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Multiprocessor system-on-chip (MPSoC) is an attractive platform for high-performance applications. Networks-on-chip (NoCs) can improve the on-chip communication bandwidth of MPSoCs. However, traditional metallic interconnects consume significant amount of power to deliver even higher communication bandwidth required in the near future. Optical NoCs are based on CMOS-compatible optical waveguides and microresonators, and promise significant bandwidth and power advantages. This paper proposes a fat tree-based optical NoC (FONoC) including its topology, floorplan, protocols, and a low-power and low-cost optical router, optical turnaround router (OTAR). Different from other optical NoCs, FONoC does not require building a separate electronic NoC for network control. It carries both payload data and network control data on the same optical network, while using circuit switching for the former and packet switching for the latter. The FONoC protocols are designed to minimize network control data and the related power consumption. An optimized turnaround routing algorithm is designed to utilize the low-power feature of OTAR, which can passively route packets without powering on any microresonator in 40% of all cases. Comparing with other optical routers, OTAR has the lowest optical power loss and uses the lowest number of microresonators. An analytical model is developed to characterize the power consumption of FONoC. We compare the power consumption of FONoC with a matched electronic NoC in 45 nm, and show that FONoC can save 87% power comparing with the electronic NoC on a 64-core MPSoC. We simulate the FONoC for the 64-core MPSoC and show the end-to-end delay and network throughput under different offered loads and packet sizes.