Fig. 1. Schematic diagrams showing (a) the structure of the proposed mode converter consisting of cross-connected symmetric Y-junctions, (b) subwavelength four- and dual-mode symmetric Y-junctions, and (c) single-mode waveguide crossing.

Fig. 2. Simulated normalized transmission spectra of the 1×4 subwavelength or conventional Y-junction for TE0, TE1, TE2, and TE3 input modes, respectively. The inset is the electric field profile of the four outputs (at 1.55 μm).

Fig. 3. Simulated distributions of the Hz field component (at 1.55 μm) of the 1×4 subwavelength Y-junction showing the phase distributions of (a)–(d) TE0, TE1, TE2, and TE3 input modes, respectively.

Fig. 4. Simulated spectra of the cross-connected converter with no voltages applied to the electrode heaters for four different input modes. The dotted lines represent the simulated results with different fabrication errors.

Fig. 5. (a) Optical microscopic image of the reference MDM system. The optical microscopic image of the reconfigurable four-mode MDM systems consisted of (b) cross- and (c) parallel-connected converters; (d) the reference system to determine the power to produce a π-phase change. (e)–(g) The SEM pictures of the subwavelength four- and dual-mode symmetric Y-junctions, and the single-mode waveguide crossing.

Fig. 6. (a) States of the phase shifters and the power consumption in a specific conversion function. (b) The normalized IL and XT profiles of the converter measured with the TE0 mode as the input mode for the specific converting state: P1 and P2 “on”. (c)–(f) The normalized IL and XT profiles of TE₀, TE₁, TE₂ and TE₃ input modes for different conversion functions.

Fig. 7. Measured average ILs, average XTs, and power consumptions for all different conversion functions of cross- and parallel-connected converters, respectively. The inset is the average power consumptions of the 16 conversion functions for cross- and parallel-connected converters.

Table 1. Mode Conversion Functions of the Thermo-Optic Cross-Connected Symmetric Y-Junctions