Ultra-high density and ultra-efficient TiO2-based chip-integrated wavelength beam splitters with an inverse design method

Wavelength beam splitter (WBS) is an important element for integrated photonic circuits. Conventional WBSs usually are based on conventional structures. However, the sizes of conventional WBSs are too large to be suitable for integrated photonic circuits. Motivated by this, the sequence quadratic program (SQP) is introduced combining with the finite element method (FEM) to realize ultra-high density and ultra-efficient chip-integrated WBSs. The SQP is used to generate iterative architectures and the FEM is adopted to monitor the optical fields of the generated architectures. And ultra-compact four-, five-, and six-channel WBSs have been implemented on TiO2 substrates of size 1.5 μm × 1.5 μm with this method. The transmission efficiencies of the four-channel WBS are 84.4%, 81.3%, 80.6%, and 81.3% at the splitting wavelengths of 504 nm, 540 nm, 578 nm, and 622 nm, and their extinction ratios (ERs) range from 11.1 dB to 24.6 dB. While those of the five-channel WBS are 89.0%, 87.6%, 81.3%, 85.1% and 87.3% at the splitting wavelengths of 478 nm, 520 nm, 556 nm, 600 nm and 678 nm, respectively. Those of the six-channel WBS are 91.9%, 88.7%, 91.2%, 86.1%, 81.4% and 86.4% at the splitting wavelengths of 502 nm, 534 nm, 564 nm, 594 nm, 620 nm and 658 nm, respectively. Our designed six-channel WBS achieves smaller size and higher efficiency than the reported chip-integrated WBSs. It is expected that it can provide a new idea to design ultra-compact integrated devices and more possibility for photonic circuits structure. •A new intelligent method is proposed, where SQP works with FEM.•The size of designed 4-, 5-, and 6-channel WBSs are 1.5μm × 1.5 μm.•The 6-channel WBS achieves the smallest size and highest efficiency.