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.

A method that significantly increases the detection efficiency of filter array-based spectral sensors is proposed. The basic concept involves a wavelength-dependent redistribution of incident light before it reaches the filter elements located in front of the detector. Due to this redistribution, each filter element of the array receives a spatially concentrated amount of a pre-selected and adjusted spectral partition of the entire incident light. This approach can be employed to significantly reduce the reflection and absorption losses of each filter element. The proof-of-concept is demonstrated by a setup that combines a series of consecutively arranged dichroic filters with Fabry–Perot filter arrays. Experimentally, an efficiency increase by a factor larger than 4 compared to a reference system is demonstrated. The optical system is a non-imaging spectrometer, which combines the efficiency enhancement module with the filter arrays, is compact ( 17.5 m m × 17.5 m m × 7.8 m m ), and integrated completely inside the CCD camera mount.

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

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.

To accurately model the specific detection characteristics of spectral sensors based on linear variable filters (LVFs) within an optical design tool, it is essential to consider crucial position-variable spectral properties, such as peak transmittance, central wavelength, half width, or slope steepness. In this context, we propose a straightforward approach, integrating a dynamic link library (DLL) containing all position-dependent spectral properties of the LVF into a commercial optical design software. Exemplary investigations are conducted for an LVF with a detection range of 450–850 nm. For ease of use, the measured position-, wavelength-, and angle-dependent transmission properties of the LVF have been described through a simple yet highly accurate model system. Moreover, to highlight the essential value of this simulation for specific applications, an efficiency-enhancing spectral module is simulated, which is an LVF-mirror arrangement characterized by a multiple-reflected beam path. The introduced optical design tool demonstrates its particular strength by enabling the optimization of the highest detection efficiency for either the short- or long-wavelength range.