Orbital angular momentum (OAM) is an intrinsic property of vortex beams characterized by helical wavefronts. Vortex beams can possess a topological charge of any integer value, with each charge being independent of the others. The superposition and multiplexing of an infinite number of topological charges can significantly enhance the channel capacity and spectral efficiency of communication systems. During the coupling process between optical fibers and waveguides, a technical challenge arises from low coupling efficiency due to mode field mismatch. This necessitates the introduction of a spot size converter (SSC) to improve coupling efficiency. Coupling OAM modes generated within optical fibers with photonic integrated circuits (PICs) holds great significance for providing high-purity mode light sources for on-chip integrated OAM communication. Furthermore, OAM facilitates subsequent signal integrated processing, thus achieving high-capacity system integration.

We propose a spot size converter that supports multiple OAM modes due to the characteristics of polymer optical waveguide materials, such as low loss, low power consumption, low refractive index, simple fabrication process, low cost, and support for large mode fields. The converter utilizes an SU-8 polymer for the core layer and SiO₂ material for the upper and lower cladding layers, featuring a regular polygonal cone structure. The spot size converter is simulated using the Finite Difference Time Domain (FDTD) method in Lumerical software.

We design a spot size converter featuring a regular polygonal cone structure. The optimized dimensional parameters are as follows: H₁=5 μm, H₂=15 μm, d₁=5 μm, d₂=2 μm, and L=350 μm, with the polygonal cone having 12 sides (Fig. 2). The mode field output from the optical fiber is coupled into the waveguide of the SU-8 polygonal cone structure which is made of polymer material, along the input direction of the coupler. This process effectively transmits and compresses the mode field carrying orbital angular momentum (Fig. 4). The output optical intensity maintains a donut-shaped distribution, and the phase retains a helical wavefront. This successfully demonstrates the feasibility of using a spot size converter to compress the mode field of the OAM beam in the fiber and achieve coupling with photonic integrated circuits (Fig. 5). The coupling efficiencies of the OAM_±1, OAM_±2, and OAM_±3 modes after the spot size converter are 90.7%, 88.4%, and 86%, respectively. For the source modes OAM_±1, OAM_±2, and OAM_±3, the mode purities are 99.26%, 99.27%, and 98.72%, respectively, with waist sizes of 4.19 μm, 4.61 μm, and 2.87 μm. After beams are passing through the spot size converter, the mode purities are 95%, 99%, and 97%, respectively, with waist sizes of 1.71, 1.91, and 1.38 μm (Fig. 6). A comparison with the method of generating OAM modes within the waveguide shows that our approach achieves higher mode purity, offering a superior quality source for subsequent information processing in PICs. The 1 dB horizontal alignment tolerances for the OAM_±1 and OAM_±2 modes with the coupler are approximately 2.4 μm and 1.8 μm, respectively. For the OAM_±3 mode, the tolerance is about 800 nm, with a 3 dB horizontal alignment tolerance of 2.7 μm. As for the 1 dB vertical alignment tolerances, they are around 2.5 μm and 1.8 μm for the OAM_±1 and OAM_±2 modes, respectively. For the OAM_±3 mode, the tolerance is about 600 nm, with a 3 dB vertical alignment tolerance of 2.6 μm (Fig. 7). The converter’s generous alignment tolerance can simplify the alignment process during the device packaging with the optical fiber.

We explore a spot size converter designed to facilitate the horizontal coupling of high-order OAM modes generated in optical fibers with planar waveguides. Utilizing high-order OAM modes produced by the superposition of LP even and LP odd modes through ±π/2 within the fiber as the source, the coupling of OAM_±l, (where l=1-3) modes and optical waveguides is simulated at 1550 nm. The results demonstrate that the spot size converters can achieve stable transmission and compression of the OAM_±1, OAM_±2, and OAM_±3 modes with coupling efficiencies of 90.7%, 88.4%, and 86% respectively. The output mode purities are 95%, 99%, and 97%, respectively. Additionally, the converter exhibits a large lateral and longitudinal alignment tolerance, reducing the technical difficulty of converter and fiber packaging, and facilitating better interconnection with photonic integrated circuits.