Transmission uniformity of diffractive parallel optical interconnect relays: a numerical analysis based on rigorous coupled wave theory

Free-space optical interconnects have been proposed as a possible solution to the interconnection bottleneck in high performance electronic systems at the board-to-board and chip-to-chip level. The optical design is shown of a VCSEL-based bi-directional optical interconnect system that has recently been implemented This clustered optical system transmits 256 channels in each direction and is based on a diffractive double minilens relay. The relay lenses have a square aperture of 750 μm, a focal length of 8.5 mm, 256 phase levels and operate at a wavelength of 850 nm. Each lens relays a 4 × 4 spot array on a 125 μm pitch. This system was originally designed using scalar diffraction theory and so it was assumed that the diffraction efficiency is a function only of the number of phase levels in the lenses and is thus identical for all beams in the array. However since each beam within the 4 × 4 array passes through a different set of zones in the lenses and since the outer zones of the lenses have smaller local periods than the central zones the diffraction efficiency experienced by each beam will not be identical, leading to transmission non-uniformity. In this paper we will apply rigorous coupled wave analysis (RCWA) to this optical system design in order to accurately determine the efficiency of the minilens as a function of the zone radius and hence calculate the variation in transmission efficiency that will result. We will also extend this analysis to faster diffractive optical systems (f/3.7) that relay larger (16 × 16) spot arrays and investigate polarization dependence.