Generation and 3D Regulation of Optical Needle Beams via Caustic Engineering

Abstract

This paper summarizes the generation and regulation of optical needle beams based on caustic engineering and angular-spectrum control. An optical needle beam can be regarded as a specially truncated Bessel-type beam with a narrow full width at half maximum, an extended depth of focus, and a strong non-diffracting core. Compared with traditional needle-beam generation methods that rely on bulky axicons, binary optical elements, or high-numerical-aperture systems, the reported approach uses a programmable phase modulation strategy to produce and regulate the beam more flexibly. The method first interprets the central focus of a Bessel beam as a degenerate caustic point, then suppresses peripheral rings to form a highly localized needle-like main lobe. By establishing the relation between the modulated angular spectrum and the desired on-axis intensity distribution, the axial intensity can be made nearly uniform, linearly increasing, or linearly decreasing over a designed propagation interval. The source material also extends the caustic-point concept from a single focal point to a sequence of focal points, providing a route for trajectory control in space. The study indicates that caustic-based optical needle beams are promising for particle manipulation, light-sheet microscopy, deep imaging, laser manufacturing, and other photonic applications requiring compact and reconfigurable beam shaping.