## 6 MZI and Waveguide Crossing Characteristics  
In this section, we present the performances of our MZI and waveguide crossing fabricated in LETI's 200 mm silicon photonics fabrication platform, with which we will plot the \(\hat{\mathcal{S}}\mathcal{R}\) values of the four topologies. We designed a \(2\times 2\) MZI circuit, Figure 8[23]. The structure has 3-dB multimode interference couplers (M1 and M2), a 250 \(\upmu\)m-length PIN diode phase shifter in each arm (PS1 and PS2), a 50 \(\upmu\)m-length heater in each arm too (H1 and H2) and a photodetector at each output-side (P1 and P2). A loop-back waveguide was added for fiber alignment and for evaluating the insertion losses of MZI. Measurements of optical power on Out1 and Out2 were performed using a fiber array feeding the input In1. The bar-state and cross-state were observed depending on the change in phase shift. For that, a current of up to 12 mA is first injected into PS1, and then the same protocol was applied to \begin{table}
\begin{tabular}{c c c} \hline \hline
**Metric** & **Value** & **Ref.** \\ \hline Waveguide crossing loss (\(Loss_{X}\)) & 0.03 dB & [21] \\ Waveguide crossing crosstalk (\(Xtalk_{X}\)) & \(<\)–40 dB & [21] \\ MZI Loss (\(Loss_{MZI}\)) & 0.8 dB & [22] \\ MZI Crosstalk (\(Xtalk_{MZI}\)) & \(-\)28 dB & [22] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: State-of-the-art loss and crosstalk of MZI and waveguide crossing. Figure 7: \(\hat{\mathcal{S}}\mathcal{R}\) of the four topologies using the state-of-the-art loss and crossing values (**a**), a significant crossing loss \(Loss_{X}=0.21\) dB (**b**) and using a non-optimized crossing crosstalk \(Xtalk_{X}=-20\) dB (**c**). PS2. The obtained results of the two-phase shifters were combined and depicted in Figure 9a. It is important to point out that our MZI operates using the two electro-optic (EO) phase shifters (PS1 and PS2), whereas the two heaters (H1 and H2) were specifically integrated to correct static phase errors. The results presented below were obtained without phase corrections, that is to say, only EO phase shifters were operated. For the best die, we measured a crosstalk of \(-33.4\) dB in bar-state, and a crosstalk of \(-31.3\) dB in cross-state. The transmission spectrum in both states and for the loop-back waveguide is depicted in Figure 9b, the insertion loss recorded in the MZI equals \(6.42\) dB at the best wavelength, but this value includes the grating coupler losses. In the loop-back waveguide, the insertion loss is \(5.11\) dB. Therefore, the MZI insertion loss is estimated near \(1.31\) dB. It is important to note that, at the wafer scale, the calculated mean value of crosstalk is \(-25\) dB, and by testing the other input (Out2 instead of Out1), the same value was found. It is noteworthy that these measurements were carried out at room temperature. We did not change the temperature to investigate its effect on the MZI, but we think that it is quite resistant to temperature changes. Waveguide crossing was also characterized; the results obtained are shown in Figure 9c. The measured crosstalk is lower than \(-42\) dB, with an insertion loss of \(0.1\) dB. Our best components in performances are summarized in Table 5.  
Compared to Table 4 values, our technology has better performances in terms of crosstalk, but lower performances in terms of loss. Figure 8: Image of \(2\times 2\) MZI circuit: M1 and M2 are 3-dB MMI couplers; the phase shifters (PS1 and PS2) are commonly grounded and symmetrically integrated; H1 and H2 are the heaters, P1 and P2 are the photodetectors. \begin{table}
\begin{tabular}{c c} \hline \hline
**Metric** & **Value** \\ \hline Waveguide crossing loss (\(Loss_{X}\)) & \(0.1\) dB \\ Waveguide crossing crosstalk (\(Xtalk_{X}\)) & \(<\)\(-42\) dB \\ MZI Loss (\(Loss_{MZI}\)) & \(\sim\)\(1.31\) dB \\ MZI Crosstalk (\(Xtalk_{MZI}\)) & \(-31.3\) dB \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Best component performances.