Figure 8.
(a1,a2) Schematic of two single grating (with period of Λ1 and Λ2, respectively) superimposed into multiple-beat modulated device [151]. An unfolded view of resulting the three beats device (a3) [151]. The far-field patterns of the two-beat grating device: (b) 1508.1 nm, topological charges combination +1 and −1; (c) 1518.9 nm, topological charges combination 0 and −2; and (d) 1530 nm, topological charges combination −3 and −1 [151]. The near-field pattern is showing in (e), and the well-defined spiral interference fringe existed at appropriate locations is showing in (f) [151].