F²BFLY: An on-chip free-space optical network with wavelength-switching

The increasing number of cores in contemporary and future many-core processors will continue to demand high through-put, scalable, and energy efficient on-chip interconnection networks. To overcome the intrinsic inefficiency of electrical interconnects, researchers have leveraged recent developments in chip photonics to design novel optical network-on-chip (NoC). However, existing optical NoCs are mostly based on passively switched, channel-guided optical interconnect in which large amount of power is wasted in heating the micro-rings and maintaining the optical signal integrity. In this paper we present an optical NoC based on free-space optical interconnect in which optical signals emitted from the transmitter is propagated in the free space in the package. With lower attenuation and no coupling effects, free-space optical interconnects have less overheads to maintain the signal integrity, and no energy waste for heating micro-rings. In addition, we propose a novel cost-effective wavelength-switching method where a refractive grating layer directs optical signals in different wavelengths to different photodetectors without collision. Based on the above interconnect and switching technologies, we propose free flattened butterfly (F²BFLY) NoC which features both high-radix network and dense free-space optical interconnects to improve the performance while reducing the power. Our experiment results, comparing F2BFLY with state-of-the-art electrical and optical on-chip networks, show that it is a highly competitive interconnect substrate for many-core architectures.