By Christopher Outwater & Van Hamersveld

Now that we know a little something about light in general, we may consider the light source needed to perform holography: the laser, which stands for light amplification by stimulated emission of radiation. The understanding of the stimulated emission of light, or how a laser works, will greatly aid in conceptualizing the holographic process.

Without the laser, the unique three dimensional imaging characteristics and light phase recreation properties of holography would not exist as we know them today. Two years after the advent of the continuous wave laser, c.1959-1960, Leith & Upatnieks (at the University of Michigan) reproduced Gabor's 1947 experiments with the laser, and launched modern holography.

A laser is a light amplifier, with very special characteristics. The laser was designed and made to work after two very useful theories had come on the scene. One is Niels Bohr's atomic theory and the other is the Quantum Theory. Niels Bohr, a Danish physicist, in the year 1913, proposed a model of the relationship between the electron and nucleus of the hydrogen atom. Bohr utilized the newly developed Quantum Theory in proposing that an electron circling the nucleus can assume certain discrete quantized levels of energy. In the lowest level, called the ground state, the electron is circling closest to the nucleus. However, if the atom is exposed to an outside source of energy the electron can be raised to a higher energy level, or an excited state, which is characterized by the electron carving a circle of greater circumference around the nucleus. It is important to note that the electron can't go just anywhere when it is excited but has to assume certain levels. Also, not just any energy would suffice in raising the electron's orbit. The energy must be equal to the energy difference between the ground state and the excited state the electron assumes. The frequency is the energy difference divided by "h" or Planck's constant. There are actually a number of different energy levels which the electron may assume but that is not essential to this explanation of how a laser works.

Energy is radiated in discrete packages, and these packages interact only on a very selective basis. There are two important reasons why lasers work. The laser depends on the very special emission characteristics of certain atoms whose electrons have been raised to the excited state. When the electron falls back down to its lower energy level (as all electrons eventually do), it in turn emits a package of electromagnetic or radiant energy which precisely equals the energy difference between the two levels, ground state and excited state. In a sense, what goes in comes out. This fact alone doesn't suffice in making a substance lase, for if too many electrons are in the ground state, the energy input would merely be absorbed by the electrons in the ground state which then might spontaneously emit a quantum of the correct size sometime in the future and that would be the end of that. We don't want to have an atom emitting its photon at just any old time, so we stimulate the atom to emit its energy package when we want it to. A package which would not be absorbed by another atom in ground state but would stimulate an atom already in an excited state to emit its own photon. In order to maintain the stimulated emission of photons which produce laser light, you must initiate and mantain a population inversion.

In lasers, electronic principles are applied to the visible portion of the spectrum. In electronics, oscillation is achieved with feedback around an amplifier. The feedback circuit determines the frequency of oscillation. In a laser, the tube of excited atoms is the amplifier. The mirror or resonator is the feedback circuit. Oscillation occurs at those wavelengths where the product of gain equals the loss, for a round trip, say starting from one mirror and coming back again. The gain of a laser is determined by population inversion, or having many more excited electrons, than electrons in the ground state i.e. electrons at their lowest energy level.

The helium-neon laser, which is probably the most common laser in use today (due to its relatively low cost) is the laser you will probably use most. The laser tube itself contains approximately 10% helium and 90% neon. Of these two inert gases, neon is the active agent in the lasing process. We could term helium the catalyst insofar as it facilitates the energy input to the neon. Before more energy is purposefully forced in the system, there is some action among the atoms and molecules comprising the gases. Some although very, very few of the electrons are already in the excited state, or upper energy levels and when they fall down, as they all tend to do, they emit a photon, only to be quickly absorbed.

The gain or loss of a photon or quantum of energy which is defined by a change in electron orbit takes place on the order of 10 to the power of 15 seconds or 100 millionths of a second.

You might ask how even some of the atoms might have electrons in the excited state if there's no energy input, i.e., before the laser is switched on. The answer is purely statistical. For example, if you have a church filled to capacity for a Sunday morning mass, say 250 people, someone has got to cough or sneeze during the sermon. If you take the number of times some two or three people cough and compare that with the amount of times everyone in the curch inhaled and exhaled without occasion, it would give you some idea of the situation in the laser tube before excitation. A few atoms are excited and then fall back to emit energy. This energy in turn goes off spontaneously to another atom whose electron almost certainly is in the ground state. The photon is absorbed. This is the key to the laser. If we have enough atoms with electrons in the excited state, the photon not only would not be absorbed, but when it did reach another excited atom it would induce it to cough up its own photon. We go from one, to two, to four, to eight, to sixteen photons very rapidly. We have achieved population inversion, i.e., many more electrons are in the excited state than in the ground state.

Remember we are considering only the helium-neon laser. It is the most economical laser and probably the one you would be using. There are other lasers such as the argon-ion laser which is able to lase in both blue and green, and better yet a mixed gas argon-krypton ion laser which is able to lase in blue, green and red. The problem is that the prices of these lasers begin at around $6,000. If you have access to these lasers, you probably would not be reading this guide anyway.

There is also the pulse ruby laser which allows you to make holograms of animate objects. In the ruby laser chromium ions locked in a sapphire host are the sources of stimulated emission. The chromium atoms are excited by a light flash from a special flashlamp.

Let's backtrack slightly and talk briefly about the job helium performs before we go on to the more mechanical aspects of laser operation. It so happens that helium has a metastable (or long lived) energy level that coincides quite well with on of the energy levels of neon which we need to obtain for lasing action to commence. Scientists discovered that it is much easier to raise helium to the excited state and let it transfer the correct energy packets to the neon when they meet inside the tube (which is at the correct pressure to assure their close acquaintance). So the helium is used as sort of a messenger, or filter, if you will, to store the correct high energy input origination from the laser power supply for the neon. Although the neon is the active ingredient in the laser, the helium greatly facilitates the process.

Virtually all we have so far, then, is a glorified light tube such as you might find lighting the streets of any late night hot spot worth its salt. The difference from this light tube stage of development to the functioning laser is essencially more of a mechanical characteristic, i.e., the precise geometrical realtionship of its optical components.

The photons are emitted from the atoms inside the tube in all different directions. However, a very small percentage, around 2%, begin traveling in a horizontal direction within the tube. They naturally stimulate already excited atoms along the way to emit their photons in the same direction. This would actually mean nothing if we did not then place mirrors at both ends of the light tube in order to induce the light to start moving back and forth along the horizontal line of the tube. Eventually this induces a large number of photons to travel in the same direction and one of the mirrors is only partially reflective which lets the light leak out.

Some of the characteristics of laser light were introduced earlier. Now we should be able to discuss the properties of the laser with this further explanation: The source of the light is the energy given off when an atom's electron falls back down toward the ground state. There is only one type of atom taking part in the actual coherent, laser light giving process; therefore, according to the law of Quantum Mechanics the energy given off by identical energy shifts in each atom must be exactly the same. In other words each photon has precisely the same amount of energy. It will also have the same frequency and wavelength, and will be coherent light. It is the mirror set-up, sometimes called the resonant cavity, which induces this fully saturated, monochromatic light to exit the tube in a straight, narrow beam, for then not only do they contain the same amount of energy in stimulated emission, but the photons travel in the same direction.

Actually the precise wavelength emitted by a laser is determined by the mirror separation: The lasing transition gives a band of wavelengths over which the laser can emit.

The diameter of the exit beam varies with bore of the tube but most helium neon beams are around 1.5mm diameter at exit and do not spread nearly as quickly as incoherent light would.

Thus laser light is coherent because it is radiated by a homogenous collection of atoms under precisely the same conditions. The mirrors at both ends make the small percentage of photons that hit the mirrors return in a straight line. This develops a cascade of light along the horizontal line of the tube. If you were to remove the laser casing you would see the same monochromatic, saturated light but the straight beam, so distinctive of laser light, would only be emitted from the end with the partially coated mirror. Now let's go on to see why we need these properties of coherent light and how we use them in holography.