Stability_of_the_Interference_Field

3.1.5 Stability of the Interference Field

To obtain high-quality holograms the stability of the holographic device during the exposure of the holographic plate must be ensured (Miler, 1978). If any part of the optical set-up during the exposure is in such motion that the interference pattern is overlapped from the maximal illumination to the minimal and vice versa, no interference structure will occur on the record and the storage medium will be equally darkened and unable of any image reconstruction after the development. This is the utmost case where a hologram is not possible to obtain.

In practice, for obtaining a quality hologram it is fully sufficient if the path change does not exceed the value of l¤10 or, at stricter requests the value of l¤20 of the wavelength of the light used during the exposure.

The limit displacements of the individual optical and mechanical components expressed in wavelengths of the light used during the exposure, that put the interference fringes in the plane of the hologram into motion, are shown in Fig. 3–17 (Abbramson, 1981).

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Fig. 3–17 Limit displacements of the optical chart components during the recording on the holographic plate

1 – laser, 2 – beam splitter, 31, 32 – mirrors, 4 – object, 51, 52 – spatial filters, λ – wavelength of laser radiation, 6 – hologram

The directions of maximal sensitivity towards displacements of the components are illustrated by the arrows. The displacements of object (4) and mirror (32) have essential influence on the quality. The displacement of spatial filters (51 a 52) has considerably minor influence. Their displacement from their relative optimal position leads to the decrease of illumination intensity of both the object and its hologram. Several authors mention also the requirement of keeping the temperature constant during the process of recording on the holographic plate (mainly in three-dimensional holograms).

If the selected construction of the table as well as of the optical and mechanical components to be used for holographic experiments do not guarantee sufficient stability of the interference field, the holographic experiments will probably not be successful. Therefore, it is suitable to ensure the inspection of the interference field stability, for example by the Michelson interferometer (Fig. 3–18).

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Fig. 3–18 Chart of Michelson interferometer for the stability of the holographic interferometer inspection

a) stable interference field
b) non-stable interference field

1– laser, 2 – micro objective, 3 – objective, 41, 42 – mirrors, 5 – ground-glass, 6 – beam splitter

The Michelson interferometer is composed of optical and mechanical components for common holographic experiments. The narrow beam coming from the laser located outside the holographic table itself is expanded and, by objective (3) transformed into the planar light wave. The semi-permeable plate (6) splits the expanded beam of parallel rays into two mutually perpendicular beams that are reflected backwards by mirrors (41), (42). A part of the first beam that passes through the semi-permeable plate (6) and a part of the second beam that is reflected from it will interfere with each other on the ground-glass (5). At suitable mutual turn of the mirrors the interference fringes of equal distance will occur on the ground-glass and we can easily observe their motion. Such a set-up of the Michelson interferometer is most convenient to create directly on the holographic table.

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