Bessel beams have irreplaceable application value in fields such as optical communication, micro-nano processing, particle micro-manipulation, medical imaging, and high-precision positioning due to their unique non-diffracting and self-healing properties. Since their first proposal in 1987, they have been a research hotspot in the optical field. Currently, the developed methods for generating Bessel beams generally have the problems of high processing precision requirements and high preparation costs. In contrast, the capillary modulation method provides a practical technical path for low-cost Bessel beam generation due to its advantages of low cost and convenient operation. However, existing studies only focus on device preparation and phenomenon observation, lacking in-depth analysis of the physical mechanism and systematic theoretical support, which restricts its further application in the field of precision optics. In this paper, a combined research method of theoretical modeling, numerical simulation and experimental verification is adopted to systematically explore the physical mechanism and parameter regulation strategy of generating high-quality Bessel beams based on capillaries.
Through theoretical analysis, a complete theoretical framework for light field transmission in capillaries is constructed, and the synergistic mechanism of anti-resonant effect and multimode interference effect is revealed: the anti-resonant effect forms an equivalent Fabry-Pérot resonant cavity in the quartz cladding of the capillary, realizing the selection and suppression of guided modes, stably confining the light energy in the air core, and providing the core physical basis for the formation of the central bright spot of the zero-order Bessel beam; the multimode interference effect constructs the transverse intensity distribution of Bessel beams with the characteristics of a central bright spot and peripheral concentric rings through the coherent superposition of multiple selected guided modes in the air core region. Numerical simulations are carried out using Comsol Multiphysics software, a two-dimensional geometric model of the capillary is established, and the central wavelength of 1550 nm in the optical communication band is selected for calculation. The simulation results are highly consistent with the light field distribution law derived from the theory, verifying the rationality of the theoretical model and the beam mode characteristics.
Through systematic experiments, the influence laws of key parameters such as capillary size (length, inner and outer diameters) and lens configuration (distance d₁ between the incident-end lens and the capillary, distance d₂ between the exit-end lens and the capillary, and detection distance d₃) on the output beam quality are explored, and the optimal parameter combination is obtained: the capillary length is selected as 8.00~12.00 cm, the critical value of d₁ is precisely controlled according to the inner diameter of the capillary, d₂ is set to twice the focal length (10.00 cm), and high-quality Bessel beam generation can be achieved when d₃≥50.00 cm. At the same time, experiments verify the non-diffracting and self-healing properties of the Bessel beam generated by this method, and clarify the evolution law of its characteristics: within the range of d₃≥50.00 cm, the transverse intensity distribution of the beam does not change with the propagation distance, and the divergence degree of the central bright spot is much lower than that of ordinary Gaussian beams; when the size of the occluder is smaller than the central bright spot, the beam can achieve nearly complete self-healing after propagating a certain distance.
In addition, the influences of capillary processing precision, laser collimation and environmental stability on beam quality are discussed in depth, and targeted regulation schemes are proposed, which effectively compensate for the processing defects of conventional low-cost capillaries. This study deepens the understanding of the light field transmission mechanism in optical fiber devices, improves the theoretical and experimental system of generating non-diffracting beams by the capillary method, and provides a quantifiable and easy-to-operate optimization scheme for the preparation of low-cost and high-performance Bessel beams. It has important application prospects in fields such as university optical teaching and precision optical measurement, and plays a positive role in the popularization and promotion of cutting-edge optical technologies.