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3D printed microoptics have become important tools for miniature endoscopy, novel CMOS-based on-chip sensors, OCT-fibers, among others. Until now, only image quality and spot diagrams were available for optical characterization. Here, we introduce Ronchi interferometry as ultracompact and quick quantitative analysis method for measuring the wavefront aberrations after propagating coherent light through the 3D printed miniature optics. We compare surface shapes by 3D confocal microscopy with optical characterizations by Ronchi interferograms. Phase retrieval gives us the transversal wave front aberration map, which indicates that the aberrations of our microlenses that have been printed with a Nanoscribe GT or Quantum X printer exhibit RMS wavefront aberrations as small as λ/20, Strehl ratios larger than 0.91, and near-diffraction limited modulation transfer functions. Our method will be crucial for future developments of 3D printed microoptics, as the method is ultracompact, ultra-stable, and very fast regarding measurement and evaluation. It could fit directly into a 3D printer and allows for in-situ measurements right after printing as well as fast iterations for improving the shape of the optical surface.

3D direct laser writing is a powerful and widely used tool to create complex micro-optics. The fabrication method offers two different writing modes. During the immersion mode, an immersion medium is applied between the objective and the substrate while the photoresist is exposed on its back side. Alternatively, when using the dip-in mode, the objective is in direct contact with the photoresist and the structure is fabricated on the objective facing side of the substrate. In this Letter, we demonstrate the combination of dip-in and photoresist immersion printing, by using the photoresist itself as immersion medium. This way, two parts of a doublet objective can be fabricated on the front and back sides of a substrate, using it as a spacer with a lateral registration below 1 µm and without the need of additional alignment. This approach also enables the alignment free combination of different photoresists on the back and front sides. We use this benefit by printing a black aperture on the back of the substrate, while the objective lens is printed on the front.

We perform extended numerical studies to maximize the overall photon coupling efficiency of fiber-coupled quantum dot single-photon sources emitting in the near-infrared and O-band and C-band. Using the finite element method, we optimize the photon extraction and fiber-coupling efficiency of quantum dot single-photon sources based on micromesas, microlenses, circular Bragg grating cavities and micropillars. The numerical simulations which consider the entire system consisting of the quantum dot source itself, the coupling lens, and the single-mode fiber, yield overall photon coupling efficiencies of up to 83%. Our work provides objectified comparability of different fiber-coupled single-photon sources and proposes optimized geometries for the realization of practical and highly efficient quantum dot single-photon sources.

Simultaneous realization of ultra-large field of view (FOV), large lateral image size, and a small form factor is one of the challenges in imaging lens design and fabrication. All combined this yields an extensive flow of information while conserving ease of integration where space is limited. Here, we present concepts, correction methods and realizations towards freeform multi-aperture wide-angle cameras fabricated by femtosecond direct laser writing (fsDLW). The 3D printing process gives us the design freedom to create 180° × 360° cameras with a flat form factor in the micrometer range by splitting the FOV into several apertures. Highly tilted and decentered non-rotational lens shapes as well as catadioptric elements are used in the optical design to map the FOV onto a flat surface in a Scheimpflug manner. We present methods to measure and correct freeform surfaces with up to 180° surface normals by confocal measurements, and iterative fabrication via fsDLW. Finally, approaches for digital distortion correction and image stitching are demonstrated and two realizations of freeform multi-aperture wide-angle cameras are presented.

Three-dimensional (3D) direct laser writing is a powerful technology to create nano- and microscopic optical devices. While the design freedom of this technology offers the possibility to reduce different monochromatic aberrations, reducing chromatic aberrations is often neglected. In this Letter, we successfully demonstrate the combination of refractive and diffractive surfaces to create a refractive/diffractive achromat and show, to the best of our knowledge, the first refractive/diffractive apochromat by using DOEs and simultaneously combining two different photoresists, namely IP-S and IP-n162. These combinations drastically reduce chromatic aberrations in 3D printed micro-optics for the visible wavelength range. The optical properties, as well as the substantial reduction of chromatic aberrations, are characterized, and we outline the benefits of 3D direct laser written achromats and apochromats for micro-optics.

In this Letter, we present a 3D-printed complex wide-angle multi-element Hypergon micro-objective, composed of aspherical lenses smaller than 1 mm, which exhibits distortion-free imaging performance. The objective is fabricated by a multi-step femtosecond two-photon lithography process. To realize the design, we apply a novel (to the best of our knowledge) approach using shadow evaporation to create highly non-transparent aperture stops, which are crucial components in many optical systems. We achieve a field-of-view (FOV) of 70°, at a resolution of 12.4 µm, and distortion-free imaging over the entire FOV. In the future, such objectives can be directly printed onto complementary metal-oxide-semiconductor (CMOS) imaging chips to produce extremely compact, high-quality image sensors to yield integrated sensor devices used in industry.

We demonstrate mass production compatible fabrication of polymer-based micro Fresnel lenses by injection compression molding. The extremely robust titanium-molding tool is structured with high precision by focused ion beam milling. In order to achieve optimal shape accuracy in the titanium we use an iterative design optimization. The inverse Fresnel lens structured into the titanium is transferred to polymers by injection compression molding, enabling rapid mass replication. We show that the optical performance of the molded diffractive Fresnel lenses is in good agreement with simulations, rendering our approach suitable for applications that require compact and high-quality optical elements in large numbers.

Planar lenses such as metalenses and diffractive lenses exhibit severe field-dependent aberrations when imaging extended objects with high numerical aperture. This problem can be overcome by stacking at least two of such devices on top of each other. In this work, we present such stacked imaging systems, namely doublets and triplets of diffractive optical elements. They are fabricated by femtosecond direct laser writing in one single step without the need for alignment in sizes of below 200 µm in diameter and 100 µm in height. The lenses allow for efficient sub µm resolution imaging at visible wavelengths combined with a full field-of-view of up to 60°. As additional benefit, our approach dramatically reduces the writing times of 3D printed lens systems to below 15 minutes.

Femtosecond 3D printing is an important technology for manufacturing nano- and microscopic optical devices and elements. However, most structures in the past have been created using only one photoresist at a time, thus limiting potential applications. In this Letter, we successfully demonstrate the combination of two different photoresists, namely, IP-S and IP-Dip, to realize multi-component three-dimensional direct laser written optics. We use the combination of IP-S and IP-Dip to correct chromatic aberrations and to realize an achromatic axicon. In a second step, we demonstrate, to the best of our knowledge, the first three-dimensional direct laser written Fraunhofer doublet. We characterize their optical properties and measure the substantial reduction in chromatic aberrations. We outline the possibilities and benefits of creating three-dimensional direct laser written multi-component structures for micro-optics.

The fabrication of 3D-printed micro-optical systems by femtosecond direct laser writing is state of the art. However, the inherent transparency of the lens mount, which is also made of photopolymer, causes a degradation of the image contrast due to stray light and scattering. Furthermore, apertures play a key role in optical design but cannot be directly integrated during 3D printing. Here, we present a superfine inkjet process for targeted filling of 3D-printed cavities in order to integrate apertures and nontransparent hulls without any alignment. Considerable contrast improvement and micro-optical systems with increased functionality are demonstrated.

We demonstrate orbital-angular momentum (OAM) light up to a topological charge of l=3 behind a single mode fiber. Femtosecond 3D direct laser writing is used to fabricate spiral phase plates of l=1,2 and 3, composed of 10 discrete steps, on the tip of single mode optical fibers. These structures efficiently convert out-coupled light from the fiber at 785 nm wavelength into optical vortex beams carrying an orbital-angular momentum of lℏper photon. Far field intensity patterns and interferograms of the OAM beams are recorded using a CCD camera. The results are in excellent agreement with numerical simulations obtained from the wave propagation method.

We introduce a modified formulation of the wave propagation method for the efficient simulation of rotationally symmetric micro-optical components. The reformulated algorithm provides an increased computational performance of approximately two orders of magnitude and strongly reduced memory requirements, in comparison to the original formulation. This enables the efficient wave optical simulation of extended micro-optical structures beyond the common thin-element approximation. As a prototypical example, we assess the modified algorithm for the evaluation of straylight induced by diffractive lenses. We find an excellent accuracy, while comparing to rigorous simulations, which justifies the ability to overcome the limitations of the thin-element approximation.

By using two-photon lithographic 3D printing, we demonstrate additive manufacturing of a dielectric concentrator directly on a LED chip. With a size of below 200 μm in diameter and length, light output is increased by a factor of 6.2 in collimation direction, while the emission half-angle is reduced by 50%. We measure excellent form fidelity and irradiance patterns close to simulation. Additionally, a more complex shape design is presented, which exhibits a nonconventional triangular illumination pattern. The introduced method features exceptional design freedoms which can be used to tailor high-quality miniature illumination optics for specific lighting tasks, for example, endoscopy.

We present a novel bimodal endoscopic imaging probe that can simultaneously provide full-field white-light video microscopy and confocal optical coherence tomography (OCT) depth scans. The two modalities rely on spectrally separated optical paths that run partially in parallel through a micro-optical bench system, which has a cross-section of only 2  mm×2.76  mm and is realized via standard silicon micromachining techniques. With a numerical aperture of 0.061, the video modality has a resolution and field of view of 9.3 and 1240  μm×1080  μm, respectively. The resolution is limited by the pixel spacing of the coherent fiber bundle, which relays the acquired image from the distal to the proximal end. A custom-designed diffractive optical element placed within the video imaging path significantly improves the image contrast by up to 45% in the medium frequency range. The OCT modality is optimized for 830 nm center wavelength, and works in a confocal arrangement with an NA of 0.018. It provides single-point depth probing at the center of the video image with a lateral resolution of 20 μm. Through its compact footprint and enhanced functionality, the probe can provide depth-resolved guiding capability for existing laparoscopes and represents a major step toward a new class of multimodal endoscopic imaging probes.

A novel type of integrated refractive-diffractive varifocal membrane lens is designed and analyzed by wave-optical methods. In contrast to other hybrid devices, the diffractive microstructure is directly imprinted onto the soft deflecting membrane, allowing for a high level of integration. Elastic deformation is taken into account by mechanical simulations with the finite element method (FEM). We show, that the superimposed structure can considerably suppress chromatic and spherical aberration. Furthermore, our algorithm is successfully applied to design a confocal hyperspectral lens.