High-order mode properties & tuning in step-index MM fibers for fiber tweezers
The abundant higher-order guided modes in multimode fibers significantly affect the spatial structure and energy distribution of the output optical field, thereby influencing the trapping and manipulation performance of fiber optical tweezers on microparticles. To investigate the mode evolution behavior in step-index multimode fibers, this study establishes a two-dimensional full-vector electromagnetic mode analysis model using COMSOL Multiphysics. Taking a 105/125 μm step-index multimode fiber as the research object, the effective refractive indices and transverse electromagnetic field distributions of guided modes of different orders are calculated by employing the Electromagnetic Waves, Frequency Domain interface and the mode analysis method. The results show that different higher-order guided modes exhibit distinctly different radial and azimuthal nodal structures, and present rich annular and multi‑lobed mode field patterns, with the mode fields mainly confined within the core region. Furthermore, by varying the operating wavelength, core diameter, and core‑cladding refractive index difference, parametric analyses are conducted on the normalized frequency, effective mode indices, and spatial structures of higher‑order modes, revealing the regulatory effects of fiber structure and optical parameters on multimode transmission characteristics. This model can be further combined with incident optical mode decomposition and mode superposition over long propagation distances, providing a numerical analysis foundation for output field shaping and trapping performance optimization in fiber optical tweezers.
