Three-dimensional ultra-compact straight-line tritter with uniform power splitting

Multiport integrated photonic devices based on waveguide optics are crucial for enabling linear optical computation and multi-photon interference on-chip, facilitating the construction of optical processor and various quantum phenomena, respectively. However, the introduction of S bends in the multiport directional couplers (DCs) occupies significant on-chip space, posing challenges for large-scale integration. In this work, a three-dimensional (3D), ultra-compact tritter was designed and fabricated using femtosecond laser direct writing. Based on a novel 3D coupling engineering strategy, both input and coupling regions were aligned in straight lines, eliminating the need for S bends. This design achieved uniform power splitting ratios of 0.34:0.32:0.34, 0.32:0.35:0.33, and 0.35:0.31:0.34 across different input ports, while its cross-sectional area is only ~2.37% of that of a traditional tritter. The proposed straight-line multiport DCs enable controlled evanescent coupling at a smaller 3D scale, further improving the integration of femtosecond laser direct-written 3D photonic circuits.