# Scalable Photonic Digital-to-Analog Converters
## 6 Discussion  
The chips were fabricated at the Advance Micro Foundry (AMF) in Singapore. In the layout design, we used the MRMs with a 22 GHz optical bandwidth from the foundry's proprietary process design kits [25] for a proof-of-the-concept demonstration. The chip was fabricated through a multi-project wafer (MPW) fabrication run, which is an experimental implementation of AMF's MRMs using a set of doping concentrations that are different from those in the PDK design. Consequently, in addition to a low modulation speed, the MRM resonance shift under reverse bias obtained in the experiment turned out to be much lower than expected, resulting in small dynamic ranges and limiting the number of bit operations. Subsequent design optimization in the ring-bus gap, the optical path length of the MRM branches, doping concentrations, and the MRM thermal heaters have revealed promising prospects for enhancing the performance of the PDAC. For the standard doping concentration in the PDK design, MRMs with 10 \(\upmu\)m radius and a 380 nm ring-bus gap, simulation results show a significant increase in modulation efficiency and speed. For this set of parameters, we simulated a 25 Gbaud PAM-4 operation with a dynamic range of approximately 7 dB (compared to an approximately 3 dB value realized previously), which shows good agreement with the PDK specifications.  
Moreover, in our first design prototype, different arms of the PDAC were not balanced, i.e., unequal optical path lengths among the MRM branches, which requires additional tuning of the phase shifters to ensure constructive interference among the modulated signals. However, this corrective measure incurred additional power consumption, contributing to  
Figure 7: (**a**) The wavelength spectrum of the 4-bit PDAC demonstrating 16 distinct intensity levels obtained for different 4-bit sequences. As for PAM modulation, the MRMs are operated in the under-coupled regime, showing an ER of about 13 dB. The vertical dashed line at 1551.15 nm represents the operating wavelength where the 16 intensity levels are distinguishable. The corresponding output power is plotted in (**b**).  
the overall energy budget. To minimize the energy cost, it is recommended to design the PDAC with all branches having an equal optical path length for uniform propagation losses in every channel, leading to better output linearity and minimum tuning requirements from the phase shifters. This will also minimize the adverse effects that may be caused by the phase noise of the laser since the random fluctuation of the phase at the laser output will propagate coherently through the PDAC MRM branches and interfere constructively at the output. Another consideration for a robust PDAC architecture is the replacement of passive power splitters, such as the MMIs in this work, with tunable directional couplers [28]. The optical signal in an MRM branch (a bit module) traverses two additional MMIs compared to that in its previous module, while the rest of the components in the module are exactly traversed once. Therefore, starting from the MSB, the optical signals in the subsequent modules encounter additional insertion loss of the MMIs, which is below 0.15 dB in the C band (1530-1565 nm). Additionally, depending on the operating wavelength, the slight imbalance of the power splitting ratio of the MMI, varying within \(\pm\)0.1 dB in the C band, can compensate for the insertion loss (by transferring relatively more power into the upper output waveguide) or further exacerbate the distribution of the optical power in undesired splitting ratios to the subsequent modules. Given the dynamic range of the LSB being 1 dB in this work, the MMI insertion loss and imbalance would not jeopardize the bit resolution significantly. The deployment of the tunable directional coupler can mitigate the risk of degradation of the bit resolution resulting from the optical component losses and imbalances.