A phase and amplitude, off-axis hologram has been synthesizedfrom three computer-generated transmission masks, using a multiple-exposureholographic recording method. Each of the masks controls onefixed-phase component of the complex hologram transmittance. The basicgrating is generated optically, relieving the computer of the burden ofdrawing details the size of each fringe. The maximum informationcapacity of the computer plotting device can then be applied to thegeneration of the grating modulation function. By this method largedigital holograms (25 mm by 25 mm) have been synthesized in dichromatedgelatin. The recording method is applicable to virtually any holographicmedium.
The modulated grating hologram was designed primarily for theapplication of spatial filtering, in which the requirement is a hologramwith large dynamic range and large free spectral range. Choice of alow-noise, high-efficiency medium such as dichromated gelatin will allowexceptionally large dynamic range. Independence of the optically-generatedcarrier grating from the computer-generated modulation functionsallows arbitrarily large free spectral range.
The performance of a holographic spatial filter will be limitedultimately by noise originating from imperfections in the holographicmedium. The characteristics of this noise are analyzed, and in thecase of a high diffraction efficiency hologram are shown to differ significantlyfrom previous analyses. The dominant noise source in hologramsof high diffraction efficiency will be scattering of the first orderor imaging wave by deformations in the hologram surface or othereffects of low spatial frequency. Experimental measurements in variouslow-noise holographic media verify these predictions.
