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25 holography lessons

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23. Adjustment of the copying scheme

Adjustment of the copying scheme is carried out in several steps.
1. Determination of a position of the photoplate before recording the reflection hologram.
2. Aligning of optical paths of the reference and signal beams.
3. Adjust the ratio between the reference and signal beams.
4. Calculation of the hologram exposure time. Because of importance of adjustment of the ratio between the reference and signal beams and importance of determination of the exposure time these operations are extracted and considered in a separate lesson.

Determination of a position of the photoplate for recording the reflection hologram.
By recording the transmission master-hologram itís practically impossible to position the object in a strictly fixed place relative to the photoplate. A person can involuntarily move before recording a portrait not to mention domestic animals, which at all donít understand the words "Attention please, Iíll just be taking a photoĒ. Even by shooting static compositions their position can depend on dimensions, content and conception of a producer. But on the reflection hologram the image should always be in the same position Ė near the centre of the photoplate. Transfer of the image forward out of the hologram plane also canít be quite arbitrary. By a significant transfer of the image forwards (more than 5 cm) sharpness of the moved forward part of the image begins to deteriorate even by illumination of the hologram by a halogen lamp. By a strong transfer of the image backwards (more than 10 cm) the same effect happens. As optimum the position of the image near the hologram plane is considered with location of the most important parts of the object for example the personís eyes by recording a portrait or flowers in a bouquet on the hologram plane where the image sharpness is maximum by any source of illumination. Therefore adjustment of the copying scheme begins from correction of position of the image restored by the transmission master-hologram in the plane of the photoplate.
As we had learned from the previous lesson the image restored by the transmission master-hologram is freely ďhangingĒ in the air and even ďupside downĒ that is itís turned upside-down in the vertical plane. This fact significantly embarrasses tune of the image on the photoplate. For preliminary adjustment a white screen is inserted into the frame instead of the photoplate and the image position is determined with the help of this screen. The frame for the master-hologram is fixed on the table in a strictly defined position, which is determined by the path of the restoring beam. Therefore itís sufficient to determine three co-ordinates of position of the frame 1, see fig., for the photographic plate relative to the frame 2 for the master-hologram: a - distance between the frames, b - the height of position of the frame relative to the table and c Ė displacement of the frames in the plane parallel to the plane of the master-hologram. If the master-hologram is copied for the first time itís necessary to make several preliminary copies and to remember the final position of the frame for the photoplate in order to place it at once into the required position. For fastening the frame of the photoplate there is a set of special washers in the complete set of the copying facility and these washers allow to lift the frame above the table surface to the required height with the accuracy of 1 cm and also there is a set of metallic angle bars with clamps for fixation of position of the frame relative to the frame for the master-hologram. The frame for the photoplate and the frame for the master-hologram should be extremely rigidly connected with each other because copying is carried out by the continuous action laser and the exposure time can reach up to 2 minutes by the maximum dimensions of the holograms 28x40 cm. Therefore the frames themselves and arrangements for their fastening are made of metal and the fastened pair of frames represents a sufficiently rigid and massive construction. After completion of setting up the frame for the photoplate preliminary adjustment of the reference and signal beams is carried out. At first the spatial filters 5, see fig. below, are removed and the not expanded beams are directed to the centre of the master-hologram and the photoplate by the mirrors 4. Then the spatial filters are set at their places and are adjusted as it was described in the Lesson 4. Operation of the spatial filters and quality of illumination of the master-hologram and the photoplate are verified with the help of the screens made of white cardboard. Illumination should be at most uniform over the whole field with minimum reducing of illuminance in the angles.

Compensation of the difference of path-lengths of the reference and signal beams.
Itís known that the coherence length of the He-Ne laser doesnít exceed 15-20 cm. If additional measures arenít taken the difference of path lengths of the reference and signal beams can approach to this value. This will bring to the following result: the contrast of the interference pattern will decrease and brightness of the holographic image will diminish significantly. And if the difference of path-lengths of the reference and signal beams exceeds the coherence length, the hologram wonít be recorded at all. Therefore equalization of the paths of the signal and reference paths is one of the most important operations of the adjustment process of the copying scheme. Itís impossible to do this for the recording scheme of the Denisyuk holograms considered in the first paragraph because both the photoplate and the object are illuminated by the same beam. For such a scheme division of the beams into the reference beam and the signal beam has a rather conditional character. In essence itís one and the same beam. Therefore by recording the Denisyuk holograms there exists a principal limitation relating to the depth of the scene being recorded. By the coherence length of the laser beam of 20 cm the scene depth canít exceed 7-8 cm. By the scene depth of 10 cm the difference of path-lengths of the reference and signal beams will be equal to 20 cm (double pass of the beam from the photographic plate to the background and backwards). At that the contrast of the interference pattern will fall down to zero and the background wonít be recorded on the hologram (it will look dark) even if it was painted by a white colour. There exists no such problem for the copying scheme being considered in this paragraph. The signal and reference beams are formed in the beam splitter 2, see second fig. of this lesson, and then they separately propagate along their optical paths marked with the green and blue dotted lines with the length values of L1 and L2 correspondingly and afterwards they meet again on the photoplate 8. At that the signal beam has the time to be transformed passing through the transmission master-hologram and reaches the photoplate already in the form of the restored image. For maximum contrast of the interference pattern the difference of path-lengths of these beams in the plane of the photographic plate should be equal to zero:

L1 = L2

This can be achieved quite simply through changing positions of the directing mirrors 4 on the paths of the reference and signal beams. In practice itís more convenient to displace position of the mirror located on the path of the reference beam. The procedure itself for measurement and compensation of the difference of path-lengths is rather simple. The elastic ruler is fastened to the casing of the beam splitter and the path-lengths of the recording beams are measured by turns. In case of need the fixture with the mirror 4 located on the rail of the optical bench is displaced to the required side whereupon the difference of path-lengths of the beams is checked again. Unfortunately such a procedure leads to displacement of the reference beam relative to the spatial filter 5. Therefore when the difference of path-lengths is finally compensated itís necessary to adjust successively the angle position of the mirror 4 and to tune the spatial filter 5 of the reference beam in order that the photoplate be again uniformly illuminated. -->