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

We report on the design and fabrication of helical cladding waveguides in Nd:YAG crystal by using femtosecond laser direct writing. These circular cladding shapes waveguides are characterized through the end-face coupling at 1064 nm, possessing good guiding properties and low bending losses. The helical waveguide structure has a minimum bending loss of 0.55 dB/cm. The mode modulation has been achieved by altering helix geometry parameters. The polarization dependence of these waveguides is kept at a relatively low level. In addition, an S-bend waveguide has been fabricated, indicating that helical structure increases bending loss by enhancing the interaction between the light and boundaries. This study paves the way for the fabrication of photonic circuits with complex 3D structures in crystals.