Optical spectrometers and sensors have gained enormous importance in metrology and information technology, frequently involving the question of size, resolution, sensitivity, spectral range, efficiency, reliability, and cost. Nanomaterials and nanotechnological fabrication technologies have huge potential to enable an optimization between these demands, which in some cases are counteracting each other. This paper focuses on the visible and near infrared spectral range and on five types of optical sensors (optical spectrometers): classical grating-based miniaturized spectrometers, arrayed waveguide grating devices, static Fabry–Pérot (FP) filter arrays on sensor arrays, tunable microelectromechanical systems (MEMS) FP filter arrays, and MEMS tunable photonic crystal filters. The comparison between this selection of concepts concentrates on (i) linewidth and resolution, (ii) required space for a selected spectral range, (iii) efficiency in using available light, and (iv) potential of nanoimprint for cost reduction and yield increase. The main part of this review deals with our own results in the field of static FP filter arrays and MEMS tunable FP filter arrays. In addition, technology for efficiency boosting to get more of the available light is demonstrated.

Optical interferometric sensors have acquired significant importance in metrology and information technology, especially in terms of their potential application in launching size, selectivity, sensitivity, resolution, spectral tuning ranges, efficiency, and cost. However, these demands are often contradictory and counteract one another, and are thus difficult to simultaneously fulfill during their interaction. This review focuses on a detailed comparison of seven different strongly miniaturized sensor concepts investigating the limits of these demands. For the visible and near-infrared spectral range, seven optical sensors were reviewed based on the following methodologies: classical optical transmission and reflection gratings, arrayed waveguide gratings, static Fabry–Pérot (FP) filter arrays, MEMS tunable FP interferometers, MEMS tunable photonic crystals, plasmonic filters, and fiber tip sensors. The comparison between the selected concepts concentrates on (i) the minimum space required for a particular spectral range, (ii) resolution, (iii) the integration in optical fiber technology, (iv) tunability to save space, (v) efficiency in using available light, (vi) multiplexing, (vii) miniaturization limits, and (viii) the potential of nanoimprint for cost reduction. Technologies for enhancing efficiency to obtain more available light and their applicability to the different methodologies were studied.

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.

We present a new method for the fabrication of diffractive and refractive micro-optical components. The method is suitable for high-quality rapid prototyping of optical components and allows the fast experimental test of designs for computer-generated holograms or refractive microstructures. Our method is based on employing a digital-multimirror device (DMD) as a switchable projection mask. The DMD is imaged into a photoresist layer using a Carl Zeiss lithography objective with a demagnification of $10:1$ and a numerical aperture of 0.32 on the image side. The resulting pixel size is $1.368×1.368μm$. In comparison with laser direct writing with a single spot, our method is a parallel processing of nearly 800,000 pixels $(1024×768pixels)$. This fabrication method can be applied to all MOEMS components. The method adds a new dimension in MOEMS processing, reducing the fabrication complexity, and improves the flexibility of process simulation and design.

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.

Spectroscopic analysis is no longer confined to science and laboratories. There is a growing demand for spectroscopic instruments with very ambitious instrumental requirements to work in extended application fields: process control for manufacturing industries, such as food¹ and pharmaceutical² industries, and screening a broad range of factors for agriculture and the environment,³ to name only a few. During the past decade, filter-based spectrometers have drawn attention, especially because of their potential for miniaturization. Filter-based spectrometers can have an extremely compact and rigid design without any moving parts, consisting of only a filter array or longitudinally variable filter placed in front of a sensor array, for example, a charge-coupled device (CCD) or complementary metal–oxide–semiconductor, to capture a spectrum in one shot. A variety of spectral filters and structures, often micrometer-sized, have been developed, such as plasmonic color filters,^4,5 photonic crystal arrays,⁶ single nanowires,⁷ absorptive filter arrays composed of either conventional colloidal quantum dots⁸ or perovskite quantum dots,⁹ Fabry–Pérot (FP) filter arrays,^{10 – 12} and linear (also called “continuously”) variable filters^{13 – 22} [linear variable filters (LVFs)]. An important benefit of such types of spectrometers compared to grating-based spectrometers is the possibility of detecting a broad spectral range with high spectral resolution despite their small size.¹² They can be implemented using a large number of narrowband filters or an appropriate gradient for the LVF. For example, the reported three FP filter arrays comprise 192 individual filters with a size of $40\times 40\mu \mathrm{m}$ and cover a spectral range between 521 and 685 nm. The full width at half maximum (FWHM) of the individual transmission curves is nearly 3 nm (ranges from 1.7 to 5 nm).¹¹