Linear Variable Optical Filter Research Articles

A general-purpose spectroscopic camera for single-board computers

A low-cost consumer-grade spectroscopic system offers potential applications in many fields. Linear variable optical filters (LVOFs) placed in front of imaging sensors are one of the most promising avenues to realize this goal. In this work, we built a spectroscopic system by depositing an LVOF directly onto the protective window of OV5647 color and OV9281 monochrome imaging sensors for the Raspberry Pi. The ¼” sensors cost $15-$80 CAD at the time of writing and have linear dimensions of only 2.7 – 3.6 mm. To achieve a suitable gradient over such small sensors, an ultra-slow shutting system was built for the physical vapor deposition of an SiO2 gradient layer between two partially transparent silver mirrors to form a miniature LVOF. The resulting devices operated as effective low-cost spectrometers that were single-moded in the visible spectrum. These simple systems were finally used to measure spectra from sample light sources, clearly illustrating the advantages and limitations of this simple and economical approach towards everyday consumer spectroscopy.

Chip-scale short-wavelength infrared InGaAs microspectrometer based on a linear variable optical filter

An extended short-wavelength infrared microspectrometer by monolithic integrarion of a 256 × 1 InGaAs linear array detector with a wedge-shaped Si/SiO2 Fabry-Perot linear variable optical filter.

Optical properties of guided-mode resonant gratings with linearly varying period

Guided-mode resonant gratings with spatially varying parameters are widely used as linear variable optical filters, and their behavior is often described using the so-called local periodic approximation, in which the structure is locally replaced by a strictly periodic grating with the period coinciding with the ``local'' period at the considered point. In this work, we investigate the optical properties of guided-mode resonant gratings with the period linearly varying along the periodicity direction. Using full-wave numerical simulations, we show that when the period change rate is relatively high (about 0.5--1 $\ensuremath{\mu}\mathrm{m}/\mathrm{mm}$ for the considered structures), the local periodic approximation becomes inapplicable, and the linewidth and the line shape of the resonances depend significantly on the period change rate. We qualitatively explain the appearance of an asymmetric non-Fano line shape with secondary maxima by analyzing the local photonic band structure of the studied varying-period gratings. For a more accurate description of such gratings, we develop a spatiotemporal coupled-mode theory, the predictions of which are in good agreement with the rigorous numerical simulation results. The validity of the derived theoretical model is additionally confirmed by a proof-of-concept experiment with a varying-period guided-mode resonant grating. The obtained results may find application in the design of compact linear variable filters based on resonant gratings with spatially varying parameters.

Design of linear variable optical filter for hyperspectral imaging

The design and simulation of a Linear Variable Optical Filter (LVOF) for the visible and near-infrared (VNIR) 450–900 nm region is described in this paper. The LVOF is designed and simulated using an all-dielectric and metal-dielectric-based induced transmission filter. We compared the design approaches based on all-dielectric and metal-dielectric filters in this paper. The LVOF filter based on the metal-dielectric method reduces design and fabrication complexities while also being cost-effective because no band rejection filters are deemed necessary. The metal-dielectric filter reported in the paper also has minimum spectral transmission of 70% as well as an FWHM of 2%–3% of central maxima.

Miniaturised Infrared Spectrophotometer for Low Power Consumption Multi-Gas Sensing.

Concept, design and practical implementation of a miniaturized spectrophotometer, utilized as a mid-infrared-based multi gas sensor is described. The sensor covers an infrared absorption wavelength range of 2.9 to 4.8 um, providing detection capabilities for carbon dioxide, carbon monoxide, nitrous oxide, sulphur dioxide, ammonia and methane. A lead selenide photo-detector array and customized MEMS-based micro-hotplate are used as the detector and broadband infrared source, respectively. The spectrophotometer optics are based on an injection moulded Schwarzschild configuration incorporating optical pass band filters for the spectral discrimination. This work explores the effects of using both fixed-line pass band and linear variable optical filters. We report the effectiveness of this low-power-consumption miniaturized spectrophotometer as a stand-alone single and multi-gas sensor, usage of a distinct reference channel during gas measurements, development of ideal optical filters and spectral control of the source and detector. Results also demonstrate the use of short-time pulsed inputs as an effective and efficient way of operating the sensor in a low-power-consumption mode. We describe performance of the spectrometer as a multi-gas sensor, optimizing individual component performances, power consumption, temperature sensitivity and gas properties using modelling and customized experimental procedures.

Thermally Reflowed Die-Attached Linear Variable Optical Filter for Mid-Infrared Volatile Organic Compounds Detection

Accidental or deliberate discharge of industrial chemicals into the environment has to be identified rapidly. Given its ease of fabrication and possibly low-cost operation, linear variable optical filter (LVOF), have been explored as on-chip mid-infrared optical engines. In this work, highly precise and replicable LVOF taper was demonstrated by sandwiching photoresist pillars between two Si-SiO2 dielectric Bragg reflector deposited Si wafers to form a LVOF. By varying the pillar height, different wedge angles were achieved. It was shown that as wedge angle varied from 0.0786° to 0.0205°, full width half maximum measured at $3.70~\mu \text{m}$ improved from 320 nm to 220 nm while transmission intensity increased by 35%. LVOF operating range was revealed to be inversely proportional to the wedge angle. Furthermore, the temperature insensitive filter was demonstrated to qualify the presence of vapor ethanol. Finally, LVOF design principles with respect to integrated linear array detectors were discussed. [2019-0101]

Deposited poly-Si as on-demand linewidth compensator for on-chip Fabry–Perot interferometer and vertical linear variable optical filter bandpass and passband manipulation

Deep reactive ion etching (DRIE) is an important process for etching vertical structures for microelectromechanical systems. Due to the sidewall profile of some photoresists as well as effects from upstream processes, bulk micromachined structures, to a certain extent, could differ from expected. Concerning photonics applications, minute deviations from the intended design might alter its optical characteristics. The most popular approach is to introduce a compensation factor during mask design. However, such a method is not robust enough to accommodate batch variations due to varying process conditions.In one particular example specific to this work, the simulated passband for a Si-air Fabry–Perot interferometer configuration was 3.67 μm. However, post DRIE the passband was measured to be 3.40 μm. To resolve this discrepancy, linewidth compensation using low-pressure chemical vapor deposition (LPCVD) poly-Si is presented. When 170 and 194 nm of poly-Si were separately deposited, the passbands redshifted to 3.54 and 3.57 μm, respectively. With the LPCVD poly-Si layer being highly conformable, the full width half maximum remains unchanged at 80 nm. An on-chip linear variable optical filter was demonstrated with a compensation of 194 nm poly-Si. It was observed that the working range redshifted from 3.0–3.9 μm to 3.3–4.5 μm.

Mid- infrared uncooled sensor for the identification of pure fuel, additives and adulterants in gasoline

The aim of the present study is to test the ability of a low-cost and portable middle infrared spectrometer based on a linear array of 1×128 of PbSe, coupled with a linear variable optical filter in the wavelength range of 3–4.5μm, for the differentiation of pure chemical substances and quality control of fuels. Potential additives and adulterants for gasoline were tested, considering the alcohols ethanol, n-butanol, n-propanol and n-hexanol as potential additives and methanol and diesel oils as adulterants. Multivariate analysis of variance (MANOVA) applied to the scores obtained in the Principal Component Analysis (PCA) was conducted to analyze the spectral data and distinguish between the individual components. For the purposes of classifying anonymous samples, the centroid of each pure substance in the canonical variables was calculated, followed by the distance calculated between new samples to such centroids, assigning the individual to the most proximate category. The results demonstrated that the technique was able to discriminate between gasoline, diesel oils and the alcohols methanol, ethanol, n-propanol, n-butanol and n-hexanol and that it had the potential to be applied in the fuel industry.

The Miniaturization of an Optical Absorption Spectrometer for Smart Sensing of Natural Gas

Natural gas is the primary energy resource in both households and industry. Due to the molecular similarity of the main constituents, i.e., hydrocarbons, composition analysis of natural gas requires a highly selective and sensitive measurement technique. Smart sensing of natural gas in a large energy grid favors the use of methods that provide low unit cost in high-volume production, such as on-chip devices that can be micromachined at the wafer level, while maintaining the performance of complex benchtop instruments. Optical absorption spectroscopy with wideband optical filters offers a comparable performance in a smaller footprint. However, the gas cell, where the absorption takes place, challenges the miniaturization of the spectrometer. This paper presents two approaches with analysis and experimental validation to integrate the gas cell and the linear variable optical filter (LVOF) at the wafer level. The first approach uses 45 $^{\circ}$ inclined mirrors to steer the light beam through the sample gas, while the second approach, the gas-filled LVOF, functionally integrates the gas cell into the resonator cavity of the filter. Both devices are self-referenced and compatible with fabrication in a CMOS process, and therefore highly suitable for smart gas sensing.

Functionalizing a Tapered Microcavity as a Gas Cell for On-Chip Mid-Infrared Absorption Spectroscopy.

Increasing demand for field instruments designed to measure gas composition has strongly promoted the development of robust, miniaturized and low-cost handheld absorption spectrometers in the mid-infrared. Efforts thus far have focused on miniaturizing individual components. However, the optical absorption path that the light beam travels through the sample defines the length of the gas cell and has so far limited miniaturization. Here, we present a functionally integrated linear variable optical filter and gas cell, where the sample to be measured is fed through the resonator cavity of the filter. By using multiple reflections from the mirrors on each side of the cavity, the optical absorption path is elongated from the physical -level to the effective -level. The device is batch-fabricated at the wafer level in a CMOS-compatible approach. The optical performance is analyzed using the Fizeau interferometer model and demonstrated with actual gas measurements.

Design and fabrication of 45° inclined mirrors for wafer-level optical absorption spectroscopy

The increasing demand for small, robust and low-cost gas sensors triggers the batch fabrication of highly selective and sensitive miniaturized devices. A linear variable optical filter (LVOF) based microspectrometer enables selectivity in a wide wavelength range, while maintaining the robustness and low cost. To achieve sensitivity in an LVOF based absorption spectrometer, a long gas cell is required. In this paper, we propose an on-chip absorption path that also serves as a gas cell, where the light beam is steered using 45° inclined mirrors. The fabrication of 45° inclined mirrors is demonstrated and optical efficiency of the system is analyzed using ray tracing.

A miniaturized optical gas-composition sensor with integrated sample chamber

A robust and highly miniaturized optical gas sensor based on optical absorption spectroscopy is presented. By using the resonator cavity of a linear variable optical filter (LVOF) also as a gas chamber, a compact and robust optical sensor is achieved. The device operates at the 15th order in 3.2–3.4μm wavelength range for distinguishing hydrocarbons. The physical cavity length at the μm-level is translated into an effective optical absorption path length at the mm-level by the use of highly reflective (R>98%) Bragg mirrors. The optical design using the Fizeau interferometer approach is described. Moreover, the CMOS-compatible fabrication method is explained. In addition to the wideband and single wavelength filter characterization, absorption of methane in the LVOF cavity is demonstrated at 3392nm and 3416.60nm wavelengths.

Compact gas cell integrated with a linear variable optical filter.

A miniaturized methane (CH(4)) sensor based on nondispersive infrared absorption is realized in MEMS technology. A high level of functional integration is achieved by using the resonance cavity of a linear variable optical filter (LVOF) also as a gas absorption cell. For effective detection of methane at λ = 3.39 µm, an absorption path length of at least 5 mm is required. Miniaturization therefore necessitates the use of highly reflective mirrors and operation at the 15th-order mode with a resonator cavity length of 25.4 µm. The conventional description of the LVOF in terms of the Fabry-Perot resonator is inadequate for analyzing the optical performance at such demanding boundary conditions. We demonstrate that an approach employing the Fizeau resonator is more appropriate. Furthermore, the design and fabrication in a CMOS-compatible microfabrication technology are described and operation as a methane sensor is demonstrated.

Implementation of CMOS-compatible Metamaterial Absorber for gas Sensing Application

The design of a metamaterial-based absorber for use in a mid-IR microspectrometer-based gas sensor is reported. The microspectrometer consists of a linear variable optical filter (LVOF) that is aligned with an array of thermopile detectors, which is fabricated on a SiN membrane and covered with the absorber. Special emphasis was put on the CMOS compatible fabrication, which resulted in an absorber design based on aluminium disk resonators and an aluminium background plane that are separated by a SiO2 layer. The fabrication process is described, and the challenges are discussed.

A Miniaturized Optical Sensor with Integrated Gas Cell

The design, fabrication and characterization of a highly integrated optical gas sensor is presented. The gas cell takes up most of the space in a microspectrometer and is the only component that has so far not been miniaturized. Using the tapered resonator cavity of a linear variable optical filter as a gas cell enables ultimate miniaturization, while maintaining robustness without any moving parts. Multiple reflections from highly reflective mirrors allow a 24-25.5μm long resonator cavity to also act as a gas cell with an equivalent optical absorption path length of 6mm.

Design, fabrication and characterization of infrared LVOFs for measuring gas composition

This paper presents the design, fabrication and characterization of a linear-variable optical-filter (LVOF) that will be used in a micro-spectrometer operating in infrared (IR) for natural gas composition measurement. An LVOF is placed on top of an array of detectors and transforms the optical spectrum into a lateral intensity profile, which is recorded by the detectors. The IR LVOF was fabricated in an IC-compatible process using a photoresist reflow technique, followed by transfer etching of the photoresist into the optical resonator layer. The spectral range between 3 to 5 µm contains the absorption peaks for hydrocarbons, carbon-monoxide and carbon-dioxide. The resulting optical absorption is utilized to measure the gas concentrations in a sample volume. Two LVOF structures were designed and fabricated on silicon wafers using alternate layers of sputtered silicon and silicon-dioxide as the high- and low- refractive index materials. These filters consist of a Fabry–Pérot resonator combined with a band-pass filter designed to block out-of-band transmissions. Finally, the filters were fully characterized with an FTIR spectrometer and showed satisfactory agreement with the optical thin-film simulations. The characterization showed a spectral resolution of 100 nm, which can be further improved with signal processing algorithms. This method makes it possible to fabricate small and robust LVOFs with high resolving power in the IR spectral range directly on the detector array chip.

Holographic Photopolymer Linear Variable Filter with Enhanced Blue Reflection

A single beam one-step holographic interferometry method was developed to fabricate porous polymer structures with controllable pore size and location to produce compact graded photonic bandgap structures for linear variable optical filters. This technology is based on holographic polymer dispersed liquid crystal materials. By introducing a forced internal reflection, the optical reflection throughout the visible spectral region, from blue to red, is high and uniform. In addition, the control of the bandwidth of the reflection resonance, related to the light intensity and spatial porosity distributions, was investigated to optimize the optical performance. The development of portable and inexpensive personal health-care and environmental multispectral sensing/imaging devices will be possible using these filters.

THE GEOSPECTRAL CAMERA: A COMPACT AND GEOMETRICALLY PRECISE HYPERSPECTRAL AND HIGH SPATIAL RESOLUTION IMAGER

Abstract. Small unmanned aerial vehicles are increasingly being employed for environmental monitoring at local scale, which drives the demand for compact and lightweight spectral imagers. This paper describes the geospectral camera, which is a novel compact imager concept. The camera is built around an innovative detector which has two sensor elements on a single chip and therefore offers the functionality of two cameras within the volume of a single one. The two sensor elements allow the camera to derive both spectral information as well as geometric information (high spatial resolution imagery and a digital surface model) of the scene of interest. A first geospectral camera prototype has been developed. It uses a linear variable optical filter which is installed in front of one of the two sensors of the MEDUSA CMOS imager chip. A accompanying software approach has been developed which exploits the simultaneous information of the two sensors in order to extract an accurate spectral image product. This method has been functionally demonstrated by applying it on image data acquired during an airborne acquisition.

Linear variable optical filter-based ultraviolet microspectrometer

An IC-compatible linear variable optical filter (LVOF) for application in the UV spectral range between 310 and 400 nm has been fabricated using resist reflow and an optimized dry-etching. The LVOF is mounted on the top of a commercially available CMOS camera to result in a UV microspectrometer. A special calibration technique has been employed that is based on an initial spectral measurement on a xenon lamp. The image recorded on the camera during calibration is used in a signal processing algorithm to reconstruct the spectrum of the mercury lamp and the calibration data is subsequently used in UV spectral measurements. Experiments on a fabricated LVOF-based microspectrometer with this calibration approach implemented reveal a spectral resolution of 0.5 nm.

Design and implementation of a sub-nm resolution microspectrometer based on a Linear-Variable Optical Filter

In this paper the concept of a microspectrometer based on a Linear Variable Optical Filter (LVOF) for operation in the visible spectrum is presented and used in two different designs: the first is for the narrow spectral band between 610 nm and 680 nm, whereas the other is for the wider spectral band between 570 nm and 740 nm. Design considerations, fabrication and measurement results of the LVOF are presented. An iterative signal processing algorithm based on an initial calibration has been implemented to enhance the spectral resolution. Experimental validation is based on the spectrum of a Neon lamp. The results of measurements have been used to analyze the operating limits of the concept and to explain the sources of error in the algorithm. It is shown that the main benefits of a LVOF-based microspectrometer are in case of implementation in a narrowband application. The realized LVOF microspectrometers show a spectral resolution of 2.2 nm in the wideband design and 0.7 nm in the narrowband design.