## 4 Experimental Setup  
### Samples  
The proposed driver was included in a \(1\,\mathrm{mm}\times 1\,\mathrm{mm}\) electronic integrated circuit (EIC) fabricated in a commercial-grade radiation-hard 65 nm CMOS technology. The circuit radiation resistance was previously tested exposing the whole EIC to X-rays at the INFN-Padova facility. Proper operation of the standalone driver was observed till a total ionizing dose (TID) of \(8\,\mathrm{MGy}\), which was the highest dose level reached during the test because of limited testing time. A 30% voltage amplitude degradation with respect to the pre-irradiation value was observed [17]. In addition, a single event upset (SEU) analysis of the same circuit was carried out in [44], proving its HEP-level radiation-tolerance. Figure 4: Proposed pseudo-differential output stage applied to LE-MZMs (**a**) and RMs (**b**). The five stage tapered chain of current mode logic (CML) buffers is also indicated, with the last pre-driving stage (colored differently), which is inductively peaked. In this work, the proposed driver was tested together with two SiPh optical modulators fabricated in a SiPh 130 nm silicon-on-insulator (SOI) process, i.e., an MZM and an RM. They were based on the corresponding building blocks provided in the foundry's process design kit (PDK) and were laid out in a \(5\,\mathrm{mm}\times 5\,\mathrm{mm}\) photonic integrated circuit (PIC) [45]. They both relied on \(450\,\mathrm{nm}\times 220\,\mathrm{nm}\) rib-waveguide phase shifters with deep-etched slabs and used standard doping levels. The MZM was \(1\,\mathrm{mm}\) long and was drawn with unbalanced arms, i.e., one arm was physically made longer, which allowed tuning the operating point by changing the laser wavelength. Each arm was sized as a coplanar waveguide (CPW), and the electrodes' layout matched the output port pattern of the full-custom driver. Although having been designed as a TW device, each MZM arm lacked an on-chip termination and effectively behaved as a lumped-element for the frequencies of interest. The associated pn-junction capacitance \(C_{\mathrm{mzm}}\) was estimated to be around \(0.5\,\mathrm{pF}\). W wafer-level radio-frequency (RF) measurements made on comparable MZM designs ensured that the electro-optical modulation bandwidth was above \(10\,\mathrm{GHz}\) with a proper termination [14]. The ring modulator had a \(7.5\,\mathrm{\SIUnitSymbolMicro m}\) radius resulting in a free spectral range (FSR) of about \(13\,\mathrm{nm}\). As shown in Figure 5, the resonance next to \(1.55\,\mathrm{\SIUnitSymbolMicro m}\) had a full-width at half-maximum (FWHM) of about \(240\,\mathrm{pm}\) and an extinction depth of nearly \(20\,\mathrm{dB}\). The spectrum was measured by feeding the device with a tunable external cavity laser (ECL) and sweeping the wavelength in the \(1510\,\mathrm{nm}\) to \(1570\,\mathrm{nm}\) range with a \(0.82\,\mathrm{pm}\) resolution. The LE capacitance \(C_{\mathrm{rm}}\) associated with the RM was expected to be on the order of a few tens of femtofarads. Electronic and photonic dies were placed on a printed circuit board (PCB) and bonded with \(40\,\mathrm{\SIUnitSymbolMicro m}\) diameter aluminum (Al) bond-wires. Tapered coplanar transmission lines were realized on the PCB to maintain the \(50\,\mathrm{\SIUnitSymbolOhm}\) impedance environment while transitioning from the centimeter-scale footprint of the high-speed SMA (SubMiniature, Version A) connectors to the \(100\,\mathrm{\SIUnitSymbolMicro m}\) bond-pads' pitch. A PCB with the same layout was used for both characterizations. The complete packaged sample is shown in Figure 6.  
Fiber-to-PIC light coupling was achieved by means of on-chip grating couplers and quasi-planar fiber-arrays. The latter were preferred to the vertical fiber couplers because of their lower encumbrance on the PIC surface [46]. Fiber arrays were manually pigtailed with the aid of micro-mechanical stages and active alignment tools.