Ultra-broadband mode converter based on phase-shifted long-period gratings operating at a dispersion turning point

We propose and demonstrate an ultra-broadband mode converter based on phase-shifted long-period gratings operating at the dispersion turning point. The device employs a sidewall-etched LPG in a polymer few-mode waveguide, where two optimized π-phase shifts broaden the resonant dip. Simulations show a -10 dB bandwidth of 745 nm for conversion between the E₁₁ and E₂₁ modes. The fabricated device shows a mode conversion efficiency higher than 15.6 dB over the wavelength range from 1285 to 1825 nm for both TE and TM polarizations. The proposed device provides a compact and fabrication-tolerant solution for mode-division multiplexing and broadband optical interconnects.