Single-photon avalanche diode (SPAD) sensors are versatile candidates for applications in low-light imaging and scenarios where high temporal resolution is crucial, like quantum imaging, fluorescence lifetime imaging, and (direct) time-of-flight methods. We demonstrate the improvement in light sensitivity by a factor of 7x for LiDAR (Light Detection and Ranging) by molding application-specific filling factor enhancing microlenses directly onto backside-illuminated SPADs. An 8"-wafer-level process is presented utilizing a mask aligner device for selective UV-curing of highly transparent polymer lenslets only in areas where SPADs are located and rinse uncured material from areas being compatible with postprocessing steps like chip dicing and electrical bonding. In addition to the optical benefits of chip-integrated lenslets, advantages arise from less system integration efforts of separately realized microlenses, especially with respect to tolerance conditions.

Regular tandem microlens arrays are well described and widely used for beam shaping and homogenization. Applying absorbing slides between the entrance and exit lenslets and channel-wise variation of the slides’ shape and size allows flexible control of the beam’s intensity profile and silhouette. The downside of absorbing slides is a significant transmission loss, limiting the achievable level of system efficiency. This work describes a more efficient method for micro-optical beam shaping with maskless irregular microlens arrays (iMLA). The iMLAs are completely absorption-free elements, enhancing the overall efficiency of the optical system. We describe basic design rules for iMLAs, including stray-light suppression, tolerancing, and modeling under consideration of manufacturing imperfections.

Echelle-inspired cross-grating spectrometers try to combine the high performance of classical Echelle spectrometers and the small footprint of compact line-grating spectrometers. Therefore, a cross-grating is used which is a superposition of two perpendicularly oriented line gratings in a single element. Highly resolved, but overlapping, diffractions orders are created by the main grating, which are separated by the cross-disperser. This powerful approach is connected to different challenges concerning the optical design, the fabrication of the cross-grating and implementation of the device. These challenges are addressed by a compact and rigid double-pass design, which utilizes the same refractive elements for collimation of the incoming beam and focusing of the diffracted light on the detector. This contribution gives an overview on the design and focusses on the implementation of the spectrometer. This includes on one hand the mounting of the cross-grating and the refractive elements in a rigid objective group and, on the other hand, the adjustment of the objective to the entrance fiber and the 2D detector. Furthermore, the implemented and calibrated instrument allows to conduct several validating experimental tests in order to proof the working principle. The spectrometer addresses a spectral range from 400 nm to 1100 nm and reaches a resolving power of 300 with an entrance pinhole diameter of 105 μm. An even higher resolving power of more than 1000 is reached with a reduced pinhole diameter of approximately 5 μm.

This contribution presents the design and implementation of a compact and robust Echelle-inspired cross-grating spectrometer which is arranged as a double pass setup. This allows use of the employed refractive elements for collimation of the incoming light and, after diffraction at the reflective crossed diffraction grating, for imaging the diffracted light onto the detector. The crossed diffraction grating combines the two dispersive functionalities of a classical Echelle spectrometer in a single element and is therefore formed by a superposition of two blazed linear gratings which are oriented perpendicularly. The refractive elements and the plane grating are arranged in a rigid objective group which is beneficial in terms of stability and robustness. The experimental tests prove that the designed resolving power of more than 300 is achieved for the addressed spectrum ranging from 400 nm to 1100 nm by using an entrance pinhole diameter of 105 µm. The utilization of a single mode fiber increases the resolving power to more than 1000, but leads to longer acquisition times.

We developed an algorithm to estimate the performance of an optical system based on the errors of its individual components. After a short training period with classical simulated systems, the performance evaluation for tolerancing could be accelerated by a factor of about three million. Additionally, we propose a probability-based sorting algorithm to combine individual, erroneous components in order to compensate for the tolerance budget within the system and increase the overall yield.

Snapshot multispectral imaging is a rising non-invasive and contact-free analysis method and technology to discriminate or identify objects based on their spectral characteristics. We demonstrate a versatile system approach for compact and real-time capable snapshot cameras for the visible (VIS) and the near-infrared (NIR) or the short-wave infrared (SWIR) wavelength range based on a micro-optical multi-aperture system and various spectral filter approaches. In addition, the manufacturing, the calibration, and the limitations of the demonstration systems are described.

We present an ultra-compact system approach for snapshot, multispectral imaging. It is based on a slanted linear variable spectral filter mounted in close proximity to the entrance pupil of a micro-optical, multi-aperture imaging system. A compact demonstration setup with a size of only 60 × 60 × 28 mm3 is developed, which enables the acquisition of 66 spectral channels in a single shot and offers a linear spectral sampling of approximately six nanometers over an extended wavelength range of 450-850 nm. The spatial sampling of each channel covers up to 400 × 400 pixels. First, the concept, the optical design and the fabrication are detailed. After the optical performance characterization, a comprehensive calibration strategy is developed and applied. An experimental demonstration is performed by acquiring the spatial and the spectral information of an imaged test scene.

We present a new concept for a fully integrated nano-optical snapshot imaging polarimeter. It consists of a nano-optical retarder and polarizer array with various orientations combined with a microlens array and crosstalk module. That facilitates a fast, robust, and compact stokes imaging polarimeter without moving parts for applications at 450 nm wavelength.

Compact cross-grating spectrometers are inspired by classical high-end Echelle spectrometers and allow to acquire a large spectrum with a high spectral resolution in a single shot. To decrease the size of an Echelle spectrometer substantially, a cross grating was employed combining both required dispersive functionalities. First, an Echelle grating diffracts light in several higher diffraction orders, while a superposed, perpendicularly oriented cross-disperser is used in first diffraction order. In this contribution, we report on the realization of a cross-grating spectrometer employing a folded reflective beam path. A toric-convex mirror was introduced allowing aberration compensation. We present the basic concept and optical design of the system and discuss the mechanical implementation and the adjustment process. Here, different laser sources and gas emission lamps are used to realize the system and derive the optical performance. In particular, the spectral resolution in the different diffraction orders is determined for the realized device.

A comprehensive schematic eye model of the hooded merganser is introduced for the first time to advance the understanding of amphibious vision. It is comprised of two different configurations, the first one modeling its visual system in air (unaccommodated state) and the second one representing the case where the eye is immersed in water (accommodated state). The model was designed using available data of former studies, image analysis and the implementation of feasible assumptions that serve as starting values. An optimization process incorporating an optical design program is used to vary the starting values with the aim of finding the setup offering the best acuity. The image quality was measured using the root-mean-square radius of the focal spot formed on the retina. The resulting schematic eye model comprises all relevant optical specifications, including aspherical geometrical parameters for cornea and lens, distances between the surfaces, the gradient index distribution of the lens, the retinal specifications and the object distance in both media. It achieves a spot radius of 4.20 mu m for the unaccommodated state, which meets the expectations derived by the mean ganglion cell density and comparison with other animals. In contrast, under water the determined spot radius of 11.48 mu m indicates an acuity loss. As well as enhancing our understanding of the vision of the hooded merganser, the schematic eye model may also serve as a simulation basis for examing similar animal eyes, such as the cormorant or other fish hunting birds.

Echelle inspired cross-grating spectrometers offer the potential to bridge the gap between classical high-end echelle spectrometers and curved-grating single-element instruments. In particular, the cross-grating approach offers the possibility to simultaneously achieve a high spectral resolution and a wide accessible spectral range in compact dimensions and without moving parts. We report on the complete realization and implementation details of an all-reflective cross-grating spectrometer based on a modified Czerny-Turner configuration including a folded beam path and a toric-convex mirror for aberration compensation. The applicability of the cross-grating spectrometer is demonstrated by test measurements including the recording of the spectra of different plant leaves. For the cross-grating spectrometer, with an accessible wavelength range between 330 and 1100 nm, a spectral resolution of 0.6 nm at 589 nm was achieved.

The concept of a new compact echelle-inspired cross-grating spectrometer is introduced, and a specific optical design is presented. The new concept aims to achieve simultaneously a high spectral resolution, a wide accessible spectral range, and compact dimensions. The essential system novelty concerns the combination of different aspects: the implementation of a crossed grating comprising both the main dispersion and order separation, a folded reflective beam path, which enables a reduction of the system volume, and the introduction of a form-adjustable mirror for aberration compensation. The exemplary optical design offers a spectral bandwidth ranging from 330-1100 nm with spectral resolution better than 1.4 nm in the fourth and 0.4 nm in the 11th order. The optical setup covers a volume of 110 mm x 110 mm x 30 mm. (C) 2019 Optical Society of America

The concept and the implementation of a compact and simplified echelle spectrometer are presented, and the working principle is demonstrated by first experimental measurements. The crucial element of the setup is a cross-grating, combining an echelle grating utilizing several higher diffraction orders (5th up to 11th) and a superposed perpendicular-oriented cross-dispersing grating. Two alternative manufacturing approaches for the cross-grating are presented and discussed. The first approach combines Talbot lithography for the deep echelle grating and interference lithography for the cross-dispersing structure. As a second approach, direct laser-beam writing was applied. The compact echelle spectrometer covers a spectral range from 380 to 700 nm and offers a spectral resolution of ∼2  nm.

To further extent our understanding of aquatic vision, we introduce a complete optical model of a goldfish eye, which comprises all important optical parameters for the first time. Especially a spherical gradient index structure for the crystalline lens was included, thus allowing a detailed analysis of image quality, regarding spot size, and wavelength dependent aberration. The simulation results show, that our realistic eye model generates a sufficient image quality, with a spot radius of 4.9 μm which is below the inter cone distance of 5.5 μm. Furthermore, we optically simulate potential mechanical processes of accommodation and compare the results with contradictory findings of previous experimental studies. The quantitative simulation of the accommodation capacity shows that the depth of field is strongly dependent on the resting position and becomes significantly smaller when shorter resting positions are assumed. That means, to enable an extended depth perception with high acuity for the goldfish an adaptive, lens shifting mechanism would be required. In addition, our model allows a clear prediction of the expected axial lens-shift, which is necessary to ensure a sufficient resolution over a large object range.

Optical systems for remote sensing commonly employ the principle of multi-/hyperspectral imaging, which is based on the acquisition of a set of two-dimensional images with distinct spectral bands in the ultra-violet, visible and/or infrared domain. Novel applications in the fields of environmental and agricultural monitoring, surveillance and biomedical inspection require miniaturized systems with high spectral and spatial sampling that furthermore enable a single shot image acquisition. However, conventional high resolution multi-spectral imaging solutions rely on bulky setups and depend on scanning techniques. In this work, we propose a multi-spectral imaging concept based on a multi-aperture system approach combined with a slanted linear variable spectral filter in order to overcome these restrictions. In particular, we demonstrate the optical design, fabrication and testing of a highly-compact, cost-effective multispectral imaging system, which exploits state of the art micro-optical manufacturing techniques on wafer level. The developed demonstration system incorporates a conventional full-frame format image sensor, a commercially available linear variable spectral filter and a customized microlens-array. In addition, a tailored baffle array is utilized for preventing optical crosstalk between adjacent optical channels. The setup enables the single-shot acquisition of 66 spectral channels with a linear spectral sampling over an extended wavelength range of 450-850 nm. The compact system with a size of only 60 x 60 x 28 mm3 provides a large field of view of 68° and a spatial sampling of 400x400 pixels per channel. Finally, we demonstrate its capabilities for advanced object classification by utilizing a customized multispectral analysis tool.

We report on an optical implementation of the teleportation protocol in the classical realm, solely based on entanglement between spatial and modal degrees of freedom of a purely classical light field.

Mode division multiplexing (MDM), where information is transmitted in the spatial modes of light, is mooted as a future technology with which to transmit large bits of information. However, one of the key issues in optical communication lies in connecting free-space to optical fiber networks, otherwise known as the 'last mile' problem. This is particularly problematic for MDM as the eigenmodes of free-space and fibers are in general not the same. Here we demonstrate a data transmission scheme across a free-space and fiber link using twisted light in the form of Laguerre–Gaussian (LG) azimuthal modes. As a proof-of-principle we design and implement a custom fiber where the supported LG modes can be grouped into five non-degenerate sets, and successfully transmit a gray-scale image across the composite link using one mode from each group, thereby ensuring minimal crosstalk.

In multi mode optical fibers degenerated modes occur, which cannot be discriminated by their propagation properties. Hence, in principle any arbitrary superposition of these degenerated modes can be used to build an equivalent set of modes. In a perturbed system this degeneracy can be broken, and as a consequence -not every superposition represents real modes of the fiber. In mode division multiplexing system this leads to high inter-modal crosstalk, if unsuitable (initially degenerated) modes are chosen. Based on the analysis of the fiber's transmission matrix, we present a measurement scheme to prove degeneracy breaking and to identify physical non-degenerated modes.