Fig. 1. (a) Schematic of the MTPBS based on multimode bus waveguides which support multiple mode channels. The modes are indistinguishable, while light can be distinguished by the polarization state. Dual-polarization MDM signals can be split into TE and TM polarizations. TE MDM signals are reflected at 90°, while TM MDM signals are transmitted straightly. The bi-trench coupler consists of a pair of total internal reflection (TIR) mirrors separated by fully etched trenches. (b) Cube PBS as an analogy of MTPBS. The cube PBS consists of a pair of right-angle prisms separated by a polarization-dependent dielectric coating on the hypotenuse of one of the prisms. P-polarized light is turned with 90°, while S-polarized light is transmitted straightly.

Fig. 2. (a) Calculated effective indices of eigenmodes in the waveguide with different widths. The square-curves and triangle-curves are for TE and TM polarizations, respectively. (b) The simulated power transmission efficiency at through port versus different trench widths for TE0 and TM0 modes by using single-trench, bi-trench, and tri-trench couplers. The FDTD numerical method and transfer matrix theory are utilized for simulation at 1550 nm as a comparison. The trench width is the same as the gap between neighboring trenches. (c) The schematics of these three kinds of trench couplers. (d) The FDTD simulated power transmission efficiency at the through port for TE0−TE6 and TM0−TM5 modes for the bi-trench coupler at 1550 nm. The simulated mode transmission efficiency when inputting (e) TE0, TE6 and (f) TM0, TM5 modes from 1500 to 1600 nm. In the legend “TM0−TM1−T,” the TM0 mode stands for the input mode, while the TM1 mode stands for the output mode. The letter “T” refers to the through port, and the letter “C” refers to the cross port.

Fig. 3. Simulated light propagation in the MTPBS for (a) TM0, (b) TM5, (c) TE0, and (d) TE6 modes at the wavelength of 1550 nm. White-solid lines indicate the location of waveguides.

Fig. 4. (a) Microscope view of the tested device with an input port and two output ports. Two tested devices with the same geometry but different mode multiplexers are needed for complete characterization. (b) Seven-TE-mode (de)multiplexer and (c) six-TM-mode (de)multiplexer are utilized to obtain single-polarization MDM signals individually. The adiabatic taper connects the TE/TM mode (de)multiplexer with the MTPBS. (d) Zoom-in view of MTPBS, where the TM grating is used to filter out the scattering light from TM polarization.

Fig. 5. (a) Schematic of ith adiabatic coupler (AC) for TE₀-TE_i mode conversion; two reversely tapered waveguides (access waveguide and bus waveguide) are placed closely to form a coupling region. The seven-TE-mode (de)multiplexer is composed of six cascaded ACs, which are used to excite TE1−TE6 modes. (b) Schematic of ith asymmetric directional coupler (ADC) to realize TM₀-TM_i mode conversion. The six-TM-mode (de)multiplexer consists of five cascaded ADCs, which are used to excite TM1−TM5 modes. (c) Detailed parameters for TE and TM mode multiplexers.

Fig. 6. Normalized spectra of the MTPBS when injecting TE0−TE6 modes. For a given inputting mode, the spectra for these seven modes at through and cross ports are measured successively. In the legend “TE1−C,” the TE1 mode stands for the output mode, while the letter “C” refers to the cross port. In the legend “Cross talk-T,” “Cross talk” refers to the maximal cross talk at the through port which mainly comes from the adjacent modes, while the letter “T” refers to the through port.

Fig. 7. Normalized spectra of the MTPBS when injecting TM0−TM5 modes. For a given inputting mode, the spectra for these seven modes at through and cross ports are measured successively. In the legend “TM1−T,” the TM1 mode stands for the output mode, while the letter “T” refers to the through port. In the legend “Cross talk-C,” “Cross talk” refers to the maximal cross talk at the cross port which mainly comes from the adjacent modes, while the letter “C” refers to the cross port.