# An \(N\)-Port Universal Multimode Optical Router Supporting Mode-Division Multiplexing
###### Abstract  
Optical network-on-chip (ONoC) is based on optical interconnects and optical routers (ORs), which have obvious advantages in bandwidth and power consumption. Transmission capacity is a significant performance in ONoC architecture, which has to be fully considered during the design process. Relying on mode-division multiplexing (MDM) technology, the system capacity of optical interconnection is greatly improved compared to the traditional multiplexing technology. With the explosion in MDM technology, the optical router supporting MDM came into being. In this paper, we design a multimode optical router (MDM-OR) model and analyze its indicators. Above all, we propose a novel multimode switching element and design an \(N\)-port universal multimode optical router (MDM-OR) model. Secondly, we analyze the insertion loss model of different optical devices and the crosstalk noise model of \(N\)-port MDM-OR. On this basis, a multimode router structure of a single-mode five-port optical router is proposed. At the same time, we analyze the transmission loss, crosstalk noise, signal-to-noise radio (OSNR), and bit error rate (BER) of different input-output pairs by inputting the 1550 nm TE\({}_{0}\), TE\({}_{1}\), and TE\({}_{2}\) modes to the router.  
multimode optical router; multimode switching element; mode (de)multiplexer; transmission loss; crosstalk  
Received: 29 September 2021  
Accepted: 18 November 2021  
Submitted: 25 November 2021  
Received: 29 September 2021  
Accepted: 18 November 2021  
Submitted: 25 November 2021
capability of ONoC. Additionally, compared with WDM, MDM only requires a laser light source of one wavelength and omits accurate wavelength control strategies [15]. Therefore, the mode division multiplexing technology is considered to be a very promising technology, which can provide scalable bandwidth growth for data centers to meet future network requirements. In recent years, many optical devices for mode multiplexing have been studied, such as mode (de)multiplexers [16; 17; 18; 19; 20], multimode bendings [21; 22], multimode waveguide crossings [22; 23; 24], multimode switches [25; 26; 27], and mode filters [28; 29].  
ONoC is composed of on-chip optical interconnects and routers. On-chip optical router, which is a switching fabric build by microring resonators (MRs) and optical waveguides, is undoubtedly one of the critical components of ONoC. It implements routing and flow control functions and exchanges data packets from the input port to the output port through the optical router unit due to photonic technology. With escalating demands for higher throughput in data centers, compared with traditional optical routers, the optical router support MDM is attractive to parallel chip multiprocessors due to their high bandwidth advantages. However, the multimode optical router (MDM-OR) has not yet formed systematic research results. Therefore, research on MDM-OR is of great significance to the construction of ONoC that supports MDM in the future.  
In this paper, based on some mode multiplexed optical devices, the MDM-OR model is proposed. Using the MDM-OR model, we can design different MDM-ORs that support TE\({}_{0}\) and arbitrary high-order mode transmission corresponding to different single-mode optical routers. Then, we explain the principle of the MDM-OR model and propose the insertion loss models of different mode multiplexed optical devices. In addition, the crosstalk noise, signal-to-noise radio (OSNR), and bit error rate (BER) of the port pairs of these MDM-ORs are analyzed. Finally, we take the optimized single-mode five-port optical router [29] as an example to show the structure of its corresponding MDM-OR. We input the TE\({}_{0}\), TE\({}_{1}\), and TE\({}_{2}\) modes of 1550 nm to the router, and the transmission loss, the crosstalk noise, the OSNR, and the BER of the port pairs of the MDM-OR are analyzed. The simulation results show that the designed MDM-OR supporting MDM has good performance.