Integration of 1D planar silica GRIN lenses in 3D optical interconnect system

Abstract

Reported is the integration of 1D planar silica graded index (GRIN) lenses in a 3D optical interconnect system to reduce free-space propagation loss. The 3D optical interconnect system consists of a pair of facing-up and facing-down 458 micro-mirrors that establish a vertical optical transmission path between stacked dies. Without the silica lens, experimental measurement shows optical power loss of 33.7dB. A pair of GRIN lenses is then integrated into the system. Optical test shows that the system performance is improved by more than 25dB.Introduction: In 3D optical interconnect systems, out-of-plane optical propagation in free space is unavoidable to create optical paths between two planar surfaces at different planar levels. Techniques for minimising the free space propagation loss by interposing various optical focusing elements include the use of a micro ball lens [1], fluidic membrane lens [2], a planar PDMS lens [3], and a polymer lens [4]. However, these methods require separate assembly processes and are not compatible with planar waveguides. To address this issue, a novel planar silica lens was proposed [5] and fabricated [6]. It consists of a parabolic refractive index profile in the silica film to focus light in the vertical plane, as well as a convex front face to focus light in the horizontal plane. To demonstrate its application in 3D optical interconnects, the micro-fabrication process challenges in integrating such a planar lens into the 3D optical interconnect system have to be overcome. These challenges stem from the different fabrication process strategies developed to form the planar lens and 3D optical interconnect system separately. In this Letter, we report that the fabrication challenges have been overcome and demonstrate the integrated system for the first time with promising optical performance. The fabrication and optical performance of a 3D optical interconnect system with only facing-up and facingdown 458 micro-mirrors will be discussed first. This will be followed by the fabrication and optical performance of the 3D optical interconnect system integrated with planar silica lens.

Optical Fibre on a Silicon Chip

“…Planar silica lens pair discussed earlier in this chapter is well suited for such purpose. The integration of this planar lens in the 3D optical interconnect system has been demonstrated [72].…”