# A Programmable ROADM System for SDM/WDM Networks
## 1 Introduction  
Optical networks, as the key Internet infrastructure in the information era, have served the explosive traffic growth successfully in the past decades. Innovations in digital signal processing (DSP) and high-order modulation signals have pushed the spectral efficiency in single-mode optical fibres (SMF) around 10 bits/s/Hz with a total capacity more than 40 Tbit/s per fibre [1]. However, the increased optical signal to noise ratio (OSNR) requirement for optical signals with advanced modulation formats reduces the transmission distance significantly. Mitigating optical nonlinearity helps improve the transmission distance but at the cost of a significant amount of computational power in the receiver. Furthermore, it has been becoming increasingly challenging to squeeze more capacity out of SMF-based wavelength-division multiplexing (WDM) technology. Space-division multiplexing (SDM) can offer significantly higher capacity by bringing another dimension in optical networks [2]. Several technologies have been demonstrated to enable SDM in optical networks by using single-mode fibre bundles, multicore fibre (MCF), few-mode fibre, or orbital angular momentum multiplexing (OAM). Most of the efforts focus on link capacity improvement without considering the implementation of switching system in SDM networks. Among these technologies, few-mode fibre and OAM technologies present strong coupling between different spatial modes, prohibiting traditional WDM switching without cumbersome mode decoupling either by high-complex and power-hungry digital signal processing or advanced optical decoupling systems. Weakly-coupled MCFs provides a promising SDM solution with more matured fabrication and availability of relevant optical components [3, 4]. Besides, MCF-based SDM networks provide full compatibility with current wide-deployed WDM systems. The latest research has demonstrated C+L band transmission of 19-core fibres for trans-oceanic communications [5]. In addition, a field-deployed MCF testbed has been installed in L'Aquila, Italy [6]. It is believed that MCF-based SDM networks provide a feasible solution for potential deployment in future optical networks. Comparing the SMF, MCFs with a large number of cores incorporated with WDM technology could potentially increase the total capacities by 10 to 20 times. To manage the growing traffic in MCF-based SDM networks, switching strategies should be reconsideredto allow efficient network resource allocation and management. As the key switching element in optical networks, ROADM systems for SDM networks will also be at the heart of the switching solution. Traditional ROADM that conducts band switching in WDM-based optical networks poses a significant challenge for efficient scaling in MCF-based SDM networks [7]. Providing flexible spectrum switching for all fibre cores not only requires a large amount of cost; it may also introduce a significant amount of spectrum fragmentation [8]. Moreover, the implementation of ROADM with full-spectrum switching requires wavelength-selective switches (WSSs) with a considerable number of ports, which poses a limitation to further scaling up. Besides, spectrum flexibility in all fibre cores becomes unnecessary, providing the node degrees are far less than the core numbers. With a sufficient number of cores in one fibre, fibre cores should be treated as a type of switching entity compared to wavelength in the conventional WDW system. Therefore, the ROADM system for SDM-WDM networks providing both wavelength and fibre core switching granularity should be adopted to allow effective and efficient resource management. In this paper, a programmable SDM/WDM ROADM system is proposed and evaluated for MCF-based SDM/WDM networks, with an extension of our previous work [9]. The proposed ROADM system is featured with programmable node synthesization and dynamic bypass-fibre-core configuration.