## 4 Experimental Setup
### Measurement Procedures  
A standard on-off keying (OOK) optical link was set up to characterize the SiPh-based transmitter with the devices under test. In order to quantify the transmission reliability, the bit error rate (BER) was chosen as a reference parameter. Optical links that were employed in HEP experiments covered at most distances on the order of hundreds of meters and, in principle, did not need to be optically amplified. To mimic the final application scenario, BER performances were evaluated at a constant optical signal-to-noise ratio (OSNR) by varying the optical power at the receiver [47]. Two different electronic dies were used for each SiPh modulator characterization. The measurements were made in distinct time slots; hence, the working conditions were not exactly the same for both cases. Figures 7 and 8 depict the experimental setups respectively built up for the MZM and the RM characterizations. Some elements were common to both scenarios. A 12.5 Gb/s BER tester (BERT) stimulated the device under test (DUT) and calculated the BER. A pulse pattern generator (PPG) encoded a pseudo-random bit sequence (PRBS) on an NRZ electrical signal with voltage levels suitable to feed the input CML stage of the driver, i.e., a symmetric 400 mV voltage swing with 1 V DC bias. A PRBS pattern \(\left(2^{31}-1\right)\)-long was chosen in order to make a repetitive measure with the largest number of bit combinations. The out-coupled modulated optical power was converted back in the electrical domain by means of a commercial 10 GHz-bandwidth module comprising a photo-detector (PD) and trans-impedance amplifier (TIA). An error detector (ED) compared the transmitter and received bit sequences and detected transmission errors. The two bit streams were synchronized by physically sharing the clock between PPG and ED. The voltage decision threshold and the sampling time on the ED were adjusted to get the lowest BER. Optical power was provided by a tunable light source (TLS) near 1.55 um. It was adjusted in order to operate the MZM in quadrature and the RM around the wavelength \(\lambda_{w}\) shown in Figure 5, which was verified to generate the largest eye opening given the actual voltage swing delivered by the driver. Since grating couplers were only sensitive to transverse-electric (TE) fields, a polarization controller (PC) was placed upstream the DUT to maximize the coupled power. In the RM characterization, a 90/10 splitter was inserted before the PD in order to continuously monitor the output signal spectrum with an optical spectrum analyzer (OSA). This allowed measuring the OSNR at the receiver and detecting eventual working point shifts during measurements. Eye diagrams were captured by connecting the PD to a 40 GHz-bandwidth oscilloscope (the BERT clock signal was used to trigger the waveform acquisition). Figure 6: Structure of the complete packaged system. The sample appears the same in both case studies described in this work. They differ only for the SiPh modulator, which is attached to the electronic integrated circuit (EIC), i.e., MZM or RM. Coplanar transmission lines convey the input differential signal to the driver circuit. Starting with a ground-signal-ground (GSG) pattern from the SMA connectors, they merge into a single ground-signal-ground (GSSG) line while approaching the EIC to match the bond-pads’ pitch. TL, transmission line. Both boards under test showed huge optical insertion losses (ILs), roughly 20 dB fiber-to-fiber IL for the MZM test chip and 40 dB in the RM case, as can be seen from Figure 5a. They were mainly attributed to accidental misalignments between the fiber array holder and the on-chip grating coupler array, probably occurred during the boards' shipping. Fiber-to-fiber ILs after the fiber array attachment were indeed measured to be around 12 dB in both cases. For this reason, some erbium-doped fiber amplifiers (EDFAs) were added to the optical signal chains. Two EDFAs were used for the RM testing because of the more severe optical losses in that specific case.