Broadband on-chip polarization mode splitters in lithium niobate integrated adiabatic couplers

We report, to the best of our knowledge, the first broadband polarization mode splitter (PMS) based on the adiabatic light passage mechanism in the lithium niobate (LiNbO₃) waveguide platform. A broad bandwidth of ~140 nm spanning telecom S, C, and L bands at polarization-extinction ratios (PER) of >20 dB and >18 dB for the TE and TM polarization modes, respectively, is found in a five-waveguide adiabatic coupler scheme whose structure is optimized by an adiabaticity engineering process in titanium-diffused LiNbO₃ waveguides. When the five-waveguide PMS is integrated with a three-waveguide “shortcut to adiabaticity” structure, we realize a broadband, high splitting-ratio (η_c) mode splitter for spatial separation of TE- (H-) polarized pump (700-850 nm for η_c>99%), TM- (V-) polarized signal (1510-1630 nm for η_c>97%), and TE- (H-) polarized idler (1480-1650 nm for η_c>97%) modes. Such a unique integrated-optical device is of potential for facilitating the on-chip implementation of a pump-filtered, broadband tunable entangled quantum-state generator.