# A Vision of 6 th Generation of Fixed Networks (F6G): Challenges and Proposed Directions
## 3 Photonics-Based Subsystems for All-Optical Processing
### All-Optical Coherent and Simplified Coherent Solutions: Challenges and Opportunities
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **Conventional** & **Conventional** & **Simplified** & **Analog** & **Low Cost/Power** & **Analog Coherent** \\  & **IMDD PAM-4** & **Coherent** & **Coherent** & **Coherent** & **Dual Polarization** & **W/Novel** & **PAM-M** \\ \hline
**LO** & No & DFB & No & DFB & No & Typical \\
**requirements** & None & Limited by eDACs & Limited by eDACs & Limited by eDACs & None \\
**Rx complexity** & PD & Power Hungry & None & oPILs & None & None \\
**DSP at Rx** & Low & Heavy & None & Low & None & Low \\
**Power** & Medium & Very High & Low & Low & Low & Low \\
**Sigaling rate** & Up to 100 Gb/s & Up to 800 Gbp/s & Up to 12 Gb/s/s//pol & Up to 800 Gb/s/s & 112 Gb/s/s//pol & 200 Gb/s/s//pol & 800 Gb/s/s \\
**Reach** & More than 20 km & More than 1000 km & Up to 2 km & Up to 10 km & N/A & Estimated up to 10 km \\
**Application** & Access/Intra-DC & Long- & Short reach/Access & Intra-DC & Intra-DC & Inter-DC/Intra-DC/Access \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of various parameters of four low-cost/-power transceiver schemes for short-reach transmission [26]. that slow (\(\sim\)10 kHz rate) precise tuning control of the phase alignments of the PMs in the cascade is required as an essential enabler for the practical polarization demux functionality of this block to which we refer to as the Analog Optical Signal Processor (AOSP). Due to the all-optical nature of signal processing in the 10-100 MHz building blocks of this transceiver, which are transparent to the ultra-high-speed optical signals modulated onto the light flowing along it, the efficient operation of the AOSP polarization demux is essentially independent of the baud rate; hence, the supported bit-rate is scalable to \(>\)800 Gb/s rates per wavelength/space lane. Moreover, multiple optical wavelengths may be served at once (polarization demultiplexed together by a single AOSP) to further scale the offered capacity. The AOSP is still followed by ADCs (slicers), albeit at a much-reduced Effective Number Of Bits (ENOB), and the DSP is reduced in cost, footprint, and power consumption, as the XY polarizations and IQ quadratures are effectively decoupled ahead of the four ADCs. Other receiver operations such as timing recovery and detection may be performed with a high-speed analog electronics stage using conventional clock and data recovery (CDR) techniques. Alternatively, these functions may be relegated to the scaled-down DSP. For basic proof-of-principle feasibility purposes, the operation of such an AOSP receiver has been demonstrated via preliminary simulations and the results are shown in Figure 6. In this particular example, operation at an aggregate rate of 400 Gb/s was considered [26]. In the simulation platform, the AOSP was constructed by considering a low-linewidth local oscillator laser and a cascade of discrete Mach-Zehnder Modulators (MZMs) and PMs for the Pol-Demux, which is not as effective as in the case where integrated elements in a Photonic Integrated Circuit (PIC) are employed. Following the all-optical AOSP-based polarization demultiplexing and phase/frequency recovery operations, the four coherently recovered analogue lanes, Ix, Qx, Iy, and Qy, display clear bipolar Pulse Amplitude Modulation 4 (PAM4) eye diagrams, which are detectable using conventional CDRs (based on all-analogue electronic symbol timing recovery and 4-level slicing implemented in merchant Silicon circuity). These initial simulation results at 400 Gb/s [26] are significant, as they validate the feasibility of the all-optical analogue signal processing-based coherent transceiver. The aforementioned all-optical solutions, besides the sufficient transmission performance, can also scale-up the capacity capabilities in order to meet the ever-growing traffic demands of the coming decade.