Multiple imaging by means of point holograms.

TLDR: This communication proposes to use an arrangement from that in Ref. 4, such that the multiplied images of the multiple imaging device do not have to be transmitted by any additional optical component.

Abstract: Multiple imaging devices are capable of producing simultaneously a number of images of one object. Several methods have been suggested to realize such an optical system. They make use of the imaging properties of a fly's-eye lens or of an array of pinholes, or they utilize birefringence. More recently, multiple image generation has been achieved by forming the convolution of the object with an array of δ functions in an ordinary spatial filtering arrangement. The complex filter is realized by the Fourier transform hologram of the cor­ responding array of point sources. The second Fourier trans­ form lens, however, has to transmit more than twice the full information of the multiplied image array. Since the most im­ portant application of multiple imaging is semiconductor device fabrication as has been discussed earlier, the resolving power has to be very high, so that up to now no lenses are available that are capable of imaging the multiple array of images of a usual etching mask. I t is for this reason that to date, images of etching masks are transferred to the semiconductor wafer by contact printing. In this communication, we propose to use an arrangement dif­ ferent from that in Ref. 4, such that the multiplied images do not have to be transmitted by any additional optical component. Par t of this communication has already been mentioned in a paper presented at a holography meeting organized by the Battelle Institute at Frankfurt /M, Germany, on 2 December 1967. The principle of operation is illustrated by Fig. 1. The hologram of the array of point sources is produced in the usual way by recording the interference pattern caused by coherent superposition of the spherical waves emanating from the ref­ erence point and the signal points the spatial positions of which correspond to the desired positions of the images to be formed. Multiple imaging is then performed in the reconstruction process by replacing the reference point source by the real image to be multiplied. This can be achieved in the well-known way by means of a lens that images the object through the hologram into the surrounding of the reference point. In this case, an additional real image of the object is formed in the surrounding of each signal point. The behavior of the hologram can be easily understood by thinking of a point hologram as of a generalized Fresnel zone plate with its well-known lenslike properties. Indeed, this hologram of the distribution of point sources may be considered as some special kind of a fly's-eye lens, in which the lenses are replaced by Fresnel zone plates. But there is a rather important difference. Contrary to lenses, the single point holograms may overlap completely without disturbing each other. For this reason, the aperture of each point hologram determining the diffraction limited size of the image point can be made equal to the aperture of the hologram of the whole point distribution provided that suitable spherical waves are used in the recording process. There is no relationship between the diameter of this aperture and the distances of the images to be formed. Limitations to the resolution of this multiple imaging device are mainly introduced by aberrations. In the special case of small plane objects, which are of dominant interest in semicon-