# 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  
Electronic Digital-to-Analog Converters, abbreviated as eDACs, are key interfaces that convert the signal from the digital to the analog domain. In high-speed digital optical transmitters, the D/O translation typically comprises costly and power consuming eDAC-based data conversion, followed by analogue optical modulation. In a conventional optical link, eDACs are used to drive multilevel analog signals into the optical modulators. Figure 11: Photonically sampled and electronically quantized ADC. The analog electrical signal is sampled through a photonic process that employs a mode locked laser, an electro-optic modulator, and a high-speed photodetector. Then the electronic ADC performs the quantization of the corresponding electrical signal. \begin{table}
\begin{tabular}{l c c c c} \hline \hline \multirow{2}{*}{**Topology**} & **Optically** & **MZM** & **MZM** & **Optically** \\  & **Clocked** & **Sampler** & **Sampler** & **Clocked** \\ \hline Sampl. Rate & 10 GS/s & 2.1 GS/s & 2.1 GS/s & 0.25 GS/s \\ \hline BW & 30 GHz & 41 GHz & - & 65 GS/s \\ \hline THD/SFDR & \(-\)39 dB/42 dBc & -/52 dBc & -/39 dBc & \(-\)38 dB/39 dBc \\ \hline @\(\text{f}_{\text{in}}\) & @32.5 GHz & @41 GHz & @10 GHz & @43 GHz \\ \hline ENOB & 5.57 (SNR) & 7 (SINAD) & 3.5 (SINAD) & 5.5 (SINAD) \\ \hline @\(\text{f}_{\text{in}}\) & @10 GHz & @41 GHz & @10 GHz & @45 GHz \\ \hline Power & 1250 mW & - & - & 506 mW \\ \hline Die Area & 4.84 mm\({}^{2}\) & - & - & 0.59 mm\({}^{2}\) \\ \hline \multirow{4}{*}{Process} & 250 nm & \multirow{4}{*}{Discrete} & \multirow{4}{*}{Integrated} & \multirow{4}{*}{250 nm photonic} \\  & Photonic & SiGe & components & silicon photonics & SiGe BiCMOS \\ \cline{1-1}  & BiCMOS & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Quantitative comparison of options for the optical sampling process of the RF signal [43]. As mentioned earlier in this chapter, from the viewpoints of performance, energy efficiency, complexity, and cost reduction, it would be advantageous to adopt direct D/O conversion and eliminate the eDAC intermediaries. This would be particularly useful with regard to ultra-high-speed photonic interconnects, as eDAC technology is even more expensive and is becoming increasingly challenging to keep scaling up data rates while keeping power dissipation at reasonable levels and maintaining the required ENOB performance. To introduce optical-to-digital conversion, let us first review the conventional way of generating multi-level optical signals. For decades, IQ modulators generating optical constellation signals remained the same, e.g., the case of 64 QAM is illustrated in Figure 12. The light split into two parallel optical paths is modulated using a pair of MZMs. Each MZM modulates the Continuous Wave (CW) optical signals according to the electrical voltage applied to its electrodes. As a consequence, for quantized electrical voltage levels applied to the MZM electrodes, quantized optical levels are generated (and are relatively phased 90\({}^{\circ}\) along the two parallel paths). However, the bandwidth of the entire transmitter is limited by the lowest bandwidth of either the DAC in the electronic driver or the MZM electrical to optical modulator transducer. In order to scale-up the capacity of a single-lane point-to-point optical tributary, one can either increase the baud rate or adopt formats with higher spectral efficiency. These solutions require either faster electronics or more complex DSP in the Rx. Either way, both options entail increased cost and power consumption. In addition, as silicon technology is approaching its physical limits, the use of faster electronics no longer seems a viable solution in terms of its upscaling rate, which is no longer able to keep up with the data rate scaling roadmap.