Beam shaping and supercontinuum generation
We experimentally observed and theoretically described for the first time the possibility of generating spiral emission (SE) by means of optical fibers. In our studies we used singlemode fibers as well as both step-index and graded-index multimode fibers. Thanks to their guiding properties, a laser beam is forced to helically propagate inside the fiber and, under opportune conditions, a spiral shaped far-field from the fiber output facet is produced. The input laser beam must be focused into the fiber cladding area with a small tilt. The tilt longitudinal and transversal angles must be chosen in order to make the wave vector projection of the incident field tangent to the air-cladding interface (see Fig.1a). Thus, the beam wave vector is along the azimuthal direction, leading to an unconventional orbital angular momentum of light that eventually gives rise to the SE. Exploiting the luminescence of the fiber defects, we could trace the beam helical trajectory (see Fig.1b). If the fiber length is opportunely chosen, the beam spreading due to diffraction and the seeded orbital angular momentum provide the proper phase profile that forms the SE (see Fig.1c). These phenomenon is purely linear and can be realized with CW sources (in Fig.1d the case of a He-Ne laser is shown). However, when employing infrared femtosecond high power pulsed sources, the SE phenomenon becomes even more spectacular. This meshes with the supercontinuum generation giving rise to the rainbow-like pattern in Fig.1e. In Fig.1d the supercontinuum spectrum is shown at different input peak powers.
Fig. 1. a) Depiction of rainbow SE geometry; b) Photoluminescence image of optical fiber side part; c) Ray optics model of the SE generation; d) True-colour image of the far-field emission from a single mode fiber coupled with a He-Ne CW laser; e) Rainbow-like SE from a grade-index multimode fiber coupled with a femtosecond laser source; f) Output supercontinuum spectra associated to e) at different input peak powers.
Numerical results confirm the experimental observations. Depending on the input position offset the beam propagates inside the fiber either clockwise or counter-clockwise. This observation, as reported in Fig.2, also show how the spiral shape is formed.
Fig. 2. Numerical results. a) Depiction of winding direction (clockwise and counter-clockwise) depending on the input position offset; b) Evolution of the Fourier transform of the beam propagating along the fiber; c) Far-field intensity in two winding conditions.
The rainbow-like SE was a previously unforeseen nonlinear phenomenon that might find application in several fields, from optical tweezers to high-order quantum entanglement, super-resolution, data storage, and nanoscale microscopy.
F. Mangini, M. Ferraro, M Zitelli, V Kalashnikov, A Niang, T Mansuryan, F Frezza, A Tonello, V Couderc, AB Aceves, S Wabnitz, “Giving light a new twist with standard optical fibres: rainbow spiral emission,” https://arxiv.org/abs/2010.00487
STEMS – Spatiotemporal multimode complex optical systems
WAVESCOPE – Wavefront Shaping System for Nonlinear Fiber-Based Microscopy and Endoscopy
WASHING – Wave shaping of optical beams for the control of light pulses by multimode fibers