Figure 2

(a) Schematic of fibre geometry and rainbow spiral emission; \(\vartheta\) is the incidence tilt of the skew ray (or zenith angle), \(\varphi\) is the azimuth angle, and \(k_t\) is the projection of the laser beam wave vector on the fibre transverse plane. (b) Schematic of set-up where typical near- and far-fields are shown. The ND filter is used to vary input power, FL stands for focus lens, and CBS for cubic beam splitter. (c) Spiral emission from CW He–Ne laser (left) or from femtosecond pulse laser (right). (d) Zoom-in of (e), dashed lines indicate core–cladding boundaries. (e) Visualization of helical beam propagation in a graded-index (GRIN) fibre, thanks to upconversion luminescence from material defects and doping. Scattered white light on the right is due to the fibre holder. (f) Same as (d) but from step-index fibre.