Objective We present a comprehensive theoretical and experimental investigation of higher-order transverse mode supercontinuum generation with doughnut-shaped intensity distribution in dual-mode photonic crystal fibers (DM-PCFs) via an axial spatial coupling scheme. Supercontinuum (SC) sources, which uniquely combine the high brightness and directionality of laser radiation with ultra-broadband spectral coverage, have established themselves as indispensable tools in modern photonics. Conventional SC research has predominantly focused on fundamental-mode pumping of single-mode PCFs, where the output maintains a Gaussian spatial profile. However, there is growing recognition that higher-order transverse modes (HOMs) in few-mode fibers offer distinct advantages for SC generation, including shorter zero-dispersion wavelengths that facilitate spectral extension toward the blue and modified nonlinear dynamics arising from multimode propagation. Furthermore, HOMs exhibiting doughnut-shaped intensity distribution possess unique spatial structures that are highly desirable for applications requiring hollow beam profiles, including super-resolution stimulated emission depletion microscopy, optical trapping, and mode-division-multiplexed optical communications. Despite these attractions, efficient and high-purity excitation of doughnut-shaped HOMs in DM-PCFs remains challenging. Traditional approaches relying on fundamental-mode offset coupling suffer from inherently low spatial overlap with the target HOM, resulting in compromised mode purity, elevated coupling loss, and reduced power conversion efficiency. To overcome these limitations, this work proposes and demonstrates an axial spatial coupling scheme specifically designed to excite doughnut-shaped higher-order transverse modes in DM-PCFs and investigates the resulting SC generation characteristics through detailed numerical modeling and experimental validation.

Methods Theoretical investigations were conducted to compare the proposed HOM axial coupling against conventional fundamental-mode offset coupling. Numerical simulations based on beam propagation methods and mode overlap integrals quantified coupling efficiency and mode excitation purity under both configurations. The results demonstrate that when a pump beam with a doughnut-shaped intensity distribution—spatially matched to the LP₁₁ mode—is launched via axial coupling, the excitation purity of the target HOM is substantially higher than that achievable through offset coupling of a Gaussian beam. The axial coupling geometry maximizes spatial overlap, significantly reducing power loss at the input facet and ensuring efficient power transfer into the guided HOM. Beyond these linear advantages, dispersion analysis reveals that the excited HOM possesses a zero-dispersion wavelength blue-shifted relative to the fundamental mode. This modified dispersion landscape promotes phase-matched generation of blue-shifted dispersive waves and alters soliton fission dynamics to favor spectral broadening toward shorter wavelengths. Consequently, the theoretical framework predicts that HOM axial coupling not only optimizes initial mode excitation but also leverages the unique dispersive properties of the higher-order mode to enhance visible SC generation by extending the short-wavelength boundary and increasing the fractional power within the visible band. Experimentally, a vortex waveplate converted a linearly polarized fundamental-mode pump laser into a second-order transverse mode with a well-defined doughnut-shapec intensity distribution. The converted doughnut-shaped pump was launched into a DM-PCF through the axial coupling configuration, where precise transverse alignment ensured optimal overlap with the fiber’s higher-order mode. As the pump pulse propagated, it underwent spectral broadening driven by self-phase modulation, stimulated Raman scattering, soliton dynamics, and phase-matched dispersive wave generation. The output SC was characterized using an optical spectrum analyzer, while spatial characteristics were analyzed across the visible and near-infrared regions using CCD cameras and bandpass filters to verify preservation of the doughnut-shaped intensity profile.

Results and Discussions The experimental results confirm the theoretical predictions and highlight the superiority of the proposed approach. The generated supercontinuum spans 435 nm to 2220 nm, bridging the visible, near-infrared, and short-wave infrared regions within a laser source. The output beam maintains a stable doughnut-shaped intensity distribution across the entire spectral bandwidth, with a measured higher-order mode purity of 77.9%. Examination of the visible spectral components confirms that the doughnut-shaped spatial profile remains intact throughout the 435–700 nm range, demonstrating that the transverse mode structure is robustly preserved throughout the nonlinear spectral evolution. Comparative experiments performed under identical pump power conditions using both the doughnut-shaped HOM and the conventional Gaussian fundamental mode unequivocally demonstrate that HOM pumping extends the short-wavelength boundary significantly deeper into the visible spectrum while increasing the overall spectral power density in the visible band. These enhancements are attributable to the shorter zero-dispersion wavelength and distinct dispersion profile of the excited HOM, which enable efficient generation of phase-matched dispersive waves at blue-visible wavelengths and support enhanced blue-edge extension. The high mode purity and low coupling loss achieved through axial coupling ensure that the majority of available pump power is efficiently channeled into the desired HOM nonlinear processes, rather than being dissipated into unguided radiation modes or generating competing fundamental-mode SC that would degrade spatial beam quality.

Conclusions In conclusion, we have proposed, theoretically analyzed, and experimentally demonstrated an axial spatial coupling scheme for the efficient generation of high-purity, doughnut-shaped higher-order transverse mode supercontinuum in dual-mode photonic crystal fibers. By employing a vortex waveplate for precise pump mode conversion and implementing on-axis spatial coupling to maximize overlap with the DM-PCF higher-order mode, we have successfully generated an ultra-broadband supercontinuum spanning 435 nm to 2220 nm with a measured mode purity of 77.9% and a stable doughnut-shaped intensity distribution maintained across all spectral components. Both theoretical comparison and experimental verification against fundamental-mode pumping confirm that the proposed approach simultaneously addresses the critical challenges of coupling loss and mode purity while leveraging the unique dispersive properties of higher-order modes to extend the visible short-wavelength boundary and enhance the visible spectral proportion. The demonstrated source is anticipated to find important applications in advanced super-resolution microscopy, optical trapping and micromanipulation, and next-generation mode-division-multiplexed optical communication systems, where the combination of ultra-broadband spectra and structured doughnut-shaped beam profiles offers distinct advantages over conventional Gaussian-beam supercontinuum sources.