Femtosecond-laser-written helical cladding waveguides: 3D fabrication and mode modulation

•Helical cladding waveguides in Nd:YAG crystal are experimentally fabricated and studied for the first time.•The fabricated helical waveguide structure has a minimum bend loss of 0.55 dB/cm.•3D waveguides exhibit greater bend losses than 2D waveguides although they have identical curvature.•Modal profiles of the waveguides are analyzed theoretically and experimentally, demonstrating mode conversion properties.