Ultra-Broadband Multimode Waveguide Bending With Dual Polarizations

As one of the key components in photonic integrated circuits, multimode bend with small size and high performance can realize high integration density and flexible layout of mode division multiplexing systems. Here, a compact and ultra-broadband multimode waveguide bend based on dual-step etching free-form curves is proposed and demonstrated. The free-form curve is a combination of a series of arcs optimized by the inverse design method. Simulations show that the proposed multimode waveguide bend with an effective radius of 14 mu m has little insertion losses (<0.04 dB) and low inter-mode crosstalks (<-20 dB) for the four TE-polarization modes (TE0-TE3) over 800 nm wavelength range (1200-2000 nm). For the fabricated device, the measured losses and crosstalks are well as the theoretical predictions in three operating bands (1310 nm/1550 nm/1950 nm bands) for all the four modes. Furthermore, it is very flexible to extend the proposed multimode bend for dual polarization. For a proof-of-concept, a dual-polarization multimode waveguide bend for supporting six mode-channels (three TE-polarization modes and three TM-polarization modes) is experimentally demonstrated, which has less insertion losses (<0.3 dB) and low crosstalks (<-18 dB) in the wavelength range from 1500 to 1600 nm. These designs can play an important role for large-scale and dense integration of polarization and mode division multiplexing systems on photonic chips.