Raman spectroscopy is of significant importance in industrial gas analysis due to its unique capability of quantitative multigas measurement, especially diatomics (N2 and H2), with a single laser. This paper presents the development of a gas analyzer system based on high pressure Raman scattering in a multipass Raman cell and demonstrates its feasibility for real-time natural gas analysis. A 64-pass Raman cell operated at elevated pressure (5 bar) is used to create multiplicative enhancement (proportional to number of passes times pressure) of the natural gas Raman signal. A relatively low power 532-nm continuous wave laser beam (200 mW) is used as the source and the signals are measured through a cooled charge-coupled device grating spectrometer (30-s exposure). A hybrid algorithm based on background-correction and least-squares error minimization is used to estimate gas concentrations. Individual gas component concentration repeatability of the order of 0.1% is demonstrated. Further, the applicability of the technique for natural gas analysis is demonstrated through measurements on calibrated gas mixtures. Experimental details, analyzer characterization, and key measurements are presented to demonstrate the performance of the technique.
This paper describes the microfabrication process and characterization of wavelength selective germanium dielectric
supported microbolometers, which should be compatible with standard microbolometer fabrication processes. Here we
have demonstrated a micro fabricated robust germanium dielectric structure layer that replaces the usual silicon nitride
structural layer in microbolometers. The fabricated microbolometers consist of a chromium resistive sheet as an absorber
layer above an air-gap/germanium dielectric structure.
Past work has discussed infrared absorption using a patterned thin resistive sheet as the frequency-selective absorber for
use in wavelength-selective long wave infrared (LWIR) microbolometer focal planes arrays. These patterned resistive
sheets are essentially slot antennas formed in a lossy resistive ground plane layer placed a quarter-wavelength in front of
a mirror. Design studies have shown that for efficient IR absorption cross-shaped slots require a lossy sheet with the
optimized sheet resistance. For realistic metal layers, however, the skin effect produces a complex surface impedance
that can be quite large in the LWIR band. In this paper we consider metal layers of thickness between one and three skin
depths as the absorber layer instead of a thin resistive sheet layer, and show that the thick metal layers can still produce
excellent absorption in the LWIR.
The use of a patterned resistive sheet acting as an infrared frequency-selective absorber is discussed. These patterned resistive sheets are a modified form of classical Salisbury Screens that utilize a resistive absorber layer placed a quarter-wavelength in front of a mirror. In contrast with previously designed planar antenna-coupled microbolometers that consist of both resistive and highly conductive metal strips (acting as antennas), the absorption layer in these structures involves a single resistive layer with patterned holes.
Although the rapid development of 2-D focal plane arrays of thermal infrared (IR) detectors has led to remarkable progress in uncooled IR imaging technology, a major limitation of these sensors is the lack of true on-chip spectral discrimination. Multi-spectral detection capabilities enable rapid, efficient and multi-dimensional scene interpretation that is especially beneficial to advanced IR imaging systems for early threat warning and target recognition applications. We propose a novel design for a monolithic micromachined array of bolometric detectors capable of multi-spectral
imaging in the long-wave IR (7-14 μm) region. The central ingredient of this approach is to employ planar multi-mode antenna structures to efficiently couple incident electromagnetic radiation to a microbolometeric sensing element that is much smaller than the IR wavelength. The wavelength selectivity of such an antenna-coupled detector can be tuned by optimizing its multiple geometric parameters. We present a planar microbolometer design that can accomplish 3-color LWIR imaging with no moving parts analogous to solid-state color videography in the visible region. The proposed effort targets applications of uncooled color IR imaging where the benefits in space, power, weight and complexity will have a significant impact.
Advanced autonomous detection of chemical warfare agents and toxic industrial chemicals has long been a major military concern. At present, our capability to rapidly assess the immediate environment is severely limited and our domestic infrastructure is burdened by the meticulous procedures required to rule out false threats. While significant advances have recently been accomplished in remote spectral sensing using rugged FTIRs and point detectors, efforts towards low cost chemical discrimination have been lacking. Foster-Miller has developed a unique waveguide spectrometer which is a paradigm shift from the conventional FTIR approach. The spectrometer provides spectral discrimination over the 3-14 μm range and will be the spectrometer platform for both active and passive detection.
Foster-Miller has leveraged its innovations in infrared fiber-optic probes and the recent development of a waveguide spectrometer to build a novel infrared sensor platform for both point and stand-off chemical sensing. A monolithic wedge-grating optic provides the spectral dispersion with low cost thermopile point or array detectors picking off the diffracted wavelengths from the optic. The integrated optic provides spectral discrimination between 3-12 μm with resolution at 16 cm-1 or better and overall optical throughput approaching 35%. The device has a fixed cylindrical grating bonded to the edge of a ZnSe conditioning “wedge”. The conditioning optic overcomes limitations of concave gratings as it accepts high angle (large FOV) light at the narrow end of the wedge and progressively conditions it to be near normal to the grating. On return, the diffracted wavelengths are concentrated on the discrete or array detector (pixel) elements by the wedge, providing throughput comparable to that of an FTIR. The waveguide spectrometer coupled to ATR probes, flow through liquid cells or multipass gas cells provides significant cost advantage over conventional sampling methodologies. We will present the enabling innovations along with present performance, sensitivity expectations and discrimination algorithm strategy.
A small, low-cost sensor capable of autonomous detection of a wide variety of chemical agents in either vapor, particulate or liquid phase is urgently needed. It now appears that this need also extends to homeland defense and the vast network of civilian security forces including police, fire, public health and emergency medical personnel. We are developing a low-cost, compact infrared Chemical Threat Monitor (CTM) that could meet this need. This palm-sized handheld instrument combines Foster-Miller's unique optical "wedge" technology coupled to novel, disposable infrared fiber optic sensors for sample collection. These technologies will be coupled to emerging high sensitivity, low-cost uncooled linear array infrared detectors optimized for this application. This combination will provide the individual user with most of the capability of today’s expensive FTIR units in a miniature robust unit that has no moving parts. In this paper we will describe the CTM device, its operation, and present preliminary results on liquid chemical agent simulants.