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Reducing the Aberrations of Holographic Images of Underwater Particles by Using the Off-axis Scheme with Normal Incidence of Object Beam

In this paper, we describe a method of recording by combining back-illumination of in-line objects with an off-axis reference beam to produce low-aberration holograms. This is advantageous since at the reconstruction stage no extra means of compensation are required. Calculations have shown that the spherical aberration does not exceed 5% for objects from 100 to 200 mum at a wavelength of 0.532 mum, the longitudinal spherical aberration does not exceed 1%, and astigmatism does not exceed 1%. A series of holograms were recorded to confirm the above predictions. Two targets consisting of arrays of squares, triangles, hexagons, rectangles and circles with dimensions of 100 mum and 200 mum were fabricated on glass plates by photolithography. Holograms were recorded with the objects placed in air or in water at various distances. After processing, holograms were replayed at lambda=0.633 mum and at lambda=0.542 mum. More than 100 images were compared with the original targets for shape, dimensions and evidence of aberrations. Corresponding particles sizes differ by no more than 10%. The above experiments allow us to conclude that use the off-axis scheme with normal incidence of object beam on holoplates can provide a reduction of aberrations without any additional compensation at reconstruction stage.