# Optical Technology for NFV Converged Networks
## 4 Tunable Filters in Silicon Photonics for Full Tunable Transceivers  
It has been discussed in Section 2 how tunable optical filters have a key role in PON networks with WDN overlay, when embedded in a WDM transceiver before the photodetector. Tunable optical filters introduce flexibility in the planning of the network and its upgrade, and enable software (SW) reconfiguration, reducing inventory costs. Presently, commercial tunable filters are mainly based on Micro-Electro-Mechanical Systems (MEMS), miniaturized electro-mechanical elements that allow wavelength selection by moving a micro-mirror. However, the power consumption of MEMS based filters may be excessive for integration in pluggable modules [12]. Furthermore, the cost of MEMS based filter is high for the scale of the application in scenarios such as 5G access networks and data centers. The high cost is due to due to their complex mechanical structures based on free space optics and 3-dimensional movements of micro-mirrors. In addition, there are few solutions allowing the fabrication of MEMS based filters through CMOS compatible processes (which are available in a standard electronic production line). This may prevent a reduction in costs even for large volume fabrication. A second solution available in commercial products is thin film filters. These are stacks of dielectric layers with thickness equal to a quarter of the central passband wavelength, where wavelength tuning is achieved by varying the incident angle of the incoming light beam. The properties of the filter are determined by the number of layers and the optical properties of the dielectrics. Commonly used materials are silica (SiO\({}_{2}\)) as the low-index layer and tantalum pentoxide (Ta\({}_{2}\)O\({}_{5}\)) as the high-index layer. The typical size is 2 mm\({}^{2}\). The three main deposition techniques are used to achieve performances compatible with DWDM filtering applications, they are: ion beam assisted deposition (IBAD), plasma-assisted deposition (PAD), and ion beam sputtering (IBS). However, the power consumption of thin film filters may be excessive for the scale of the application scenario and the cost of thin film filters (due to the fabrication process and the cost of the controls associated to incidence angle variation) is relatively high for 5G access networks [46]. In addition, thin film filters with tunable functions cannot be integrated in a silicon photonic chip with standard CMOS compatible processes, and the footprint of the filter is large compared to the total area of a photonic chip. For this reason there is the need to explore new solutions for the realization of tunable optics that are compliant with size and consumption requirements of, e.g., common pluggable modules, such as SFP28 [47] and are compatible with production in volumes at a cost comparable with the current module price. Silicon photonics provides a viable solution to produce tunable filters with small footprint and reduced consumption that are potentially integrable in small form factor transceivers. Furthermore, the compatibility with CMOS production lines in use for integrated electronics allows the reduction in production costs with respect to commercial filter solutions based on MEMS and thin film filters. Potentially, SOAs can be integrated for increased power budget in an efficient way [30,31]. The requirements for the use of tunable optics in the scenarios outlined in Section 2 are to operate on a wide spectral range of deployed WDM network: to achieve full functionality the offer of channels should be increased from the actual number of 4-8 channels [15] to about 20 bidirectional channels. As discussed in Section 2, the spacing of the channels should be 100 GHz with a minimum filter BW of 37 GHZ. For the deployment of WDM on the existing system apparatus, the filtering function should not impact the power budget of the line, therefore optical amplifiers such as SOA may be included in the chip design to compensate for the insertion loss introduced by the filter.