The Fraunhofer Institute for Photonic Microsystems (IPMS) will present a miniaturised near-infrared (NIR) spectrometer at Photonics West 2013 on 2-7 February at the Moscone Centre in San Francisco, California (US), one of the trade shows for optics technology. This device enables analysis of gaseous, liquid and solid materials immediately on site instead of taking samples for lab analysis. The optical bench is smaller than a sugar cube and, thus, might fit into mobile devices as well as automated process analysis tools.
Whether it is freshness testing of food, separation of plastics in recycling, analysis of raw materials in the pharmaceutical industry or measurements of the composition of liquids in automobile applications: in each case it is the time efficient qualitative and quantitative determination of the materials involved according to type and concentration. Spectroscopy, the analysis of light scattered or attenuated by the material, is predestined for these types of applications. Measurements take place without direct contact and are non-destructive to the samples.
Despite these advantages and while today’s market does offer many spectrometers for lab use, there are few systems for simple on-location measurements. There are two major reasons for this: First, most of these instruments use one-dimensional (1D) or two-dimensional (2D) photodiode arrays of III-V semiconductors, such as indium-gallium-arsenide (InGaAs), which inflate costs; and second, they are voluminous for technological reasons. With miniaturised grating spectrometers this will change, says Dr. Heinrich Grüger, who is responsible for the Sensors and Actuators Systems business unit at Fraunhofer IPMS, where the spectrometer has been developed. “Our new hybrid-integrated system has a volume of 2.1 cubic centimetres (cm³) only, which is about 30% smaller than a common sugar cube. Due to its small outline and low power consumption of only a few milliwatts (mW), it is [suitable] for integration into mobile measurement instruments and in situ measurements in facilities and buildings.”
With Fraunhofer IPMS scanning grating spectrometers, the light is diffracted on a movable optical grating realised in a microelectromechanical systems (MEMS) technology. The device utilises a resonantly driving principle of the grating, which enables it to scan a spectrum with a single highly sensitive detector instead of an array arrangement. The central element of the spectrometer is a MEMS chip, which has been developed by Fraunhofer IPMS and measures just 9.5 × 5.3 × 0.5 cubic millimetres (mm³). The scientists realise this MEMS scanner featuring the diffraction grating and both optical slits directly on silicon wafers. A single 6-inch (~15cm) silicon wafer is large enough for chips for hundreds of spectrometers on it. Applying hybrid-integration of the MEMS wafer with a spacer and an optical mirror substrate of identical size would mean that hundreds of spectrometers can be manufactured instantly. They are aligned and assembled and then separated into individual spectrometers. This means that the scientists don’t have to adjust the mirrors, gratings, slits and detectors piece by piece as required with conventional spectrometers.
Dr. Grüger estimates that it will be about three to five years before the system will be ready for introduction to the market. Nevertheless, he and his team will be presenting the first fully functional model of this mini-spectrometer to the broad public at Photonics West in February 2013. The system, which will be demonstrated in an application example of plastics identification, performs measurements in the wavelength range from 950 nm to 1900 nm. This spectral range opens interesting applications for organic compound evaluation as for example portable analysis instruments for the food industry, mobile medical and pharmaceutical analysis instruments, in situ quality testing or warning and monitoring systems for security or facility management applications.
Photo: Scanning grating spectrometer: as big as a sugar cube