Remote leak detection of gases such as the homonuclear molecules (N2, H2, etc.) and noble gases (He, Ar etc.) is still an issue for tunable laser spectroscopy (TLS) because these gases do not have infrared absorption bands. In order to detect a leak in air, the gas displacement of the ambient air is used as an indirect indication of the leak. So, the unique idea is to measure the reduced oxygen concentration by a standoff laser spectrometer at an emission wavelength of 761 nm. The advantage of oxygen as indicator gas is the stable concentration level with respect to low spatial and temporal fluctuations. The challenge of the standoff detection is to analyze the small relative transmission change for weak light intensity scattered by the background. Furthermore, a remote measurement technique for high-level oxygen concentration on ppm level resolution is demonstrated. Here the combination of a high performance distributed feedback laser at 761 nm and high end sophisticated electronics for driver and data acquisition is required and designed. With the direct absorption spectroscopy, the concentration change of 2000 ppm within a 1 cm plume size (10 ml/min flow, ambient room conditions) corresponds to a transmission change in order of 2E-4 has been resolved on a low absolute power level of few micro watts in 1m distance. The detection limit corresponds to a nitrogen leakage rate of 0.1mbar·l/s which is comparable to ordinary remote detection systems for methane leakages.
Step by step, US and European legislations enforce the further reduction of atmospheric pollution caused by automotive
exhaust emissions. This is pushing automotive development worldwide. Fuel efficient diesel engines with SCRtechnology
can impede NO<sub>2</sub>-emission by reduction with NH<sub>3</sub> down to the ppm range. To meet the very low emission limits of the Euro6 resp. US NLEV (National Low Emission Vehicle) regulations, automotive manufacturers have to
optimize continuously all phases of engine operation and corresponding catalytic converters. Especially nonstationary
operation holds a high potential for optimizing gasoline consumption and further reducing of pollutant emissions. Test
equipment has to cope with demanding sensitivity and speed requirements. In the past Fraunhofer IPM has developed a
fast emission analyzer called DEGAS (Dynamic Exhaust Gas Analyzer System), based on cryogenically cooled lead salt
lasers. These systems have been used at Volkswagen AG‘s test benches for a decade. Recently, IPM has developed
DEGAS-Next which is based on cw quantum cascade lasers and thermoelectrically cooled detectors. The system is
capable to measure three gas components (i.e. NO, NO<sub>2</sub>, NH<sub>3</sub>) in two channels with a time resolution of 20 ms and 1
ppm detection limits. We shall present test data and a comparison with fast FTIR measurements.
Sensitive and fast identification of drugs or drug precursors is important and necessary in scenarios like baggage or
container check by customs or police. Fraunhofer IPM is developing a laser spectrometer using external cavity quantum
cascade lasers (EC-QCL) to obtain mid-infrared (IR) absorption spectra in the wavelength range of the specific
vibrational bands of amphetamines and their precursors. The commercial EC-QCL covers a tuning range of about 225
cm<sup>-1</sup> within 1.4 s.
The system could be used for different sample types like bulk samples or liquid solutions. A sampling unit evaporates the
sample. Because of small sample amounts a 3 m long hollow fiber with an inner volume smaller than 1ml is used as gas
cell and wave guide for the laser beam.
This setup is suitable as a detector of a gas chromatograph instead of a standard detector (TCD or FID). The advantage is
the selective identification of drugs by their IR spectra in addition to the retention time in the gas chromatographic
column. In comparison to Fourier Transform IR systems the EC-QCL setup shows a good mechanical robustness and has
the advantage of a point light source. Because of the good fiber incoupling performance of the EC-QCL it is possible to
use hollow fibers. So, a good absorption signal is achieved because of the long optical path in the small cell volume
without significant dilution. In first laboratory experiments a detection limit in the microgram range for pseudo
ephedrine is achieved.
Periodic silicon nanostructures can be used for different kinds of gas sensors depending on the analyte concentration.
First we present an optical gas sensor based on the classical non-dispersive infrared technique for ppm-concentration
using ultra-compact photonic crystal gas cells. It is conceptually based on low group velocities inside a photonic crystal
gas cell and anti-reflection layers coupling light into the device. Experimentally, an enhancement of the CO<sub>2</sub> infrared
absorption by a factor of 2.6 to 3.5 as compared to an empty cell, due to slow light inside a 2D silicon photonic crystal
gas cell, was observed; this is in excellent agreement with numerical simulations. In addition we report on silicon nanotip
arrays, suitable for gas ionization in ion mobility microspectrometers (micro-IMS) having detection ranges in principle
down to the ppt-range. Such instruments allow the detection of explosives, chemical warfare agents, and illicit drugs, e.g., at airports. We describe the fabrication process of large-scale-ordered nanotips with different tip shapes. Both silicon microstructures have been fabricated by photoelectrochemical etching of silicon.
The monitoring of acetylene (C<sub>2</sub>H<sub>2</sub>) concentrations is important for many chemical processes. Industrial trace gas
measurements are usually performed using gas chromatographs (GC) which have time constants of several minutes.
Optical analyzers are expected to yield faster response times with lower maintenance costs. We investigated the use of
quantum cascade laser (QCL) spectroscopy in the 14μm range for the sensitive and fast detection of C<sub>2</sub>H<sub>2</sub>. This spectral
range is favorable, as it avoids spectral interferences by other components which could be present in typical process
gases. We developed new custom DFB QCLs and characterized their spectral properties. We determined the
performance of our QCL gas analyzer setup and demonstrate a noise equivalent concentration of 10 ppb in 20 s average
Sensitive and fast detection of explosives remains a challenge in many threat scenarios. Fraunhofer IPM works on two
different detection methods using mid-infrared absorption spectroscopy in combination with quantum cascade lasers
(QCL). 1. stand-off detection for a spatial distance of several meters and 2. contactless extractive sampling for short
The extractive method is based on a hollow fiber that works as gas cell and optical waveguide for the QCL light. The
samples are membranes contaminated with the explosives and real background. The low vapor pressure of TNT requires
a thermal desorbtion to introduce gaseous TNT and TATP into the heated fiber. The advantage of the hollow fiber setup
is the resulting small sample volume. This enables a fast gas exchange rate and fast detection in the second range. The
presented measurement setup achieves a detection limit of around 58 ng TNT and 26 ng TATP for 1 m hollow fiber.
TATP - an explosive with a very high vapor pressure in comparison to TNT or other explosives - shows potential for an
adequate concentration in gas phase under normal ambient conditions and thus the possibility of an explosive detection
using open path absorption of TATP at 8 μm wavelength. In order to lower the cross sensitivities or interferents with
substances with an absorption in the wavelength range of the TATP absorption the probe volume is checked
synchronously by a second QCL emitting beside the target absorption wavelength. In laboratory measurements a
detection limit of 5 ppm*m TATP are achieved.
Stand-off and extractive explosive detection methods for short distances are investigated using mid-infrared laser spectroscopy. A
quantum cascade laser (QCL) system for TATP-detection by open path absorption spectroscopy in the gas phase was developed. In
laboratory measurements a detection limit of 5 ppm*m was achieved. For explosives with lower vapor pressure an extractive hollow
fiber based measurement system was investigated. By thermal desorption gaseous TATP or TNT is introduced into a heated fiber.
The small sample volume and a fast gas exchange rate enable fast detection. TNT and TATP detection levels below 100 ng are
feasible even in samples with a realistic contaminant background.
We present an overview of the current status of laser diodes used in remote sensing application including novel
laser types such as single mode emitting DFB lasers operating at wavelengths up to 3 μm and quantum cascade
lasers for mid infrared absorption spectroscopy. In particular we will focus on applications of these devices in the
frame of safeguard measures and home security.
A grey body emitter based on a microcavity array with Pt-heater on the backside is presented. The microcavity array is
made by electro-chemical etching of silicon. It has been shown in a previous work, that this emitter has especially in the
spectral region >8 μm significantly higher emissivity than commercial available emitters.
Due to the thin-film technology of MEMS-based emitters, these types can be typically operated with a maximum
temperature of 700°C to 800°C. Higher temperature causes degradation of the heater. But higher temperatures also mean
an enhancement in radiation power and thus open a wider area of application.
The presented work shows a temperature enhanced thermal emitter with a ceramic heater passivation. Short time tests
show the possibility of a maximum temperature of 1000°C.
The part of light emitted by the microcavities in comparison to the whole device as well as the influence of the pore size
concerning the emitted spectral range is investigated. The results are the basis for a redesign of the microcavity array for
an enhancement of the geometry tuned emissivity.
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.
The sensitivity of an infrared gas sensor depends on the interaction length between radiation and gas, i.e. a reduction in
cell size generally results in a reduced sensitivity, too. However, low group velocity regions in the bandstructure of
photonic crystals should enable the realization of very compact gas sensors. Using photonic crystals based on
macroporous silicon experimental results with CO<sub>2</sub> show an increase of the gas sensitivity in the photonic crystal
compared to an empty cell of same dimensions. For practical applications the results are compared with gas
measurements using conventional multireflection cells and hollow fiber setups.
Terahertz Time-Domain Spectroscopy (THz-TDS) is used to investigate water and soot contaminations in oils, exhibiting different dilution modes. For synthetic polyglycol oils, the water is dissolved due to the polar behavior of the oil, whereas in non polar mineral oils the water-oil compound forms an emulsion. This behavior is modeled with an effective medium approximation (EMA) formalism. Small soot agglomerates are remaining in suspension when mineral oils are polluted with soot particles. In this case, the absorption spectrum is dominated by scattering effects. Due to the small particle size of the soot agglomerates compared to the THz wavelength, coherent scattering is the dominant process.
The water content in polyglycol oils is investigated by Terahertz Time-Domain Spectroscopy (THz-TDS). These oils are able to dissolve a certain percentage of water. Changes in the absorption coefficient and refractive index are observed related to the amount of water added to the pure oils. Comparison of the experimental data with predictions based on Beer-Lambert and Lorentz-Lorenz-theory, respectively, exhibits an excellent agreement. Analyses with Fourier Transform Infrared (FTIR) Spectroscopy reveal sensitivity similar to the THz-TDS experiment. THz-TDS may offer powerful tools to quantitatively determine the water concentration in petroleum products.
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 CO<sub>2</sub> measurements as an example.
Light emitting devices for the infrared spectral region are used in a lot of application fields. In the mid infrared (MIR) region, where a lot of gases show strong absorptions, the optical output power of inexpensive emitters in the relevant wavelength range is too low. An optically pumped emitter for the MIR region around 4 μm based on narrow gap semiconductors is demonstrated. The pumping takes place using inexpensive near-infrared (around 1 μm) high power continuous wave (cw) semiconductors laser. The radiation is converted by the narrow gap semiconductor into the MIR region as spontaneous emission. Molecular beam epitaxy (MBE) grown IV-VI lead chalcogenide-based compounds, especially PbSe, are applied for frequency conversion. The structural and optical quality of these thin film materials is characterized mainly by X-ray defraction measurements (XRD) and photo luminescence (PL) spectroscopy. For high radiation efficiency the outcoupling of the light is enhanced by surface structuring. Useful structures generating high photoluminescence intensity are characterized by IR imaging with an IR camera system being sensitive in the spectral region of interest. Due to the high pumping powers the device design-especially the thermal management of the active PbSe film-plays an important role. We will present a preparation technique for optically pumped, surface structured PbSe emitters in transmission geometry exploiting the transparency of the substrates and glues in the relevant wavelength region. The measured total emission power of the emitters exceeds 0.5 mW. Using an optimised design total emission powers up to 2 mW were achieved.
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.
Metal oxide gas sensors are widely used for different applications and operate normally at high temperatures between 300°C and 600°C. Such high temperatures are mainly needed to speed up the desorption of molecules from the gas sensor surface. Goal of the reported investigations is the reduction of the operating temperatures of such devices by the influence of radiation on the gas adsorption/desorption process. Therefore, the influences of radiation on the gas sensing mechanisms at surfaces of different metal oxides (SnO<sub>2</sub>, ZnO, WO<sub>3</sub> and Cr<sub>2-x</sub>Ti<sub>x</sub>O<sub>3+z</sub>) have been studied for different wavelengths. The experiments were carried out at an operating temperature of 130°C as well as at room temperature. As radiation sources LEDs emitting at different wavelength were used. The sensor response to NO<sub>2</sub>, CO, NH<sub>3</sub> and H<sub>2</sub> has been measured with and without illumination. The investigations have shown that light mainly influences the photo-activation of electron-hole pairs, which results in an increasing of the electrical conductivity of the illuminated metal oxide. The observed influences of photoadsorption and photocatalytic effects are small compared to the photoelectric effect. Only a weak increase of the NO<sub>2</sub> sensitivity during illumination has been measured.
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.
The bandstructure of photonic crystals offers intriguing possibilities for the manipulation of electromagnetic waves. During the last years, research has mainly focussed on the application of these photonic crystal properties in the telecom area. We suggest utilization of photonic crystals for sensor applications such as qualitative and quantitative gas and liquid analysis. Taking advantage of the low group velocity and certain mode distributions for some ~k-points in the bandstructure of a photonic crystal should enable the realization of very compact sensor devices. We show different device configurations of a photonic crystal based on macroporous silicon that fulfill the demands to serve as a compact gas sensor.
We present a novel hybrid light emitting device design based on a standard InAlGaAs/GaAs high-power laser diode array chip as a pump source and a narrow-gap PbSe-layer as active optical material. Maximum cw output powers of more than 1.1 mW and slope efficiencies of 0.4 mW/A are obtained at 25 °C. The external power efficiency amounts to 3.5×10<sup>-2</sup> %. The emission wavelength is 4.2 μm, with a half width of 770 nm (50 meV). Details about the optimization of the emitter material and device design are discussed as well.
Since the invention of the Quantum Cascade Laser (QCL) a decade ago an impressive progress has been achieved from first low temperature pulsed laser emission to continuous wave operation at room temperature. Distributed feedback (DFB) lasers working in pulsed mode at ambient temperatures and covering a broad spectral range in the mid infrared (MIR) are commercially available now. For many industrial applications e.g. automotive exhaust control and process monitoring, laser spectroscopy is an established technique, generally using near infrared (NIR) diode lasers. However, the mid infrared (MIR) spectral region is of special interest because of much stronger absorption lines compared to NIR. The status of QCL devices, system development and applications is reviewed. Special emphasis is given to the situation in Europe where a remarkable growth of QCL related R&D can be observed.
The bandstructure of photonic crystals offers intriguing
possibilities for the manipulation of electromagnetic waves.
During the last years, research has mainly focussed on the
application of these photonic crystal properties in the telecom
area. We suggest utilization of photonic crystals for sensor
applications such as qualitative and quantitative gas and liquid
analysis. Taking advantage of the low group velocity and certain
mode distributions for some k-points in the bandstructure
of a photonic crystal should enable the realization of very
compact sensor devices. We show different device configurations of
a photonic crystal based on macroporous silicon that fulfill the
demands to serve as a compact gas sensor.
We report on the development of epitaxial thin film materials for optical pumped light emitting devices in the wavelength range of 4-5 μm. The active layers are lead selenide (PbSe) thin films grown by molecular-beam epitaxy (MBE) on single crystalline, infrared transparent BaF<sub>2</sub> substrates. The electrical properties of the layers were determined by van der Pauw Hall measurements. A dependency of the PL intensity on the dopant type and carrier concentration was found. To increase the output power, layers with antireflection coatings were grown and characterized by Fourier-transform infrared (FTIR) spectroscopy and photoluminescence (PL) measurements. A further possibility to increase the extraction efficiency is surface texturing. Infrared imaging and PL measurements at samples with different surface structures, prepared by wet chemical etching, are presented. To improve the heat dissipation, which is a problem of optical pumped devices due to the small efficiency and pump densities up to some kW/cm<sup>2</sup>, the BaF<sub>2</sub> substrates were removed and the active layers were transferred to different heat sinks with significantly higher thermal conductivities. Afterwards the PL intensities were compared among each other.
An optically pumped emitter for the mid-infrared region around 4 µm based on narrow gap semiconductors is demonstrated. The pumping takes place in the near-infrared around 1 μm and the radiation is converted by the narrow ap semiconductor into the MIR region as spontaneous emission. IV-VI lead chalcogenide-based compounds, especially PbSe and III-V InAsSb-based quantum well systems are applied for frequency conversion. These materials are grown by MBE and characterized mainly by photo luminescence spectroscopy. For a high radiation efficiency the outcoupling of the light is enhanced by surface structuring. Useful structures generating high photoluminescence intensity are characterized by IR imaging with an IR camera system being sensitive in the spectral region of interest.
We present pulsed operation of index-coupled distributed feedback quantum cascade lasers based on the GaInAs/AlInAs/InP materials system emitting at a wavelength around 5.4 μm. The emission is single mode in the entire investigated temperature range between 240K and 350K with a side mode suppression ratio larger than 27 dB. These devices are employed in a fast gas detection experiment for the quantitative detection of nitric oxide. With the present measurement system minimum noise equivalent concentrations between 16.7 ppbv and 23.3 ppbv are obtained, corresponding to minimum detectable optical densities between 4.7•10<sup>-5</sup> and 6.5•<sup>10-5</sup>.
This paper describes the development of a technological process that allows the integration of gas sensors and thin film CMOS circuitry. The main technological points for the integration of gas sensors in a standard CMOS process and the effects on the full process will be described. A set of preliminary tests have been done prior to the definition of a CMOS compatible process for µ-structures and a complete test of sensitive materials are presented in order to determine the materials with most appropriate annealing temperatures for CMOS compatibility. Results show that gas sensors can be integrated with their circuitry, although require of special materials and non-CMOS standard processing as post-processing sequences.
The use of catalytic converters in cars with gasoline engine results in a tremendous reduction of the emission of pollutant gases. The optimal operation of the exhaust treatment systems is being checked and maintained periodically, but there is always a significant percentage of cars with a malfunction of the catalytic converter causing a substantial percentage of the total emission. Roadside emission monitoring of individual cars in the running traffic could be used to indicate these gross polluters, arrange maintenance of their vehicles and thus reduce total emission. Present monitoring systems use non- dispersive IR spectroscopy. Other systems are based on mid- IR diode laser spectroscopy offering a higher signal to noise ratio, higher selectivity for detection of specific compounds and better optical quality for long open path measurements, but these systems depend on liquid nitrogen cooling. In this work a compact mid-IR (MIR) laser diode system for roadside measurements will be presented, that is cooled thermoelectrically using a Peltier element. Sensitivity and time resolution of the system have been determined and found to be suitable for detection of single gross polluters in the running traffic. The presented system demonstrates the feasibility of high sensitive, selective and fast field MIR laser diode spectroscopy together with ruggedness and low maintenance expense.