H. Sagberg

Micromechanical gratings for visible and near-infrared spectroscopy

We present a micromechanical grating array that acts as a configurable optical filter for low-cost and compact visible and near-infrared spectrometric sensors. We show how the grating array can be used either as a fast scanning monochromator or as a diffractive filter, and that the expected signal-to-noise ratio is approximately equal for the two modes of measurement. The free spectral range of the filter can be matched to a defined spectral measurement region, so that we can optimize the relationship between spectral resolution and electromechanical complexity. We have numerically studied diffraction efficiency and errors in filter shape. Finally, we fabricated a small configurable grating array and present measurement results that demonstrate electrostatic filter modulation.

Optical microphone based on a modulated diffractive lens

The authors present an optical microphone based on a diffractive lens. The distance between the diffractive lens and a reflecting microphone membrane determines the light intensity at the focal point of the lens. With this design, an integrated micromachined microphone transducer can be mass produced at low cost. They describe the sensing principle, calculate the intensity at the focal point, and show how it depends on the membrane position. The authors present results from measurements with a prototype setup which proves the measurement principle and has excellent properties when compared to an expensive condenser microphone.

Reconfigurable near-infrared optical filter with a micromechanical diffractive Fresnel lens

We have fabricated and characterized a micromechanical Fresnel lens that serves as a reconfigurable filter for near-infrared spectrometry, for the analysis of gas, liquids, or solids. The lens consists of silicon segments that can be individually actuated. Moving the segments vertically controls the spectrum of the light that is focused on a detector. The holographic pattern on top of the segments permits both focusing of the light and spectral control. We have demonstrated experimentally the desired spectral control of the filter.

Algorithms for the synthesis of complex-value spectral filters with an array of micromechanical mirrors.

We study optical spectral filter synthesis with arrays of piston-actuated micro-mirrors. We propose two algorithms for the calculation of the positions of the micro-mirrors, giving us control of both the amplitude and phase of the synthetic filter. Both algorithms for filter synthesis are explored in an analytic version and in numerical searches for the least deviations between the target and the synthesized filter. We measure the quality of the filter both in terms of the deviations and in filter transmissivity, and present results of numerical simulations for a wide selection of target filters. We find that numerical searches can sometimes yield considerable improvement in the filter synthesis compared to the analytic approximation.

Configurable spectral filter with an array of diffraction grating

This paper presents the design, fabrication, and characterization of a configurable diffractive filter capable of high optical resolution within a narrow spectral range. Based on measurements of spectral reflectance from this device, and from corresponding set of fixed diffractive elements, this type of spectral filter is well suited for spectroscopy applications.

Tunable diffractive filters for robust NIR and IR spectroscopy

Three different types of specialized spectrometers have been implemented. These sensors are well suited for gas detection or other industrial applications where long-term stability is a key requirement. These sensors are based on tunable diffractive filters that were developed in the past and that we have now adapted to fit into the presented spectrometers, each of them developed to perform a dedicated measurement. The diffractive filters share the common feature that a single surface hologram integrates the functions of lenses, beam splitters and spectral filters. In order to perform a compensated measurement, it is possible to switch between several filter functions, which can be done without the need for complicated or precise mechanical control. These features make it possible for the spectrometers to be made robust against long-term drift, which eliminates the need for frequent recalibration and maintenance. In addition, the simplicity of the sensor assembly makes the sensors well suited for a wide range of applications; the requirement is either a low production cost or robustness to harsh industrial environment. The sensors are or have been industrialized, with application mainly for gas spectroscopy. Results in terms of sensitivity and long-term drift were obtained in the field and are presented.

A micro-mechanical fresnel lens for spectral filtering

In this paper, there has been a recent interest for MEMS-based adjustable or re-configurable diffraction filters. Reconfigurable diffraction filters are well adapted to NIR spectroscopy, with applications such as identification of polymers and detection of carbon monoxide in air. A cost-effective implementation of a NIR spectrometer was proposed by Lovhaugen. The system uses a focusing static Diffractive Optical Element (DOE). The set-up minimizes the number of elements in the system, as no lens is needed

High performance low cost microphones utilising micro-optical technology

Two micro-optical microphone principles have been realised. The first is based on a self-aligning Fabry-Perot cavity with a LED. The second is based on a diffractive lens, a reflecting membrane and a fiber.