The coherent optical field diffracted through a circular aperture can produce optical tubular structures single-closed or twice-closed, by choosing the suitable experimental conditions. Both structures can be generated on the same optical set-up only by changing the experimental parameters involved on it, i.e. the aperture radius, the on-axis observation point, the wavelength and the on-axis source position. Such structures, generated in this way, have strong intensity gradients at their edges and their dimensions can be controlled in order to make them appropriate structures for optical trapping of particles, for instance.