# Silicon Integrated Nanophotonic Devices for On-Chip Multi-Mode Interconnets
## 5 Reconfigurable Multi-Mode Photonics  
Reconfigurability is a very desirable attribute in MDM systems. A reconfigurable MDM is developed to select any desired mode channels and deploy them at will. This allows for a dynamic  
Figure 18: Upper panel: the schematic for photonic inverse-design. Lower panel: the schematic for inverse-designed mode (de)multiplexer [85], multi-mode bend [175], multi-mode crossing [204] and multi-mode splitter [220]. and flexible MDM network where carriers can be optimally utilized. For single-mode photonics, there are two main streams of reconfigurable devices, i.e., non-blocking switches [201; 202; 232; 233; 234; 235; 236] and reconfigurable optical add-drop multiplexers (ROADM) [237; 238; 239]. The counterparts in multi-mode photonics are mode switches [240; 241; 242; 243; 244; 245; 246; 247; 248; 249] and mode ROADM [250; 251], as summarized in Figure 19.  
The mode switches can re-allocate the signal mode relation directly in the optical realm without add/drop operations. The simplest case is a mode exchanger that swaps two-channel signals carried by TE\({}_{0}\) and TE\({}_{1}\)[240]. The proposed structure consists of two symmetric Y-junctions with a phase shifter in between. The controllable TE\({}_{0}\)-TE\({}_{1}\) exchange can be achieved by tuning the phase difference between two arms. The measured extinction ratio is as large as ER \(\approx\) 24 dB as TE\({}_{0}\) and TE\({}_{1}\) are switched. Such mode exchange can also be realized by applying some analogous schemes, such as ones based on adiabatic couplers [241], MMIs [242; 243] and long-period gratings [244]. However, all these structures can only work for a two-channel MDM. In [245], a general architecture was proposed for scalable mode switches, as illustrated in Figure 19a. For a mode switch with four channels, the inputted TE\({}_{0.3}\) modes are firstly demultiplexed into four waveguide channels with TE\({}_{0}\), which are then arbitrarily routed by a non-blocking single-mode switch (with Benes or Spanke-Benes fabrics) and reverted to TE\({}_{0\text{-}3}\) modes by a mode multiplexer. Generally, the N-mode switch can be easily obtained by cascading an N-channel mode demultiplexer, an N \(\times\) N non-blocking switch and an N-channel mode multiplexer. Furthermore, by using a MN \(\times\) MN fabric, the modified mode switch is also capable of arbitrarily distributing M \(\times\) N modes among M multi-mode waveguides. The mode switching is also achievable for the WDM/MDM or PDM/MDM hybrid systems by exploiting tunable MRRs [246; 247; 248] or polarization-selective switches [249]. The mode ROADM can add/drop signals carried by any desired mode channels. This functionality allows light paths to be set up and taken down dynamically as needed in the MDM network. In 2015, [PERSON] et al. reported an asymmetric-MRR-based structure that can select and extract a single WDM/MDM hybrid signal from the multi-mode bus waveguide [252]. However, in the strict sense, this design is not a mode ROADM since it is unable to upload carriers. To the best of our knowledge, the first mode ROADM was reported in [250]. The device consists of two four-channel mode (de)multiplexers and four parallel Mach-Zehnder interferometer (MZI) switches, as shown in Figure 19b. The incident TE\({}_{0\text{-}3}\) modes are firstly transferred to different waveguide channels with TE\({}_{0}\). When the switches are off, the hitless signals will propagate through. A mode demultiplexer is then utilized to restore the higher-order modes. On the other hand, when the switches are on, the MDM  
Figure 19: The configurations for (**a**) mode switch [245] and (**b**) mode reconfigurable optical add-drop multiplexers (ROADM) [250]. The insets show the mapping relation between signals and modes. Here, a four-channel MDM system is illustrated as an example. signals can be added and dropped at different ports of MZI. The experimental results show low losses (IL \(\approx\) 1-5 dB) and high extinction ratios (ER \(\approx\) 15-20 dB) over a 30-nm wavelength band.