For optical neural networks implemented with computer-generated planar holograms the space–bandwidth product of the hologram is a major consideration. Off-axis holograms can be fabricated with a single binary transmission mask. However, the carrier frequency greatly increases the space–bandwidth product. On axis-holograms use a lower space–bandwidth product to encode interconnections but require a multilevel phase transmission profile. Significant errors can result during the fabrication of multilevel phase structures. With modification of the on-axis geometry the effects of the fabrication errors can be reduced while a lower space–bandwidth product per interconnection is retained. The interconnection accuracy, the diffraction efficiency, the and sensitivity to fabrication errors are compared for the off-axis, the on-axis, and the modified on-axis diffraction geometries.
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