## 6 Technical Challenges and Future Directions
### Technical Challenges  
Energy efficiency constitutes a major challenge for network operators, as it can minimize the operational expenditures of network providers, as well as lower the environmental impacts by minimizing carbon emissions. As the majority of the energy is consumed by the ONUs in the FTTH context [39], novel transceivers have to be introduced. A cost-effective solution to this challenge is the exploitation of D/O conversion instead of using the eDAC. This solution incorporates a low-cost coherent transceiver that is tailored to access domains, as the DSP module is extracted, providing significant gains in terms of power efficiency and cost. Capacity scaling is another major challenge, as it can offer high capacity to the end users in a flexible and scalable manner. This challenge is not trivial to solve, as the current fiber transmission technology has reached the nominal plateau of 100 Tb/s per fiber; however, in real links, the transportation capacity is significantly lower, as mainly the C and lesser the L-band are engaged. The exploitation of amplification bands beyond C and L is an efficient solution that can provide greater connectivity and/or greater capacity as extra channels are transmitted [90; 91; 92; 93; 94]. In particular, the low loss fiber amplification bands such as O, E, S, C and L can be exploited in order to support transportation capacity significantly greater than 200 Tb/s for long-haul transmission and higher than 450 Tb/s for data center interconnections [95; 96]. In order to realize a MB system, a significant amount of research has to be carried out both at a network and component levels, in order to develop novel and cost-effective optical amplifiers, transceivers, etc. Another possible solution is space division multiplexing (SDM), which is mainly based on bundles of SMF fibers and can directly lead to an increased overall transportation capacity [97; 98; 92]. Optical communication modules are severely under-integrated, providing a low channel count (four or eight per module), in profound contrast to the high level of integration reached in switching ASICs capable of handling hundreds of +50 Gb/s lanes. This limits the number of modules that can be hosted on switch facelplates (facelplate density) and poses limitations in bandwidth and power consumption due to electronic SERDES (serializers/deserializers) that are present between the switching ASICS and the faceplate-mounted optical modules. The elimination of multiple power-hungry and costly electronic DACs by introducing all-optical counterparts will bring down the required cost and power savings at the digital part of the switches, assisting also at the increase of faceplate densities. An oDAC is essentially an optical multilevel constellation generator (e.g., a source of optical PAM4 or PAM8 in one-dimension (1D) or optical QAM constellations in 2D). Unlike conventional eDAC + modulator optical transmitter structures, oDACs should be driven right from the digital input bitstream (preferably generated by state-of-the-art CMOS at low voltage swings) without internally resorting to intermediary electrical multilevel signals, thus eliminating the problematic ultra-high-speed eDACs. As a consequence, oDACs are expected to play a major role in the next generation of optical communications systems as they enable the generation of the highest capacity signals while being the most energy efficient for supporting Tb/s interconnections. Fiber remote sensing based on coherent optical frequency domain reflectometry (COFDR) is a novel family of optical sensors that can attain ranges of various km, allowing to monitor in real-time and continuously critical parameters such as temperature,acoustics/vibrations, strain, etc. over the entire length of an optical fiber.