# Polarization Effect on the Performance of On-Chip Wireless Optical Point-to-Point Links
## 1 Introduction  
The performance improvement of future computer architectures is also related to overcoming the communication bottleneck at the inter- and intra-chip level. In fact, the communication among different processing units can limit the overall performance of the system. The extensive research efforts toward more efficient communication are motivated by the constant growth of the computational requirements of data-intensive applications. Networks-on-chip (NoCs) are the most assessed forms of technology that allow efficient communication through electrical links and switches and among different cores and/or processing units [1, 2]. However, latency, multi-hop communication, and scalability are seen as potential limits to a continuous increase in the performances of electrical NoCs. Therefore, alternative solutions are being investigated, ranging from the integration of an optical communication layer to the wireless interconnection at millimeter wave frequencies [3, 4, 5, 6]. Optical Networks-on-chip (ONoCs), i.e., the optical \"wired\" technology, are based on the integration into computing architectures of an optical layer, housing signal routing and processing components such as switches and filters [7, 8, 9, 10]. Optical communication is characterized by high bit rates and low latencies, and it is incredibly promising for overcoming the communication bottleneck. In general, the signal routing in the ONoCs is achieved by employing resonant or non-resonant components [11, 12, 13]. Resonant devices, such as micro-ring resonators (MRRs) or photonic band gap components, are intrinsically narrow band devices. The resonance condition occurs at given wavelengths, which depend on the geometrical and on the optical characteristics of the device (e.g., geometrical size, periodicity, refractive index, etc.) [14, 15, 16, 17, 18]. The use of many resonant devices in the same network requires a fine tuning of the resonances (e.g., by thermal tuning) to guarantee efficientcommunication between the different network nodes, especially when multiple resonant wavelengths are associated to multiple communication channels [12; 19; 20]. This can result in a considerable increase in the overall power budget and in the device complexity due to the tuning circuitry. Differently from resonant components, non-resonant ones, such as Mach-Zehnder Interferometers (MZIs), can achieve a broadband behavior and can allow the simultaneous switching of multiple communication channels [13; 21]. Therefore, Dense Wavelength Division Multiplexing (DWDM) communication can be implemented to reach high aggregate data rates. As the network size scales up and the number of transmitters and receivers increases, many routing devices are required, resulting in higher losses and crosstalk, especially due to waveguide crossings. A recently proposed technology, which is an alternative to ONoCs, is based on Wireless Networks-on-chip (WiNoCs). This technology offers the possibility of simplifying the network topological constraints and of reducing the inter-router hops while guaranteeing low-latency and broadcast communication [6; 22; 23; 24]. Moreover, the on-chip integrability of wireless networks is allowed by the use of mm- or THz-wave signals, which require very small antennas. However, the integration of miniaturized antennas (e.g., graphene antennas [25]) is not yet fully mature technology, and many research challenges are still open. On-chip wireless optical communication is an emerging technology that can contemporarily exploit, at the chip level, the advantages of optical and of wireless communications [26; 27; 28; 29; 30]. The combination of optical wired and wireless communications can help to reduce the network complexity and the number of switching elements necessary for linking all the transmitters and all the receivers. Moreover, the possibility of reducing the network topological constraints can lead to the exploration of new communication paradigms.