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Network-on-chip (NoC) can improve the performance, power efficiency, and scalability of chip multiprocessors (CMPs). However, traditional NoCs using metallic interconnects consume a significant amount of power to deliver high communication bandwidth required in the near future. Optical NoCs are based on CMOS-compatible optical waveguides and optical routers, and promise significant bandwidth and power advantages. In this work, we review different designs of 5×5 and 4×4 optical routers for mesh or torus-based optical NoCs, and compare them for cost of optical resources and optical power loss. Besides, we use a 8×8 mesh-based optical NoC as a case study and analyze the thermal-induced power overhead while using different optical routers. Results show that the number of switching stages in an optical link directly affects the total optical power loss under thermal variations. By using passive-routing optical routers, the maximum number of switching stages in a XY-routing path is minimized to three, and the thermal-induced power overhead in the optical NoC is less than the matched networks using other routers.