Lasers with cylindrically symmetric polarization states are predominantly based on whispering-gallery modes characterized by high angular momentum and dominated by azimuthal emission. Here, a zero-angular-momentum laser with purely radial emission is demonstrated. An axially invariant, cylindrical photonic-bandgap fibre cavity8 filled with a microfluidic gain medium plug is axially pumped, resulting in a unique radiating field pattern characterized by cylindrical symmetry and a fixed polarization pointed in the azimuthal direction. Encircling the fibre core is an array of electrically contacted and independently addressable liquid-crystal microchannels embedded in the fibre cladding. These channels modulate the polarized wavefront emanating from the fibre core, leading to a laser with a dynamically controlled intensity distribution spanning the full azimuthal angular range. This new capability, implemented monolithically within a single fibre, presents opportunities ranging from flexible multidirectional displays to minimally invasive directed light delivery systems for medical applications.
Comparison of radial and whispering-gallery modes. Schematic drawing of the radially emitting fibre laser structure and energy density plot for a high-Q TE0n fibre cavity laser mode. The outgoing, radially directed red arrows denote the direction of laser emission
Preform and fibre structure.