# 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  
A tremendous amount of research has been conducted on Analog-to-Digital Conversion from the beginning of modern telecommunications. The reason is apparent: The information to be exchanged among the users via the Internet is predominantly, almost exclusively, digital. Although research is focused on both electronic and photonic realizations of ADCs, heretofore only the electronic ones have been commercialized. More specifically, due to the steady decrease in transistor size (technology node), commercial solutions have exclusively focused on the development of the electronic option: eADCs. Unfortunately, in the last 10 years, transistors' speed and size have started approaching their limits [37]. In particular, [PERSON]'s survey in 2021 [37] highlights that energy consumption and jitter are the main bottlenecks, which have mainly arisen from the minimization of the technology node. Figure 9 shows the Energy per Nyquist sample as a function of the Signal to Noise plus Distortion Ratio (SNDR), which is one of the most well-known FoM metrics that evaluates the performance of an ADC. SNDR is measured when the frequency of operation is set to Nyquist Rate/2. In this figure, the lines for Jitter = 0.1 psrms/1 psrms designate the performance that a fictitious sampler with only the specified jitter numbers (no other nonidealities such as quantization noise, etc.) would achieve. On the other hand, accurate Analog to Digital (A/D) conversion of wideband multi-GHz signals has always been a problem as the use of eADCs is far from optimal and not sufficiently scalable looking forward. The reason is that digitizing analog signals in the range of 100 GHz and beyond is practically impossible because of the lack of electronic devices, which can sample the input signal with sufficient low aperture jitter at this rate, as it is shown in Figure 9. These facts kept the research of photonic ADCs active for almost 30 years, however, without yielding a practical realization at sufficient SNDR. The origins of photonic ADCs date back to 1970 when [PERSON] and [PERSON] [38] were working on optical sampling of Radio Frequency (RF) signals notwithstanding that the different main targets of that work. Photonic ADCs can be classified into four broad categories as per Figure 10 [39]: In particular, photonic-assisted ADCs use optics in order to replace the track-and-hold operation on the RF input signal and perform both sampling and quantization in the electronic domain. Photonic sampled ADCs and photonic quantized ADCs use photonic technology to sample or quantize the signal (but not both), respectively. The combination of sampling and quantizing the signal with photonics is a different category that enables the optical sampling of the intensity angle. Figure 8: Coherent transceiver alternatives for the different applications based on the cost/power consumption and transmission length. Figure 9: ADC Performance Survey 1997–2021 [37]. Illustration of the operational frequency of published ADCs vs. the SNDR FoM. With the red solid/dotted lines is shown the Jitter FoM. As the range of the different approaches for constructing a photonic ADC is huge, the most useful category is the one aiming to overcome the most significant bottleneck of high-speed eADCs. That category is the photonic-sampled ADCs, which overcome the electronic jitter limitation. A general overview of photonic ADCs can be found in the excellent review by [PERSON] [39]. Heretofore, there have been just three State-of-the-Art photonic-sampled ADCs implemented and tested, based on a common fundamental principle of operation. The first one was proposed by a group of researchers at MIT in 2012 [40], whereby they implemented a discrete time-to-wavelength mapping approach (an approach first proposed by [PERSON] in [41]).