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

Wave length

The wave length λ (in nanometer - nm) defines the colour of light. The relation between colour of light and wave length is similar to the relation between intensity of light and amplitude. Therefore different colours of light do have different wave lengths. The human eye can perceive light with a wave length between about 400 nm and 700 nm.

"Lichtquellen, wie die Sonne oder die Glühlampe, senden ein Gemisch aller möglichen Wellenlängen, d. h. Farben aus. Ein derartiges Gemisch von Farben empfinden wir als weißes Licht. Aber selbst, wenn uns Licht einfarbig grün oder rot erscheint, wie etwa das Licht einer Verkehrsampel, enthält es normalerweise noch mehrere, wenn auch nur wenig unterschiedliche Wellenlängen."

[Light sources like the sun or a bulb emit a mixture of all kinds of wave lengths, i. e. colours. Such a mix of colours occurs to the human eye as white light. But even light occurs plain green or red - like light from a traffic light - it normally contains several (although few) different wave lengths.]

Fig. Partition of white light into different-coloured spectra by prism

As the first ever Newton carries out a test as shown in the figure above and publishes his observations in 1704 in his book about optics. About 200 years later physicists like Niels Bohr and Arnold Sommerfeld succeed in assigning different colours of light to different spectral lines with specific wave lengths.

Fig. Wave lengths of visible light

Therefore red light (650 nm, or 6.500 Å - Ångström) is long-wave light, while violet light (430 nm, or 4.300 Å) is short-wave light. Light with a wave length of more than 650 nm is called infra-red. Light with a wave length of less than 430 nm is called ultra-violet. Light of the helium-neon gas laser - which plays an important role later - has got a wave length of 623,8 nm and appears to the human eye as red light.

Every single lightwave illustrated in Fig. Wave lengths of visible light is monochromatic - consists of just one colour of light (respectively has got the same wave length). As coming entries will show, is "[...] monochromatic light [...] of basic importance in holography [...]".