Femtosecond laser direct writing (FsLDW) three-dimensional (3D) photonic integrated circuits (PICs) can realize arbitrary arrangement of waveguide arrays and coupling devices. Thus, they are capable of directly constructing arbitrary Hamiltonians and performing specific computing tasks crucial in quantum simulation and computation. However, the propagation constant β$\beta$ is limited to a narrow range in single-mode waveguides by solely changing the processing parameters, which greatly hinders the design of FsLDW PICs. This study proposes a composite waveguide (CWG) method to increase the range of β$\beta$, where a new single-mode composite waveguide comprises two adjacent circular waveguides. As a result, the photon propagation can be controlled and the variation range of β$\beta$ can be efficiently enlarged by approximately two times (Δβ∼36 cm−1$\mathrm{\Delta }\beta \sim 36{\mathrm{cm}}^{-1}$). With the CWG method, we successfully realize the most compact FsLDW directional couplers with a 9 μm pitch in a straight-line form and achieve the reconstruction of the Hamiltonian of a Hermitian array. Thus, the study represents a step further toward the fine control of the coupling between waveguides and compact integration of FsLDW PICs.

中文翻译：

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复合波导对激光写入光子芯片的光子传播控制

飞秒激光直写（FsLDW）三维（3D）光子集成电路（PIC）可以实现波导阵列和耦合器件的任意排列。因此，它们能够直接构造任意哈密顿量并执行在量子模拟和计算中至关重要的特定计算任务。然而，仅通过改变处理参数，传播常数β$\beta$被限制在单模波导中的一个窄范围内，这极大地阻碍了 FsLDW PIC 的设计。本研究提出了一种复合波导 (CWG) 方法来增加β$\beta$的范围，其中新的单模复合波导包含两个相邻的圆形波导。结果，可以控制光子传播并且β的变化范围$\beta$可以有效地放大大约两倍 ( Δ β ∼ 36 cm − 1 $\mathrm{\Delta }\beta \sim 36{\mathrm{cm}}^{-1}$ )。利用CWG方法，我们成功地以直线形式实现了最紧凑的9μm间距FsLDW定向耦合器，并实现了Hermitian阵列的哈密顿量重构。因此，该研究代表了对波导之间耦合的精细控制和 FsLDW PIC 紧凑集成的进一步发展。