## 4 Photonic Integrated WDM Cross-Connect  
Based on the schematic shown in Figure 3, a PIC integrating four optical modules each with four WDM channels (without a wavelength combiner) are designed, as shown in Figure 4. The chip has been realized in a multi-project wafer (MPW) in the Jeppix platform with limited space of the cell (\(6\;\mathrm{mm}\times 4\;\mathrm{mm}\)). Each of the four identical modules processes one of the four WDM inputs. At the input of each module, an 800 \(\upmu\)m booster SOA is employed to compensate the 6 dB losses of the 1:4 splitter and, partially, the AWG losses at the WSS. The passive 1:4 splitter is realized by a cascading \(1\times 2\) multimode interferometer (MMI), with the outputs connected to four identical WSSs, respectively. The AWGs of the WSS are designed with a free spectral range (FSR) of \(15\;\mathrm{nm}\), which has been tailored to fit the limited cell size offered in the MWP. The quantum well active InGaAsP/InP SOA gates have a length of \(350\;\mathrm{\upmu m}\). The input and output facets of the chip are anti-reflection coated. The chip includes a total of 112 elements (four booster SOA pre-amplifiers with DC bias, 64 gate SOAs with RF bias, 32 AWGs, and 12 \(1\times 2\) MMI couplers). The light-shaded electrodes are wire bonded to the neighboring PCBs to enable the control of the SOA gates. Lensed fibers have been employed to couple the light in and out of the chip. Spectral tuning of the AWGs is, in principle, possible by using heaters on top of the AWG waveguides with a limited tuning range within the channel (not implemented in this design). The parameters and static performances, including the operational bandwidth of the SOAs, central wavelength, as well as the passband for the AWGs, and crosstalk between different channels have been characterized by employing the experimental setup shown in Figure 5. Four optical input channels spaced by 500 GHz, from \(\lambda 1=1532\;\mathrm{nm}\) to \(\lambda 4=1544\;\mathrm{nm}\), are launched into input port 1 of the photonic chip to characterize the chip. As the optical modules are identical, we have assessed the operation of one optical module to characterize the switching operation. The optical power of the WDM input channels was \(2\;\mathrm{dBm}/\mathrm{channel}\) (see also Figure 5). The input SOA was biased with \(100\;\mathrm{mA}\) of current. The temperature of the chip was maintained at \(25\;^{\circ}\)C. A polarization controller  
Figure 3: Schematic of the WDM cross-connect based on Semiconductor Optical Amplifiers (SOAs). was employed at the input of the chip due to the high polarization-dependent loss of the quantum well SOAs. First we assess the operation of the single WSS. One WDM channel at a time is statically switched at the WSS output by enabling one of the four SOA gates at a time. The current applied at each of the SOA gates was 62 mA. The four measured spectra at the WSS output (output 1 of the chip) are shown in Figure 6. An on/off switch ratio higher than 30 dB was measured. The optical power at the chip output for Channel 1 and Channel 3 was \(-9\) dBm. Considering 6 dB/facet coupling loss, a 1 dB on chip gain is estimated. However, Channels 2 and 4 have 10 dB of extra losses. Inspection of the chip reveals that two waveguides after the AWG demux are not fully resolved, which leads to substantial optical power coupling between the channels. The coupled signals at different wavelengths are filtered by the output AWG, explaining the extra losses. This can be solved in the next fabrication run. In the second experiment, Channel 1 has been switched to one of the output ports at a time by enabling one of the SOA gates of the four WSSs at a time. To measure the cross-talk between the WSSs output ports (here we define the cross-talk as the ratio between the optical power of a single channel at the desired output port and the optical power at the other output ports), Figure 7 shows the optical spectra recorded at the four outputs when Channel 1 is only switched at output 1.