## 5 Results and Discussion  
Measured BERs as a function of the input power \(P_{\mathrm{pd}}\) on the photo-detector are shown for different data rates in Figure 9 for the MZM testing case. The whole system worked up to a bit-rate of \(1.5\,\mathrm{Gb/s}\), while BER floors started to appear around \(1.7\,\mathrm{Gb/s}\), and a completely compromised transmission functionality was captured already at \(2\,\mathrm{Gb/s}\), as shown in Figure 10.  
The results for the RM-based system are instead shown in Figures 11 and 12. BER floors here became visible only above \(5.5\,\mathrm{Gb/s}\), and data transmission at \(5\,\mathrm{Gb/s}\) with BER below \(10^{-9}\) was confirmed to be achievable with optical power levels on the receiver side (\(P_{\mathrm{pd}}\)) greater than \(-4\,\mathrm{dBm}\). The eye diagrams in Figure 11 were all taken in the condition \(P_{\mathrm{pd}}=-6\,\mathrm{dBm}\) to be comparable. In that situation, the cases corresponding to the \(4\,\mathrm{Gb/s}\) and \(5\,\mathrm{Gb/s}\) data rates did not represent error-free scenarios, as confirmed by the BER plot, and their apparent narrow eye aperture was therefore justified. Moreover, in line with the reported OSNR levels at the receiver, a larger noise accompanied the optical levels in Figure 11 with respect to the MZM case. It could be attributed to the usage of an additional EDFA in the RM test, which determined a greater in-band ASE-related noise. In [17], we already stated that the unexpected results recorded in the MZM case were packaging-dependent, even because error-free transmissions till \(5\,\mathrm{Gb/s}\) were verified from fully electrical measurements made on a standalone driver installed on a testing PCB. As can be seen from the detail in Figure 9, a few bond wires indeed traversed the whole chip and potentially induced undesired couplings between the EIC input/output ports. The eye diagrams in Figure 10 indeed present non-smooth rising/falling edges and multiple traces, which were a clear manifestation of a strong inter-symbol interference (ISI). Instead, the wire bonding arrangement in the RM case appeared clean, and the full \(5\,\mathrm{Gb/s}\) speed rating was achieved for the RM case. This proved the pseudo-differential output stage's correct applicability to single-ended modulators. Figure 10: Eye diagrams on the PD output port in the MZM case study (scale \(20\,\mathrm{mV/div}\)). Figure 9: BER performances as a function of the received power \(P_{\mathrm{pd}}\) for the MZM-based transmission system. The obtained data rates were relatively low compared to those for data center purposes (\(>\)25 Gb/s [7]) and even with respect to state-of-the-art laser diode-based HEP optical links (5 to 10 Gb/s [15]). However, as extensively reported in Section 3, many circuital degrees of freedom (DoFs) have been used to address the radiation hardening requirements, sacrificing speed performances. The proposed circuit had indeed a radiation tolerance compliant with the levels foreseen for the next upgrade of the LHC apparatus, i.e., 8 MGy, while the laser diode-oriented gigabit laser driver (GBLD) operation was guaranteed till 1 MGy at most [48]. As already shown in [44], it is important to note that the proposed driver architecture could sustain a 5 Gb/s transmission to these extreme radiation levels, though undergoing a reduction in the output voltage swing. The voltage amplitude versus TID trend appeared to be slightly smooth, and only a 30% decrease was captured at 8 MGy TID [17]. Even though transistor parameters would continue to degrade, it was likely that the driver could operate at even higher doses. The speed limit of the proposed driver was not entirely imposed by the chosen architecture and the connected SiPh load, but was rather due to the components that surrounded the signal path. A major role was played by the electro-static discharge (ESD) protection circuits deployed around each pad. A building-block solution was chosen because of its recognized radiation hardness despite presenting a capacitance greater than 0.5 pF.