# Automatic Initialization Methods for Photonic Components on a Silicon-Based Optical Switch
## 5 Overall Performances of the IRIS Device  
Figure 7 reports the performances measured by the internal photo-diodes after each photonic stage of the IRIS chip. In this case we showed the device operating in the add mode, i.e., the input signal was injected from the local port to be added to one line port, as depicted in the left panel the figure. A tunable laser was used as input light source. The photonic components along the pathway were aligned according to the proposed optimization routine. The curves were normalized to the optical signal collected by the photo-diode, after the input grating coupler (PD 0 in the schematics on the left). As a first step, the light passed through the interleaver. The black curve represented the transmission spectrum at one of the two output arms (PD 1). Notice that the multispectral signal in the input was de-interleaved into the single odd channels. The red curve was acquired after the switching node which was turned to the on-state (PD 2). The MRR-based node accounted for 2 \(\div\) 3 dB optical losses. The blue curve showed how the AWG affected the transmission, reducing the optical power by another 3 dB (PD 3). The effect of fabrication inaccuracies on AWGs and the possible ways to correct them could be found in [19; 33]. Finally, the green curve was collected after the second interleaver, just before the output grating coupler (PD 4). Let us note that these measurements were acquired with a single channel being injected into the chip. When more channels were simultaneously launched at the local ports, additional inter-channel interference could arise. This crosstalk analysis has been used to fix the isolation requirements of the optical components during the chip design [11]. Experiments in a fully aligned optical switch showed zero power penalties in the presence of 1, 2, or 3 interferences, with respect to the single channel transmission BER curve [15]. Figure 8 showed the overall fiber-to-fiber performances of the IRIS switch. Here the experimental setup consisted of a tunable laser source as input and an InGaAs photodetector fiber-coupled to the output grating coupler (sketched on the left of the figure) as the output monitor. In the figure, the 12 optical channels, which had been acquired one at a time, are superimposed. The optical signal at the InGaAs detector was normalized to the signal from the tunable laser. The IRIS chip demonstrated total insertion losses of 23.8 \(\pm\) 0.5 dB. These losses come partially from the optical propagation within the device (about 9 \(\div\) 10 dB, as is clear from Figure 7, green curve). The total length of the connection waveguides varied from about 1.2 cm (in the case of the shortest path) to almost 2 cm (in the case of the  
Figure 7: Signal collected by the on-chip photo-diodes in different locations of the photonic chip. The optical pathway on the left outlined the functioning of the IRIS (Integrated reconfigurable silicon photonics based optical switch) switch in the add mode. The different noise levels in the right plot were due to the difference between the various photodiodes, each one with its own performances. longest path). The rest was due to the coupling (almost 7 dB per grating coupler). It is worth noting that the far-field envelop effects were negligible, as shown in Figure 8.  
For the purpose of scalability, insertion losses had to be decreased. First, one could reduce the losses by improving the alignment of the fiber arrays with the grating couplers. In fact, fabrication errors had caused twice as large losses per grating, with respect to the design. Second, one can introduce active components by heterogeneous integration, such as optical amplifiers, in order to compensate the on-chip device losses. A more detailed discussion on this issue is given in [15]. Focusing on the channel isolation, i.e., the difference between the intensity of the optical signal within the colored slots and the plateau of spurious signal out of the channels, we got a value of \(30.1\pm 0.8\) dB.