There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

We present a universal statistical model for texture images in the context of an overcomplete complex wavelet transform. The model is parameterized by a set of statistics computed on pairs of coefficients corresponding to basis functions at adjacent spatial locations, orientations, and scales. We develop an efficient algorithm for synthesizing random images subject to these constraints, by iteratively projecting onto the set of images satisfying each constraint, and we use this to test the perceptual validity of the model. In particular, we demonstrate the necessity of subgroups of the parameter set by showing examples of texture synthesis that fail when those parameters are removed from the set. We also demonstrate the power of our model by successfully synthesizing examples drawn from a diverse collection of artificial and natural textures.

/pdf/a-parametric-texture-model-based-on-joint-statistics-of-2p5nbgtfy6.pdf

A Parametric Texture Model Based on Joint Statistics of Complex Wavelet Coefficients

There have been three basic approaches to optical tomography since the early 1980s: diffraction tomography, diffuse optical tomography and optical coherence tomography (OCT). Optical techniques are of particular importance in the medical field, because these techniques promise to be safe and cheap and, in addition, offer a therapeutic potential. Advances in OCT technology have made it possible to apply OCT in a wide variety of applications but medical applications are still dominating. Specific advantages of OCT are its high depth and transversal resolution, the fact, that its depth resolution is decoupled from transverse resolution, high probing depth in scattering media, contact-free and non-invasive operation, and the possibility to create various function dependent image contrasting methods. This report presents the principles of OCT and the state of important OCT applications. OCT synthesises cross-sectional images from a series of laterally adjacent depth-scans. At present OCT is used in three different fields of optical imaging, in macroscopic imaging of structures which can be seen by the naked eye or using weak magnifications, in microscopic imaging using magnifications up to the classical limit of microscopic resolution and in endoscopic imaging, using low and medium magnification. First, OCT techniques, like the reflectometry technique and the dual beam technique were based on time-domain low coherence interferometry depth-scans. Later, Fourier-domain techniques have been developed and led to new imaging schemes. Recently developed parallel OCT schemes eliminate the need for lateral scanning and, therefore, dramatically increase the imaging rate. These schemes use CCD cameras and CMOS detector arrays as photodetectors. Video-rate three-dimensional OCT pictures have been obtained. Modifying interference microscopy techniques has led to high-resolution optical coherence microscopy that achieved sub-micrometre resolution. This report is concluded with a short presentation of important OCT applications. Ophthalmology is, due to the transparent ocular structures, still the main field of OCT application. The first commercial instrument too has been introduced for ophthalmic diagnostics (Carl Zeiss Meditec AG). Advances in using near-infrared light, however, opened the path for OCT imaging in strongly scattering tissues. Today, optical in vivo biopsy is one of the most challenging fields of OCT application. High resolution, high penetration depth, and its potential for functional imaging attribute to OCT an optical biopsy quality, which can be used to assess tissue and cell function and morphology in situ. OCT can already clarify the relevant architectural tissue morphology. For many diseases, however, including cancer in its early stages, higher resolution is necessary. New broad-bandwidth light sources, like photonic crystal fibres and superfluorescent fibre sources, and new contrasting techniques, give access to new sample properties and unmatched sensitivity and resolution.

/pdf/optical-coherence-tomography-principles-and-applications-17y32cabbm.pdf

Optical coherence tomography - principles and applications

Optical wireless communication (OWC) refers to transmission in unguided propagation media through the use of optical carriers, i.e., visible, infrared (IR), and ultraviolet (UV) bands. In this survey, we focus on outdoor terrestrial OWC links which operate in near IR band. These are widely referred to as free space optical (FSO) communication in the literature. FSO systems are used for high rate communication between two fixed points over distances up to several kilometers. In comparison to radio-frequency (RF) counterparts, FSO links have a very high optical bandwidth available, allowing much higher data rates. They are appealing for a wide range of applications such as metropolitan area network (MAN) extension, local area network (LAN)-to-LAN connectivity, fiber back-up, backhaul for wireless cellular networks, disaster recovery, high definition TV and medical image/video transmission, wireless video surveillance/monitoring, and quantum key distribution among others. Despite the major advantages of FSO technology and variety of its application areas, its widespread use has been hampered by its rather disappointing link reliability particularly in long ranges due to atmospheric turbulence-induced fading and sensitivity to weather conditions. In the last five years or so, there has been a surge of interest in FSO research to address these major technical challenges. Several innovative physical layer concepts, originally introduced in the context of RF systems, such as multiple-input multiple-output communication, cooperative diversity, and adaptive transmission have been recently explored for the design of next generation FSO systems. In this paper, we present an up-to-date survey on FSO communication systems. The first part describes FSO channel models and transmitter/receiver structures. In the second part, we provide details on information theoretical limits of FSO channels and algorithmic-level system design research activities to approach these limits. Specific topics include advances in modulation, channel coding, spatial/cooperative diversity techniques, adaptive transmission, and hybrid RF/FSO systems.

/pdf/survey-on-free-space-optical-communication-a-communication-239v68hfyw.pdf

Survey on Free Space Optical Communication: A Communication Theory Perspective

In free-space optical communication links, atmospheric turbulence causes fluctuations in both the intensity and the phase of the received light signal, impairing link performance. We describe several communication techniques to mitigate turbulence-induced intensity fluctuations, i.e., signal fading. These techniques are applicable in the regime in which the receiver aperture is smaller than the correlation length of fading and the observation interval is shorter than the correlation time of fading. We assume that the receiver has no knowledge of the instantaneous fading state. When the receiver knows only the marginal statistics of the fading, a symbol-by-symbol ML detector can be used to improve detection performance. If the receiver has knowledge of the joint temporal statistics of the fading, maximum-likelihood sequence detection (MLSD) can be employed, yielding a further performance improvement, but at the cost of very high complexity. Spatial diversity reception with multiple receivers can also be used to overcome turbulence-induced fading. We describe the use of ML detection in spatial diversity reception to reduce the diversity gain penalty caused by correlation between the fading at different receivers.

/pdf/free-space-optical-communication-through-atmospheric-48mj86nu81.pdf

Free-space optical communication through atmospheric turbulence channels

Atmospheric aerosol effects are often overlooked in target acquisition studies. Typically, performance models only consider extinction and turbulence within the prediction processes. The aerosol modulation transfer function (MTF) is included in range acquisition algorithms to determine how scattering and absorption effects change the target identification predictions. We modeled the aerosols as monodisperse water droplets comparable to a tenuous fog or mist. Integrating the aerosol MTF into the system MTF gives the opportunity to utilize the night vision integrated performance model to predict the target identification range with aerosol contributions. The aerosol MTF is a function of range, water droplet composition, wavelength, and aperture size. The analysis focuses on these variables with an emphasis on wavelength dependence to characterize mid-wave and long-wave performance. Results show that the mid-wave systems have a substantial diffraction advantage over long-wave systems. Only in the limit of increasing optical depths do the mid-wave and long-wave performance models begin to converge, verifying that the aerosols can be the limiting factor for target identification.

Effects of aerosol modulation transfer function on target identification

A novel near ultraviolet (UV) gas-filled detector which is operated in the prebreakdown regime is described. The detection mechanism is photoionization of the excited state. Modulated continuous UV light was used to study experimentally the detector responsivity and the spectral response for typical electrode materials such as Mo, Ti, W and Ta at around 200 nm wavelength. Of the materials investigated, molybdenum yielded the highest relative responsivity. This is attributed to the secondary electron emission and, in particular, to the initial electron energy after secondary emission, which contributes to internal signal gain and to improved quantum efficiency through increased excitation. For molybdenum electrodes, the detector performance was studied with various electrode geometries. Of the electrode configurations considered, two straight parallel wires which are located parallel to the incoming UV photon direction yields the best responsivity. This is attributed to maximum electric field nonuniformity and photon interaction depth between the electrodes. Effective quantum efficiency, including detector internal gain, is found to be about 5570% per photon at 200 nm wavelength. This leads to an effective yield of 9 A W-1, including internal gain, at 200 nm wavelength. The noise equivalent power NEP is estimated to be approximately 10-15 W Hz-12/.

https://iopscience.iop.org/article/10.1088/0957-0233/5/5/012/pdf

A near UV envelope detector in the prebreakdown regime based on photoionization of excited gas atoms

Millimeter waves imaging systems have many applications in medicine, communications, homeland security, and space
technology. The advantages of MMW includes high penetrates thru almost every dielectric materials and no known
ionization hazard for biological tissue, and low atmospheric scattering compared to infrared and optical rays. The Glow
Discharge Detector (GDD) is a low cost room temperature MMW detector based on a commercial neon indicator lamp.
Previously the GDD was proven to be very sensitive and inexpensive MMW radiation detector capable of direct and
heterodyne radiation detection. Focal Plane Arrays (FPA) of GDD pixels were constructed and experimentally tested. In those experiments, the change in the DC bias current of the GDD was measured as function of the incidence MMW radiation. In this work the up-conversion of MMW radiation to visual light is demonstrated. By using a CCD/CMOS camera and GDD FPA a faster, more sensitive, and very inexpensive MMW and THz imaging system can be implemented. Also it is shown that by using the up converting method a real time 2D and 3D images can be obtained. 3D
imaging system can provide the topography of the object plane in addition to intensity reflected from the image. Preliminary experimental results of an imaging system based on a unique quasi optical system, an 8×8 GDD's focal plane array (FPA), and a CCD camera is demonstrated. Also it is shown that the MMW images can be obtained in Real time.

Inexpensive and simple MMW imaging using optical detection of light emitted from glow discharge detectors

Simple filters used to restore blurred images require knowledge of the point spread function (PSF) of the blurring system. Unfortunately such knowledge is usually not available when the blur is caused by relative motion between the camera and the scene. Various methods addressing this problem were developed in the last four decades. These methods can be divided into two types: direct methods whereby the restoration process is performed in a one step fashion, and indirect methods whereby the restoration process is performed by an iterative technique. Direct methods usually require identification of the PSF as a first step, and then use it to restore the blurred image with a simple filter. Lately, a new direct method was developed. As a result of this development, direct restoration methods (given only a single blurred image) are studied and compared in this paper for a variety of motion types. Various criteria such as quality of restoration, sensitivity to noise and computation requirements are considered.

Comparison of direct methods for restoration of motion-blurred images

Absorption edge measurements indicate hardly any change in long wavelength detection cutoff of GaAs shallow-junction surface-emitting LED's as a result of pressure effects This supports the concept that vacuum-induced significant emission wavelength tuning of these devices is a result of surface effects rather than changes in E g  Also, possible slight decrease of long wavelength cutoff in these devices via gamma irradiaition, while emission wavelength is shifted significantly to longer wavelengths, proves that γ-irradiation spectral tuning of them does not derive from bulk bandgap changes and supports the concept that emission wavelength increase in these shallow-junction devices is a result of effects of γ-irradiation on surface emission rather than on E g in the bulk

Norman S. Kopeika

Papers

Effects of aerosol modulation transfer function on target identification

A near UV envelope detector in the prebreakdown regime based on photoionization of excited gas atoms

Inexpensive and simple MMW imaging using optical detection of light emitted from glow discharge detectors

Comparison of direct methods for restoration of motion-blurred images

Long wavelength bulk absorption cutoffs in GaAs light-emitting diodes exhibiting vacuum and gamma-irradiation emission wavelength tunability