The primary object presented in this contribution is the miniaturization of a displacement sensor system with the potential for high accuracy measurements and for cost-effective production in polymers. The measurement of linear displacements can be performed by different methods e.g. magnetoresistive, potentiometric, electromagnetic or inductive encoder systems. For movements in the millimeter range and above the most precise systems are based on optical methods.
The displacement measurement of our sensor system uses the intensity modulation of two amplitude gratings, moving relative to each other and illuminated by a LED. To increase the system resolution and the signal quality the grating/detector combination is divided into four areas which are phase shifted to each other. The grating period is 25 μm with a geometrical accuracy below 1 μm. The amplitude gratings have been processed on a glass substrate lithographically. Applying electro-discharge machining a miniaturised optical bench for the passive alignment of the optical and the opto-electronic components has been realised.
The sensor has an overall size of 6x4x3 mm<sup>3</sup> and is designed for the future replication in one single polymer part. In combination with an electronic interpolation the sensor will be capable for a sub-micrometer accuracy.
The optical application of electrowetting-on-dielectric (EWOD) using thin dielectric layers is the focus of this paper. An optical switching configuration is designed with transparent indium-tin-oxide (ITO) electrodes and glass substrates as well as a transparent dielectric film between the liquid and the electrodes. A polyimide layer with a thickness of 0.5 μm to 1.5 μm and a top coating of 0.5 μm PTFE as hydrophobic surface has been used as dielectric film. Experimentally we present a significant change in the contact angle up to 56° applying a voltage of 155 V. The wettability of the surface can be controlled and a liquid flow is achieved by applying a voltage below 100 V. The saturation of contact angle is described by a model including a contact angle dependent resistance of the dielectric layer as fitting parameter and a constant resistance of the water droplet. The resistances have been confirmed by independent measurements. Based on these results optical switching has been performed by the principle of total reflection. Thereby the refractive index of the optical beam path is changed between the total reflection condition at an interface and transmission. This operation is realised by moving a water droplet between two glass plates. Within this concept addressable operations of a liquid on a fluidic chip and the integration of optical guiding and switching of light is possible. The application of EWOD for optics can be fundamental for the integration of micro-optics and fluidics in one device and the development of new micro-opto-electro-mechanical systems (MOEMS).
A miniaturized optical spectrometer module has been developed and realized in polymer by injection moulding. The spectrometer is designed for the visible (VIS, 380 nm-750 nm) and near infrared spectral (NIR, 680 nm-1100 nm) range. The assembled module has a size of a match box with a spectral resolution (Rayleigh criterion) of <7 nm /10 nm for the visible and <7 nm/8 nm for the near infrared spectrum depending on the pixel width of the used detectors. The stray light has been reduced well below 0.5 % for the VIS-module (VIS: filter OG550, measured at 500 nm) and NIR-module (NIR: filter RG850, measured at 790 nm). To avoid a wavelength shift caused by a thermal expansion of the system, a passive temperature compensation unit is designed. As a result of this the temperature shift between -40 °C and +70 °C can be reduced to <0.03 nm/K. To guarantee a flexible application of the spectrometer the measurement signal is coupled into the spectrometer by a fibre to free-space coupling unit with a 90° beam deflection. In order to use injection moulded components for optical sensors, mould inserts with a high optical quality are required. A toroidal optical mirror with an average surface roughness of Ra<20 nm and a radial shape accuracy as high as 0.2 % (0.1 mm) and optical gratings for the visible and near infrared spectral range with a planarity of 4 μm/cm and an absolute diffraction efficiency as high as 80 % can be fabricated. LIGA-technology, ultra-precision machining and electro-forming processes are applied. All optical elements have been replicated in polycarbonate (PC) with comparable characteristics. The spectrometer set up is based on a modular concept. This enables a high position accuracy of the elements to each other (few tens of μm) and a variation of specification (wavelength and resolution).