# A Programmable ROADM System for SDM/WDM Networks
## 2 Relevant Works  
MCF-based SDM networks are designed to integrate with the traditional WDM technologies by adding another dimension. Multicore fibre is a typical approach to introduce multiple spatial paths into a fibre from an SDM fibre point of view [3]. The transmission distance and maximum cores number in MCFs are significantly limited by both cross talk and attenuation. Several compensating methods have been proposed. For example,crosstalk-aware spectrum defragmentation [11] was proposed to remedy the issue of spectrum fragmentation in the SDM optical network, and the EDFAs in switching nodes can compensate for the attenuation. In SDM/WDM networks, fully flexible spectrum switching would be a straightforward solution to use network resources efficiently [7; 12]. The fully flexible ROADM will require WSSs, the key switching device in ROADMs, to provide switching ports more than the total fibre core numbers in all ingress fibres. [PERSON] [13] explained how to create very large-scale ROADM, but the requirement of WSS port number is very high. With the increased core numbers offered by MCFs, WSSs with such high port counts become unpractical. Therefore, traffic aggregation emerged as an option to reduce switching complexity. Besides, the work in [14] confirmed that traffic aggregation in space and spatial can reduce the routing complexity and save a considerable amount of switching components. One approach for traffic aggregation is to provide network resources as superchannel signals either in space or spectral [15]. Regarding space superchannel signals, several cores can be grouped together to perform core switching as shown in [16]. However, such coarse granularity in multiple core levels prohibits deployment in optical networks, which require flexibility in the channel level according to the client interface speed. By combining with spectra and cores, spatial superchannels can be set up over multiple cores [12]. Compared with the traditional spectral superchannel signals, the spatial superchannel signals require specially designed WSSs for switching operations. By taking account of the recent standardisation of 400 GbE, the spatial superchannels over multiple cores will not be foreseen for any commercial deployment in a few years. Another approach to handle SDM switching is to offer both core switching and wavelength switching in a collaborative approach. In 2017, we proposed the concept of core bypassing in [17], to reduce the scale of the ROADM for better hardware utilization. The followed work evaluated the design at the network level and further reduced the required port count of the WSSs in the ROADM by cascading two-level WSSs [9]. [PERSON] [18] recognized core bypassing as a key enabling technology for the massive SDM era. However, the work separates the whole network into a pure SDM network and a traditional WDM network. It is believed that integrating SDM and WDM networks with fibre core bypassing will achieve better switching performance. Therefore, several key enabling technologies are listed here to take full advantage of combined fibre core bypassing and traditional WDM in SDM/WDM networks. 1. The bypass cores should be integrated with the WDM system that consists of the residual cores with traditional ROADM switching. The integration allows the bypass core to serve as a part of optical links. Without the integration, bypass cores will impact network flexibility, especially when core numbers are low. The integration requires connecting bypass cores back to the ROADM system at the end of the bypass cores. 2. The bypass cores should be reconfigurable at each node. The configurability of bypass cores enables adaptability of networks to accommodate different network traffic and support the migration from SMF-based networks to MCF-based optical networks. Firstly, variations of network traffic during the life cycle of optical links require fibre cores to be reconfigured as either bypass cores or WDM cores to optimize network performance.