Free-space optical link realized with microlensed components

Higher computer clock speeds will require alternate technologies to overcome the performance limitations of backplane electrical interconnections. One such method is to use parallel free-space beams for board-to-board interconnects. We demonstrate a free-space optical link using 980 nm vertical-cavity lasers (VCLs) as transmitters and back-side illuminated double-pass Schottky diodes as receivers. These devices are integrated on-chip with refractive microlenses, resulting in components that can be used directly in systems, without the need for external optics. A single-mode dielectrically-apertured VCL of diameter 3.1 μm integrated with a microlens, has a far-field divergence half-angle of ~1 degree, allowing for an interconnect length of ~5 mm. VCLs of this size have bandwidths ~15 GHz at powers ~1 mW, suitable for high-speed optical interconnects. We have studied the tolerance of the free-space link to mechanical misalignments and to fabrication variations by evaluating the power throughput and crosstalk from adjacent channels positioned on a 250 μm pitch. The misalignment tolerances were also evaluated experimentally. We achieved data transmission at 400 Mbit/s with bit error rate (BER) <10^-12 through the free-space system with microlensed components. The data rate is presently limited by the packaging, not the inherent bandwidth of the VCL. We have also demonstrated data transmission at 3 Gbit/s with BER<10^-12 by launching signal from the microlensed VCL directly to a fiber-coupled high-speed receiver