# A Vision of 6 th Generation of Fixed Networks (F6G): Challenges and Proposed Directions
## 3 Photonics-Based Subsystems for All-Optical Processing
### Photonic ADCs: Challenges and Opportunities
The second alternative resulted from a collaboration of researchers from Rohde and Schwarz and IHP in 2017 [42], which demonstrated a monolithically integrated time-interleaved topology to relax the timing constraints at the highest sampling rates. The third and most recent one was proposed by the Institute of Integrated Photonics of Aachen University [42], demonstrated two monolithically integrated topologies; one based on time-interleaving, whereas the second one was based on frequency interleaving. Although the target here is not to elaborate on the principle of operation of these implementations, we discuss the most important aspect, namely the optical sampling of the RF signal [43]. The main components of an optically sampled system are shown in Figure 11 and comprise a stable pulsed laser, an optical modulator, and a detector/integrator. The basic concept is that the ultra-short pulses of the laser actuate the modulator over an ultra-short and ultra-stable (low-jitter) time-window, such that the average amplitude of the analog signal over that window, an excellent approximation of a point-sample, is mapped on the amplitude of the optical signal. This mapping of the amplitude is converted into electrical variation by the photodetector and eventually quantized in the electronic domain (thus the sampling is performed by optics, the quantizing is electronic). The most significant advantage is that the overall jitter induced by the laser pulses is much smaller compared with the jitter created in its electronic counterpart. For clarification, the sampling rate is just the Mode-Locked Laser (MLL) pulse repetition period. As demonstrated in [40], with a single laser, several repetition periods can be implemented by adding a multiplexer, a dispersive fiber, and a demultiplexer. A comparative analysis of various options for the optical sampling process is summarized in Table 3.  
In Table 3, a trade-off between the sample rate and the bandwidth of the optical sampling processes for RF signals is evident across these four alternatives. Alongside this trade-off, it should be pointed out that different processes to manufacture the PIC have been employed. Both bipolar complementary metal-oxide-semiconductor (BiCMOS) and silicon photonic integration may be considered. All four variants have successfully implemented a photonic-sampled ADC, with the latest option exceeding 60 GHz bandwidth and achieving the lowest power consumption and die area. These four variants validate the concept of photonic ADC and establish its great potential to enable systems aiming  
Figure 10: Four major classes of photonic ADCs with their different variants. to process signals in the microwave region with sufficient accuracy, made possible by the sampling photonic technology. In conclusion, we deem it important to point out that as a first step, the major bottleneck of high-speed ADCs may be eliminated based on photonic technology, exceeding the performance of an electronic counterpart. The next step towards achieving a fully optically enabled ADC is to conceive a feasible method that implements photonic quantization. Although practical implementations have not been realized yet, some photonic sampled architectures through optical quantization architectures hold promise to exceed their electrical counterparts [44], as well as to reduce power consumption due to the use of optical elements. Nevertheless, a significant amount of additional work lies ahead along the road towards realizing an all-optical ADC with reduced power consumption. Moreover, the photonic ADCs surveyed above have electronic inputs and outputs. A worthy future objective would be to establish the feasibility of an oADC with optical input (carrying the information modulated signal) and electronic digitized output, embedded in a future all-optical polarization and IQ demultiplexing front-end of a coherent receiver, in order to pave the way for truly all-optical low-energy consumption systems.