Inverse design of gradient-index volume multimode converters

Graded-index optical elements are capable of shaping light precisely and in very specific ways. While classical freeform optics uses only a two-dimensional domain such as the surface of a lens, recent technological advances in laser manufacturing offer promising prospects for the realization of arbitrary three-dimensional graded-index volumes, i.e. transparent dielectric substrates with voxel-wise modified refractive index distributions. Such elements would be able to perform complex light transformations on compact scales. Here we present an algorithmic approach for computing 3D graded-index devices, which utilizes numerical beam propagation and error reduction based on gradient descent. We present solutions for millimeter-sized elements addressing important tasks in photonics: a mode sorter, a photonic lantern and a multimode intensity beam shaper. We further discuss suitable cost functions for all designs to be used in the algorithm. The 3D graded-index designs are spatially smooth and require a relatively small refractive index range in the order of 10(-2), which is within the reach of direct laser writing manufacturing processes such as two-photon polymerization. Published by Optica Publishing Group under the terms of the Creative Commons Attribution 4.0 License.

Automated summary

Graded-index optical elements can shape light precisely and in specific ways. Recent advances in laser manufacturing offer possibilities for realizing three-dimensional graded-index volumes. This study presents an algorithmic approach for computing these elements, along with suitable cost functions for different designs.