Håkon Sagberg

TUNABLE BAND-PASS FILTER USING RF MEMS CAPACITANCE AND TRANSMISSION LINE

In this paper we present the design and fabrication of an RF MEMS tunable band-pass fllter. The band-pass fllter design uses both distributed transmission lines and RF MEMS capacitances together to replace the lumped elements. The use of RF MEMS variable capacitances gives the ∞exibility of tuning both the centre frequency and the band-width of the band-pass fllter. A prototype of the tunable band-pass fllter is realized using parallel plate capacitances. The variable shunt and series capacitances are formed by a combination of parallel plate RF MEMS shunt bridges and series cantilevers. The fllter operates at C-X band. The measurement results agree well with the simulation results.

Spectral uniformity of two- and four-level diffractive optical elements for spectroscopy.

We present simulations and characterization of gold coated diffractive optical elements (DOEs) that have been designed and fabricated in silicon for an industrial application of near-infrared spectroscopy. The DOE design is focusing and reflecting, and two-level and four-level binary designs were studied. Our application requires the spectral response of the DOE to be uniform over the DOE surface. Thus the variation in the spectral response over the surface was measured, and studied in simulations. Measurements as well as simulations show that the uniformity of the spectral response is much better for the four-level design than for the two-level design. Finally, simulations and measurements show that the four-level design meets the requirements of spectral uniformity from the industrial application, whereas the simulations show that the physical properties of diffraction gratings in general make the simpler tw level design unsuitable.

Two-state Optical Filter Based on Micromechanical Diffractive Elements

We have designed a robust two-state filter for infrared gas measurement, where the filter transmittance alternates between a single bandpass function, and a double-band offset reference. The device consists of fixed and movable diffractive sub-elements, micromachined in the device layer of a bonded silicon on insulator (BSOI) wafer. Switching between the two states of the filter is obtained by actuation of the movable sub- elements between idle and pull-in positions, which affects the interference of reflected light The characteristics of the filter are defined by a diffractive microrelief pattern etched on top of the sub-elements and by the position of the movable sub-elements at pull-in, the latter mechanically defined by the buried oxide layer. Thus, no accurate electrical control is needed to operate the filter. The first test components operate at 2 mum wavelength using a displacement of 500 nm and an actuation voltage of 5 V. No sticking or change in filter characteristics have been observed after repeated pull-in operations. The simplicity of fabrication and operation is likely to make the two-state filter an attractive component for sensors such as non-dispersive infrared gas detector.

Wireless Infrared Gas Sensor

Infrared hydrocarbon gas detectors are essential for safety, but the requirement for cabled power complicates installation. A new low-power optical design based on a micro-opto-electromechanical system gives several years of reliable battery operation.

Tunable Lowpass Filter with RF MEMS Capacitance and Transmission Line

We have presented an RF MEMS tuneable lowpass filter. Both distributed transmission lines and RF MEMS capacitances were used to replace the lumped elements. The use of RF MEMS capacitances gives the flexibility of tuning the cutoff frequency of the lowpass filter. We have designed a low-pass filter at 9–12 GHz cutoff frequency using the theory of stepped impedance transmission lines. A prototype of the filter has been fabricated using parallel plate capacitances. The variable shunt capacitances are formed by a combination of a number of parallel plate RF MEMS capacitances. The cutoff frequency is tuned from C to X band by actuating different combinations of parallel capacitive bridges. The measurement results agree well with the simulation result.

Discrete dipole approximation: a numerical tool for subsurface sensing

We present a method to simulate scattering of microwaves in realistic soil representations. Our results will be used by the designers of a new microwave antenna for humanitarian mine clearance being developed within the European Project PICE. We aim to identify scenarios giving false alarm signals and search for means to avoid these conditions. We use the Discrete Dipole Approximation (DDA), a method originally developed to study the scattering of light by particles in interstellar dust. The DDA simulates the scattering objects by a lattice of electric dipoles and calculates the total scattering as the contribution of all the dipoles in the lattice to the total field. It is a very suitable method to study electromagnetic scattering by objects of irregular shape and by clusters of them. We have successfully applied DDA to the analysis of light scattering by clusters of many thousands of particles, simulating pigments in paper coatings.

Optical MEMS filter for gas spectroscopy

An optical MEMS filter for infrared spectroscopy has been designed, fabricated, and tested. The filter is a deformable hologram which combines focusing and optical filtering into one element. By modulating sections of the hologram, transmission spectra adapted to the absorption lines of a gas can be generated, allowing accurate measurements of gas concentration. The device is fabricated by micromachining grooves in silicon on top of an electrostatic actuator.