Jan Grahmann

Integrated position sensing for 2D microscanning mirrors using the SOI-device layer as the piezoresistive mechanical-elastic transformer

Position feedback of resonant scanning micromirrors plays a key role for various applications like portable laser projection displays or scanning grating spectrometers. The SOI device layer without an additional surface implantation is used for the piezoresistive sensor design. It assures the full compatibility to microscanner technology and requires no additional technological efforts. The necessary asymmetry of the current field density is achieved by the geometrical design of the sensor and its contacting. Integrated 2D position sensors with amplitude sensitivities of 0.42mV/V° were fabricated. FEA simulation and measured data correlates well with variations of ≤ 20.4%.

Widely tunable quantum cascade lasers for spectroscopic sensing

In this paper recent advances in broadband-tuneable mid-infrared (MIR) external-cavity quantum cascade lasers (EC-QCL) technology are reported as well as their use in spectroscopic process analysis and imaging stand-off detection of hazardous substances, such as explosive and related precursors. First results are presented on rapid scan EC-QCL, employing a custom-made MOEMS scanning grating in Littrow-configuration as wavelength-selective optical feedback element. This way, a scanning rate of 1 kHz was achieved, which corresponds to 2000 full wavelength scans per second. Furthermore, exemplary case studies of EC-QCL based MIR spectroscopy will be presented. These include timeresolved analysis of catalytic reactions in chemical process control, as well as imaging backscattering spectroscopy for the detection of residues of explosives and related precursors in a relevant environment.

Bi-resonant scanning mirror with piezoresistive position sensor for WVGA laser projection systems

Fraunhofer IPMS developed a new type of small-sized scanning mirror for Laser projection systems in mobile applications. The device consists of a single crystal mirror plate of 1 mm diameter in a gimbal mounting enabling a bi-resonant oscillation of both axes at a resonance frequency of about 100 Hz and 27 kHz respectively. The mechanical scan angle (MSA) achieved is ± 7° for the slow and ± 12° for the fast axis. The mirror angle position and phase can be read out via two piezo-resistive sensors located at the torsion axes. In order to allow for a minimum device size of the resonantly driven slow axis the sensor of the inner fast axis was connected by a new kind of thin silicon conductors. Those are created by means of an etch stop in TMAH etch and kept as thin as possible in order to reduce their contribution to the mechanical stiffness of the mirror-supporting structures. This new system enables to lead six (or even more) independent electrical potentials onto the moving parts of the device, whereas the mechanical properties are mainly determined by only 2 torsion axes. The devices were subsequently characterized and tested. Technology details, simulation results, pictures of the device and the new conductor structures as well as measurement results are presented.

Large MOEMS diffraction grating results providing an EC-QCL wavelength scan of 20%

Experimental results of a large scanning grating with a diameter of 5mm and 1 kHz scan frequency are discussed. An optical diffraction grating is fabricated on a mirror single crystal silicon plate to scan the first diffraction order in the MIR-wavelength range over a quantum cascade laser facet. Special emphasis is on the development of the grating technology module to integrate it with high accuracy and reproducibility into the IPMS AME75 process flow. The principle EC-QCL setup with the scanning grating is described and first measurement results concerning laser output power and tuning range are presented.

Tunable External Cavity Quantum Cascade Lasers (EC-QCL): an application field for MOEMS based scanning gratings

In situ process information in the chemical, pharmaceutical or food industry as well as emission monitoring, sensitive trace detection and biological sensing applications would increasingly rely on MIR-spectroscopic analysis in the 3 μm - 12 μm wavelength range. However, cost effective, portable, low power consuming and fast spectrometers with a wide tuning range are not available so far. To provide these MIR-spectrometer properties, the combination of quantum cascade lasers with a MOEMS scanning grating as wavelength selective element in the external cavity is addressed to provide a very compact and fast tunable laser source for spectroscopic analysis.

Laser projector solution based on two 1D resonant scanning micro mirrors assembled in a low vertical distortion scan head

A Scan Head package including two 1D resonant electrostatic driven micro scanning mirrors with piezoresistive position detection was developed. The scanning frequency of the slow and the fast axis is 100Hz and 29,05kHz, allowing WVGA-resolution. Thereby the Scan Head design reduces vertical distortion strongly and can potentially be assembled automatically. In addition FPGA based video processing electronic was developed to improve the sorting of the picture information corresponding to the Lissajous figure with the objective of high picture contrast and a homogeneous brightness.

Real-time spectroscopic sensing using a widely tunable external cavity-QCL with MOEMS diffraction grating

We present spectroscopic measurements performed with an EC-QCL combining a broadly tunable quantum cascade laser chip with a tuning range of more than 300 cm-1 and a resonantly driven MOEMS scanner with an integrated diffraction grating for wavelength selection in Littrow configuration. The grating geometry was optimized to provide high diffraction efficiency over the wide tuning range of the QCL, thus assuring high power density and high spectral resolution in the MIR range. The MOEMS scanner has a resonance frequency of 1 kHz, hence allowing for two full wavelength scans, one up and the other downwards, within 1 ms. The capability for real-time spectroscopic sensing based on MOEMS EC-QCLs is demonstrated by transmission measurements performed on polystyrene reference absorber sheets as well as on gaseous samples of carbon monoxide. For the latter one, a large portion of the characteristic CO absorption band containing several absorption lines in the range of 2070 cm-1 to 2280 cm-1 can be monitored in real-time.

MEMS-mirror based trajectory resolution and precision enabled by two different piezoresistive sensor technologies

Two new technological process flows for the piezoresistive position detection of resonant and quasistatic micro scanning mirrors were developed to increase sensitivities by a factor of 3:6 compared to former sensors, improve signal to noise ratio of the sensor signal and to allow controlled feedback loop operation. The sensor types use differently doped and deposited silicon. One is based on single crystal silicon with a pn-junction to isolate the active sensor area from the device layer silicon, the other one is based on a deposited and structured polysilicon. The sensor characteristics are compared including light, temperature dependence and reliability results.

Integrated piezoresistive position detection for electrostatic driven micro scanning mirrors

We have been developing a piezoresistive position detection for scanning micro mirrors in order to combine high position resolution with the capability of monolithic integration. In comparison to our formerly published results, the sensor sensitivity was strongly enhanced by implanting a 1 μm thick p-doped layer of NA ≈ 1017 cm-3 into the lowly p-doped SOI device layer of NA ≈ 1015 cm-3. This sensitivity was even further improved by at least a factor of 3 by a novel sensor design, allowing to couple more mechanical stress into the sensor structure.

Real-time spectroscopy enabled by external cavity QCLs with MOEMS diffraction gratings

In this contribution, we report on real-time mid-IR spectroscopy enabled by rapidly tunable External Cavity Quantum Cascade Lasers (EC-QCLs). High speed spectral scanning in a Littrow-type resonator is realized by employing a resonantly driven micro-opto-electro-mechanical-systems (MOEMS) grating as wavelength selective element. Oscillating at a frequency of 1 kHz with mechanical amplitudes of up to 10°, the MOEMS grating is able to cover the whole spectral range provided even by broad-gain QCL chips in just 500 μs. In addition to the high spectral scanning frequency, the MOEMS approach also allows for a miniaturized and rugged design of the EC-QCL. An evaluation of this laser source with regard to spectral reproducibility of consecutive scans, pulse intensity noise, and spectral resolution will be given. Furthermore, we present spectroscopic measurements in backscattering as well as in transmission geometry, demonstrating the real-time capability in different scenarios.