Fig. 1. Global data generated annually^[1]

Fig. 2. Evolution trend of data center network. (a) Traditional data center network; (b) disaggregated optical transmission network; (c) disaggregated and optical switching networks

Fig. 3. Integrated optical switching devices on different material platforms. (a) Planar silica integrated optical switching device^[11]; (b) III-V material integrated optical switching device^[12-13]; (c) Ti∶LiNbO₃ optical switching device^[14]; (d) optical switching device on TFLN-OI^[15]; (e) PLZT waveguide integrated optical switching device^[16]; (f) silicon MEMS integrated optical switching device^[17]; (g) silicon optical switching device^[18]

Fig. 4. Development of silicon thermo-optic switching devices^[37-46]

Fig. 5. Development of silicon electro-optic switching device^[18,47-54]

Fig. 6. Common networks used for constructing large-scale integrated optical switching devices. (a) Benes network; (b) PILOSS network; (c) S&S network; (d) double layer network

Fig. 7. Optimization approaches for optical switching units. (a) Switching unit with cascaded DC couplers^[47]; (b) exchanged switching unit at output^{end[57]; (c) switching unit with bent waveguide DC couplers[58]; (d) switching unit with adiabatic DC couplers[59]; (e) switching unit with adjustable splitting ratio[66]; (f) switching unit with array of MZIs[61]; (g) double-MZI switching unit[20]; (h) nested-MZI switching unit[62]; (i) switching unit with phase pre-bias[18]; (j) switching unit with wide waveguide phase shifters[63]; (k) calibration-free MZI switching unit[64]; (l) switching unit with three-section waveguide phase shifters[65]; (m) PIN diode switching unit vertically stacked with micro-heater[52]; (n) PIN diode switching unit with micro-heater in series[54]}

Fig. 8. Integrated optical crossing devices. (a) Crossing waveguide with three-mode synthesis^[73]; (b) crossing waveguide with inverted tapers^[74]; (c) crossing waveguide optimized with PSO^[75]; (d) subwavelength grating crossing waveguide^[76]; (e) crossing waveguide with lateral subwavelength nanostructures^[77]; (f) inverse-designed crossing waveguide^[78]; (g) ultra-compact crossing waveguide^[79]; (h) crossing waveguide with curved anisotropic metamaterial^[80]; (i) star-crossing waveguide^[81]; (j) three-dimensional crossing waveguide device^[82]

Fig. 9. Several types of grating couplers. (a) Polysilicon grating^[84]; (b) double-etched grating^[85]; (c) grating with subwavelength structure^[86]; (d) grating with bottom reflector^[87]; (e) Si-SiN dual-layer grating^[88]; (f) one-dimensional inverse design grating^[89]; (g) two-dimensional multi-layer inverse design grating^[90]; (h) plasmonic grating^[91]

Fig. 10. Edge couplers. (a) Edge coupler with SiN waveguide^[92]; (b) edge coupler with two-layer Si waveguide^[93]; (c) edge coupler with metamaterial structure^[94]; (d) edge coupler using silica-based connecting device^[95]

Fig. 11. Some other types of optical coupling methods. (a) 45° curved micro-mirror^[99]; (b) vertically curved Si waveguide coupler^[100]; (c) coupling with microlenses on the backside^[103]; (d) photonic wire coupling^[107]

Fig. 12. Silicon optical switching devices integrated with on-chip optical amplifier. (a) Integration of optical amplifiers through flip-chip bonding^[109]; (b) integration of optical amplifiers through chip facet butt-coupling^[111]

Fig. 13. Heterogeneous integrated devices via molecular bonding and epitaxy growth. (a) Integrated quantum dot laser via molecular bonding^[115]; (b) integrated quantum dot laser via epitaxial growth^[114]

Fig. 14. Impact of crosstalk on the testing of switching units in Benes network

Fig. 15. Calibration methods for optical switching units: (a) Setting up monitoring points at the four ports of each switching unit^[52]; (b) using integrated detectors to monitor two output ports of each switching unit^[119]; (c) monitoring point setting based on Benes network characteristics^[18]

Fig. 16. System-level calibration methods for optical switching devices. (a) Multi-port input switching unit calibration method^[121]; (b) system calibration method based on pulse response^[122]

Fig. 17. Optical packaging methods. (a) Packaging with grating array coulping; (b) packaging with edge coupler array

Fig. 18. Electrical packaging methods. (a) Monolithic integration of electronic and photonic devices^[51]; (b) wire welding at the edge of the optical chip^[42,56]; (c) fan out the electrical interface through the interposer^[41]; (d) photoelectric hybrid package on glass substrates^[123]

Fig. 19. Three-dimensional packaging schematic of the optical switching chip