High-speed interconnects are highly demanded in data centers and high-performance computing, and the use of optical interconnects to replace electrical interconnects has been proposed and widely studied. For medium-range board-to-board communications, free-space based optical interconnects provide the additional advantage of reconfigurability and flexibility, compared with traditional waveguide or fiber-based counterparts. However, the bit rate and interconnection range in free-space optical interconnects are typically limited. In this paper, we propose a carrierless-amplitude-phase (CAP) modulated and space-time coded free-space optical interconnect scheme to overcome these limitations. Up to 80 Gb/s (2 × 40 Gb/s) interconnection with reconfigurability and flexibility is experimentally demonstrated for a maximum error-free distance exceeding 50 cm, showing about 65% improvement over previous demonstrations. The receiver sensitivity of the space-time coded system is measured as well, and results show that the space-time-block code (STBC) is capable of improving the receiver sensitivity and suppressing the inter-channel crosstalk. The impact of air turbulence, which exists in typical interconnect environments, is also experimentally investigated and results show that its impact on the proposed scheme is negligible. The proposed reconfigurable free-space optical interconnects with CAP and STBC can be further scaled up to provide higher speed and further extended interconnection range.

High-Speed Reconfigurable Free-Space Optical Interconnects with Carrierless-Amplitude-Phase Modulation and Space-Time-Block Code

We experimentally demonstrate an elliptical-aperture multimode diversity receiver to mitigate the impairment from anisotropic turbulence, which is emulated by employing an electrically-controlled heater. The anisotropy gradually decreases as the flow-level of the introduced airflow from fans increases. In contrast to circular-aperture reception, the elliptical-aperture multimode diversity receiver can effectively resist beam wander under anisotropic turbulence, reducing outage probability from 14.63% to 0.38% for detecting a dual-polarization 30-Gbaud QPSK signal. Without the aid of an adaptive optics system, this work demonstrates the feasibility of realizing an anisotropic-turbulence-resistant FSO link using multimode and multi-aperture diversity reception employing digital coherent combining techniques. 

Free-Space Optics Communications Employing Elliptical-Aperture Multimode Diversity Reception Under Anisotropic Turbulence

This paper reports development and demonstration of free space optical (FSO) interconnect for short distance high speed data transfer (> 1.5 Gbps) between payload subsystems to enable replacement of physical electrical harness. Pair of compact optical transceiver connectors have been developed using 850nm AlGaAs vertical cavity surface emitting laser (VCSEL) module and GaAs photodiode. Firstly, characterization test setup has been developed for FSO and free space optical data transfer has been verified at 2 Gbps using developed optical connectors placed at 300mm spacing with known digital data pattern generated through Intel Stratix-V FPGA development board. Subsequently, free space transfer of image data from 70MP CMOS sensor with Bit Error Rate (BER) ≈ 10−12 has been achieved through FSO connectors. This confirms the suitability of indigenously developed optical interconnects for use in imaging payload subsystems instead of physical electrical harness.

Free Space Optical Interconnect for Short range Data Transfer Between Imaging Payload Subsystems

To provide more flexibility in inter-board communication in blade chassis, we developed a Free Space Optical Interconnect (FSOI) system for short range high speed data transfer. We designed robust and low footprint components compliant with both a use in milaero environment and an operation up to 5 and 10 Gbps. The 5Gpbs configuration demonstrated large tolerance to misalignment between emitter and receiver modules: +1.5/-1 mm along optical axis, lateral tolerance of +/-1 mm and angular tolerances of +/-1.5°. Reliable performances have been demonstrated over a temperature range from -30°C to 80°C and constraint environment as thermal and damp heat cycles and vibrations. Increase the data rate of the FSO device one step beyond up to 10 Gbps requires dealing with mode partitioning troubles generated by the use of VCSEL lasers. We designed and evaluated an improved opto-mechanical combination to overcome this drawback. The resulting device shows error free 10 Gbps data transfer while keeping large tolerance to Tx/Rx misalignments.

Daniel Mousseaux

Papers

Free Space Optical Interconnect (FSOI) modules for short range data transfer applied to board to board high rate communication