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25 holography lessons

Glossary FAQ


4. Pinhole

It is necessary to expand a laser beam for the object or photoplate to be exposed. It is better to use for this purpose 10-fold, 20-fold and 40-fold microscope objectives. The greater fold of the objective (the less focal distance), the greater expansion of the beam. However, if there are some specks of dust on the lenses of the objective, the laser beam diffracts and partly scatters. In this case parasite interference pattern appears in the form of contrast concentric rings, see photo. This pattern disturbs homogeneity of the photoplate's or object's exposure and quality of the recorded hologram deteriorates. A pinhole is used to eliminate this lack. It is a diaphragm with a small hole located in the objective focal plane, see fig. Profile of the laser beam is described by Gauss function, it is not focused as a dot in the focus of the microscope objective (this is idealization of geometric optics), but it is shrunk up to a definite small size. Light scattered on the specks of dust will spread at an angle with an optical axis, so that it will be collected at some distance from the axis in focal plane. The size of hole is selected in such way the main laser beam passes through the hole in the diaphragm and light scattered on the specks is blocked. In that case a clean homogeneous beam will come out of the pinhole. Practice of the pinhole usage presents the following approximate dependence of the diaphragm hole size and the objective fold:

- 30 micron is for 10-fold objective.
- 20 micron is for 20-fold objective.
- 15 micron is for 40-fold objective.

Quality of the objective and precision of the diaphragm manufacture are very important for proper work of the pinhole. Presence of the objective aberration and non-round form of the diaphragm hole can dramatically worsen filtration of the laser beam. It is better to produce the diaphragm hole by means of lithography on the thin copper foil and check quality of the hole using a microscope. However, some virtuosos make a hole on the aluminium chocolate foil with a sharp needle. To adjust a pinhole precisely, the diaphragm should move along the optical axis and two coordinates perpendicularly to the optical axis by means of micrometer screws, see photo.
Adjusting of the pinhole is executed in the following way. Direct the laser beam to the centre of the photoplate (or to the centre of the object). Press a glass plate to the face of the microscope objective on the side of the laser and fix it in such way that the laser beam incidents on the centre of the input lens and the beam reflected from the glass plate returns back into the laser. (This operation is centering of the objective along the optical axis.) Then move the diaphragm along the optical axis and place it near the objective's focal plane. If the diaphragm moves very close to the focus you can see a "coarse grain" reflection of the beam from the foil. Reflected light has a very mobile granular structure. Then take a piece of white paper, turn off the light and move the diaphragm in the plane perpendicular to the optical axis. Try to "catch" the beam passed through the hole. The first time may be not successful, you need some experience. At the beginning of tuning, the beam shape behind the pinhole will not be ideal because the diaphragm is not in the objective focal plane and the beam is cut with the hole edges, see photo above. You should adjust the diaphragm position thoroughly and obtain complete passing of the beam through the hole. The beam will have round profile without edge effects, see photo.

R. J. Collier, K. B. Burckhardt, L. H. Lin "Optical Holography", Academic Press, New York, 1971.