Altti Akujärvi

Novel miniaturized hyperspectral sensor for UAV and space applications

In many hyperspectral applications it is beneficial to produce 2D spatial images with a single exposure at a few selected wavelength bands instead of 1D spatial and all spectral band images like in push-broom instruments. VTT has developed a new concept based on the Piezo actuated Fabry-Perot Interferometer to enable recording of 2D spatial images at the selected wavelength bands simultaneously. The sensor size is compatible with light weight UAV platforms. In our spectrometer the multiple orders of the Fabry-Perot Interferometer are used at the same time matched to the sensitivities of a multispectral RGB-type image sensor channels. We have built prototypes of the new spectrograph fitting inside of a 40 mm x 40 mm x 20 mm envelope and with a mass less than 50 g. The operational wavelength range of built prototypes can be tuned in the range 400 - 1100 nm and the spectral resolution is in the range 5 - 10 nm @ FWHM. Presently the spatial resolution is 480 x 750 pixels but it can be increased simply by changing the image sensor. The hyperspectral imager records simultaneously a 2D image of the scenery at three narrow wavelength bands determined by the selected three orders of the Fabry-Perot Interferometer which depend on the air gap between the mirrors of the Fabry-Perot Cavity. The new sensor can be applied on UAV, aircraft, and other platforms requiring small volume, mass and power consumption. The new low cost hyperspectral imager can be used also in many industrial and medical applications.

Optical transmission performance of a surface-micromachined Fabry–Pérot interferometer for thermal infrared

We developed a surface-micromachined tunable Fabry?P?rot interferometer for the thermal infrared spectral range of wavelengths 7?12??m. In this paper, we present the device performance in terms of the optical transmission and the tunability. The device represents the first layout that proved successful in terms of the manufacturing process yield (about 80%). The optical transmission over the wavelengths from 3 to 20??m is presented with the emphasis on analysing the first-order transmission peak. The transmission band width and the peak height are compared using the existing theory for this type of an interferometer. The deviation from an ideal performance is resolved and partly explained through the known structural unidealities.

Miniaturized hyperspectral imager calibration and UAV flight campaigns

VTT Technical Research Centre of Finland has developed Tunable Fabry-Perot Interferometer (FPI) based miniaturized hyperspectral imager which can be operated from light weight Unmanned Aerial Vehicles (UAV). The concept of the hyperspectral imager has been published in the SPIE Proc. 7474, 8174 and 8374. This instrument requires dedicated laboratory and on-board calibration procedures which are described. During summer 2012 extensive UAV Hyperspectral imaging campaigns in the wavelength range 400 - 900 nm at resolution range 10 - 40 nm @ FWHM were performed to study forest inventory, crop biomass and nitrogen distributions and environmental status of natural water applications. The instrument includes spectral band limiting filters which can be used for the on-board wavelength scale calibration by scanning the FPI pass band center wavelength through the low and high edge of the operational wavelength band. The procedure and results of the calibration tests will be presented. A short summary of the performed extensive UAV imaging campaign during summer 2012 will be presented.

MOEMS miniature spectrometers using tuneable Fabry-Perot interferometers

New tuneable MOEMS filters have been developed to cover the spectral range from 400 to 750 nm. Compared with previous MEMS based visible light filters, these Fabry-Perot Interferometers (FPIs) have increased transmission (90%), spectral resolution of ∼ 4 to 9 nm, and larger aperture diameter (2 mm), which allows them to be used in spectral imaging devices. We present the fabrication process and characterization of tuneable MOEMS FPIs for central wavelengths of λ = 420  nm and λ = 670  nm. Miniature imaging spectrometers have potential novel applications in diagnostics and health care, bioprocess, and environmental monitoring, process analytical instrumentation, and water-quality analysis.

MOEMS Fabry–Pérot interferometer with point-anchored Si-air mirrors for middle infrared

We studied how a micromachined Fabry–Perot interferometer, realized with wide point-anchored Si/air-gap reflectors, performs at the middle-infrared. A computational analysis of the anchor mechanical behavior is also presented. Compared with solid-film reflectors, this technology features better index contrast, which enables a wider stop band and potentially higher resolution. In this work, we investigate whether the performance is improved according to the index-contrast benefit, or whether the mechanical differences play a role. For comparison, we manufactured and characterized another design that applies solid-film reflectors of Si/SiO2 structure. This data is exploited as a reference for a middle-infrared interferometer and as a template for mapping the performance from the simulation results to the measured data. The novel Si/air-gap device was realized as a non-tunable proof-of-concept version. The measured data is mapped into an estimate of the achievable performance of a tunable version. We present the measured transmission and resolution data and compare the simulation models that reproduce the data. The prediction for the tunable middle-infrared Si/air-gap device is then presented. The results indicate that the interferometer’s resolution is expected to have improved twofold and have a much wider stop band compared with the prior art.

PICASSO VISION instrument design, engineering model test results, and flight model development status

PICASSO - A PICo-satellite for Atmospheric and Space Science Observations is an ESA project led by the Belgian Institute for Space Aeronomy, in collaboration with VTT Technical Research Centre of Finland Ltd, Clyde Space Ltd. (UK) and Centre Spatial de Liège (BE). The test campaign for the engineering model of the PICASSO VISION instrument, a miniaturized nanosatellite spectral imager, has been successfully completed. The test results look very promising. The proto-flight model of VISION has also been successfully integrated and it is waiting for the final integration to the satellite platform.

MOEMS FPI sensors for NIR-MIR microspectrometer applications

This paper presents near- and mid- infrared (NIR-MIR) wavelength range optical MEMS Fabry-Perot interferometers (FPIs) developed for automotive and multi-gas sensing applications. MEMS FPI platform for NIR-range consist of LPCVD (low-pressure chemical vapour) deposited polySi-SiN λ/4-thin film Bragg reflectors, with the air gap formed by sacrificial SiO2 etching in HF vapour. Characterization results for the NIR MFPI devices for λ = 1.5 – 2.0 μm show resolution of 15 nm at the optimization wavelength of 1750 nm. We also present a MIR-range MEMS FPI for λ = 2.5 – 3.5 μm, which utilizes silicon and air in within the Bragg reflector structure to provide a high contrast for improved resolution. Characterization results show a FWHM (Full Width Half Maximum) of 20 nm in comparison to the 50 nm resolution provided by earlier MEMS FPIs realized for hydrocarbon sensing with conventional CVD-thin film materials. The improved resolution and the extended operation region shows potential to enable simultaneous sensing of CO2 and multiple hydrocarbons.

MEMS Fabry-Perot interferometer with Si-air mirrors for mid- and thermal infrared

We studied how a surface-micromachined Fabry-Perot interferometer, realized with Si / air-gap distributed Bragg reflectors, would perform at the middle-infrared wavelengths. Compared with traditional solid-film pairs, this Si-FPI technology features better index contrast, which enables wider stop band and potentially higher resolution. Four different designs of interferometers were prepared and compared. Two designs apply the solid-film reflectors of Si/SiO2 structure. Their data is exploited as a reference of a middle-infrared interferometer and, as a template for mapping the performance from the simulation results to the measured data. The third design operates at the thermal infrared and it was our first embodiment with the Si/air-gap mirrors. The performance, reported earlier, is here referred to for estimating the technology scalability down to shorter wavelengths. Finally, we realized a non-tunable proof-of-concept version of the Si/air-gap technology for middle infrared. The measured data is mapped into an estimate of the achievable performance of a tunable version. We present the transmission and resolution data and argument the simulation models that reproduce the data. The prediction for the tunable middle-infrared Si-FPI is then presented. The results indicate that such a device is expected to have two-fold better resolution and a clearly wider stop band, compared with the prior art.

Hand-held MEMS hyperspectral imager for VNIR mobile applications

This paper presents a novel miniaturized hand-held hyperspectral imager for VNIR range of λ = 600 – 900 nm based on MEMS Fabry-Perot interferometer (MFPI) technology. In recent years, tunable MFPI optical filters have been utilized to demonstrate sensors for mobile applications, including CO2 smartphone sensor for mid infra-red region and hyperspectral iPhone for visible spectrum. This hand-held sensor module targets the VNIR range in order to enable food sensing, while utilizing low-cost camera technology to enable potential volume scalability for future sensing applications. The sensor module is wirelessly connected to a mobile device, which enables further application algorithms development and cloudbased solutions.

VTT's Fabry-Perot interferometer technologies for hyperspectral imaging and mobile sensing applications

VTT’s Fabry-Perot interferometers (FPI) technology enables creation of small and cost-efficient microspectrometers and hyperspectral imagers – these robust and light-weight sensors are currently finding their way into a variety of novel applications, including emerging medical products, automotive sensors, space instruments and mobile sensing devices. This presentation gives an overview of our core FPI technologies with current advances in generation of novel sensing applications including recent mobile technology demonstrators of a hyperspectral iPhone and a mobile phone CO2 sensor, which aim to advance mobile spectroscopic sensing.