Tomographic refractive index profiling of direct laser written waveguides

The fabrication of complex integrated photonic devices via direct laser writing is a powerful and rapidly developing technology. However, the approach is still facing several challenges. One of them is the reliable quantitative characterization of refractive index (RI) changes induced upon laser exposure. To this end, we develop a tomographic reconstruction algorithm following a modern optimization approach, relying on accelerated proximal gradient descent, based on intensity images only. Very recently, such algorithms have become the state of the art in the community of bioimaging, but have never been applied to direct laser written structures such as waveguides. We adapt the algorithm to our concern of characterizing these translation-invariant structures and extend it in order to jointly estimate the aberrations introduced by the imaging system. We show that a correct estimation of these aberrations is necessary to make use of data recorded at larger angles and that it can increase the fidelity of the reconstructed RI profiles. Moreover, we present a method allowing to cross-validate the RI reconstructions by comparing en-face widefield images of thin waveguide sections with matching simulations based on the retrieved RI profile. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License.

Automated summary

The study developed a tomographic reconstruction algorithm using modern optimization methods to quantitatively characterize refractive index changes induced by laser exposure. By jointly estimating the aberrations introduced by the imaging system, the fidelity of the reconstructed RI profiles was enhanced. Cross-validation of the RI reconstructions was achieved by comparing actual waveguide section images with simulated images.