Holograms, as a concept, have awed many with their ability to provide credibility to any document or product. In this article, Balasubramaniam A V traces the origin of holography and applications where it can be deployed

Some of the questions about holography which come to mind immediately might serve as a good starting point for our discussion. They are: ‘What is a hologram? And how does holography work?’ Note that the process is referred to as holography while the plate or film itself is referred to as a hologram. The terms holograms and holography were coined by Dennis Gabor (the father of holography) in 1947. The word hologram is derived from the Greek words ‘holos’ meaning whole or complete and ‘gram’ meaning message. Older English dictionaries define a hologram as a document (such as a last will and testament) hand-written by the person whose signature is attached.

The theory of holography was developed by Dennis Gabor, a Hungarian physicist, in the year 1947. His theory was originally intended to increase the resolving power of electron microscopes. Gabor proved his theory not with an electron beam, but with a light beam. The result was the first hologram ever made. The early holograms were legible, but plagued with many imperfections because Gabor did not have the correct light source to make crisp, clear holograms as we can today nor did he use the off axis reference beam which we will describe later. What was the light source he needed? The LASER, which was first made to operate in 1960.

Laser light differs drastically from all other light sources, man-made or natural, in one basic way which leads to several startling characteristics. Laser light can be coherent light. Ideally, this means that the light being emitted by the laser is of the same wavelength, and is in phases. These might be new terms for some of you, so let us form an analogy that might clarify the term coherence.

Let’s say that you are flying over a freeway at rush hour, and directly below you is a long tunnel that all the cars must go through. Nothing is strange about the fact that all different styles and makes of motor vehicles emerge from the tunnel at differing velocities. A Cadillac at 75 mph, a Volkswagen at 45 mph, a motorcycle at 60 mph, a truck at 40. The distances between vehicles also vary. Thus you have different types of vehicles at varying speeds, and at constantly changing distances between each other. But then something very strange takes place; you see that more and more 1973 Cadillac Coupe de Villes are emerging.

No, wait, look! All the cars coming out of the tunnel are 1973 Cadillac Coupe de Villes, gold with tinted windows, exactly alike, (a situation not totally uncommon in some carefully chosen Southern California suburbs). Not only are they the same year, make and colour, but they are all travelling at exactly the same speed and all bumper to bumper, never changing. So that if you just happened to have a stopwatch to hand you would find that the cars are exiting at a rate of one car per second. If you were to leave, or more likely, pass out from the fumes, you would observe upon reawakening that the exit rate of the cars is still exactly one car per second. The cars are in phase.

The way in which coherent light is emitted from a laser is analogous. Keep in mind that although absolute 100 percent coherence is rarely, if ever, attained, there are certain types of lasers readily available which have sufficient coherence to make excellent off axis holograms.

The light emitted from a laser is all exactly the same type, or make, depending on the characteristics of the substance which is lasing. It is important to remember that the frequency of laser light is unvarying and that in the same medium, all light, i.e. light of different wavelengths of frequency, travels at the same speed.

It’s true that all electromagnetic radiation, including the very small portion we call visible light, travels in a vacuum at the approximate finite speed of 186,000 miles per second. (Note: The velocity of light in a vacuum is one of nature’s constants and is referred to by the letter c). Light waves, can oscillate at different frequencies and with correspondingly different wavelengths so that for any given amount of time, say one second, a greater number of shorter wavelengths of (blue) light would be emitted from a laser than longer wavelengths of (red) light. This does not mean that different wavelengths travel at different speeds. Again to the freeway analogy: given the same speed and same distance between cars, more Volkswagens (short wavelength) than Cadillacs (long wavelengths) would pass by a point in the same amount of time.

Now is a good time to define some terms used previously but that you will see throughout this explanation. Wavelength, usually symbolised by the Greek letter for lambda, and frequency, symbolised by the Greek letter v pronounced nu, have a reciprocal relationship v = C. The shorter the wavelength, the higher the frequency and vice versa. The amplitude is the height or intensity of the wave. For example, a laser rated at 5 mW (milliwatts or thousandth of a watt) would give off light at the same frequency and wavelength as another laser of the same type rated at 1 mw. But the wave heights of the 5 mw laser light would be five times higher than that of the 1 mw laser.

The wavelength is the distance from one crest to the next; this is also one cycle. It seems logical that we would need some constant measure of time in order to count the cycles. This constant unit of time is usually one second. Thus the term cycles per second, or cps, which is often referred to as Hertz or Hz (in honour of the German Physicist Heinrich Rudolph Hertz, who discovered radio waves). The oscillation frequency of electromagnetic radiation in the visible region is approximately 10 Hz. Wavelengths of visible light are between 400 and 700 nanometers or billionths of a meter in length.

We have described light as energy that travels through space in a wave form. For our purposes in talking about holography this is the case. However, the theory of light has unfolded miraculously, involving such great minds as Issac Newton, Thomas Young, Christian Huygens, Max Planck, Niels Bohr and, of course, Albert Einstein. Still, the dual characteristic of light remains one of the many puzzles of nature. The particle wave problem which we refer to, was clarified somewhat in the year 1900 when Max Planck proposed that all electromagnetic energy is radiated in discrete packages which he called quanta, or singular quantum. Einstein later confirmed Plank’s theory via the photoelectric effect and used the word photon to refer to these energy packages. Scientists today refer to light sometimes as particles (photons or quanta) and other times as continuous waves depending on the situation or experiment. The problem is not with nature but with our models or concepts of nature. It is always very important to remember not to let your idea or model of the way anything should be usurp the place in your mind of the way it is or might be. That place should always be open for new information whether it agrees with theory or not.

Light travels in a wave form. More precisely, a transverse wave form. The crests and troughs of the waves (which in the case of light are electromagnetic fields) are rising and falling in a direction at right angles to the direction of travel. A swell or wave in the ocean is a good example of transverse wave motion. You’ll notice that the rising and falling action of the wave is at right angles to the direction of travel.

A simple proof of the wave theory was first demonstrated by an English physician named Thomas Young in 1802. He forced the light from a single light source to pass through a narrow slit and then forced that same light to pass through two more narrow slits placed within a fraction of an inch of each other. The light from the two slits fell on a screen. Surprisingly he saw not just the simple accumulation of the light from both slits on the screen, but a pattern of light and dark lines. He believed the pattern was the result of the mixing of the waves of light emanating from the respective slits.

At that time, it was very difficult for the many justifiably avid fans of Issac Newton to incorporate this new discovery into Newton’s particle theory of light. Newton tried to explain optical phenomena such as refraction and reflection in terms of gravitational-like effects. As it turned out later, in a way, Newton’s theory was given partial confirmation by the Quantum Theory.

The lines or ‘fringes’, which Young saw, we call the interference pattern of the two light waves. When a crest interferes with a crest it is positive, or constructive interference, resulting in a bright spot. On the other hand, when a crest meets a trough we have a dark area or destructive interference.

As mentioned earlier, light waves oscillate at approximately 10 Hz, or a million billion times per second. There is no machine known to man sensitive enough to record the individual fluctuations additive effect of the light waves of which at each second 10 to the power of 15 wavelengths are interacting on the screen. This number, like so many numbers you may encounter in such fields as physics, astronomy and electronics, is incomprehensible. Yet, precise measurement is part and parcel of the advancement of science. Suffice it to say that one billion seconds, for example, equals roughly 30 years, and 10 to the power of 15 seconds is one millions times that.

Applications of Holography:

Holograms made with X-rays or ultraviolet light can record images of particles smaller than visible light, such as atoms or molecules. Holography’s unique ability to record and reconstruct both light and sound waves makes it a valuable tool for industry, science, business, and education. The following are some applications:

Double-exposed holograms (holographic interferometry) provide researchers with crucial heat-transfer data for the safe design of containers used to transport or store nuclear materials.

Holography is used to depict the shock wave made by air foils to locate the areas of highest stress. These holograms are used to improve the design of aircraft wings and turbine blades.

Holography is used to store data on disks. Holographic data storage is the best type of data storage technology that can meet the requirements of today and also tomorrow.

The author is a final year MCA student at Crescent Engineering College