# A Vision of 6 th Generation of Fixed Networks (F6G): Challenges and Proposed Directions
## 3 Photonics-Based Subsystems for All-Optical Processing
### Photonic DACs: Challenges and Opportunities
The basic goal of a parallel design is to generate optical multilevel PAM signals using only binary driving digital signals. As shown in Figure 13(a), in this implementation, 1-bit gates are the fundamental building blocks. These blocks either let the optical signal pass through the specific path or block it and can be practically implemented using MZMs. Then, by adding all the optical paths using a combiner, optical signals with different amplitude levels can be generated. Figure 13(b) illustrates how a PAM8 generator can be implemented using this architecture. In practice, the variable splitters and combiners are identical and bit gates are put just in parallel, demonstrating the high scalability of this architecture. In that aspect, it is possible not only to scale the parallel paths but also the electronic PAM (ePAM) levels that drive the MZMs in order to scale the attainable data rate. It is theoretically established that by using ePAM4 in each of the two parallel paths via a two-way o-DAC, it is possible to scale the constellation up to optical 256-QAM [59]. Moreover, extensive analysis has been performed in [49] in order to demonstrate the significance of splitting and combining ratios to realize such a design. Also, in [49], control and calibration functionalities were added along the optical paths by inserting slow phase modulators. Accounting for mismatch in splitting or combining ratios occurring in the manufacturing processes, or due to environmental disturbances, this current design becomes far more robust compared with that of previously published work. Other optical DAC solutions based on (a) silicon ring modulators [49; 50], (b) III-V-on-Si electro-absorption modulated distributed feedback laser [51], and (c) polarization division multiplexing [53; 54] have been also proposed. The previous generations of MZM devices might not be suited for short-reach interconnects as typically large transmission line structures are required. With ring resonators, the system becomes too dependent on temperature fluctuations and needs control systems to guarantee its stable operation. Silicon microring modulators are already implemented but they are limited to 80 Gb/s rates even with DSP on both the transmitter and receiver sides, something that contradicts the notion of the DSP-free architecture. Generally, from all the experiments demonstrated heretofore, we may conclude that although optical DAC implementations have been proposed, the results show that there is no particular implementation yielding considerably better results compared with the others. If one thing is clear, it is that control and calibration of different kinds of mismatches like phase mismatches or energy mismatches will play a key role for any type of deployed modulator. It goes without saying that the path for reducing energy consumption and achieving better performance in the near future acknowledges that electronics have come to their limits and the future seems to be purely optical. Although these designs are promising, the research community still seeks optimal scaling options for ultra-highspeed optical modulator-based transmitters, facing multiple challenges precluding a breakthrough in these directions.