Photon propagation control on laser-written photonic chips enabled by composite waveguides

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 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 , 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 can be efficiently enlarged by approximately two times (Δ β ∼36 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.