Laser

The term LASER is an acronym for Light amplification by Stimulated Emission of Radiation and describes a physical principle of optical amplification; not the device itself, even the term commonly has to serve as a description for the device.

There is a whole bunch of different lasers: gas laser, solid-state laser, dye laser, semiconductor laser, excimer laser, lasers for every visible colour of light, for ultra-viloet and infra-red light, laser for continuos operation and impulse laser (generates short light impulses, like flashlight). Even not each of the lasers is suitable for making holograms, they all have one thing in common: they generate coherent light, with radiation, behaving as going out from one single point. Vice versa the light rays can be concentrated back to one single point.

In holography usually Helium-Neon gas lasers (He-Ne) are used. The are quite cheap and easy to use. Its mechanics should be should be explained here.

He-Ne lasers consist of a glass tube, which is filled with a mixture of Helium and Neon under low pressure. The tube is sealed. On both transparent ends is a mirror attached. One reflecting light 100% and one reflecting light 98%. First the Helium atoms gets stimulated by energy input. They bump into the Neon atoms. Thereby they get transferred into a higher energy state. When falling back into their previous energy state, they set photons free. These photons stimulate surrounding atoms, and so forth. The emerged light begins to move between the two mirrors back and forth. For the two mirrors are fitted exactly many times over the wave length off each other, the amplitudes adds form wave to wave and amplifies. Finally light becomes so bright, that it passes out as coherent light the laser on that side with 98% reflecting mirror.

Fig. Gas laser

Gasentladungsröhre: Glass tube

Spiegel: Mirror

halbdurchlässiger Spiegel: transparent Mirror

Spannungsquelle: voltage source

Laserlichtbündel: bunch of laser light

Coherence

An Interference pattern can not be generated by random light source. Ordered conditions of illumination are needed.

To illustrate this, the example of two stones in entry on interference hitting the surface of the water should assist once again. A constant pattern solely occurs under the condition of synchronization and phasing. For the stones wouldn't hit the water simultaneously or for several stones would hit the water uncontrolled, of course wave fronts would meet and join to a new wave front, but this one wouldn't be constant. This behaviour of waves goes for both, water and light waves.

Viewing figures Partition of white light into different-coloured spectra by prism and Wave lengths of visible light against this backdrop, it becomes obvious that sun light or the light of a candle or an ordinary lamp - like any other light occurring in nature - is just very limited or even not at all suitable to generate constant interference pattern. All these lights consist of several light colours with several different wave lengths. There are superpositions, but they are high complex and chaotic, that no observable interference pattern could be generated with it.

This special property light has to have to generate such an interference pattern is called coherence. Coherent light is ordered and sorted light, which does not propagate in all directions in space, but in just one direction and that consists of waves with just one wave length, i. e. which is monochromatic. As yet is the laser the only source of coherent light.

Light

As fundamental light is for the recording and the reconstruction of a hologram, as fundamental is the knowledge of what light is and the understanding of how it works. The coming posts are about:

• history of light
• wave nature of light (amplitude, wavelength, frequency)
• interference
• coherence
• diffraction
• laser

Even these coming entries are not about phantasms, they would like to provide a service giving everyone an understanding of the properties of light, who isn't much into it or into physics and mathematics.

Here is a track called Lightwave by VNV Nation, going perfectly with the entries to come - by title and even musically.