Fabrication of 45-Degree Integrated Mirrors for Three-Dimensional Board-Level Optical Interconnects Realizing Efficient Light Coupling

Efficient (low-loss) and robust technologies for optical in- and out-coupling from integrated waveguides at different interconnection levels is a prerequisite. A suitable beam delivery scheme is required in out-of-plane coupling configurations to reduce the losses originated from the beam divergence along the free space optical path. In this work, to realize a high coupling efficiency of inter-chip connection for three-dimensional (3-D) optical interconnects, mechanical dicing process was conducted to fabricate out-of-plane mirrors for light beam turning. A 45-degree optical interface for light 90-degree beam turning into/out of the waveguide with up-turned mirrors were demonstrated. Characterizations of the surface topographies and sloping angles revealed that relatively smooth 45-degree interface was achieved. Surprisingly, the angle deviation of the 45-degree reflector was within ± 1 -degree, and the surface roughness was approach to 170 nm. A theoretical model specified for the optical link was developed to theoretically calculate the link loss of the 3-D optical interconnects based on the waveguide with 45-degree mirrors. The coupling loss of the 45-degree interface at the Tx and Rx side was experimentally investigated. The results showed that the total link loss was 7.77 dB for a 110-mm-long waveguides and the excess loss calculated for the mirror coupling was 2.56 dB for the Tx side and 2.86 dB for the Rx side. Besides, the Fresnel reflection originated from the horizontally fiber and the 90-degree end face was 0.7 dB. Misalignment tolerance results indicated that the fabricated 45-degree end face beam turning structure has relatively loose alignment requirements for the optics in the X-direction. The measured of 1 dB lateral alignment tolerance amounted to ± 17 µm in the X-direction. The functional optical performance indicated that the up-turned mirrors were suitable for practical printed circuit boards (OPCB) applications where an optical I/O-interface to couple light in or out of waveguides to surface-mounted VCSEL or photodiode was required. This work presented here also provided a promising technology to implement optical interconnects on PCBs, for realizing the anticipated high coupling efficiency 3-D interconnected optical polymer waveguide links.