Multilevel Phase Structures For Array Generation

Array generators are optical systems which split a single beam from a laser source into a one- or two-dimensional array of beams. We are interested in the generation of spot arrays for illuminating two-dimensional arrays of optical logic devices in an optical digital computer. A variety of solutions to this problem has been offered, among them the use of computer generated diffraction gratings [1,2,3], holographically produced lenslet arrays [4] and phase contrast methods based on spatial filtering [5]. Binary Dammam gratings and arrays of lenslets have also been employed by Veldkamp et al. [6,7] for the inverse task of coherent coupling of the output beams of several laser diodes. In the following, we want to discuss two different techniques for array generation. The first one is an extension of the theory of binary phase gratings to multilevel phase gratings. The use of multiple phase levels helps to increase the efficiency of the array illuminator. The second method employs lenslet arrays. However, as opposed to the method described in reference [4] in which the lenslets simply focus an extended laser beam down to many small spots, we perform an optical Fourier transformation of the lenslet array. This technique offers the possibility to generate very small spots because the full aperture is used. It also helps to reduce the problem with the nonuniformity of the array due to the effect of the spatially nonuniform intensity profile of the illuminating laser beam. Both techniques will be discussed in more detail in sections 3 and 4. First, we start with a short description of the fabrication of multilevel phase structures.