# Optical Technology for NFV Converged Networks
## 4 Tunable Filters in Silicon Photonics for Full Tunable Transceivers
This is a fundamental implementation point since the integration of SOA must be compatible with volume production. Issues related to SOA integration on silicon were widely discussed in Section 3. Different approaches are possible for the design of the silicon photonic filter in the TRX. In the following we consider different TRX architectures with alternative options for the filter elements and the method employed for coupling with the input-output fiber. Figure 5 shows two possible architectures for a tunable transceiver including fiber coupling, a polarization diversity scheme, optical amplifiers and add and drop tunable resonators. The WDM input is split into two sub-bands that correspond to the portion of the band dedicated to the transmission channels from the transmitter (TX) and the reception channels at receiver (RX). \begin{table}
\begin{tabular}{c c c} \hline \hline
**System Parameters** & **Value** & **Notes** \\ \hline Data Rate & 25.78125 Gbit/s & \\ \hline Modulation format & NRZ OOK & \\ \hline DWDM channel spacing & 100 GHz & \\ \hline DWDM channels & 191.4–193.5 THz Downstream & 1550–1570 nm \\  & 194.0–196.1 THz Upstream & \\ \hline TRX Form factor & SFP28 & \\ \hline TRX Power consumption & 1.5 W & \\ \hline BER & 5 10–5 & \\ \hline Rx Parameters & \\ \hline Min channel input power & \(-\)18 dBm & \\ \hline Min Optical Channel BW (\(-\)0.5 dB) & 37 GHz & \\ \hline Interchannel cross-talk & \(-\)20 dB & Defined as in [48]; all DWDM channels & \\  & & on; all channels set at equal power. \\ \hline Min OSNR at photodiode (0.1 nm) & 25 dB & Dependent on SOA noise figure \\ \hline Min PDL & 1 dB & \\ \hline Tx parameters & & \\ \hline Min Tx output power & 0 dBm & Launch power in fiber \\ \hline Min extinction ratio & 6 dB & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Optical parameters for a 25 Gbit/s tunable transceiver. Figure 5: Tunable transceiver architectures with polarization diversity schemes and fiber coupling with integrated band splitter (**a**) and external band splitter (**b**). EC: edge coupler, PS: polarization splitter, PR: polarization rotator, F: add and drop resonator filter, DPGC: dual polarization grating coupler, PIC: photonic integrated circuit. In this scheme the two sub-bands represent two halves of the operating BW of the transceiver. This operation can be performed by an element integrated in the photonic chip (PIC), as in Figure 5a, or an external band splitter, as in Figure 5b, e.g., a commercial band splitter based on thin films [49]. The advantage of the external splitter is in the reduction in the BW that must be coupled to the photonic chip, which can represent an advantage in the fabrication of the fiber coupler. Additionally, integrated band splitters are a type of photonic element that has not yet been implemented. However, the design of integrated solutions is desirable since the use of an external band splitter might increase the footprint and consumption of the tunable transceiver. The resonators accomplish the filtering function, adding or dropping a resonant wavelength that can be tuned, e.g., via thermo-optic variation of the refraction index of the resonator [50]. These can be micro ring resonators (MRR), comprising multiple micro rings [51], reflector resonators based on Bragg Gratings, such as Distributed Feedback Bragg Reflectors (DFBR) [52] which comprise coupled Bragg gratings and a multi-mode interferometer (MMI), or ring assisted Mach-Zehnder interferometers [53]. The requirement for these resonators is to have a free spectral range (FSR) that is larger than the operating range of the filter so to select a single wavelength within the range. This requirement is challenging for the design since it may require tight fabrication tolerances to the solutions based on micro rings, with precise control of the coupling in the add and drop waveguides [54].