Optical broadband directional couplers (BDCs) are indispensable components for providing wavelength-insensitive and flexible optical splitting in the construction of functional photonic integrated circuits (PICs). The existing BDC device structures are usually required to determine specific design parameters for different waveguide structures and operating wavelength bands. To circumvent this dilemma, here we present a novel optimization procedure to realize a compact BDC by using the asymmetric curved waveguide structure. The versatile particle swarm optimization (PSO) technique is adopted to determine the optimal device parameters of the compact and broadband asymmetric curved directional couplers (ACDCs) for different coupling ratios. In order to reduce the computational complexity in the optimization, the 3D ACDC is first converted to an equivalent 2D structure by using the modified effective index method (MEIM). The device parameters of the equivalent 2D ACDC are optimized by the PSO with the objective function of a wavelength flattened coupling ratio. Afterward, the optimized 2D structure is converted to the 3D one by including the waveguide thickness. To cope with the approximation error by the MEIM, the 3D ACDC is further fine-tuned by sweeping one of the device parameters with the full 3D simulation but keeping all of the other optimal parameters obtained from the PSO intact. As a result, a DC with broad bandwidth of 100 nm is obtained over the wavelength range from 1.50 µm to 1.60 µm with a very small coupling length of 6 µm. The semi-optimized ACDC is used to construct an unbalanced Mach-Zehnder interferometer (MZI) and a Sagnac loop mirror (SLM), both of which show high extinction ratios of >25 dB over a broad wavelength range with low excess loss.

光学宽带定向耦合器（BDC）是必不可少的组件，用于在功能性光子集成电路（PIC）的结构中提供对波长不敏感且灵活的光分离。通常需要现有的BDC器件结构来确定不同波导结构和工作波长带的特定设计参数。为了解决这个难题，在此我们提出一种新颖的优化程序，以通过使用非对称弯曲波导结构来实现紧凑的BDC。采用通用粒子群优化（PSO）技术来确定不同耦合比的紧凑型和宽带非对称弯曲定向耦合器（ACDC）的最佳设备参数。为了减少优化中的计算复杂度，首先使用修改后的有效索引方法（MEIM）将3D ACDC转换为等效的2D结构。等效2D ACDC的设备参数通过PSO进行了优化，其目标函数是波长平坦的耦合比。之后，通过包括波导厚度将优化的2D结构转换为3D结构。为了应对MEIM的逼近误差，可以通过使用完整的3D仿真扫描设备参数之一，同时保持从PSO获得的所有其他最佳参数不变的方式，对3D ACDC进行进一步的微调。结果，在1.50 µm至1.60 µm的波长范围内获得了100 nm宽带宽的DC，耦合长度仅为6 µm。