

5. 2 Data storage A double-sided, double-layer DVD can hold 15.9 GB of data, which is about eight hours of movies [13]. These conventional storage mediums meet today's storage needs, but storage technologies have to evolve to keep pace with increasing consumer demand. In order to increase storage capabilities, scientists are now working on a new optical storage method, called holographic memory, that will use the volume of the

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recording medium for storage, instead of only the surface area. Holographic memory offers the possibility of storing 1 terabyte (TB) of data in a sugar-cube-sized crystal. The basic components that are needed to construct a holographic data storage systems are: laser, beam splitter, mirrors, lenses, lithium-niobate crystal or photopolymer and detector [14]. Light from a single laser beam is split into two beams, the signal beam (which carries the data) and the reference beam. The hologram is formed where these two beams intersect in the recording medium. The process for encoding data onto the signal beam is accomplished by a device called a spatial light modulator (SLM). The SLM translates the electronic data of zeros and ones into an optical “checkerboard” pattern of light and dark pixels. The data are arranged in an array or page of over one million bits. The exact number of bits is determined by the pixel count of the SLM. At the point where the reference beam and the data carrying signal beam intersect, the hologram is recorded in the light sensitive storage medium. A chemical reaction occurs causing the hologram to be stored. By varying the reference beam angle or media position hundreds of unique holograms are recorded in the same volume of material. In order to read the data, the reference beam deflects off the hologram thus reconstructing the stored information. This hologram is then projected onto a detector that reads the entire data page of over one million bits at once. This parallel read out of data provides holography with its fast transfer rates. More details about the process of data recording and occurring problems you can read in a last year seminar from colleague Kavčič [15].

Figure 15: This scheme shows us how information is stored in a holographic data storage system [14].

There are still many technical problems that need to be solved. If there are too many holograms stored on a crystal, and the reference laser used to retrieve a hologram is not perfectly aligned, a hologram will pick up a lot of background from the other holograms stored around it. It is also a challenge to pack all of these components in a low-cost system.

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Figure 16: tapestry™300r, the world’s first commercial holographic drive and media from the

InPhase Technoogies [14].

5.3 Security holograms Security holograms are the most common type of holograms used in many industries [6]. Security hologram products like stickers, labels, cards are found on a lots of products and packaging, certificates, tickets, passes, bank notes and many kinds of identification and membership cards. Security holograms are an essential method of protection against forgery. They are produced on special materials and with more complicated techniques resulting that products cannot be copied by standard printing techniques or replicated by colour copiers or computer scanning equipment. As a result, the use of security holograms virtually guarantees product authenticity. More details are hard to find because the exact procedures are confidential otherwise the forgery wouldn’t be a problem. But we can mention two most common types of security holograms.

Figure 17 and 18: Security holograms on banknotes and credit cards [6]. 2D / 3D "hologram" images These are by far the most common type of hologram - and in fact they are not holograms in any true sense of the words. The term "hologram" has taken on a secondary meaning due to the widespread use of a multilayer image on credit cards and driver licenses. This type of "hologram" consists of two or more images stacked in such a way that each is alternately visible depending upon the angle of perspective of the viewer. The technology here is similar to the technology used for the past 50 years to make red safety night reflectors for bicycles, trucks, and cars. These holograms display a unique multilevel, multi-colour effect.

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Electron-Beam lithography These types of hologram are originated using highly sophisticated and very expensive electron-beam lithography system and this is the latest technology. This technique requires development of various algorithms for designing optical elements that shapes scattered radiation patterns. Such type of hologram offers features like the viewing of four lasers at a single point, 2D/3D raster text, switch effects, 3D effects, concealed images, laser readable text and true colour images.

Figure 19: VOID Pattern Released tamper evident holograms will leave 'VOID' when peeled off. These kind of holograms enhance your hologram security a lot because only few company can

make good pattern released [16] .

5.4 Smart holograms Smart holograms are a new branch of holography usage resulting from extensive researches in the field of “portable” diagnostics and new sensors. In a smart hologram [17], a suitable complementary receptor is attached to the polymer matrix of the photographic emulsion. When a particular substance (called the analyte) reacts with a polymer physical or chemical changes occur, and polymer responds to them. This means that smart polymer will generate observable changes in the wavelength (i.e. colour), intensity (brightness) or encoded image of the reflection hologram. The active component in such a “smart” hologram is normally a 3D polymer network called a hydrogel, which is a material that is very good at absorbing water and can swell anywhere up to 1000% of its original volume. By incorporating a hologram throughout the volume of a hydrogel, holographic gratings can be fabricated that respond to, for example, humidity, water, solvents, dissolved gases, ions, metabolites, drugs, antibiotics, sugars or enzymes. The structure of the interference fringes indicate the extent of hydrogel expansion accompanied with visible changes. Combined with their rapid response times, smart holograms offer great potential for monitoring changes in analyte concentrations in real time.

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Figure 20: A green smart hologram sprayed with ethanol to simulate a Breathalyser test for

alcohol on expired breath turns blue when the polymer contracts. The frames were obtained at 20 ms intervals [17]..

One of the simplest application of holographic sensors are measurements of water quantity in immiscible liquids. Immersing gelatin-based holograms in “wet” solvents cause water from the bulk solution to move into the holographic phase. The hydrogel swells and consequently imposes increase in the diffraction wavelength, which is directly proportional to the water containing in the sample. Holographic sensors thus can serve as power-free sensors of water activity in the petrochemical, food, textile, electronics and pharmaceutical industries, where water contamination can cause physical or financial damage. The potential of smart holograms is huge. First product a moisture-sensitive sensor called H2No has been developed to control water contamination in aviation fuel. Water contamination can cause the fuel to freeze at high altitude, which can block filters with potentially disastrous consequences. It is usually identified by performing a somewhat subjective “clear-and-bright” test, in which a hazy visual appearance indicates water contamination. On the other hand, this device — a hand-held syringe that contains a holographic sensor some 1 cm in diameter — instantly generates an easily discernible red cross if the water content of the fuel is above acceptable limit of 30 ppm. Example of this test is in figure 21. Other devices in development are blood-glucose monitor, monitor for architectural glazes and ion detectors.

Figure 21: The green circle on a H2No sensor from Smart Holograms changes to a red cross if the

water contamination in aircraft fuel is above a level of 30 ppm [17].

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5.5 Dynamic holography In static holography, recording, developing and reconstructing occur sequentially and a permanent hologram is produced. There also exist holographic materials which do not need the developing process and can record a hologram in a very short time. This allows us to use holography to perform some simple operations in an all-optical way. Examples of applications of such real-time holograms include phase-conjugate mirrors ("time-reversal" of light), optical cache memories, image processing (pattern recognition of time-varying images), and optical computing. A particularly promising application is optical phase conjugation. It allows the removal of the wavefront distortions a light beam receives when passing through an aberrating medium, by sending it back through the same aberrating medium with a conjugated phase. This is useful for example in free-space optical communications to compensate for atmospheric turbulence. It is possible, using nonlinear optical processes, to exactly reverse the propagation direction and phase variation of a beam of light [18]. The reversed beam is called a conjugate beam, and thus the technique is known as optical phase conjugation. One can interpret this nonlinear optical interaction as being analogous to a real-time holographic process. In this case, the interacting beams simultaneously interact in a nonlinear optical material to form a dynamic hologram (two of the three input beams), or real-time diffraction pattern, in the material. The third incident beam diffracts off this dynamic hologram, and, in the process, reads out the phase-conjugate wave. In effect, all three incident beams interact (essentially) simultaneously to form several real-time holograms, resulting in a set of diffracted output waves that phase up as the "time-reversed" beam. In the language of nonlinear optics, the interacting beams result in a nonlinear polarization within the material, which coherently radiates to form the phase-conjugate wave.

Figure 22 Comparison of a phase conjugate mirror with a conventional mirror. With the phase conjugate mirror the image is not deformed when passing through an aberrating element twice

[18].

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6. Conclusion There are many more areas where we apply holography and in this seminar just some are presented. Although principle of holography is rather simple it helps to create demanding creations such as holographic memory. Some methods are already well established, although there are also new ideas occurring and evolving, like in the case of smart holograms. All in all holography is very useful branch present all around us about which we haven’t heard a last word yet.

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Reference [1] P. Hariharan, Optical Holography: Principles, techniques and applications (Cambridge University Press, Cambridge, 1996) [2] http://www.fou.uib.no/fd/1996/h/404001/ (quoted 2.5.2009) [3] http://laser.physics.sunysb.edu/~dbennett/optics2/optics2.htm (quoted 2.5.2009) [4] http://kmr.nada.kth.se/wiki/Main/PointFocus (quoted 25.5.2009) [5] C. Outwater and V. Hamersveld, Guide to Practical Holography (Dimensional Arts Inc., 1995) [6] http://www.hologramsuppliers.com (quoted 2.5.2009) [7] http://www.humanproductivitylab.com (quoted 2.5.2009) [8] http://suppliers.jimtrade.com/10/9803/76114.htm (quoted 2.5.2009) [9] http://weblogs.newsday.com/entertainment/tv/blog/cnn/ (quoted 2.5.2009) [10] H. Fein, Indust. Phys. 3, 37, (1997) [11] Vito Babič, Holografija, (Modrijan, Ljubljana, 1996) [12] H. Timmers, Holographic Interferometry of a Stressed Pop Can (http://www.wooster.edu/physics/JrIS/Files/Timmers_Web_article.pdf ) (quoted 2.5.2009) [13] B. J. Thompson, IBM J. Res. Develop. 44, 341 (2000) [14] http://www.inphase-technologies.com/default.asp (quoted 2.5.2009) [15] B. Kavčič, Holografsko shranjevanje podatkov, 2007 (available on http://mafija.fmf.uni-lj.si/seminar/seminar.php) [16] http://www.hlhologram.com/pattern1.htm (quoted 2.5.2009) [17] C. Lowe and C. Larbey, Phys. World, 21 No. 2, 21 (2008) [18] http://www.absoluteastronomy.com/topics/Nonlinear_optics (quoted 2.5.2009)