451715.fig.002a
(a)
451715.fig.002b
(b)
451715.fig.002c
(c)
451715.fig.002d
(d)
Figure 2: (Colour online) shows the contour plot of the total intensity distribution in the plane near the focus for the focusing system with 3 belts BPF. (a) Single-ring focus, (b) double-ring focus, (c) double-ring focus with large separation of axial distance, and (d) triple-ring focus.