# LightR: A Fault-Tolerant Wavelength-Routed Optical Networks-on-Chip Topology +
## 1 Introduction  
Stimulated by recent breakthroughs in silicon photonics, optical networks-on-chip (ONoCs) have emerged as a next-generation solution to overcome the bandwidth and energy limitations of the electrical interconnects in multiprocessor system-on-chip (MPSoC) [1; 2]. As the name suggests, ONoCs use optical signals to transmit data [2]. Taking advantage of the wavelength-division multiplexing (WDM) technology and the ultra-low propagation delay of light in silicon, ONoCs promise to meet the high bandwidth demands while maintaining low latency and power [3]. Current ONoC architectures can be classified into two categories: control-networks-based and wavelength-routed [3]. On control-networks-based ONoCs, before a sender (master) can transmit data to a receiver (slave), a signal path needs to be reserved through an additional control network [4; 5]. On the other hand, wavelength-routed ONoCs (WRONoCs) fix collision-free signal paths between all master-slave pairs at the time of the design so that all masters can communicate to all slaves simultaneously [6; 7; 8; 9; 10]. Therefore, WRONoCs are free from the energy and latency overhead for arbitration and are gaining increasing research interest. Typically, the WRONoC design is divided into two consecutive steps: a topological and a physical design. A WRONoC topology specifies the interconnection and configuration of the network components, and a physical tool implements the interconnection of the inputtopology on a layout plane [11]. Figure 1a shows a simple WRONoC topology, where one master sends signals to three slaves. The signals sent from the master are modulated on three different wavelengths, represented by the blue, red, and green arrows. The signals travel along the same waveguide until they are demultiplexed by different optical switching elements (OSEs). Figure 1b shows one typical structure of the 2-input \(\times\) 2-output OSEs, called a crossing switching element (CSE). A 2 \(\times\) 2 CSE consists of a pair of orthogonal waveguides and two microring resonators (MRRs) configured to be on-resonance with the wavelength \(\lambda_{i}\). As shown in Figure 1c, when signals on \(\lambda_{i}\) enter the CSE, they are coupled to the MRR and experience a 90\({}^{\circ}\) change in their propagation directions. On the other hand, when signals on the wavelengths other than \(\lambda_{i}\) enter the CSE, they will pass through the CSE and keep their propagation directions, as shown in Figure 1d. Due to the complexity of the manufacturing process, MRRs are susceptible to fabrication errors [12; 13; 14]. Defective MRRs can cause malfunctions and even data loss in WRONNoCs, which lowers the fabrication yield. For example, if the MRR in OSE\({}_{1}\) shown in Figure 2 is defective and fails to resonate with its designed wavelength \(\lambda_{i}\), the signal on \(\lambda_{i}\) will fail to reach Slave\({}_{1}\), causing data loss. Therefore, enhancing the reliability of WRONNoCs is of great importance. However, current WRONoC topologies have rarely considered fault tolerance. For most WRONoC topologies, only one fixed signal path is reserved for each master-slave pair that requires communication. When an MRR along the designated signal path is defective, there is no available resource to re-arrange a new path for the signal. To the best of our knowledge, RobustONoC [15] was the only work that considered fault tolerance in the  
Figure 1: (**a**) A simple WRONoC topology. (**b**) A 2\(\times\)2 CSE structure. (**c**) On-resonance signals change their propagation directions. (**d**) Off-resonance signals pass through the CSE without direction change. Figure 2: The MRR in OSE\({}_{1}\) is defective and the signal on \(\lambda_{i}\) fails to be coupled to the MRR. WRONoC topology design. For a given topology, RobustONoC modifies the topological structure by inserting backup MRRs and waveguides to enable the deviated signals to return to their planned paths. However, RobustONoC requires roughly twice the number of MRRs in the original topology as backups.