Exploiting several key characteristics of quantum cascade (QC) lasers, including wide tunability and room-temperature operation, the Quantum Cascade Laser Open-Path System (QCLOPS) was designed for the detection of a range of trace gases and for field deployment in urban environments. Tunability over a wavelength range from 9.3 to 9.8 µm potentially provides the capability for monitoring ozone, ammonia, and carbon dioxide, a suite of trace gases important for air quality and regional climate applications in urban environments. The 2008 Olympic Games in Beijing, China drew attention to air quality problems in urban environments. Prior to and during the Olympic games, regional air quality modifications through factory shutdowns, car restrictions, and construction halts in Beijing and its surrounding areas created a unique test bed for new sensor technologies such as the QCLOPS sensor. We report the design of this novel, open-path air quality sensor and the results of both laboratory tests and field trials during the 2008 Olympic Games in Beijing, China.
Open-path quantum cascade laser (QCL) systems are being developed for remote environmental monitoring applications
for detection of small levels of toxins or pollutant gases in ambient air. In monostatic systems that rely on topographic
backscatter, the surface reflection of the target becomes important. To address the feasibility of natural targets in an
open-path geometry, we present the backscattering measurements of common urban building materials (aluminum,
natural stones, ceramic wall tiles and concrete block) using a distributed feedback (DFB) pulsed QCL. Real surface
roughness in the materials was taken into account. In particular, oblique scattering cases which are often unavoidable in
field measurements were also investigated. The QCL measurements were evaluated with a FTIR system in which wide
frequency range (2.8μm - 25μm) measurements were possible. These results were applied to a total link model to define
the potential and range of an open path QCL chemical sensor system.
Indentifying and quantifying ambient aerosols are important for air-quality applications. Unlike trace gases where
chemical spectral signatures are sharp and well defined, aerosol spectral signatures are broader and highly overlapping.
Therefore separation of aerosols into different size classes requires very broad spectral coverage from the visible (VIS)
to mid-infrared (MIR). In this paper, we investigate the feasibility in using a VIS (0.65μm) diode laser combined with a
suitable pulsed high power Quantum Cascade Laser (4.6μm) to obtain backscatter measurements that can be used to
isolate fine and coarse mode aerosol fractions. Based on realistic source characteristics, we study the information content
in the spectral extinction using different combinations of extinction measurements using Least Squares Minimization
applied to a wide range of aerosol multimode mixtures obtained using realistic models obtained from the Optical
Properties of Aerosol and Clouds (OPAC) model. This model is especially convenient since the optical spectral
extinction and backscatter spectra are evaluated over a wide wavelength range from 250nm to 40μm. In particular, we
find that with the latest QCL systems, it is possible to achieve signal to noise ratio (SNR) values ~10 with suitable
temporal and spatial averaging for aerosol layers ~1.5km making it suitable for PBL layer studies.
KEYWORDS: Signal to noise ratio, Quantum cascade lasers, Sensors, Ozone, Absorbance, Absorption, Error analysis, Receivers, Data acquisition, Chemical analysis
Remote sensing of enemy installations or their movements by trace gas detection is a critical but challenging military
objective. Open path measurements over ranges of a few meters to many kilometers with sensitivity in the parts per
million or billion regime are crucial in anticipating the presence of a threat. Previous approaches to detect ground level
chemical plumes, explosive constituents, or combustion have relied on low-resolution, short range Fourier transform
infrared spectrometer (FTIR), or low-sensitivity near-infrared differential optical absorption spectroscopy (DOAS). As
mid-infrared quantum cascade laser (QCL) sources have improved in cost and performance, systems based on QCL's
that can be tailored to monitor multiple chemical species in real time are becoming a viable alternative. We present the
design of a portable, high-resolution, multi-kilometer open path trace gas sensor based on QCL technology. Using a
tunable (1045-1047cm-1) QCL, a modeled atmosphere and link-budget analysis with commercial component
specifications, we show that with this approach, accuracy in parts per billion ozone or ammonia can be obtained in
seconds at path lengths up to 10 km. We have assembled an open-path QCL sensor based on this theoretical approach at
City College of New York, and we present preliminary results demonstrating the potential of QCLs in open-path sensing
applications.
Low power Mid-IR laser light exhibits much lower attenuation in propagation through the New York metro area when compared to Near-IR wavelengths. Depending on the type of atmospheric extinction we record a reduction of up to 800% in the exponential Beer-Lambert coefficient for Mid-IR light compared to Near-IR, thereby demonstrating the possibility of significantly increased deployable range and SNR of current communication systems by utilizing the Mid-IR spectrum.
We present and analyze transmission data from an outdoor collinear, coaxial, multi-wavelength laser test bed comparing 1.31&mgr;m, 1.55&mgr;m and 8&mgr;m through outdoor atmospheric fog and rain over a 550 m free space optical link across the Stevens Institute of Technology campus. This is achieved using lasers with average power ranging from 1 mW (Mid-IR QCL) to tens of milliwatts which have been normalized under lock-in detection.
We also present corroborating results from an indoor fog experiment simulating various fog types. Here we have also deconstructed Beer's attenuation coefficient and distinguish the contribution of scattering and absorption with a purpose-built polar nephelometer. Using Mie predictions we determine and measure the extent by which a Mid-IR system scatters light less under fog than a traditional Near-IR one, hence accounting for the performance enhancement in the metro-air test bed. We conclude finally that the Kruse-Mie prediction of insignificant Mid-IR-over-Near-IR-gain is strongly in error.
These experiments report the first qualitative observation of dependency
of typical fog transmission on pulse length and the first systematic measurement
of scattering amplitude and pattern for fs-pulse propagation through strongly
scattering (fog) medium. Also, we include the first reported measurement of
enhancement of fs-pulse propagation in real FSO environment. We also provide a classical analysis of the Mie-Scattering predictions,
and experiments which demonstrate a different transmission behavior than is
predicted. This proves only in part that the enhanced fs performance is due to a
different scattering formula than is expected; absorption is normally a value
which is measured by default. Once the transmission and scattering properties of
a stable substance are known (usually determined in two different
measurements), the difference is the absorption. This is particularly hard to do in
simulated fog, as the simulation itself is unstable, as is real fog. However, there is
no current measurement or claim which indicates this enhanced propagation is
related to different physics of absorption of Ultrafast Pulses by the atmosphere.
Graphic Data Systems Corporation (GDS Corp.) and Intellignet Graphics Solutions, Inc. (IGS) combined talents in 1995 to design and develop a MicroGDSTM application to support field investiations of crime scenes, such as homoicides, bombings, and arsons. IGS and GDS Corp. prepared design documents under the guidance of federal, state, and local crime scene reconstruction experts and with information from the FBI's evidence response team field book. The application was then developed to encompass the key components of crime scene investigaton: staff assigned to the incident, tasks occuring at the scene, visits to the scene location, photogrpahs taken of the crime scene, related documents, involved persons, catalogued evidence, and two- or three- dimensional crime scene reconstruction. Crime scene investigation, reporting, and reconstruction (CSIRR$CPY) provides investigators with a single applicaiton for both capturing all tabular data about the crime scene and quickly renderng a sketch of the scene. Tabular data is captured through ituitive database forms, while MicroGDSTM has been modified to readily allow non-CAD users to sketch the scene.
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