# Optical Technology for NFV Converged Networks
## 5 ROADM in Silicon Photonics
Method (1) has the advantage of requiring electronic circuits with limited complexity but it requires more power to stabilize the whole PIC temperature. Method (2) requires control circuits with higher complexity, with respect to approach (1), but has the advantage of requiring very low power, of the order of few mWatt per MRR. Additionally, the package is simplified by the absence of the TEC. In the design of the mini-ROADM the approach (1) was preferred for its implementation simplicity. The temperature stabilization circuit required 1-2 W power to provide an accuracy of 0.1 C and a response time of the order of 1 ms.  
Each structure in the mini-ROADM has a polarization diversity architecture, with a double-polarization grating coupler at the input that adapts the random polarization of the input signal to the device's main propagation mode. At the output ports, the two polarization components are recombined before coupling to the optical fibers. A previous version of this device was realized with a single polarization, as it was made insensitive to the input polarization with an integrated polarization controller [62,63]. However, with a single polarization controller the operation requires a low PMD, since different WDM channels, though launched with equal polarization, arrive with random polarization alignment at the input like port. Therefore, a dual polarization architecture is preferred. The PIC of the mini-ROADM prototype has a size of 5 \(\times\) 5 mm (see Figure 11b). The PIC is mounted on a control board (see Figure 11c) to provide functionalities, such as the adjustment of MRR heater currents for performing channel add or drop, and setting the propagation direction of each individual wavelength at the 1 \(\times\) 2 switch placed at the add port (Figure 11a). Each switch reverts the propagation direction, e.g., for traffic protection in case of fiber break. This allows to implement well-known ring protection mechanisms without duplication of the optical interfaces at each add or drop port. Figure 11: (a) Block diagram of the realized mini-ROADM, (b) photonic chip, (c) control board. The control board is equipped with two Erbium Doped Fiber Amplifiers (EDFA) with a gain of 23 dB and maximum output power of 20 dBm, one for each bus in function of the working direction of the wavelengths along the network. The high gain and output power compensate the high coupling loss of the prototype, due to the non-ideal grating coupler fabrication process. They can be considerably lower in an engineered device. Integrated SOAs can be also considered but they have the limitations discussed in Section 2. The spectrum of one the MRR is shown in Figure 12a, for a channel passing through the MRR and a dropped channel. The measured drop loss is 1.8 dB, excluding the polarization grating coupler losses. The maximum drop loss difference from the first to the last channels is about 1.5 dB. The resonance frequency is plotted in Figure 12b versus different voltage values applied on the MRR heater. The curve in Figure 12b shows that the efficiency of tunability is 35 \(\upmu\)W/GHz. In Figure 12c is shown the spectral output of a single channel in three different operating conditions: (i) all heaters are switched off ('no heaters through/drop' in the legend) (ii) only the channel heaters are enabled, all the other channels are switched off ('only double ring heaters through/drop' in the legend); and (iii) both the channels heaters and all the other channels heaters are enabled ('all heaters through/drop' in the legend). With the current silicon photonics technology, it is possible to expand the number of WDM channels up to 24 but supporting denser frequency spacing (e.g., 50 GHz) would require technological advances to improve the wavelength stability of the micro-rings.