# Optical Technology for NFV Converged Networks
## 5 ROADM in Silicon Photonics  
As reported in Section 2, low cost, reduced size ROADMs are a key technology for providing the right level of automation in the access network segment. Tunable filters can be used to operate the wavelength selection in a ROADM [58]. When used in ROADMs, tunable optical filters relieve operators from deploying and storing many variants of fixed optical add drop multiplexers (OADM) where each fixed OADM corresponds to a specific group of wavelengths by replacing the fixed OADMs with a single reconfigurable device. This leads to advantages in network planning simplification and saving of costs for the acquisition and maintenance of backup components, which are necessary to cope with possible failures, as failures can be addressed with a single tunable device. Different WSS technologies exist, based on Microelectromechanical Mirrors (MEMS), Liquid Crystal (LC) on glass and Liquid Crystal on Silicon (LCoS) [59; 60]. LCoS technology dominates current WSS shipments because it can support flexible channel plans. However, LCoS devices are polarization dependent, they need a polarization diversity configuration. Additionally, cost, size, port isolation, and crosstalk remain major design challenges. ROADMs architectures based on WSS, that reduce cost and fiber connections, with respect to traditional solutions, are reported in [61]. However, they are far to meet the requirements of the network scenarios presented in Section 2, for which a truly low-cost ROADM node is essential. Silicon photonics are the best candidates for such networks due to its characteristics of easy integration with control electronics, miniaturization, mass producibility, and potential for high yield and low cost in the well-established CMOS production infrastructure. In [1; 25; 62] examples of system-on-chip ROADMs, referred as Mini-ROADM, are presented. An example of architecture for the Mini-ROADM is depicted in Figure 11.  
This device has two-line ports, referred as east and west line port, for operation over two fibers rings. It presents two independent structures, one used for adding WDM channels to send the output line port, and the other used for dropping channels from the input line port. In a prototype realized at Ericsson Research, each structure comprises 8 ports to add and drop an equal number of 200 GHz spaced WDM channels in C band. A similar design can be generalized to a higher number of 100 GHz spaced channels. Such a two-way Mini-ROADM can be also enhanced to operate over a larger number of ways: for instance, a 4-way ROADM can be realized by connecting 4 of these structures [63]. In the Mini-ROADM, optical channels are added and dropped by silicon micro-ring resonators (MRR) as wavelength-selective switching elements that also acts as an optical filter. Each wavelength switching element is implemented by two-coupled MRRs with a slight resonance offset, set to obtain 40 GHz optical BW at \(-1\) dB. The measured adjacent channel isolation is larger than 20 dB. Silicon photonics MRRoptical filters have multiple advantages respect to other types of filter technologies since they are low cost, have low power consumption, and have a  
Figure 10: Spectral response of the RX filter according to MC simulations. very small footprint in the range of micrometers. However, they are resonant devices, quite sensitive to temperature variations, and for operating correctly in the system the wavelength resonance needs to be stabilized. Two possible methods can be adopted for MRR resonance stabilization:  
1. Use of a temperature control circuit to keep the temperature of the entire PIC at a pre-determined value by using an off-chip thermo-electric cooler (TEC). The TEC is used as the actuator that is driven by a feedback control (usually a PID), together with an integrated temperature sensor
2. Use of an active tuning circuit to lock the MRR resonance to the carrier wavelength by using a micro-heater as actuator on top of the MRR.