# A Vision of 6 th Generation of Fixed Networks (F6G): Challenges and Proposed Directions
## 3 Photonics-Based Subsystems for All-Optical Processing
### Reconfigurable Photonics  
Generally, PICs have been extensively investigated as platforms for optical fiber communications. Over recent decades, the basic driving force behind creating lower-cost and more robust photonic circuits has been the increase in data rates in order to meet the ever-increasing demand of end-users. The main objective of PICs is to process optical signals efficiently and at low latency, over a low-cost and low-power platform. These requirements cannot be simultaneously satisfied by simply considering digital solutions, due to the inevitable severe tradeoffs in terms of energy efficiency and processing speed electronics. This fact, along with observing the track record of electronics re-programmability capabilities, which turned out to be highly valued in practical use-cases, suggests exploring reconfigurability for photonic circuits as well. This concept attracted the attention of researchers and start-ups, and extensive efforts have been invested in reconfigurable and programmable photonic circuits [60; 61; 62]. The most promising architectural concept is that of the 2D hexagonal Tunable Base Unit (TBU) mesh. The TBU is basically a tunable coupler constructed by a balanced Mach-Zehnder Interferometer (MZI) [60]. Upon investigating all the parameters and analyzing the TBU in great detail, the functionality of different topologies based on this basic unit was shown to be realizable. A plethora of photonic functions has been demonstrated through the 2D hexagonal mesh architecture, such as basic tunable Infinite Impulse Response (IIR) and Finite Impulse Response (FIR) filters with feed-forward and feed-backward capabilities and multiple input/output optical linear transformers [60]. Although research on such technology and its applicative benefits is still relatively immature and various drawbacks are still to be addressed, it appears that the successful development of robust, industrial-grade reconfigurable photonics will keep evolving and, as a consequence, pave new ways to streamline design and implementation of an all-optical analog platform supporting a plethora of applications. Automated approaches may enable photonic mesh designs capable of supporting more than just simple photonic functions, e.g., filters. In particular, optical DACs have been proposed in [22], endowed with reconfigurability--which may be traced to the modularity of their design, down to the basic building block level, comprising elements such as \"1-bit gates\" or the 2:2 Mode Converters as per Figure 15a. Moreover, even more \"exotic\" photonic circuits may be implemented over reconfigurable optics, as is the case with waveband selective switches to be discussed in Section 5, to provide processing of optical signal at a very wide bandwidth (over S + C + L amplification bands), routing its spectrum portions in several bands as shown in Figure 15b. Significantly, the reduced waveband count and filter sharpness allow one to meet the filtering requirements with multi-tap FIR optical filters, implemented as adaptive lattice filters on a photonically integrated circuit as opposed to bulkier dispersive free-space optics arrangements with LCoS channel selection. Moreover, the design of direct FIR filters (digital and their optical brethren) based on MZI structures like the TBU is well understood and known to require more stages than IIR filters [61]. Due to the large FSR requirement of the implemented filters (~140 nm), IIR filters of low loss would be hard to implement. The spectral resolution (i.e., the transition bandwidth) of an FIR filter would scale with the FSR  
Figure 15: (**a**) One of the two basic building blocks of the o-DAC [22]; (**b**) general architecture of the waveband selective switch, consisting of an adaptive filtering stage and an output port switch; (**c**) structure of photonic lattice filter consisting of repeating blocks of tunable couplers and unit delay.