Optical Radiation Sources in Holography

2.3 Optical Radiation Sources in Holography

Holography uses light interference for recording of three-dimensional images, therefore, it requires the use of coherent light sources whereas the whole device must be compact, protected against shakings from its surroundings and usually requiring the obscuration of the working site. Holography needs "ideal" optical sources with a high level of coherence to observe the interference and light diffraction phenomena. Both the space and time coherences are concerned. The more pointed the source the bigger the space coherence, and the more monochromatic the light the bigger the time coherence. The ideal monochromatic source is the laser – the generator of electromagnetic radiation in the optical area of the wavelengths.

Lasers are the sources of light characterised by large intensity, monochromaticity, coherence and small divergence of radiation. The abbreviation laser is derived from the English name of Light Amplification by Stimulated Emission of Radiation, which represents the device where the light amplification by stimulated emission occurs.

The laser consists of three basic parts: work (active) surroundings, exciting source and resonance system.

The laser ensures both the time and spatial coherences that make it possible to obtain a stable and distinct structure in the plane of the hologram. It also ensures a sufficient performance corresponding to the light wavelength and measured in any point and any time interval.

The formerly manufactured lasers produce only light of a single frequency (wave-light), therefore, they produce monochromatic light. At present, for example the Coherent company manufactures lasers producing "white" light. Moreover, the laser radiation coming from a laser is coherent (all waves are mutually in a phase). It means that the light propagates from its source by regular waves, therefore, it remains concentrated in a narrow beam even to long distances.

In our experiments we used the helium-neon laser (He-Ne) that radiates red light with the wavelength of  0,6328.10^–6 m, with continuous exposure. It works with the performance of several mW (15–100 mW). The coherent wavelength of He-Ne lasers is about 15 to 30 cm.