We present investigations of a new miniaturized NIR spectrometer
with a size of only 1088 cm3, and a MOEMS-scanninggrating
chip as a main element. It works currently in a spectral range of
1200 to 1900 nm with a resolution of less than 10 nm using only one
single InGaAs diode as a detector. One entire spectral measurement is
done within 6 msec, calculated by a digital signal processor, which is
included in the spectrometer. The MOEMS-scanning-grating chip is resonantly
driven by a pulsed voltage of up to 36 V, has a grating plate 3
3 mm2, and reaches deflection angles of ±8 deg at 25 V. Control and
investigation of the deflection angle, the static deformation, the spectral
efficiency, and the mechanical shock resistance are key parameters to
reach the spectrometer specifications. Results of these measurements
and their influence on the spectrometer are discussed. Special etch control
structures to monitor the fabrication process of the grating structure
in the nanometer range, which can be easily done by microscopic inspection,
are also presented.
Silicon micro machining once headed into two directions: MEMS (micro electro mechanical systems) based sensors
like accelerometers and gyroscopes on the one hand, MOEMS (micro opto electro mechanical systems) based
actuators like scanner mirrors on the other hand. Now both directions meet again: A tilt compensated projector
module uses a two dimensional excited scanner mirror as well as accelerometers and gyroscopes.
The projector module can have a minimum size of 30 x 15 x 15 mm3 with a monochrome red laser source (λ = 635
nm). It reaches a resolution of 640 x 480 pixels (VGA) and a frame rate of 50fps. Colour projection requires lager
size due to the lack of compact green laser sources. The tilt and roll angles are measured statically by a three axes
accelerometer, fast movement is detected dynamically by three single axis gyroscopes. Thus tilt of the projection
systems was compensated successfully. The dynamic range was set to 300 x 300 pixels for sufficient system
dynamic.
Furthermore the motion detection was used to achieve control and input device functions. The first demonstration
and test system consists of a projector mounted at the axis of a PC racing wheel together with the additional inertial
measurement unit (IMU) system. It was shown that projection and input function work well together. Using this
approach, new possibilities for hand-held devices arise in the close future.
Spectroscopy in the infrared region is today an important application to measure, control and investigate liquids or
gases in industrial, medical or environmental applications. We have developed a small, transportable NIRspectrometer
with a size of only 120 x 80 x 80 mm3, and a MOEMS-scanning-grating chip as main element. The
scanning-grating chip is resonantly driven by a pulsed voltage of only 36V, has a mirror aperture of 3 x 3 mm2 and
reaches maximum deflection angles of +/- 11o. The NIR-micro-spectrometer works currently in a spectral range of
1200 - 1900 nm with a resolution of less than 10 nm using only one single InGaAs-diode as detector. Additionally,
scanning grating chips have been already developed for spectral ranges of 900 - 1800 nm and 1250 - 2500 nm. One
entire spectral measurement is done within 6 milliseconds, calculated by a digital signal processor, which is included
in the spectrometer. Results can be either displayed by special computer software or directly by a graphical user
interface. In this paper, we will focus on the control of the grating fabrication process, which can be done by
microscopy, using new control structures. A time-consuming control with SEM (Scanning electron microscope) is
no longer needed. Furthermore the characterization of the fabrication process and its consequence on the
spectrometer properties will be discussed, as well as the characterization of the scanning grating chip itself
(frequency, movement, static deformation, spectral efficiency...). Characteristic measurement results of an argon
calibration lamp, which shows the performance of the NIR-micro-spectrometer, will be presented as well.
In recent years, Micro Opto Electro Mechanical Systems (MOEMS) have been reached more and more importance in technical applications. This is caused by the increased reliability of micro systems combined with the reduction of costs by high volume production. In this paper, we will present a resonant scanning grating chip with high diffraction efficiency, developed for the NIR region (900 - 2500 nm), which is based on our resonant micro scanning mirror. The grating was additionally applied to the silicon mirror plate by a chemical wet etch process. Therefore, three different fabrication technologies have been developed, showing high efficiencies in the first diffraction order. Compared to investigations with direct structured gratings in the reflective aluminium surface, gratings with up to 714 lines/mm could be fabricated combined with an improved process parameter control. These new resonant driven scanning gratings are still compatible to the scanning mirror fabrication process. They have a large surface of 3x3 mm2 and resonant frequencies of down to 150 Hz, which results in a lower demand on the bandwidth of the electronic read out, when applied to a spectrometer set-up. The maximum mechanically scan angle of the grating mirror plate could be increased to +/- 12° at a driving voltage of 36 V. First measurement results and an improved design of a micro spectrometer, working with only one single InGaAs-Detector in a spectral range of 900 to 2500 nm will be presented and discussed.
Further optimization of the agricultural growth process and quality control of perishable food which can be fruits and vegetables as well as every kind of meat or milk product requires new approaches for the sensitive front end. One possibility is reflectance or fluorescence spectroscopy in a wide wavelength range. By now broad usage is hindered by costs, size and performance of existing systems. MOEMS scanning gratings for spectrometers and translational mirrors for Fourier Transform spectroscopy enable small robust systems working in a range from 200nm to 5μm. Both types use digital signal processors (DSPs) capable to compute the spectra and execute complex evaluation and decision algorithms.
The MOEMS chips are realized by anisotropic etching of a silicon on insulator (SOI) substrate. First the backside silicon and buried oxide is removed by a wet process then the front side structure is realized by dry etching. Depending on the bearing springs a silicon plate up to 3 x 3 mm2 wide and typically 30μm thick can be driven resonantly to rotational or translational movement. Combined with additional optical components and appropriate detectors handheld Czerny-Turner or Fourier Transform spectrometers have been realized and tested.
Results of first measurements of reflection spectroscopy on model substances have been performed with both system types in the NIR range. Measurements on real objects like tomatoes or apples are intended for a wider wavelength range. Future systems may contain displays and light sources as well as data storage cards or additional interfaces.
KEYWORDS: Sensors, Signal processing, Mirrors, Near infrared, Spectroscopy, Digital signal processing, Microopto electromechanical systems, Infrared sensors, Light emitting diodes, LED displays
The examination of spectra in the NIR range is necessary for applications like process control, element analysis or medical systems. Typically integrated NIR spectrometers are based on optical setups with diffraction grating and detector arrays. The main disadvantage is price and availability of NIR array InGaAs-based detectors. The implementation of a scanning grating chip realized in a MOEMS technology which integrates the diffractive element makes it possible to detect spectra with single detectors time resolved. Either simple InGaAs photodiodes or cooled detectors may be used.
The set up is a shrinked Czerny-Turner spectrometer. The light is coupled in by an optical fibre. After focussing the light passes the scanning grating moving at 150-500 Hz in a sinusoidal way. There it is split off in the different wavelength, the monochrome intensity is caught by a second mirror and led to the detector. The detector signal is amplified by a transimpedance stage and converted to digital with 12 bit resolution. The main part of the signal processing is done by a digital signal processor, which is used to unfold the sinusoidal position and calculate the final spectra. The data rate can be up to 3 MHz, then a spectrum is acquired every 2ms by using a 500Hz Mirror. Using the DSP, the spectrometer can operate autarkic without any PC. Then the spectrum is display on a 160 x 80 pixel graphic LCD. A keypad is used to control the functions. For communication a USB port is included, additional interfaces can be realized by a 16-pin expansion port, which is freely programmable, by the system firmware.
KEYWORDS: Spectroscopy, Sensors, Mirrors, Signal processing, Silicon, Digital signal processing, Microopto electromechanical systems, Near infrared, Indium gallium arsenide, Infrared radiation
Optical spectroscopy is a common tool for many applications. Micro systems most often use fixed gratings and array detectors. In the infrared wavelength range above the limit for Si-detectors (1100nm) and Ge-detectors (1700nm) respectively, this is either very expensive or almost impossible. Micro opto electro mechanical systems (MOEMS) offer very promising options. A movable grating can be realized by a silicon chip, using the technology of a well established scanner mirror chips in combination with the realization of a reflective grating either through etching of the aluminium mirror layer or even a more sophisticated technology. The patented resonant drive realizes a mechanical angle of ±7° with CMOS compatible voltages of approximately 20V. This technology leads to the realization of a set up close to a classical Czerny-Turner spectrometer using a single
detector only. The device offers the capability to be scaled down to the size of a cigarette box. The spectrometer presented here was adjusted to 900...2500nm range. The scanning grating chip has either 500, 625 or 714 lines/mm. As detector serves a fast InGaAs photodiode, read out through a 12 Bit AD converter. The sinusoidal
movement is unfolded by a signal processor (TI TMS320F2812) which also computes the spectrum. Acquired data can be shown by a display or transmitted to a host PC. System tests have been performed using infrared LEDs. Wavelengths have been 1300, 1400 or 1550nm for example.
The spectrometer is working accurately. First result of micro shaped grating structures to enhance the sensitivity are presented.
Micro Opto Electro Mechanical Systems (MOEMS) reach more and more importance in technical applications. They are smaller than conventional devices, less expensive when fabricated in higher numbers and offer new options concerning reliability and measuring methods. Resonant movable micro-mirrors produced as single crystalline chips with CMOS-compatible technologies provide a broad field of applications. In this paper, we will present different micro-mirrors, which are developed by the Fraunhofer IPMS in Dresden, Germany. They have different layouts and are thus suitable for several applications. Fabricated 1D-mirrors with mechanical angles of ± 16° can be used for laser deflection in bar-code-scanners, 2D-mirrors with different sizes and frequencies are suitable for imaging, displaying etc. Furthermore processes to apply diffractive structures on the micro-mirror surface were developed, showing an increased efficiency in the first diffraction order. Thus a micro-spectrometer has been built up, working in a wavelength range of 900-2500 nm. Due to the Czerny-Turner set-up, only one fast single InGaAs-photodiode is required.
KEYWORDS: Spectroscopy, Mirrors, Diodes, Near infrared, Digital signal processing, Microopto electromechanical systems, Diffraction gratings, Light emitting diodes, Data processing, Signal processing
In the last few years the importance of Micro Opto Electro Mechanical Systems (MOEMS) increased significantly in technical applications. This is caused by the possibility of combining micro optical elements with micromachining technology that makes it feasible to develop new systems with high volumes and low prices. In this article, we report on the realization of a NIR (near infrared) spectrometer in the range of 900 - 2000 nm using MOEMS technology. It is based on a scanning mirror chip, which mirror plate is structured with a diffractive aluminium layer on top. This offers the possibility to fabricate a spectrometer, which needs only one single InGaAs detector photo diode. In contrast to common CCD arrays, the obtained resolution is only limited by the performance of the spectrometer (entrance slit, exit slit, focus length, diffractive element). The scanning grating chip operates at a frequency of 500 Hz, at an optical scan range of ± 4°. The whole spectrometer has a size of 90 x 60 x 50 mm. For first investigations of the performance, IR LEDs (light emitting diode) with 1300, 1450 and 1550 nm wavelength have been measured.
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