This paper aims to evaluate the performance of several supervised machine learning methods as to determine the best algorithm for detecting explosives with multispectral imagery. Ocean Thin Films SpectroCam with 8 interchangeable band pass filters is used to collect images. The stack of 8-dimensional data cube can be obtained and subsequently analyzed with various machine learning algorithms. We specifically study four classifiers: Convolutional Neural Network, Support Vector Machine, Quadratic Discriminant Analysis, and Linear Discriminant Analysis. We examine and compare the accuracy of the four classifiers’ performance in the application of detecting trace C4 material. Our results show that the Support Vector Machine and Convolutional Neural Network classifiers achieve the best overall accuracy, although they have the longest training time.
This paper presents the algorithms to detect trace chemicals using a multi-wavelength camera. Multispectral images of the chemical and the background were collected using the Ocean Thin Films SpectroCam. The camera has an integrated motor with 8 filter color wheels and 8 interchangeable custom band pass filters in the spectral range of 200–900 nm. Since chemicals have their unique spectral reflectance, the stack of 8-dimensional image data was obtained and subsequently analyzed to develop algorithms that can uniquely identify the area where a chemical is present. In this study, we primarily used RDX, 1,3,5-Trinitroperhydro-1,3,5-triazine, the explosive component in C4. The aim of this study was to investigate the potential of the multispectral imaging system and the accuracy of the model in determining C4 chemical.
We present simulations on temperature dependence of the threshold current density (Jth) in InGaN-AlGaN and ZnCdSe-ZnMgSSe quantum wire lasers having exciton transitions. We find that Jth in a quantum wire laser is insensitive to temperature variation when exciton binding energies are in the range of 30 to 50 meV. This behavior is similar to quantum dot lasers having free carrier transitions. We show that the temperature dependence of the threshold current density is greatly reduced in lasers having exciton transitions in comparison to the ones having free carrier transitions. The reduced dependence of Jth on the temperature is attributed to the inherent confinement of electron-hole pairs within the exciton radius along the wire axis, whereas the quantum wire cross section provides confinement of carriers in the plane perpendicular to the wire axis. The large exciton binding energies give rise to high exciton density of states and narrow spectral line width, thus confining the carriers into a pseudoquantum dot-like structure. The effect of exciton localization due to the randomness in quantum wire fabrication, causing exciton inhomogeneous line broadening, is also discussed.
Previously we have shown that error diffusion neural networks (EDNs) find local minima of frequency-weighted error
between a binary halftone output and corresponding smoothly varying input, an ideal framework for solving halftone
problems. An extension of our work to color halftoning employs a three dimensional (3D) interconnect scheme. We cast
color halftoning as four related sub-problems: the first three are to compute good binary halftones for each primary color
and the fourth is to simultaneously minimize frequency-weighted error in the luminosity of the composite result. We
have showed that an EDN with a 3D interconnect scheme can solve all four problems in parallel. This paper shows that
our 3D EDN algorithm not only shapes the error to frequencies to which the Human Visual System (HVS) is least
sensitive but also shapes the error in colors to which the HVS is least sensitive. The correlation among the color planes
by luminosity reduces the formation of high contrast pixels, such as black and white pixels that often constitute color
noise, resulting in a smoother and more homogeneous appearance in a halftone image and a closer resemblance to the
continuous tone image. The texture visibility of color halftone patterns is evaluated in two ways: (1) by computing the
radially averaged power spectrum (2) by computing the visual cost function.
This is a generalization, to color images, of earlier results
on two-dimensional monochromatic halftoning with error diffusion
neural networks (EDNs). Previously, we have shown that EDNs find
local minima of frequency-weighted error between a binary halftone
output and corresponding smoothly varying input, which is an ideal
framework for solving halftone problems. We cast color halftoning
as four related subproblems: the first three are to compute good
binary halftones for each primary color and the fourth is to simultaneously
minimize frequency-weighted error in the luminosity of the
composite result. We show that an EDN with a three-dimensional
(3D) interconnection scheme can solve all four problems in parallel.
The 3D EDN algorithm not only shapes the error to frequencies to
which the human visual system (HVS) is least sensitive but also
shapes the error in colors to which the HVS is least sensitive—
namely it satisfies the minimum brightness variation criterion. The
correlation among the color planes by luminosity reduces the formation
of high contrast pixels, such as black and white pixels that often
constitute color noise, resulting in a smoother and more homogeneous
appearance in a halftone image and a closer
resemblance to the continuous tone image.
A description of a novel Y-branch photonic crystal laser diode operating at 1.55 µm is presented. Optical gain calculations within an InGaAsP-InP multiple-quantum-well (MQW) gain medium predict a threshold current of 0.5 mA, which is lower than a typical ridge laser. The resonant structure of the beamsplitting laser is modeled using a scattering matrix approach incorporating the effects of multiple internal reflections on the cavity mode structure and mode spacing. Phase-matching conditions are derived, taking into account all the different resonant substructures. The results from the phase-matching analysis show an operating mode that reduces threshold current beyond that of a geometrically similar conventional ridge laser. This Y-branch photonic crystal laser, when cascaded as a multichannel resonant structure, produces numerous optical intensity levels that can be used in an optical analog-to-digital converter (ADC).
A Y-branch photonic crystal laser diode operating at 1.55 μm is presented. The photonic band gap of the three-layered device was calculated by the use of simulation software. The propagation constant was determined from simulation. In addition, a scattering matrix approach was used to model the resonant structure of the beam splitting laser, incorporating the effects of internal reflections on the cavity mode structure and mode spacing. Phase matching conditions were explored, taking into account all the different resonant sub-structures. The results from the phase matching analysis lead directly to an operating mode that reduces threshold current beyond that of the conventional ridge laser. Optical gain calculations in an InGaAsP-InP MQW structure show a threshold current of 0.5mA. This device could be configured to work as a combined optical amplifier and beam splitter. The paper ends with a discussion of how this would be possible.
Quantum wire lasers and modulators offer superior performance
over their quantum well counterparts. We present simulations of
an integrated InGaAs-InP quantum wire laser-modulator structure operating
at 1.55 mm. In the case of quantum wire lasers, we have computed
the optical gain as a function of current density for wires having widths
ranging between 60 and 100 Å. For example, the threshold current density
of 61 A/cm2 is computed for a wire with a width of 80 Å. In the case
of quantum wire modulators, we compute the changes in the absorption
coefficient and index of refraction due to an external electric field to
implement electroabsorptive and electrorefractive optical modulators.
For example, the absorption coefficient changes (Da/a) by 450% when
an applied electric field changes from 30 to 60 kV/cm for an 80 Å quantum
wire. The corresponding change of refractive index is about 11%. A
structure integrating an edge-emitting laser with an in-line type electroabsorptive
or electrorefractive modulator is presented. The quantum
wires are designed to operate the laser at a wavelength that corresponds
to the Stark effect tuning of the modulator. We can maximize the
changes in electroabsorptive and electrorefractive modulators by choosing
the right combination of wire dimensions, operating wavelengths, and
electric fields.
Quantum wire lasers and modulators offer superior performance over their quantum well counterparts. This paper presents simulation of an integrated InGaAs-InP quantum wire laser-modulator structure operating at 1.55 μm. In the case of quantum wire lasers, we have computed the optical gain as a function of current density for wires having widths ranging between 60-100 Å. For example, the threshold current density of as low as 61 A/cm2 is computed for a wire with a width of 80 Å. In case of quantum wire modulators, we compute the changes in the absorption coefficient and index of refraction due to an external electric field ranging between 0-120kV/cm. For example, the percentage of absorption changes (Δα/α) between 30kV/cm and 60kV/cm applied electric field is about 450% for a 80 Å quantum wire. The changes in electro-absorption or electro-refraction can be maximized by choosing optimum combination of wire dimensions, operating wavelength and electric field to obtain lasing and modulation.
This paper describes novel multiple quantum well optical modulators in which the optical modulation is achieved by field dependent birefringence in strained MQW layers. The modulator operates under normal incidence. In our study, field dependent birefringence is obtained by straining the MQW layers using Surface Acoustic Waves. Computations of refractive index change, absorption coefficient and contrast ratio are presented. The devices are being fabricated using lattice matched InGaAsP/InGaAsP layers on InP substrates. The polarization-sensitive birefringent spatial light modulators are different from conventional multiple quantum well SLMs and self electro-optical devices which generally utilize electro- absorption.
The exciton binding energies in InGaN-AlGaN quantum wire are calculated to be 30-60 meV as wire width reduces from 150 angstrom to 50 angstrom. This high binding energy results in large exciton densities, making optical transitions due to excitons dominant over free electrons and holes. Optical gain and threshold current densities in InGaN-AlGaN based multiple quantum wire lasers are computed including the effect of strain and dislocations. The calculated threshold current density for a defect free compressively-strained quantum wires laser, such as realized on sapphire or SiC substrate, are shown to yield an ultra-low threshold current density of 148 A/cm2 and 1,600 A/cm2 in the presence of dislocations. The exciton transitions assist in lowering the threshold current density which is adversely affected by the presence of dislocations and surface states. This shows an improvement over our computed value as well as the experimental data reported by Nakamura et. al. for quantum well lasers.
A model to calculate the optical gain due to excitonic transitions is developed and used to calculate unstrained as well as strained ZnCdSe/ZnMgSSe multiple quantum well (MQW) lasers. Strain induced changes in energy band gap and effective masses of light and heavy holes are included in the gain coefficient and threshold current density calculations. The theoretical simulations are matched with the experimental data on compressively strained ZnCdSe-ZnSSe devices grown on GaAs substrates. Our calculations predict lower threshold current density for the tensile strained Zn.8Cd.2Se-Zn.2Mg.8S.03Se.97 quantum well lasers grown on InP substrates. Unlike the III-V strained layer quantum well lasers, the contribution to gain coefficient due to excitonic transitions is predominant in II-VI systems as the exciton binding energy is larger by a factor of 5. This results in a primary role for excitons in lasing, which has been verified experimentally.
This paper describes the usage of a novel asymmetric resonant tunneling structure (RTS) to obtain low voltage drop contacts to pZnSe and other wide energy gap semiconductors. Generally, the contact to p-ZnSe is achieved by the formation of a Schottky barrier or by forming a graded layer interface to pZnSe. These techniques have been employed in fabricating ZnSe based blue-green lasers, reported during the past few years using structures grown by Molecular Beam Epitaxy. Most of these approaches result in contact voltage drops ranging from 6-30 volts for a typical current above laser threshold. In the case of pGaAs-pZnSe (or pZnCdSe) interface, the presence of a large valence energy band offset results in high voltage drop due to a rectifying interface. The use of asymmetric resonant tunneling structure(s) at these heterointerfaces is shown to result in a significantly low (0.4 volts) voltage drops at current densities above threshold (approximately equals 600 A/Cm2). The incorporation of asymmetric resonant tunneling structures is also proposed for lasers on InP substrates. It is a generic technique which can be used for realizing low resistance contacts in material systems where standard contact techniques produce poor results. This technique is applicable for both p- and n-type heterojunction interfaces.
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