Albert Mendoza

Novel polarization-sensitive micropulse lidar measurement technique

Polarization-sensitive detection of elastic backscattered light is useful for detection of cloud phase and depolarizing aerosols. The U.S. Department of Energys Atmospheric Radiation Measurement Program has deployed micropulse lidar (MPL) for over a decade, but without polarized detection. Adding an actively-controlled liquid crystal retarder provides the capability to identify depolarizing particles by alternately transmitting linearly and circularly polarized light. This represents a departure from established techniques, which transmit exclusively linear polarization or exclusively circular polarization. Mueller matrix calculations yield simple relationships between the well-known linear depolarization ratio delta(linear), the circular depolarization ratio delta(circ), and this MPL depolarization ratio delta(MPL).

Optical response of oriented and highly anisotropic subwavelength metallic nanostructure arrays

Here we describe the optical response of highly anisotropic subwavelength coatings with Au structures based on the open-ring-resonator and fabricated via nanoimprint lithography and metal sputtering. This approach allows fabrication of dense arrays of oriented nanostructures over large areas with a resonance in the visible wavelength range. Nanostructures are wire-like, with a nanoscale L-shaped cross section approximately 70 nm in width. The coatings exhibit a resonant transmission response that is highly angle and polarization dependent. Experimental results are presented along with complementary numerical modeling results predicting the resonance shift with corresponding changes in fabrication parameters.

Visible hyperspectral imaging for standoff detection of explosives on surfaces

There is an ever-increasing need to be able to detect the presence of explosives, preferably from standoff distances of tens of meters. This paper presents an application of visible hyperspectral imaging using anomaly, polarization, and spectral identification approaches for the standoff detection (13 meters) of nitroaromatic explosives on realistic painted surfaces based upon the colorimetric differences between tetryl and TNT which are enhanced by solar irradiation.

Subwavelength resonant nanostructured films for sensing

We present a novel subwavelength nanostructure architecture that may be utilized for optical standoff sensing applications. The subwavelength structures are fabricated via a combination of nanoimprint lithography and metal sputtering to create metallic nanostructured films encased within a transparent media. The structures are based on the open ring resonator (ORR) architecture which has a characteristic resonance frequency. Any perturbation of the nanostructured films due to chemical or environmental effects can shift the resonant frequency and provide an indication of the external stimulus. This shift in resonance can be interrogated remotely either actively using either laser illumination or passively using hyperspectral or multispectral sensing. These structures may be designed to be either anisotropic or isotropic, which can also provide polarization-sensitive interrogation. Due to the nanometer scale of the structures, they can be tailored to be optically responsive in the visible or near infrared spectrum with a highly reflective resonant peak that is dependent solely on structural dimensions and material characteristics. We present experimental measurements of the optical response of these structures as a function of wavelength, polarization, and incident angle demonstrating the resonant effect in the near infrared region. Numerical modeling data showing the effect of different fabrication parameters such as structure parameters are also discussed.

Passive fully polarimetric W-band millimeter-wave imaging

We present the theory, design, and experimental results obtained from a scanning passive W-band fully polarimetric imager. Passive millimeter-wave imaging offers persistent day/nighttime imaging and the ability to penetrate dust, clouds and other obscurants, including clothing and dry soil. The single-pixel scanning imager includes both far-field and near-field fore-optics for investigation of polarization phenomena. Using both fore-optics, a variety of scenes including natural and man-made objects was imaged and these results are presented showing the utility of polarimetric imaging for anomaly detection. Analysis includes conventional Stokes-parameter based approaches as well as multivariate image analysis methods.

Visible/near-infrared hyperspectral sensing of solids under controlled environmental conditions

We describe the use of a wind tunnel for conducting controlled passive hyperspectral imaging experiments. In recent years, passive hyperspectral detection of solids, minerals and ores has emerged as a very useful technique, for example for classifying land types, mineral deposits, and agricultural practices. Such techniques are also potentially useful for detecting explosives, solid-phase chemicals and other materials of interest from a distance so as to provide operator safety. The Pacific Northwest National Laboratory operates a wind tunnel facility that can generate and circulate artificial atmospheres whereby certain environmental parameters can be controlled such as lighting, humidity, temperature, aerosol and obscurant burdens. By selecting the appropriate fore-optics and sample size, one can conduct meaningful experiments under controlled conditions at relatively low cost when compared to typical field deployments. We will present recent results describing optimized sensing of solids over tens of meters distance using both visible and near-infrared cameras, as well as the effects of certain environmental parameters on data retrieval.

Atmospheric Sounder Spectrometer for Infrared Spectral Technology (ASSIST) Handbook

The Atmospheric Sounder Spectrometer for Infrared Spectral Technology (ASSIST), like the AERI, measures infrared spectral zenith radiance at high spectral resolution.

Three-dimensional speckle imaging employing a frequency-locked tunable diode laser

We describe a high accuracy frequency stepping method for a tunable diode laser to improve a three dimensional (3D) imaging approach based upon interferometric speckle imaging. The approach, modeled after Takeda, exploits tuning an illumination laser in frequency as speckle interferograms of the object (specklegrams) are acquired at each frequency in a Michelson interferometer. The resulting 3D hypercube of specklegrams encode spatial information in the x-y plane of each image with laser tuning arrayed along its z-axis. The specklegrams are processed by Fast Fourier Transformation (FFT) along the z-axis of the hypercube and the center of the peak in the resulting power spectrum for each pixel encodes its surface height. Alternatively, Takeda’s method can be followed which uses the phase of the FFT, unwraps it, and determines the surface height encoded in the slope of a line fitted to the phase. Wraparound of modulations above the Nyquist limit results in ambiguity in the optical path difference (OPD) between test and reference surfaces. Wraparound also amplifies measurement noise caused by errors and jitter in frequency stepping the illumination laser. By locking the laser frequency to successive cavity modes of a reference confocal interferometer, tuning is precisely controlled resulting in dramatically improved imaging quality/. We present laboratory data of before and after results showing enhanced 3D imaging resulting from precise laser frequency control.

Three dimensional imaging with multiple wavelength speckle interferometry

We present the design, modeling, construction, and results of a three-dimensional imager based upon multiplewavelength speckle interferometry. Speckle imaging used in non-destructive evaluation is well-known but requires a precisely acquired reference image and can measure excursions only within the 2π ambiguity range determined by the illumination wavelength. Our approach is based upon earlier efforts pioneered by Takeda, but with updated illumination, imaging, and processing tools, in which a surface under test is illuminated with tunable laser light in a Michelson interferometer configuration. A speckled image is acquired at each laser frequency step creating a data hypercube. Interference between the reference wavefront and light from the object causes the amplitude of the speckles to cycle with laser tuning. Fourier transforming the hypercube in the laser frequency dimension reveals periods that map heights of surface features. Height resolution is determined by the maximum tuning range of the laser, which for our 16-nm tuning range provides approximately 18 micron resolution without any efforts at interpolation. The largest height without wraparound depends on the smallest tuning steps, which for our laser is 15 cm for 0.002 nm (1 GHz) tuning steps. In this way, objects with large discontinuous steps or holes can be imaged without confusion. Also, due to the illumination beam being normal to the surface under test, shadowing is eliminated. To inform our design and better understand our system’s limitations, we have developed extensive numerical models based upon Monte Carlo ray tracing in which speckle patterns are produced after scattering from model surfaces by coherent summing of rays at the detector plane. Data acquired by the system as well as modeling results will be shown.

Final Report for SERDP Project RC-1649: Advanced Chemical Measurements of Smoke from DoD-prescribed Burns

Abstract : Project RC-1649, Advanced Chemical Measurement of Smoke from DoD-prescribed Burns was undertaken to use advanced instrumental techniques to study in detail the particulate and vapor-phase chemical composition of the smoke that results from prescribed fires used as a land management tool on DoD bases, particularly bases in the southeastern U.S. The statement of need (SON) called for (1) improving characterization of fuel consumption and (2) improving characterization of air emissions under both flaming and smoldering conditions with respect to ... volatile organic compounds, heavy metals, and reactive gases. The measurements and fuels were from several bases throughout the southeast (Camp Lejeune, Ft. Benning, and Ft. Jackson) and were carried out in collaboration and conjunction with projects 1647 (models) and 1648 (particulates, SW bases).