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.