S. Hartwig

Micromachined IR-source with excellent blackbody like behaviour (Invited Paper)

There are several micro sized thermal emitters commercially available, but compared with an ideal black body radiator, their emissivity and thus the emitted radiation is moderate. This was the motivation to develop a novel type of micromachined thermal IR-emitters. The main difference compared with common thermal micro emitters is the use of 2D structured bulk silicon. The regular ordered macropores of the emitters are obtained by electrochemical etching of prepatterend silicon substrates. Typical pore diameter of the fabricated photonic-crystal-like structures are in the range of 2.5 μm to 30 μm. The macroporous silicon shows a black-body-like emission profile for a wide wavelength range.

A highly sensitive IR-optical sensor for ethylene-monitoring

Precise and continuous ethylene detection is needed in various fruit storage applications. The aim of this work is the development of a miniaturised mid-infrared filter spectrometer for ethylene detection at 10.6 μm wavelength. For this reason optical components and signal processing electronics need to be developed, tested and integrated in a compact measurement system. The present article describes the proposed system set-up, the status of the development of component prototypes and results of gas measurements performed using a first system set-up. Next to a microstructured IR-emitter, a miniaturised multi-reflection cell and a thermopile-array with integrated optical filters and microstructured Fresnel lenses for the measurement of ethylene, two interfering gases and one reference channel are proposed. Recently a miniaturised White cell as absorption path is tested with various commercial and a self-developed thermal emitter. First ethylene measurements have been performed with commercial twofold thermopile detectors and a Lock-in-amplifier. These showed significant absorption at an ethylene concentration of 100ppm. For the detection module different types of thermopiles were tested, first prototypes of Fresnel lenses have been fabricated and characterised and the parameters of the optical filters were specified. Furthermore a compact system electronics for signal processing containing a preamplification stage and Lock-in-technique is in development.

Linewidth measurement of free-running, pulsed, distributed feedback quantum cascade lasers

In gas spectroscopy with distributed feedback quantum cascade lasers (DFB-QCLs), linewidth is an important measurement parameter. However, in standard usage, these lasers are used in pulsed operation, so that the laser linewidth can no longer be assumed to remain constant over the length of the pulse. For the instantaneous measurement of such dynamic linewidths, a suitable measurement technique was still lacking. Frequently, pulse lengths of 5 to 10 ns are used to obtain laser linewidths that will be suitable for gas measurements. In these experiments, standard photoconductive or photovoltaic mid-infrared detectors with bandwidths in the 1 MHz range are employed, so that the linewidth is averaged over the tuning range of the QCL. The linewidth averaging could be reduced by shorter pulses, but for pulses shorter than 5 ns, the Fourier limitation increases the linewidth again. We present a technique to measure the dynamic laser linewidth continuously within a laser pulse of 30 ns to several μs. Due to the u...

Hollow fibers for compact infrared gas sensors

Hollow fibers can be used for compact infrared gas sensors. The guided light is absorbed by the gas introduced into the hollow core. High sensitivity and a very small sampling volume can be achieved depending on fiber parameters i.e. attenuation, flexibility, and gas exchange rates. Different types of infrared hollow fibers including photonic bandgap fibers were characterized using quantum cascade lasers and thermal radiation sources. Obtained data are compared with available product specifications. Measurements with a compact fiber based ethanol sensor are compared with a system simulation. First results on the detection of trace amounts of the explosive material TATP using hollow fibers and QCL will be shown.

Optimizing Gas Sensors Based on Quantum Cascade Lasers and Photonic Bandgap Hollow Waveguides

In the present study, bending losses in conventional hollow waveguides (internally Ag/AgI coated) and in photonic bandgap (PBG) hollow waveguides (HWG) are compared based on studies via FT-IR spectroscopy and quantum cascade lasers (QCL). To date, literature on bending losses in hollow waveguides focuses on conventional HWG structures (e.g., silica structural tube with internal Ag/AgI coating), whereas the results discussed here compare relative bending losses in novel photonic bandgap waveguides, a new type of HWG progressively more integrated in gas sensors, versus conventional HWGs for the first time. Photonic bandgap waveguides are expected to exhibit lower polarization-dependent relative bending losses due to radiation propagation via omnidirectional reflection, in contrast to conventional HWGs. Accordingly, photonic bandgap waveguides offer superior flexibility and robustness against bending losses in coiled configurations rendering them promising structures for next-generation miniaturized QCL-based HWG gas sensors.

Fresnel lenses: study and fabrication in silicon technology for medium-IR applications

Diffractive Fresnel Lenses (FL) were designed, fabricated and tested. The lens aims for increasing the sensitivity of a Non-Dispersive InfraRed (NDIR) silicon based optical gas system, focusing as much radiation as possible onto the detector. The studied wavelengths are 10.6μm and 3.4μm, which are the main absorption lines for ethylene and ethanol respectively. The lens diameter (5mm) and the focal length (4mm) are fixed by the detector package. Those diffractive lenses are compatible with the planar nature of silicon microtechnology. A theoretical study about the global lens efficiency as a function of the technological constrains and the process complexity has been carried out. Using only three photolithographic masks, eight quantization steps can be etched and a theoretical lens efficiency of 95% can be achieved. Once the devices were fabricated, the focal length and the spot size have been measured.

A compact optical ethylene monitoring system

In various fruit storage applications precise and continuous ethylene detection is needed. The aim of this work is the development of a miniaturised mid-infrared filter spectrometer for ethylene detection at 10.6 &mgr;m wavelength. For this reason optical components and signal processing electronics were developed, tested and integrated in a compact measurement system. The present article describes the optical components, the integration of the optical system, electronics and results of gas measurements. Next to a Silicon-based macroporous IR-emitter, a miniaturised absorption cell and a detector module for the simultaneous measurement at four channels for ethylene, two interfering gases and the reference signal were integrated in the optical system. Optical filters were attached to fourfold thermopile-arrays by flip-chip- technology. Silicon-based Fresnel multilenses were processed and attached to the cap of the detector housing. Because of the high reflection losses at the silicon-air surface the Fresnel lenses were coated with Antireflection layers made of Zinc sulphide. For the signal processing electronics a preamplification stage and a Lock-in-board has been developed. First ethylene measurements with the optical system with miniaturised gas cell, Silicon-based IR-emitter, a commercial thermopile detector and the self-developed system electronics showed a detection limit of smaller than 20ppm.

New infrared sources for breath analysis

Infrared breath analysis is used in diagnostics of respiratory diseases, pulmonary function testing, and for metabolic studies. With selective and highly sensitive instruments exhaled trace gas concentrations can be related to specific diseases. For many applications also a time resolution below 0.1s is needed. Frequently, performance is limited by the IR source. New developments offer solutions even for compact instruments. Different setups employing quantum cascade lasers (QCL), VCSELs, and a new optically pumped IR emitter are compared focusing on CO2 measurements as an example.

Schnelle Gasspektroskopie mit gepulsten Quantenkaskadenlasern für die Luftgütemessung (Fast Gas Spectroscopy with Pulsed Quantum Cascade Lasers for Air Quality Measurement)

Abstract Quantenkaskadenlaser (QCL) sind neuartige Laserstrahlquellen im mittleren Infrarot (MIR, 3–30μm). Damit können kompakte und hochempfindliche Laserspektroskopiesysteme ohne aufwändige Kühlung aufgebaut werden. Mit gepulsten QCLs sind Messraten im Bereich 10–100 kHz möglich. Bei Messungen mit den Gasen CO2, NO und NH3 werden Nachweisgrenzen unter 1 ppm·m erreicht. Das Verfahren eignet sich für offene Messstrecken und liefert eine simultane Wegmessung.

Charakterisierung von Hohlfasern für kompakte Infrarot-Gasmesszellen Characterization of Hollow Fibres for Compact Infrared Gas Measurement Cells

Zusammenfassung Die Nachweisempfindlichkeit von infrarotoptischen Gassensoren ist von der Wechselwirkungsstrecke zwischen Gas und Strahlung abhängig — d. h. eine Verkleinerung der Absorptionsstrecke bedeutet zugleich eine geringere Nachweisempfindlichkeit. Einer Miniaturisierung sind damit Grenzen gesetzt. Die Verwendung von gasdurchströmten Hohlfasern ist eine Möglichkeit, kompakte Gassensoren mit geringen Gasvolumina bei relativ langer Wegstrecke zu realisieren. Verschiedene Hohlfasertypen wurden charakterisiert und auf ihre Eignung als Gassensor untersucht. Simulation und experimentelle Ergebnisse zur Realisierung eines kompakten Alkoholsensors auf Hohlfaserbasis zeigen sehr gute Übereinstimmung.