MODTRAN7, a polarimetric extension of the MODTRAN6 atmospheric radiative transfer model, is being developed. The vectorized MODTRAN7 will provide band model (BM), correlated-k (Ck), and line-by-line options for computing Stokes vectors. The radiative transfer problem is being solved for Isotropic and Symmetric Media (ISM) using the basic phenomenology described in the classic text by Mishchenko, Travis and Lacis, “Multiple Scattering of Light by Particles”1. VDISORT, a vectorized version of the DISORT scalar model currently in MODTRAN6, will compute the Stokes vectors for 1-D atmospheres. The MODTRAN method for extracting spherical refractive path contributions from the plane parallel scattering models will be adapted for the polarimetric model. The upgrade is to include new polarimetric optical properties for both the existing aerosol and cloud models within MODTRAN and for recently developed cirrus cloud and dust particulate data. A new algorithm has been developed that enables Generalized Spherical Function (GSF) expansion coefficients to be accurately computed to very high order. MODTRAN has already been restructured to generate Stokes vector data for single scatter solar/lunar applications and validated against the NASA Goddard Space Flight Center model, 6SV.
Spectra of the optical constants (n, k) of a substance are often obtained by comparing spectroscopic measurements of a bulk sample with a simulation model. The reflectance method requires a sample with a perfectly smooth surface to give unbiased values of n and k. The ellipsometric method generates n and k spectra which are accurate in general but which sometimes generate errors over limited spectral ranges when simulating polarized reflectance. We propose a new hybrid method to calculate reliable spectra of n and k. The proposed method uses both ellipsometric and s-polarized reflectance measurements and takes into account the potential roughness of the sample’s surface with the help of a specularity factor. The proposed method provides n and k that better simulate polarized reflectance measurements and applies to isotropic bulk samples with either smooth or rough flat surfaces. We provide demonstrations in the infrared spectral region with a smooth sample and a rough sample.
The performance of thermal electro-optic infrared sensors (EO/IR) may be limited in certain specific circumstances, particularly for the detection and identification of targets embedded in an isothermal scene, i.e. when there is insufficient thermal contrast between the targets against their surrounding background. Such situations generally occur at the beginning and end of the day, but can also happen at any time during the day. One way to cope with this limitation is to employ EO/IR sensors that are sensitive to the polarization states of light. With this intention, Defence Research and Development Canada (DRDC) has developed thermal infrared multispectral and hyperspectral polarimetric imaging systems and spectral algorithms to extract the polarized radiance components of targets of interest, and use this additional information to enhance detection and identification while reducing false alarm rate. This paper presents experimental results from measurements using ground-based multispectral and hyperspectral polarimetric imaging sensors to acquire the polarized radiance of targets set up at multiple orientation angles with respect to the sensors lineof-sight (LOS). The objectives of the experiments were to study the phenomenology of polarized surface radiance in the Long-Wave Infrared (LWIR) and assess the effect of different materials on the resulting s-polarized and p-polarized spectral components. Experimental results show the advantages of thermal multispectral and hyperspectral polarimetric imaging sensors over conventional unpolarized ones to discriminate targets against their background, particularly during thermal cross-over periods.
We report and analyze recent results obtained with the MoDDIFS sensor (Multi-option Differential Detection and Imaging Fourier Spectrometer) for the passive polarization sensing of liquid contaminants in the long wave infrared (LWIR). Field measurements of polarized spectral radiance done on ethylene glycol and SF96 probed at distances of 6.5 and 450 meters, respectively, have been used to develop and test a GLRT-type detection algorithm adapted for liquid contaminants. The GLRT detection results serve to establish the potential and advantage of probing the vertical and horizontal linear hyperspectral polarization components for improving liquid contaminants detection.
MoDDIFS (Multi-option Differential Detection and Imaging Fourier Spectrometer) is a DRDC Valcartier technology built around a differential Fourier Transform Infrared (FTIR) spectrometer optimized for optical subtraction in the long wave infrared (LWIR). MoDDIFS is a dual use hyperspectral prototype offering two fore-optics configurations: "long range", specialized for the detection of small quantities of gaseous substances, and "polarization", built to investigate liquids and powders spills. We report and present a preliminary analysis of a series of measurement tests made with the polarization configuration. The tests were performed under indoor and outdoor environments. Different liquid and solid substances were deposited on different types of surfaces. Many liquid targets and some solid materials produce a noticeable linearly-polarized signal, with a more or less characteristic spectral modulation. For the liquids, the behavior of the observed radiance spectrum seems more predictable when the liquid is thick, or when it is deposited at any thickness on non-absorbing and weakly-reflective substrates. The behavior of the radiance spectrum observed becomes more complex when a thin layer of the liquid is deposited on a smooth and strongly-reflective substrate, or on an absorbing substrate. The parameter chosen to analyze the relative amount of polarization is the degree of linear polarization. When its value is noticeable, the polarized hyperspectral radiance measurements bring additional information on both targets and the backgrounds, as compared to standard unpolarized hyperspectral measurements. The tests performed can then help assess the materials for which the detection and the identification will be improved with polarized measurements.
Among the multiple techniques proposed to obtain continuous tuning of a laser, many involve a wavelength selective element that needs to be rotated in order to select the output wavelength. Such methods usually require more optical components than the technique proposed in this paper. Our group uses the line spacing variation in a custom-engineered chirped grating to achieve a mode-hop free modification of the output wavelength of a semiconductor laser by a simple translation of the grating. The useful beam out-coupled through the 0th-order is propagating in a constant direction for any tuned wavelength. The variation in the line space also induces a focal effect in the axis perpendicular to the grooves; it is exploited to compensate a part of the beam divergence in the external cavity. Moreover, a transverse focal effect from the holographic grating is introduced for improved reinjection in the laser chip. With this simple configuration, it has been possible to obtain a continuous tuning of a semiconductor laser over 10 nm at an average wavelength close to 1540 nm. The output power of such a laser was near 8 mW and quite stable over all the tuning range. Fine adjustments are still made to obtain a greater tuning range.
The passive standoff monitoring of vapor precursors emanating from a location under surveillance can provide relevant
information on the nature of products fabrication. Defence Research & Development Canada Valcartier recently
completed the development and field-validation of a novel R&D prototype, MoDDIFS (Multi-option Differential
Detection and Imaging Fourier Spectrometer), to address this remote sensing application. The proposed methodology
combines the clutter suppression efficiency of the differential detection approach with the high spatial resolution
provided by the hyperspectral imaging approach. This consists of integrating a differential CATSI-type (Compact
ATmospheric Sounding Interferometer) sensor with the imaging capability of the Hyper-Cam infrared imager. The
MoDDIFS sensor includes two configuration options, one for remote gas detection, and the other for polarization
sensing of surface contaminants. This paper focuses on the infrared spectral detection of gases. A series of measurements
done with MoDDIFS on selected laboratory solvents in vapor form are analyzed and discussed.
The passive standoff detection of vapors from particular explosives and precursors emanating from a
location under surveillance can provide early detection and warning of illicit explosives fabrication. DRDC
Valcartier recently initiated the development and field-validation of a novel R&D prototype, MoDDIFS
(Multi-Option Differential and Imaging Fourier Spectrometer) to address this security vulnerability. The
proposed methodology combines the clutter suppression efficiency of the differential detection approach
with the high spatial resolution provided by the hyperspectral imaging approach. This consists of
integrating the imaging capability of the Hyper-Cam IR imager with a differential CATSI-type sensor. This
paper presents the MoDDIFS sensor methodology and the first investigation results that were recently
obtained.
The focusing properties of orthogonal optical systems that include a varied line-space grating with curved lines
can be analyzed efficiently with the ray matrices presented in this paper. These matrices are obtained by comparing the
true optical path length truncated to the second order and the eikonal function (the phase of the kernel appearing in the
Fresnel-Kirchhoff diffraction integral) expressed in terms of ABCD-matrix elements.
Mode-hop-free operation of an extended-cavity diode laser is achieved with an original technique. A varied
line-space diffraction grating is used as an external coupler. Continuous tuning is obtained by a simple linear translation
of the grating. As of now, the measured continuous tuning range is as wide as 14 nm near 1568 nm with a relatively
constant output power.
We present a new concept of optical spectrometer based on a plane holographic grating having a spatially variable period (chirped grating) used as the component for spectral selection. The chirped grating is made through a holographic technique. We have developed a mathematical model to predict the properties of the grating, as a function of the writing beam parameters. The experimental results confirmed the predictions from the model. The spatially variable period provides focusing properties to the grating which eliminate the need for the usual curved mirrors in the spectrometer. Once calibrated, this spectrometer has a resolution comparable to the one of a high-resolution commercial device, considering their different dimensions. This comparison is achieved with the help of a performance criterion we have introduced. The new concept would make it possible to reduce the optical dimensions of usual spectrometers approximately by half, for a given resolution. The aberrations produced by the chirped grating, the spectral range that can be analyzed and the positioning control of the detected image will be discussed.
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