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25 holography lessons

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16. Adjustment of the optical system

Adjustment of the optical arrangement of the pulse holographic setup can be conventionally divided into two parts tuning of the pulse laser and adjustment of the illuminating light beams.

Tuning of the pulse laser is executed if the laser light beam energy has decreased or distribution of intensity across the beam section has deteriorated. Testing is executed in the following original way. Its known that if the ordinary photographic paper or film is exposed by a very intensive light beam reduction of the metallic silver will take place without any development (photolysis). This property of a photographic paper allows very easily verifying quality of the laser beam both inside the laser by its formation in the infrared range and at the output from the laser.
A strip of photographic paper with a width of 3-4 cm is cut out and it is placed on the path of the laser beam. When the beam hits a photographic paper, a characteristic click is heard and a dark spot - a trace of the beam appears by which both the form of the beam section and approximately its energy could be easily determined. In the fig.1(a, b) the photos of the beam in the most important points at the output from the master oscillator and at the output from the laser are shown, and at that the photographic paper at the output from the laser was exposed from the side of the substrate because of a great energy of the output beam otherwise the beam structure would be lost because of over-exposure. In the fig. 1(c) the beam with a disturbed distribution of intensity because of dustiness of the output mirror of the master oscillator resonator is shown. It's impossible to record a high-quality hologram using such a beam and careful wiping the appropriate laser element is needed. In this case measurement of the laser beam energy should be executed with the help of a special instrument measuring the light pulse energy.

In contrast to the Denisyuk scheme where only one writing beam is used various variants of the object illumination are possible in the pulse setup. For a standard shooting e.g. portraits or classic compositions its sufficient to form three beams one reference beam and two beams illuminating the object. Scheme of formation of the reference beam is shown in the fig. and includes the following elements the pulse laser 1, the beam splitter 2, the mirror 3, the spatial filter 4, 5 and the lens system 6 (the second highly aperture lens is fixed on the shooting cabin, and it can be seen in the third fig. of the lesson 13) for magnification of the beam sizes up to the plate format. The beam splitter 2 represents a plane-parallel glass plate. It means that only several percent of the laser beam energy are spent for formation of the reference beam. The rest of energy is used for illumination of the object. The spatial filter has an unusual structure the lens 4 has a focus of about one meter. Its connected with the fact that for a short-focus length or for a microscope objective the energy density of the beam in the focus is so high that an electric disruption of the air can occur. In this connection the laser beam characteristics (spectrum, coherence) deteriorate abruptly. Accordingly the diameter of aperture of the diaphragm 5 of the spatial filter is rather big 1 mm. By the same reason the first lens 6 of the beam expander is negative and doesnt focus the laser beam at all. Adjustment of the reference beam is reduced to testing operation of the spatial filter and position of the reference beam on the photoplate. The beam should pass through the diaphragm orifice exactly in the centre not touching the edges and should uniformly illuminate the photoplate plane. Scheme of formation of the reference beam is located on the same table where the pulse laser is situated, see photo above. Scheme of formation of the beams illuminating the object is shown in the fig. above. The splitter 1 divides the beam in two beams of approximately equal intensity, which are expanded by the negative lenses 2 and mirrors 3 and directed to the glass diffusers 4. The light diffused by the diffusers illuminates the object being shot from two sides. Scheme of formation of the beams illuminating the object is located on the special table on the front wall of the shooting cabin, see photo. -->