# Beyond 100G: All-Optical Processor for High-Capacity Access\(\sim\)Networks
## 1 Introduction  
The post-pandemic era has placed enormous strain on modern networks that strive to adapt to a rapidly changing landscape. The relentless traffic growth, driven by the explosion of bandwidth-intensive applications, such as high-definition video streaming, cloud computing, and increasingly sophisticated AI services, alongside the proliferation of connected devices has pushed the telecommunication networks to their ultimate operational limits. Amidst this unprecedented digital transformation, access networks, the vital link connecting end-users to the core of the telecommunication network, are currently strained under the weight of escalating data traffic and the need for universal, seamless connectivity. The adoption of the 50G PON standard signifies progress in meeting these demands through the use of high-speed IM/DD transceiver solutions, yet the industry and academia are collectively navigating the discussion towards 100G or even beyond. Two paths emerge. The first involves the adoption of a new generation of IM/DD transceivers capable of supporting 50 Gb/s Non-Return-to-Zero (NRZ) signals and extending to 100 Gb/s by means of four-level Pulse Amplitude Modulation (PAM4). The implementation of IM/DD systems offers simplicity and cost and power efficiency and has been traditionally considered as the ideal solution for the \"last mile\" of network connectivity (10G and 25G PON standards). Nevertheless, as the speed requirements of the access networks increase beyond 100G, the cost-performance benefit of IM/DD systems becomes questionable due to the severe effect of chromatic dispersion, which limits the PON reach. Various approaches have been proposed to compensate for the induced impairments utilizing sophisticated electrical equalization techniques [1; 2; 3; 4] and neural-network-based approaches [5; 6; 7; 8; 9; 10]. Additionally, solutions involving different types of amplification [11; 12], alternative modulation schemes [13], or hybrid approaches [14; 15; 16] have been demonstrated. However, all these approaches increase the overall system complexity as well as the power consumption. The second path is the employment of coherent solutions, which are increasingly penetrating applications traditionally dominated by IM/DD technology [17]. The integration of coherent optics into a PON offers specific advantages, and extensive research has been conducted in this direction [18; 19; 20; 21]. However, the adoption of coherent technologies in the next generation of PONs is not without significant challenges, the most important among these being the implementation cost. Given that PON equipment involves the end-users, any increase in implementation cost is amplified exponentially. The incorporation of an additional laser for the LO, the increase in the number of photodetectors used, the more sophisticated modulator designs, power-hungry DSP algorithms, and increased electronics complexity not only escalate costs but also augment the power consumption and the overall system complexity. Furthermore, upstream burst mode detection becomes notably more intricate with coherent optics compared to IM/DD approaches. These challenges underscore the necessity for careful consideration and strategic planning in navigating the transition towards the next generations of PON technologies. The complexity and power consumption of electrical and machine learning approaches, particularly as PON technologies evolve towards higher speeds, have highlighted the potential advantages of optical processing equalization techniques. The exploration of optical methods, long considered for their efficiency and performance benefits, has been intensified to mitigate the limitations of traditional electrical solutions, such as reliance on power-hungry DSP algorithms and complex electronic circuits, while maintaining similar, if not better, performance. Various approaches have been explored, making optical signal processing a promising candidate for the next generations of PONs [22; 23; 24; 25; 26; 27].