Jarkko Antila

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.

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.

Aalto-1 - An experimental nanosatellite for hyperspectral remote sensing

In this paper we describe the Finnish Earth Observation nanosatallite project Aalto-1. The Aalto-1 is a 4 kg student satellite, based on CubeSat standards. The satellite is designed to carry the worlds smallest remote sensing imaging spectrometer for Earth Observation and several other payloads.

Hydrocarbon gas detection with microelectromechanical Fabry-Perot interferometer

VTT Technical Research Centre of Finland has developed microelectromechanical (MEMS) Fabry-Perot interferometer (FPI) for hydrocarbon measurements. Fabry-Perot interferometer is a structure where is two highly reflective surfaces separated by a tunable air gap. The MEMS FPI is a monolithic device, i.e. it is made entirely on one substrate in a batch process, without assembling separate pieces together. The gap is adjusted by moving the upper mirror with electrostatic force, so there are no actual moving parts. The manufactured MEMS FPIs have been characterized. The tuning wavelength range of the MEMS FPI is 2.8-3.5 μm and its spectral resolution is 50-60 nm. VTT has designed and manufactured a handheld size demonstrator device based on the technology presented in this abstract. This device demonstrates gas detecting by measuring cigarette lighter gas and various plastic materials transmission spectra. The demonstrator contains light source, gas cell, MEMS FPI, detector and control electronics. It is connected to a laptop by USB connection, additional power supply or connection is not needed.

MEMS and piezo actuator-based Fabry-Perot interferometer technologies and applications at VTT

Miniaturized spectrometers covering spectral regions from UV to thermal IR are of interest for several applications. For these purposes VTT has for many years been developing tuneable MEMS-based and more recently piezo-actuated Fabry-Perot Interferometers (FPIs). Lately several inventions have been made to enter new wavelengths in the VIS range and enlarge apertures of MEMS devices and also extending the wavelength range of piezo-actuated FPIs. In this paper the background and the latest FPI technologies at VTT are reviewed and new results on components and system level demonstrators are presented. The two FPI technologies are compared from performance and application point of view. Finally insight is given to the further development of next generation devices.

Spectral imaging device based on a tuneable MEMS Fabry-Perot interferometer

The trend in the development of single-point spectrometric sensors is miniaturization, cost reduction and increase of functionality and versatility. MEMS Fabry-Perot interferometers (FPI) have been proven to meet many of these requirements in the form of miniaturized spectrometer modules and tuneable light sources. Recent development of MEMS FPI devices based on ALD thin film structures potentially addresses all of these main trends. In this paper we present a device and first measurement results of a small imaging spectrometer utilizing a 1.5 mm tuneable MEMS FPI filter working in the visible range of 430-580 nm. The construction of the instrument and the properties of the tuneable filter are explained especially from imaging requirements point of view.

Bragg reflectors for large optical aperture MEMS Fabry-Perot interferometers

This paper presents the fabrication of large-aperture low-pressure chemical-vapour deposited (LPCVD) Bragg reflectors utilizing low-stress polysilicon (PolySi) and silicon-rich silicon nitride (SiN) λ/4-thin film stacks. These structures can function as the upper mirror in a MEMS FPI device. High aspect-ratio mirror membranes were successfully released for 5 - 10 mm diameter range by sacrificial SiO2 etching in HF vapour. Optical simulations are presented for the Bragg reflector test structures designed for FPIs operating in the NIR range and the properties such as release yield and mechanical stability of the released LPCVD deposited polySi-SiN mirror membranes are compared with similar released atomic layer deposited (ALD) Al2O3-TiO2 λ/4-thin film mirror stacks. The realization of these Bragg reflector structures is the first step in the process integration of large-aperture MEMS FPI for miniature NIR imaging spectrometers, which can be applied to a variety of applications ranging from medical imaging and diagnostics to spaceand environmental monitoring instrumentation.

Compact large-aperture Fabry-Perot interferometer modules for gas spectroscopy at mid-IR

VTT has developed Fabry-Pérot Interferometers (FPI) for visible and infrared wavelengths since 90’s. Here we present two new platforms for mid-infrared gas spectroscopy having a large optical aperture to provide high optical throughput but still enabling miniaturized instrument size. First platform is a tunable filter that replaces a traditional filter wheel, which operates between wavelengths of 4-5 um. Second platform is for correlation spectroscopy where the interferometer provides a comb-like transmission pattern mimicking absorption of diatomic molecules at the wavelength range of 4.7-4.8 um. The Bragg mirrors have 2-4 thin layers of polysilicon and silicon oxide.

Aalto-1: a hyperspectral Earth observing nanosatellite

This paper introduces the Aalto-1 remote sensing nanosatellite, which is being built under the coordination of The Department of Radio Science and Engineering of Aalto University School of Electrical Engineering. The satellite is a three unit CubeSat, and it will be mostly built by students. The satellite platform is designed to house several payloads, and the main payload of the Aalto-1 mission will be the worlds smallest hyperspectral imager while secondary payloads being a compact radiation monitor and an electrostatic plasma brake for de-orbiting.

Miniaturized spectral imager for Aalto-1 nanosatellite

The Aalto-1 is a 3U-cubesat project coordinated by Aalto University. The satellite, Aalto-1, will be mainly built by students as project assignments and thesis works. VTT Technical Research Centre of Finland will develop the main Earth observation payload, a miniaturized spectral imager, for the satellite. It is a novel highly miniaturized tunable filter type spectral imager. Mass of the spectral imager will be less than 400 grams, and dimensions will be approximately 80 mm x 80 mm x 45 mm. The spectral imager is based on a tunable Fabry-Pérot interferometer (FPI) accompanied by an RGB CMOS image sensor. The FPI consists of two highly reflective surfaces separated by a tunable air gap and it is based either on a microelectromechanical (MEMS) or piezo-actuated structure. The MEMS FPI is a monolithic device, i.e. it is made entirely on one substrate in a batch process, without assembling separate pieces together. The gap is adjusted by moving the upper mirror with electrostatic force. Benefits of the MEMS FPI are low mass and small size. However, large aperture (2-10 mm) MEMS FPIs are currently under development, thus it is not yet known if their performance is adequate. The piezo-actuated FPI uses three piezo-actuators and is controlled in a closed capacitive feedback loop. The drawback of the piezo-actuated FPI is its higher mass. However, it has a large aperture which enables a shorter exposure times. Selection of the FPI type will be done after thorough evaluation. Depending on the selected FPI type, the spectral resolution of the imager will be 5 - 10 nm at full width at half maximum and it will operate in the visible and/or near infrared range.