# 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  
After a long period of utilizing coherent systems for long-haul transmission, the need to exploit the same benefits in terms of offered data rates for shorter links has become  
Figure 4: Conventional optical transceiver. On the transmitter side, the electronic circuit employs the DSP, DAC, and DRIVER functionalities, whereas on the receiver side, the electronic circuit employs the ADC, TIA-AGC, and DSP functionalities. The photonic circuit performs the modulation of light on the transmitter side and photodetection on the receiver side. apparent. Following this path, a lot of research has been conducted lately to unravel coherent transceivers with the power consumption and cost penalties that they are attached to. In this context, two main alternatives have been extensively investigated in the literature: (a) The simplified coherent and (b) the analog coherent. In order to offer optimized performance for short links and high energy efficiency, both discard many of the digital signal processes that conventional coherent transceivers employ. The all-analog (electronic and photonic) signal processing techniques have been utilized to partially replace some of the processes of DSP and power-hungry electronics (e.g., DACs, ADCs, and electrical phase-locked loops). This is different than the lite-coherent approach where the main objective is to optimize the DSP algorithms in order to reduce the power consumption. The differences between analog and simplified coherent can be mostly found on the optical frontend, where analog coherent systems remain the same compared with the conventional architecture, while in simplified coherent systems, different architectures than those of the conventional ones have been proposed to exploit the benefits of coherent transmission. Furthermore, solutions for scaling up the capacity using direct detection schemes have been proposed as a simpler alternative. In [24, 25], using a polarization demultiplexing scheme, it is described how to exploit two different polarization multiplexed intensity-modulated signals. Thus, direct detection can offer twice the rate once an analog polarization demultiplexer is realized. Along these lines, we can extend the benefits of IMDD by employing a novel PAM-M modulator that has been proposed recently using a parallel scheme of MZMs in order to relax the requirements of the eDACs [26]. In conclusion, even though simplified coherent solutions have been much more investigated and are presented in the upcoming paragraphs, a quantitative comparison between the conventional schemes, analog coherent, simplified coherent, and the two very similar architectures that share strong similarities between analog and simplified is introduced and tabulated in Table 2.  
The all-optical receiver has been initially presented in [26] (see Figure 5 for its high-level design), indicating the possibility of replacing the DSP chip with an all-optical analog front-end that can achieve most of the required functionality. The novel required Rx Blocks of this scheme are: Analog optical carrier phase recovery utilizing an Optical Phase-Locked Loop (OPLL) based on a low-linewidth Laser [23, 27], to optically decouple the two I and Q quadratures. Analog optical polarization demultiplexing circuit [28], decoupling all optically the two signal polarizations. The OPLL could be implemented at a bandwidth of \(\sim\)10-100 MHz based on a high-bandwidth photodetector and a low-linewidth semiconductor laser, operating the local oscillator laser as a Current Controlled Oscillator (CCO). The Analogue Polarization Demultiplexing (PolDMUX) circuit block, depicted in Figure 5 in the polarization controller inset, consists of a cascade of several 2 \(\times\) 2 matrix stages and is implemented based on directional couplers and phase modulators (PMs), as was experimented in [29]. It is noted