Tomographic refractive index profiling of direct laser written waveguides

Abstract

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.

Direct laser-written aperiodic photonic volume elements for complex light shaping with high efficiency: inverse design and fabrication

“…This profile must be precisely known, as it serves as the basis for the numerical design process. The RI profile characterization is performed in a home-built tomographic microscope 19 . The RI distribution of each voxel is assumed to be invariant along the z direction.…”

“…Our method allows for obtaining high diffraction efficiencies of up to 80%, many times more than what could previously be achieved 17 . This is made possible by employing precise tomographic voxel characterization 19 in combination with a design algorithm based on numerical beam propagation and the use of adaptive optics to ensure space-invariant voxel shapes throughout the full body of the APVE. We experimentally demonstrate the design and manufacture of three different, highly integrated APVEs that are optimized for intensity shaping, spatial mode, and wavelength multiplexing.…”

Design and manufacturing of efficient computer-generated volume holograms in glass

“…This profile must be precisely known, as it serves as the basis for the numerical design process. The RI profile characterization is performed in a home-build tomographic microscope [18]. The RI distribution of each voxel is assumed to be invariant along the z-direction.…”

“…This may be attributed to small remaining differences between the assumed and real voxel profiles at length scales of a few hundred nanometres. Such small irregularities will especially affect shorter wavelengths and are likely to be overlooked by our tomographic voxel characterization method, which employs the same wavelength for inspection and a smoothing total variation norm regularizer [18]. The simulated and experimentally measured transmission values T for all three readout wavelengths are shown in table 2.…”

“…Our method allows for obtaining high diffraction efficiencies of up to 80%, more than six times more than what could previously be achieved [17]. This is made possible by employing precise tomographic voxel characterization [18] in combination with a design algorithm based on numerical beam propagation and the use of adaptive optics to ensure space invariant voxel shapes throughout the full body of the CGVH. We experimentally demonstrate the design and manufacture of three different, highly integrated CGVHs that are optimized for intensity shaping, spatial mode and wavelength multiplexing.…”