Construction of space-invariant fan-out holograms by copying Dammann grating onto dichromated gelatin

One of the components essential to optical computing systems is a spatially invariant optical interconnect. Such a device is necessary, for example, in the distribution of laser power to an array of optical logic gates. Although there are several possible ways of achieving this, the use of a Dammann grating provides a particularly attractive solution. A Dammann grating is basically a 2-D binary phase grating which, when used in conjunction with a focussing lens, produces an array of equally spaced spots. By optimising the grating structure it is possible to distribute equal power into a central N×N block of these spots. They thus have a component which performs an equal intensity, N×N fan-out. Advantages arise if one fabricates a conventional holographic copy of such a grating. For instance it is possible to combine the focussing lens and grating into one, thin film, element. This simplifies the complexity of any practical system. There also exists the possibility of modifying the original wavefront during recording, for example removing unwanted higher orders. The authors discuss preliminary experimental results obtained while attempting to fabricate such HOE´s