25 holography lessons Glossary FAQ

5. Beam polarization

It is well known that light which is reflected from the glass plate depends on the angle of incidence: the larger angle, the larger degree of reflection: from 4% at a normal incidence up to 100% at sliding incidence, when the beam is practically parallel with the plate, see the graphical chart, upper curve.
Laser radiation is coherent and polarized. Light reflection in the photoplate for coherent light has very complicated nature because of multiple inner reflections, see fig. below. The beams which are reflected from the lower and upper planes of the photoplate interfere and this causes parasite interference pattern that makes worse the picture. Since there are some small variations of photoplate's width, this pattern looks like strips on the tree cut and this picture is called "dendriform", see the photo below.
Vector of the laser beam polarization is located in the plane which is perpendicular to the direction of beam spreading. If this vector of polarization is located in the incidence plane (the incidence plane is the plane formed by normal to the photoplate and the beam axis), there is a special angle - Brewster angle. The beam reflections from the photoplate disappear for Brewster angle, see the graphical chart, lower curve. The beam passes through the photoplate without reflection! Brewster angle depends on refraction index of the photoplate's glass substrate and it is calculated according to the following formula:

tg(A) = n,

where n is refraction index of the photoplate, A - angle of the beam incidence in air.
Refraction index of the glass, which is used for the photoplate substrate, is equal to 1.52. Gelatine layer has approximately the same refraction index. The equation gives the result:

A = 56.60

Reference beam should fall on the photoplate exactly under this angle, in order to obtain pure holographic image, see figure.
It's not difficult to adjust Brewster angle. You may use polarizer and determine the plane where is located the vector of laser polarization. If you need, it is possible to turn the plane by rotating the laser itself (if its design allows this) or turn the plate of polarization using a semi-wave plate. In our case, polarization vector of He-Ne laser radiation is located in the vertical plane therefore the recording beam is formed in the vertical plane too (see Lesson 1). Then put an ordinary glass plate under the laser beam and observe reflected light on a sheet of white paper. Black paper should be placed under the plate, and the light in a room should be switched off, because reflected light is not intensive. If you change the angle of photoplate illumination, you'll see that intensity of reflected light varies also. Brewster angle is the angle under which intensity of reflected light will be minimal or disappear completely.

References
M. Born, E. Wolf, " Principles of Optics", New York.
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