Diffractive axicons for quasi-achromatic, propagation-invariant tubular beams

This work presents an optical system that generates a quasi-achromatic, propagation-invariant tubular light field whose diameter remains constant over meters of propagation and for all investigated wavelengths. The system combines two diffractive axicons of identical grating period, where the phase offset of the binary axicon (DOE1) determines whether a single-ring or double-ring structure appears in the Fourier plane by controlling the interference between the +1^st and -1^st diffraction orders. Beyond the Fourier plane, we analyze the full axial evolution of the beam. Numerical simulations and experiments show that the transverse ring diameter remains invariant during propagation. The results demonstrate strong agreement between two-dimensional simulations and measurements, confirming the quasi-achromatic and propagation-invariant behavior of the generated tubular light field.