Spectrometer Uses Micromachined Grating
Daniel S. Burgess
In the pursuit of compact spectrometers for a variety of industrial applications, researchers at Fraunhofer Institut für Photonische Mikrosysteme in Dresden, Germany, have developed a device based on a micromechanical scanning grating. They suggest that the work may yield handheld products with a better combination of range and resolution than today's low-cost, fixed-grating spectrometers.
Heinrich Grüger, a division head at the institute, explained that the device configuration is a variant of the Czerny-Turner setup, in which a motor tilts a dispersive element, typically a grating, and the position of the element defines the wavelength of the monochromatic light emerging from the exit slit. The new spectrometer, however, employs a micromachined grating that is steered with a patented, 20-V resonant electrostatic drive (Figure 1). The grating is operated in a scanning mode at a resonance frequency of between 500 Hz and 2 kHz, depending on the size of the element, enabling the device to acquire a spectrum in milliseconds.
Figure 1. A spectrometer developed at Fraunhofer Institut für Photonische Mikrosysteme uses a micromachined scanning grating as a dispersive element. In this variant of the Czerny-Turner configuration, a 20-V electrostatic drive steers the grating, enabling system integration using low-cost CMOS processes. Images courtesy of Heinrich Grüger.
As with the Czerny-Turner design, a single photodetector behind the exit slit measures the intensity of the monochromatic signal. Fixed-grating spectrometers, in contrast, employ detector arrays, thus increasing costs.
The dispersive element is fabricated in a micro-optoelectromechanical systems process from bonded silicon on insulator wafers (Figure 2). Aluminum is deposited on the mirror plate through a photomask to form the grating structure -- 500 grooves per millimeter in the 2 x 2-mm prototype (Figure 3). The scanning range of the element can be up to 60°, and the spectral range of the device is approximately 500 nm.
Figure 2. The 2 X 2-mm dispersive element is micromachined from bonded silicon on insulator wafers. Aluminum deposited through a photomask forms the grating structures on the mirror plate.
Because the electrostatic drive is compatible with CMOS processes, the researchers are working to miniaturize the device by integrating the grating with a driving application-specific integrated circuit, an analog-to-digital converter, and a controller or digital signal processing chip. Currently, the position of the grating is calculated off-chip employing a computer that is equipped with an analog-to-digital card.
Figure 3. A close-up of a prototype element illustrates its 500-groove-per-millimeter grating and comblike drive structures.
To characterize the performance of the spectrometer, the scientists captured a spectrum that included the output of three laser sources. Measuring the 632.8-nm line of a HeNe laser, they calculated that the device displayed a resolution of 0.7 nm full width half maximum, in good agreement with theoretical calculations.
Immediate applications of the new spectrometer will be in industrial processes such as air-quality monitoring or sorting plastics for recycling, using an infrared photodetector in the design, Grüger said. Others might include the analysis of exhaust gases from diesel engines in automobiles and from oil furnaces in home heating systems, which would lower unit costs as the number of devices produced grows.
Economies of scale are central to the researchers' plan. "From my point of view, the combination of a wide and selectable range and a reasonable price is a key to the market," he said. "As is typical for every new system, the applications that offer higher economical possibilities will be served first, and with decreasing prices, further applications will arise."
One shortcoming of the device is that it requires relatively high signal intensities because the mechanical properties of the materials involved limit the gratings to sizes of up to 3 x 3 mm, and thus lower the throughput. The use of ultralow-noise photodetectors could compensate for this, but the added expense might negate the cost savings of the micromachined approach.
Although low signal intensities would not be an issue in the targeted applications, Grüger said that the scientists are working to optimize the grating design of the spectrometer for higher throughputs.
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