# 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
This is why one of the key goals of the research community is the reduction in the energy-per-bit by conceiving a new optical transceiver architecture able to attain direct digital to optical domain conversion, the optical DAC (oDAC), eliminating power-hungry electronics and specifically doing away with conventionally expected future eADCs featuring three or more bits given their scalability problem in rate-vs.-energy-efficiency. The oDAC will still be driven by electronic D/A interfaces, but those will be highly efficient and scalable--the drivers of the oDAC will comprise arrays of decoupled 1 bit (NRZ) or, at most, 2 bits (PAM4), where signal formats are relatively mature, more energy efficient, and more amenable to upscaling than higher-order ADCs, which are about to be pushed beyond their limits in the upcoming optical interconnects roadmap. Over the last decade, the research activity regarding the oDAC has been mainly focused on'serial' structures, which are based on partitioning the electrodes of the basic MZM (Figure 13). The so-called Segmented Mach-Zehnder Modulator (SEMZM) [45, 46, 47, 48, 49, 50, 51, 52, 53] breaks up the MZM electrodes pair along the two MZM waveguides into multiple (shorter) electrically isolated segments. Each modulation-segment amounts to an elementary 1-bit modulation gate driven by a separate binary signal. The summation of the optical phases  
Figure 12: Conventional 64-QAM architecture where the light is split into two parallel optical paths and is modulated using a pair of MZMs. induced by the array of 1-bit segments along the device generates a multilevel output optical signal. Accordingly, Figure 13 depicts an exemplary S = 7 segment thermometer-weighted SEMZM optical structure, aiming to implement oDAC functionality. The contiguous MZM electrodes are partitioned into S segments. Each segment acts as a \"partial MZM\" with a push-pull drive. Hypothetically assuming a single segment is electrically driven (the other segments grounded or removed), we would obtain a \"partial\" MZM modulator with a short electrode pair, inducing just a fraction of the maximum possible peak phase modulation, as attained when all segments are active. When all the segments are modulated in unison, driven by properly synchronized binary signals, the total differential phase along the device may assume a multiplicity of values, inducing, by interference in the output coupler, multiple optical amplitude (or power) levels at the device output. As an alternative approach, a \"parallel\" design that employs electroabsorption modulators (EAMs) instead of MZMs has been also proposed in [54]. The compact size of the EAMs and their ease of integration with silicon photonics make them a real lucrative option. However, various works examining the'multi-parallel' oDAC functionality have been proposed, e.g., via arraying in parallel a pair of 1-bit (OOK) [54] or 2-bit (QPSK) [55, 56, 57, 58] modulation gates, fed after splitting a common optical source, with their outputs superposed in an optical combiner. The latest and state-of-the-art'multi-parallel' oDAC structure is proposed in [22] introducing an architecture, which, via the combination of identical parallel paths and a variable splitter and combiner, is able to generate both direct and coherent detection constellations (Figure 14a). Figure 14: (**a**) The generalized parallel architecture proposed in [22] and (**b**) an implementation of a PAM8 generator that comprises a variable 1:3 splitter, a fast MZM array to modulate the light, and a variable 3:1 combiner. The slow PM array is a crucial element for the control and calibration of the o-DAC. Figure 13: Segmented MZM (SEMZM) 7 segments oDAC architecture-optimized thermometer-weighted DAC Structure for generation of PAM8 signals [22]. This SEMZM comprises seven segments; each segment is described as a yellow box. In red, the optical waveguide that light passes through and is modulated by each segment appropriately is shown.