This paper presents the design and implementation of compact, fully axial wavelength-scanning systems for infrared spectroscopy. The proposed systems are based on an optical design that employs a hyperchromatic approach and a simple on-axis structure with a point detector. Two implementations are investigated: a purely refractive system combining highly and weakly dispersive elements and a hybrid system incorporating both diffractive optical elements (DOEs) and refractive components. Wavelength scanning is achieved by axially shifting an optical element, with the required displacement being significantly smaller than the corresponding axial spectral range. The hybrid spectrometer exceeds the performance of the purely refractive system, achieving a nearly constant resolution of up to 30 nm over the 1000-2200 nm wavelength range. A minimum equivalent Abbe number of 0.22 and a corresponding axial chromatic spread of 90 mm enables this characteristic. These results are achieved in a compact design, requiring an adjustment range of only 4.3 mm for the tunable optical component-approximately 1/20 of the axial chromatic spread. The findings underscore the feasibility of this axial wavelength-scanning system for NIR spectrometry, providing a compact and cost-effective alternative to existing technologies with potential applications across various scientific and industrial fields.

Filter-based spectral systems are highly competitive due to their compactness, simplicity, and well-defined spectral characteristics. However, their primary drawback remains low detection efficiency. This work explores various strategies to enhance detection efficiency. While an additional row of beamsplitters can significantly improve illumination, alternative folded beam path designs—eliminating the need for beamsplitters—prove to be far more effective. Additionally, a novel approach utilizing a freeform mirror is introduced, enabling differential adjustment of detection efficiency across different spectral regions. For the first time, a comprehensive comparison of these strategies is presented.

Hybrid polymers combine the benefits of inorganic and organic material properties, offering superior thermal, mechanical, and chemical stability, making them ideal for optical applications. This study focuses on the fabrication and characterization of antireflective (AR) structures within hybrid polymers using reactive ion etching (RIE). The etching process produces nanopillars with controlled heights, achieving excellent AR performance across a broad spectral range from 450 nm to 2 µm. Optical characterization, including angle-resolved transmission and reflection measurements, shows that the structured samples maintain high transmission efficiency and reduced reflectance at varying incidence angles. Thermal stability tests reveal that the AR structures preserve their optical properties after exposure to temperatures up to 250 °C. Higher temperatures cause significant material yellowing, which is attributed to changes in the bulk material rather than damage to the structured surface. Hydrophobicity measurements show significant water repellency in structured samples, with contact angles more than twice those of unstructured layers. These findings highlight the potential of hybrid polymers with moth-eye-inspired nanostructures for high-performance, durable optical components in demanding environments.

In contrast to conventional optical systems, which are optimized for wavelength-independent imaging, hyperchromats aim for strongly wavelength-dependent focal lengths. In this contribution, the design parameters of hyperchromatic two-lens optical systems were derived that provide strong axial color splitting expressed by extremely low equivalent Abbe numbers. These systems have been investigated for compositions of either pure refractive or all diffractive lenses, as well as hybrid configurations thereof. First, lens doublets made of cemented elements are considered and the variables affecting the equivalent Abbe number of the system are investigated. In particular, the influence of the focal lengths of the individual lenses and the Abbe numbers of the selected lens materials are taken into account. The best parameter-sets were determined by paraxial numerical simulations for different cemented configurations. To ensure a simple implementation, especially to avoid exotic or potentially harmful materials, only readily available inorganic standard glasses were considered. In the next phase of this investigation an air gap was inserted between the two lenses, which is an additional influence parameter on the equivalent Abbe number. Following the paraxial considerations, selected two-lens configurations were transferred to the non-paraxial domain and refined using optical design software, also taking aberrations into account. To further reduce achievable equivalent Abbe numbers, an aspherical surface was introduced to compensate for spherical aberrations. Finally, for the refractive doublets an equivalent Abbe number of 2.4 was achieved, which corresponds to only 12% of the smallest Abbe number of the selected materials. This result was even surpassed by the hybrid hyperchromat, resulting in an extraordinary minimum equivalent Abbe number of -0.6 that is more than five times smaller than the Abbe number of diffractive lenses.

This study explores the design and optimization of cascaded double-hyperchromatic optical systems (i.e., 2×2 lenses), focusing on achieving an extremely linear axial spectral decomposition characterized by an exceptionally low equivalent Abbe number. The investigation involves two double hyperchromats, considering both purely refractive systems and hybrid configurations that combine refractive and diffractive elements. For purely refractive systems, alternating focal length signs of divergent and collective lenses are crucial to achieve significant axial chromatic dispersion. In hybrid systems, the position of the diffractive optical element (DOE) and the selection of focal lengths play key roles in obtaining extremely low equivalent Abbe numbers. The optimized systems demonstrate absolute equivalent Abbe numbers of 0.983 for purely refractive and 0.65 for hybrid systems—more than four times lower than the absolute Abbe number of a single diffractive element. Notably, even systems using standard materials exhibit significantly low equivalent Abbe numbers of 2.5 and 1.4 for pure refractive and hybrid configurations, respectively. These results offer promising opportunities for improving optical applications based on axial spectral decomposition, overcoming previous limitations of axial chromatic spreading.

This paper presents an advanced exposure system that, we believe, for the first time, enables annular lithography to create micro- and mesostructures on curved surfaces. The principle of ring creation is based on an axicon zoom system whereas the exposure tool exhibits several features that enhance its performance over previous models. Its optical design features a high numerical aperture, resulting in narrow ring widths and high resolution. Additionally, the ring diameter, which can be varied between 400 µm and 8 mm, remains constant along the optical axis due to telecentric imaging. Further functional components include an observation unit for alignment and monitoring, as well as an integrated autofocus system. In addition to determining the exposure ring width (∼ 10 µm), diffractive lenses on planar substrates as well as on a spherical lens with a radius of curvature of 68.8 mm were structured. The periods of the diffractive structure varied between 60 µm and 460 µm.

This study demonstrates the concept of an angle-variable compact spectral module. As a key feature, the filter-based module enables highly efficient wavelength-selective light detection by applying the reflective beam path according to the origami example. It was accomplished through inclined mirrors, which allow for different incident angles on the wavelength separating interference filters used in a robust assembly with no moving parts. To experimentally verify the concept, a wavelength range between 550 and 700 nm was detected by 11 spectral channels. These initial results showed the potential to develop easily scalable and application-tailored sensors, which can overcome conventional filter-based sensor approaches that use upright or fixed-angle illumination.

This work provides a comprehensive analysis of the maximum chromatic axial split of two-element hyperchromats, with the distance between the two lenses being a key variable. Purely refractive and diffractive systems are considered, as well as hybrid layouts combining refractive and diffractive elements. In order to achieve extreme chromatic axial splitting and accordingly a minimum equivalent Abbe number for lens combinations, a three-step procedure was used. In the first paraxial step, purely optical quantities such as focal lengths of the lenses, inter-lens distances and dispersion properties of the lenses were investigated. In the second step, which also takes place in the paraxial domain, additional geometric boundary conditions such as the radii, diameters and thicknesses of the lenses are taken into account. The results of this step serve as an input for the final optimization using optical design software, which derives practical solutions for minimum equivalent Abbe numbers with diffraction-limited image quality. As a significant result, the comparison with directly cemented lens doublets shows that the introduction of a distance between the elements allows for a much stronger chromatic decomposition for refractive, diffractive and also hybrid combinations. Quantitatively, the minimum equivalent Abbe number for refractive systems is reduced from 2.5 (without spacing) to 1.79 (with spacing). For hybrid combinations, a corresponding reduction from 0.4 to 0.29 is achieved.

This paper presents concept, optical design, and the implementation of a novel, to the best of our knowledge, lithographic exposure tool for the fabrication of rotationally symmetric meso- and microscale optical structures using a variable ring-shaped light distribution. Compared to the conventional lithographic technique of direct writing in Cartesian coordinates, which is intrinsically suboptimal for the fabrication of rotationally symmetric optical structures, this approach allows for fast exposure and avoids disturbing stitching effects. The diameter of the exposure ring varies between 1.6 and 6.5 mm, and the ring width measures ∼ 75 µ m full width at half-maximum for all diameters. The basic capabilities of the exposure tool are demonstrated by the fabrication of exemplary meso- and microscale structures such as diffractive axicon elements, phase rings, Fresnel zone plates and zone plate arrays.

This contribution addresses an alternative lithographic technique for the tailored fabrication of rotationally symmetric meso- and microscale optical components. A variable ring-shaped light distribution is created by an axicon-pair based zoom-concept and can be used for the manufacturing of single optical components and array elements as well. First, design considerations of the basic axicon system and the achievable system characteristics are discussed. In particular, minimum and maximum ring diameter depending on axicon angle variations and displacement distance of employed axicons as well as potential deviations from the telecentricity condition are considered. Additionally, further aspects concerning the system implementation are presented, e.g. the achievable resolution which is dependent on the entrance pinhole. Finally, the performance of the system is presented by demonstrating the fabrication of exemplary meso- and microscale structures.

This paper demonstrates a method to significantly enhance the detection efficiency of filter-based spectral sensors without the use of additional dichroic optics for spectral preselection. The fundamental principle is that light reflected from one interference filter or filter segment can be used consecutively, reducing the overall system losses. The proof-of-concept is presented using two compact optical modules. The first module uses 10 individual filters between 520 and 800 nm, and the second is capable of continuous spectrum acquisition between 450 and 825 nm using a linear variable filter (LVF) as a key element. An efficiency increase factor of up to approximately 100 compared to a common system, where the entire LVF is directly illuminated, was demonstrated.

Hyperchromatic systems are characterized by strong longitudinal chromatic aberrations that are quantitatively described by very small equivalent Abbe numbers. In this contribution, doublet systems are systematically studied with the aim of obtaining extreme values for the equivalent Abbe numbers. Both purely refractive combinations and hybrid systems of diffractive and refractive components are considered. Chromatic axial splitting is determined as a function of the optical powers of the individual components as well as the dispersion properties of the materials involved. In order to determine actual implementable configurations for extremely small equivalent Abbe numbers, a systematic ray-trace analysis is performed in addition to paraxial studies, taking into account geometric constraints on lens curvatures and considering also complete, continuous dispersion curves. As extreme values for systems with appropriate imaging quality, an equivalent Abbe number of υ~=-2.5 is obtained for the purely refractive approach, and υ~=0.4 for the hybrid case, which is more than 8 times smaller than the absolute value of a single diffractive lens.