Three-dimensional on-board optical interconnects enabled by femtosecond direct laser writing in polymer

Femtosecond direct laser writing has recently become a well-known technique of three-dimensional (3D) fabrication for micro-/nano- optics. 3D free-form structures are realized by two-photon polymerization after washing away the unexposed material in developer bath. This paper introduces a new concept to fabricate integrated optical waveguides over large distance of several cm for on-board interconnects. By employing a novel host-guest photopolymer, high-index-contrast patterns are built deep inside the resist, and wet etching is therefore not necessary. Objective with working distance of 1.7 mm ensures deep penetration of laser focus spot (~ 1 mm) and large writing field (4 inches) without stitching. Measured near-field intensity at the end facet of sample waveguides showed single-mode Gaussian profiles. We further demonstrated how voxel size and refractive index profile can be adjusted using scanning speed and laser intensity. Moreover, we changed the voxel shape by a field aperture in front of the objective to avoid high number of parallel writing. Some potential applications of this concept are 3D arrays of optical waveguides, optofans, or splitters.