All-optical hybrid active-passive waveguide selective mode converter.

Waveguide-structured mode converters are essential in integrated photonics for multidimensional optical control, directly impacting the capacity and reliability of optical communication, interconnects, and mode-division multiplexing. Passive designs achieve fixed mode conversion by tailoring waveguide geometry and refractive index profile; while active control expand the flexibility through external stimuli that modulate waveguide properties for mode conversion. However, most devices confine mode conversion to the adjustment of intensity distribution under a single polarization state and lack the functionality for selective mode conversion. To address this limitation, we propose an all-optical hybrid active-passive waveguide selective mode converter that integrates a passive waveguide architecture with an active photothermal polypyrrole (PPy) material for thermally regulated mode manipulation. In the passive state, TE00 converts to TE01 with TM00 unchanged; in the active state, TM00 converts to TM01 with TE00 preserved, demonstrating selective mode conversion capability for both TE and TM modes. The proposed device achieves insertion losses below 0.032 dB and mode crosstalk under -21 dB, with extinction ratios exceeding 23.6 dB across all operating states at a switching power consumption of 4.79 mW. This mode converter enhances both the performance and capacity of optical communication systems and enables polarization-selective multimode control in all-optical networks.