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 NO2-emission by reduction with NH3 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, NO2, NH3) 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-1 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.
The monitoring of acetylene (C2H2) 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 C2H2. 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.
Spectroscopic concentration measurements of oxygen at high pressures are limited by the effect of pressure
broadening and line mixing. These effects strongly depend on the gas mixture in which the oxygen concentration
has to be determined as the pressure broadening coefficients of different gases vary over a large range. Line
broadening coefficients of the oxygen a-band for a large number of different gases are well known in the literature,
but up to now there is, to our best knowledge, no experimental data available which describes the line broadening
of oxygen in hydrogen. In respect to a possible application for online-monitoring of oxygen in hydrogen electrolysis
we have measured the pressure broadening coefficient of the oxygen P9P9 line in hydrogen and compared with
the theoretical model. To confirm the result, also measurements of the well known broadening coefficient of
oxygen in helium were accomplished. Measurements were obtained using laser absorption spectroscopy with
vertical-cavity surface-emitting lasers in a Herriott-Cell with 15 m path length adapted for vacuum pressures.
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.
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.
A new, synchronously pumped picosecond OPO for CARS microscopy is presented. It is based on non-critically
phasematched interaction in LBO pumped by a frequency-doubled modelocked Nd:Vanadat laser at 532 nm.
Within the parametric process a tuneable pair of two different wavelengths in the NIR range is generated (Signal <680
...990 nm, Idler 1150...>2450 nm). In this system they are extracted from the cavity at the same mirror and therefore
propagating collinear at the same beam path. Due to the mechanism of their generation there is no jitter between Signal
and Idler. Though the wavelengths are different the GVD is negligible for this picosecond pulse duration. As a result the
two pulse trains are spatially and temporally perfectly matched.
The pulses generated are close to transform limit with about 5-6 ps pulse duration, excellent beam quality (M2 < 1,1) and
high pointing stability. The output power for Signal and Idler is about 1 W each @ 4 W pump power. The tuning
mechanism is split into two parts - temperature tuning for rough variations and fast angular BRF tuning for the fine
adjustment of the output wavelength.
The perfect spatial and temporal overlap make the described OPO an ideal and nearly hands-free laser source for CARS
microscopy with a tuneable energy difference 1,400 ... >10,000 cm-1. The absolute wavelength range is resulting in high
penetration depth and low photo damage of the analyzed samples.
Finally some CARS-images are presented and the latest results and methods for further sensitivity enhancements are