Analysis of two-dimensional spatial data is important for aerial surveys of crops and soils, reconnaissance, medical diagnosis, geographical information systems, and many other domains. Partitioning of images is helpful in the processing and analysis of spatial data. Investigations have shown that splitting an image into sub-images and compressing each sub-image using fewer calculations leads to a faster and more efficient method for the compression of the main image. The relationship between partitioning, spatial information, and the ease of compressing is explored in this paper.
Chaos has the deterministic and unpredictable aspects useful for secure communications. Chaos encrypted optical signals provide high degree of security in free space as well as in optical fibers, but the effects of propagation through these mediums were not explored. In this paper, the evolution of Lyapunov exponent of optical chaos encrypted signal in optical fibers is studied. To study the effects of fiber channel, three different chaotic optical encryption schemes are studied and compared.
Chaotic optical signals may be generated by using an acousto-optic system in the Raman-Nath mode. The bistability in the Raman-Nath mode is realized by detecting the intensity of the diffracted optical beams and feeding back the electronic signal to drive the acousto-optic cell. If the parameters of the system are chosen ”judiciously”, the intensity of the output beams oscillates in a chaotic manner in time. The conditions for generating chaos from various diffraction orders of the Raman-Nath cell are derived by calculating the Lyapunov exponents. If the Lyapunov exponent is a positive number then the system generates chaos; otherwise the system remains stable and produces a well-defined output signal. When the Lyapunov exponent for a RamanNath diffraction order is known the physical parameters are calculated so that chaos is produced in the output of the system in that diffraction order.
The presence of atmospheric turbulence in the free space causes fading and degrades the performance of a free space optical (FSO) system. To mitigate the turbulence-induced fading, multiple copies of the signal can be transmitted on a different wavelength. Each signal, in this case, will undergo different fadings. This is known as the wavelength diversity technique. Bit error rate (BER) performance of the FSO systems with wavelength diversity under strong turbulence condition is investigated. K-distribution is chosen to model a strong turbulence scenario. The source information is transmitted onto three carrier wavelengths of 1.55, 1.31, and 0.85 μm. The signals at the receiver side are combined using three different methods: optical combining (OC), equal gain combining (EGC), and selection combining (SC). Mathematical expressions are derived for the calculation of the BER for all three schemes (OC, EGC, and SC). Results are presented for the link distance of 2 and 3 km under strong turbulence conditions for all the combining methods. The performance of all three schemes is also compared. It is observed that OC provides better performance than the other two techniques. Proposed method results are also compared with the published article.
The non-invasive, painless, radiation-free and cost-effective infrared breast thermography (IBT) makes a significant contribution to improving the survival rate of breast cancer patients by early detecting the disease. This paper presents a set of standard breast thermogram acquisition protocols to improve the potentiality and accuracy of infrared breast thermograms in early breast cancer detection. By maintaining all these protocols, an infrared breast thermogram acquisition setup has been established at the Regional Cancer Centre (RCC) of Government Medical College (AGMC), Tripura, India. The acquisition of breast thermogram is followed by the breast thermogram interpretation, for identifying the presence of any abnormality. However, due to the presence of complex vascular patterns, accurate interpretation of breast thermogram is a very challenging task. The bilateral symmetry of the thermal patterns in each breast thermogram is quantitatively computed by statistical feature analysis. A series of statistical features are extracted from a set of 20 thermograms of both healthy and unhealthy subjects. Finally, the extracted features are analyzed for breast abnormality detection. The key contributions made by this paper can be highlighted as — a) the designing of a standard protocol suite for accurate acquisition of breast thermograms, b) creation of a new breast thermogram dataset by maintaining the protocol suite, and c) statistical analysis of the thermograms for abnormality detection. By doing so, this proposed work can minimize the rate of false findings in breast thermograms and thus, it will increase the utilization potentiality of breast thermograms in early breast cancer detection.
Generation of chaos from nonlinear optical systems with an optical or electronic feedback has been studied for several
years. Such chaotic signals have an important application in providing secure encryption in free-space optical
communication systems. Lyapunov exponent is an important parameter for analysis of chaos generated by a nonlinear
system. The Lyapunov exponent of a class of a nonlinear optical system showing a nonlinear transfer characteristics of
the form sin2(x) is determined and calculated in this paper to understand the dependence of the chaotic response on the
system parameters such as bias, feedback gain, input intensity and initial condition exciting the optical system. Analysis
of chaos using Lyapunov exponent is consistent with bifurcation analysis and is useful in encrypting data signal.
Generation of chaos from acousto-optic modulators with an electronic feedback has been studied for several years. Such chaotic signals have an important application in providing secure encryption in free-space optical communication systems. Lyapunov exponent is an important parameter for analysis of chaos generated by a nonlinear system. The Lyapunov exponent of an acousto-optic system is determined and calculated in this paper to understand the dependence of the chaotic response on the system parameters such as bias, feedback gain, input intensity and initial condition exciting the cell. Analysis of chaos using Lyapunov exponent is consistent with bifurcation analysis and is useful in encrypting data signals.
Propagation of a monochromatic Gaussian beam through a stack of alternating layers of positive-refractive-index
dielectrics and negative-refractive-index metamaterials is analyzed using paraxial ray-optics approach.
Expressions for the change of the spot-size of the Gaussian beam are derived. Sensors for measuring parameters
that affect the thickness or refractive index of the metamaterials can be developed based on the change of the
spot-size.
KEYWORDS: Chaos, Interference (communication), Receivers, Transmitters, Acousto-optics, Bragg cells, Heterodyning, Linear filtering, Bistability, Signal to noise ratio
Signal encryption and recovery using chaotic optical waves has been a subject of active research in the past 10 years. Since an acousto-optic Bragg cell with zeroth- and first-order feedback exhibits chaotic behavior past the threshold for
bistability, such a system was recently examined for possible chaotic encryption using a low-amplitude sinusoidal signal
applied via the bias input of the sound cell driver 1, 2. Subsequent recovery of the message signal was carried out via a
heterodyne strategy employing a locally generated chaotic carrier, with threshold parameters matched to the transmitting
Bragg cell. The simulation results, though encouraging, were limited to relatively low chaos frequencies and sinusoidal
message signals only. In this paper, we extend the previous work by (i) increasing the chaos frequency using appropriate
parameter control; (ii) carefully examining the system sensitivity to three system parameters, viz., feedback delay,
feedback gain, and dc bias level; (iii) examine signal recoverability relative to shifts in the three parameters mentioned
above relative to the transmitter; and (iv) determining the robustness of such a system relative to the primary transmitter
parameters. Additionally, we consider also the effect of the additive bandpass noise (obtained from white Gaussian noise
in the simulator) on signal recovery in such a system from a performance standpoint. It is also conjectured that signal
recovery can be effected by passing the modulated light through a second sound cell in a matched chaotic regime. This
aspect is also under investigation.
In this paper we describe a free-space optics (FSO) based mobile sensor network that is not subject to RF interference
common to wireless sensor networks. FSO-based mobile sensor networks can potentially be used in applications where
security of communication, including freedom from susceptibility to jamming, is important. The design of nodes
containing multiple transceivers each composed of an LED and an angle-diversity array of identical photo detectors is
discussed in this paper. Depending on the number of photodetectors in the array and the angular field of view of each
photo detector we may obtain an increase in the signal to noise ratio of the overall optical communication system.
Transmission and reflection spectra of periodic and random stacks comprising positive index materials and
metamaterials have been extensively studied. In this paper we investigate the effectiveness of periodic stacks of
PIM/NIM for use as a sensor. The transfer matrix method is used to find the transmittance and reflectance.
Differences between the zero average refractive index bandgap and Bragg bandgap are illustrated. It is shown how
these bandgaps can be used as the basis for designing sensors with minimal cross-sensitivity.
We discuss the design of an acousto-optic cell based free space optical communication link where the data
beam is made secure through chaos encryption. Using external signal modulation of the diffracted light from
a hybrid acousto-optic cell chaos (or directly via incorporation in the sound-cell driver's bias voltage)
encryption of data is possible. We have shown numerically that decryption of the encoded data is possible by
using an identical acousto-optic system in the receiver.
This paper proposes an FSO-based mobile sensor network that is not subject to RF interference common to wireless
sensor networks. FSO-based mobile sensor networks can potentially be used in a battlefield where security of
communication, including freedom from susceptibility to enemy-induced jamming, is important. The paper discusses the
design of nodes containing multiple transceivers composed of LEDs and photo detectors. Results of initial experiments
are included. The work reported in this paper is part of an ongoing investigation on mobile FSO networks, including the
design of efficient protocols that can allow the mobile sensor nodes to function as a mesh network permitting
information exchange among nodes directly and, possibly, through an intermediate node.
A non-contact and low-cost nanomaterial based fiber-optic sensor is developed for measuring large values of electric
currents. The magnetic field, generated by the electric current, changes the refractive index of a liquid in which
nanomaterial particles are suspended. The change of refractive index is converted to a change in the intensity of light
transmitted in an evanescent field based fiber optic sensor. The change in the intensity is proportional to the magnitude
of the electric current and thus the current can be measured by measuring the resultant change in the intensity of light.
We propose a new method for using a MEMS device for measuring the power and spot size of a laser beam. The device
consists of a doubly-clamped single crystal silicon micro-beam. A laser beam incident on the microstructure exerts an
optical pressure on it and consequently the micro-beam gets strained. Analysis and simulations show that the laser
power and spot size can be determined by measuring the strains at two different positions on the microstructure.
KEYWORDS: Telecommunications, Free space optics, Computer security, Sensors, System identification, Information security, Network security, Modulation, Radio optics, Transmitters
A free space optics based identification and interrogation system has been designed. The applications of the proposed
system lie primarily in areas which require a secure means of mutual identification and information exchange between
optical readers and tags. Conventional RFIDs raise issues regarding security threats, electromagnetic interference and
health safety. The security of RF-ID chips is low due to the wide spatial spread of radio waves. Malicious nodes can read
data being transmitted on the network, if they are in the receiving range. The proposed system provides an alternative
which utilizes the narrow paraxial beams of lasers and an RSA-based authentication scheme. These provide enhanced
security to communication between a tag and the base station or reader. The optical reader can also perform remote
identification and the tag can be read from a far off distance, given line of sight. The free space optical identification and
interrogation system can be used for inventory management, security systems at airports, port security, communication
with high security systems, etc. to name a few. The proposed system was implemented with low-cost, off-the-shelf
components and its performance in terms of throughput and bit error rate has been measured and analyzed. The range of
operation with a bit-error-rate lower than 10-9 was measured to be about 4.5 m. The security of the system is based on the
strengths of the RSA encryption scheme implemented using more than 1024 bits.
Surface plasmon resonance based sensors are most useful in measuring the refractive indices of biochemicals. In such
sensors a beam of light obliquely incident at an interface of glass and metallic thin film excites resonant plasmon waves in
the metal if the angle of incidence or the wavelength is selected properly. The resonance conditions are changed by the
refractive indices of any material in contact with the metal film. When resonance occurs the light beam is absorbed
strongly. We can easily show that the phenomenon of surface plasmon resonance in such a system acts as a high quality
spatial notch or band rejection filter.
An optical fiber, stripped partially out of its cladding is used to sense refractive index of a liquid to a precision to fifth
place of decimal. The dependence of the light output of the sensor on the refractive index of the test liquid is nonlinear.
The light output of the sensor depends on the thickness to which the cladding is stripped. It shows both positive and
negative slope with increasing refractive index of the test liquid. The slope of the plot of sensor output against liquid
refractive index shows a change of sign at around the fiber refractive index. The sensor is unaffected by the presence of
absorption and is insensitive to the chemical nature of the solute. The sensor is sensitive in the whole of the tested range
of refractive indices 1.33 to 1.52. Experiments that show the significance of cladding modes in sensing are described.
Singly-clamped micron-sized cantilevers actuated by optical radiation pressure exerted by a laser are analyzed. An expression for optimum point of actuation giving the maximum amount of deflection is obtained.
Surface plasmon resonance based optical sensors can be realized in integrated planar optical system on a glass substrate using mirrors and diffractive optical elements etched on the surfaces. Such a realization gives new capabilities for interrogating the sensor and for enhancing the response through multiple interactions and new possibilities for all-optical processing of the optical signals in the sensor.
Parallel optical interconnections which replace metallic transmission lines with optical fibers or free space channels pro-vide high throughput, easy system integration, and low latency. These systems are used in making multiprocessor based supercomputers, telecommunication exchange switches and terminals, optical information processors and computers. A first-order model for the decrease in coupling efficiency between elements of two linear arrays of a free-space, parallel optical interconnect owing to misalignments or offsets in packaging is developed. Such an array interconnect consists of an array of optical sources, such as, optical fibers or VCSELs and an array of photo-receptors, such as, optical fibers, micromirrors or photodetectors. The coupling efficiency between source and receptor elements is modeled in terms of the sizes of the array elements, inter-element spacing and distances. The coupling efficiency is subject to degrading in-fluence of six varieties of random offsets, which may occur during the alignment, and fixing of the two arrays in a pack-age. We then determine first order approximations of the effects of these offsets. Our paper presents simple analytical formulas useful for a quick design of array-based parallel optical system packages and estimation of overall system per-formances. The formulas developed are useful for design and packaging of any optoelectronic processing system.
Alignment tolerance can be improved if instead of a large photodetector an array of several photodetectors is used to receive the power in each light beam used for interconnection. The design formulae for optically selecting the number of photodetectors, the center-to-center spacing between the photodetectors, and the radii of the photodetectors in a focal plane array are derived as a function of the spot sizes and diffraction patterns of the beams. Since photocurrents generated by several smaller photodiodes are summed together it is possible to enhance the signal to noise ratio without suffering a degradation in the overall bandwidth in the interconnect.
A model for power distribution from a series of radiative taps on multimode optical fibers is presented. This model can be used for designing and packaging tapped fiber based optical devices and subsystems: distributed sensors, delay lines, signal processors, add-drop multiplexers, and array illuminators. Case studies on analyzing a distributed fiber optic pressure sensor and designing an array illuminator with an equal amount of power from each tap are discussed.
Parallel optical interconnections which replace metallic transmission lines with optical fibers or free space channels provide high throughput, easy system integration, and low latency. Such interconnects are used in the design of multiprocessors and telecommunication central office switches and routers. In all parallel optical interconnects we need to couple a set of laser beams coming out of an array of sources or passing through an array of optical devices to another array of optical devices. Owing to diffraction laser beams spread spatially. So some optical devices in the path of laser beam may receive power from adjacent channels. The power form adjacent channels gives rise to crosstalk noise. In this paper we quantify the amount of optical crosstalk that can corrupt a channel in 2D rectangular arrays of parallel optical interconnects. The worst case signal to crosstalk power ratio in an array of interconnects is calculated as a function of the sizes of the array elements, inter-element spacing and distances. From the analytical results in this paper it is possible to determine guidelines on packaging optical interconnects, free-space or optical fiber-based. The effects of built-in offsets on the crosstalk power can be quantified. The results of this paper are also useful in optimizing the design of various types of extrinsic optical sensors in which cross-coupling or crosstalk is the basis of the sensing process.
Design of simple, low cost optical sensors for continuous liquid level measurements is discussed. The sensor consists of only a source and a photo-receptor placed in a cylindrical tube containing the liquid. Various parameters of this simple package are selected through diffraction calculations.
Small samples of radioactive atoms can now be concentrated with magneto- optic laser traps into a very small space and cooled to microdegree temperatures. The first application of these new techniques to atoms with radioactive nuclei offers new possibilities for nuclear physics measurements. The general applicability of these techniques of minute quantities of other materials is discussed.
Optoelectronic realization of adaptive filters and equalizers using fiber optic tapped deIay lines and spatial light modulators has been discussed recently. We describe the design of a single layer fiber optic Adaline neural network which can be used as a bit pattern classifier. In our realization we employ as few electronic devices as possible and use optical computation to utilize the advantages of optics in processing speed, parallelism, and interconnection. The new optical neural network described in this paper is designed for optical processing of guided Iightwave signals, not electronic signals. We analyzed the convergence or learning characteristics of the optically implemented Adaline in the presence of errors in the hardware, and we studied methods for improving the convergence rate of the Adaline.
It is desirable to design an optical interconnect so that it has high bandwidth, signal-to-noise ratio, and alignability. Examining the relationship between the alignability and bandwidth and the alignability and signal-to-noise ratio shows that increasing the alignability can lead to a simultaneous increase in the signal-to-noise ratio, but a decrease in the bandwidth. To find a design which balances these opposing effects, the product of alignability, signal-to-noise ratio, and bandwidth is plotted versus the normalized detector size of the interconnect. The peak of this curve yields the optimum detector size for the given physical interconnect design. Using this detector size, the interconnect's signal power and receiver parameters can be altered, as needed, to yield the required bandwidth and signal-to-noise ratio.
The nature of the diffracted Gaussian'' beam from a photorefractive hologram under the influence of electronic noise is investigated by determining the signa1-tonoise ratio and spatial degradation index of the output. 1.
Many ideas on opto-electronic realizations of various neural networks have
been described over the last few years. Most of them use analog optical
devices and have been shown to work satisfactorily as simple content
addressable memories. An important question is how much of the faults
and defects in the storage and retrieval systems, that is, the spatial and
temporal errors in the spatial light modulators or the photorefractive
arrays and the detector arrays these neural network realizations can
tolerate. One measure of fault tolerance is the probability of correct
retrieval or classification when the network is used as a content
addressable memory. Using recent theories we will derive upper bounds
to the probability of errors in various popular opto-electronic neural
network realizations as a function of the size, capacity, and the magnitude
of spatial and temporal errors. Guidelines on the design and
implementations of opto-electronic neural networks will be derived.
Adaptive infinite-impulse response (IIR) filters can be used for a variety of digital communication and digital signal processing problems, such as, linear prediction, adaptive notch filtering, adaptive differential pulse code modulation, channel equalization, and adaptive array processing [1 ] . In such applications an adaptive IIR filter can provide a better performance than the standard adaptive FIR filters because hR filters can track both zeroes and poles. Also to achieve a specified level of performance an IIR filter requires fewer coefficients than the corresponding FIR filters [1]. Many types of optoelectronic adaptive FIR filters based on fiber optics or acousto optics have been developed over the last few years [2,3 ]. Design and fabrication of optoelectronic adaptive IIR signal processors is a goal of our research.
Planar integration of diffractive-reflective optical systems around a glass substrate is an important concept for efficient, reliable, and compact packaging of optical interconnects. Realization of joint transform correlators in planar integrated packages is described. Such compact correlator packages can be combined with other integrated packages on an optoelectronic breadboard to build complex optical processors for various target recognition applications.
Acousto-optic (AO) cells are widely used in real-time optical processing of RF signals because of their high computational throughput resulting from large modulation bandwidths and large parallel channel capacity. In this paper we discuss how an AO cell can be used to build a general purpose infinite impulse response (IIR) filter for processing sampled data or discrete time signals. The advantages of the system are high speed real time filtering of discrete time RF signals with sampling frequencies in the 10 MHz to a few GHz range. IIR filters are attractive because they can provide a good magnitude characteristics for a smaller number of taps than finite impulse response filters can. In our realization the filter coefficients are stored in spatial light modulators (SLMs) and thus can be changed in time to build adaptive IIR filters for various applications.
Good alignment of the light beams and devices is necessary for maximum efficiency and reliability of an optical interconnection system. An optical interconnect which is difficult to align will be costly to manufacture and deploy. A measure of the ease with which a single beam of light and a device a basic optical interconnect can be aligned has been developed. This measure the alignability is a function of the beam size the spot size and the overall cost measure associated with the alignment. Practical optical interconnects consisting of several optical beams and devices such as optical crossbar switches can be modeled as series and parallel combinations of basic optical interconnects. The overall alignability can be obtained from the alignabilities of the component interconnects. A set of formulas for calculating component interconnects involving Gaussian and/or uniform beams and devices are derived. Using these formutas the alignability of several optical interconnection schemes and design and packaging guidelines can be developed.
An efficient optical adaptive equalizer has been developed for the optical processing of guided lightwave signal waveforms corrupted by time-varying dispersion and attenuation effects. The optical equalizer is based upon a tapped delay line filter built by laser ablative chemical etching. The least-mean-square algorithm is used to adapt an equalizer''s weights in real-time as the optical channel varies. The equalizer is designed for incoherent lightwave systems carrying binary, on/off keyed, digital data.
A CW CO2 laser is used to etch micron-scale taps in the cladding of multimode silica optical fibers. The smooth surfaces of the etched grooves directionaly radiate a small fraction of the propagating light energy which can be easily and precisely controlled via the laser parameters. The radiation pattern from the tap is in reasonably good agreement with initial theoretical calculations. This technique allows computer controlled rapid and inexpensive fabrication of high quality fiber optical taps which are needed to realize new fiber optical systems.
A new improved version of the arithmetic Fourier transform algorithm is presented. This algorithm computes the Fourier
coefficients of continuous -time signals using the number-theoretic technique ofMobius inversion. The major advantage of
this algorithm is that it needs mostly addition operations, except for a few real multiplications. The improved version can
be realized efficiently on integrated circuit chips and optical parallel processors using tapped delay lines.
Realization of direct forms infinite-impulse response digital filters using various radiatively-tappered fiber optic delay lines is discussed. Such filters are attractive because they can provide better magnitude characteristics in their frequency response for a smaller number of taps than finite-impulse response filters can. These realizations use spatial light modulators and thus can be incorporated into optical adaptive filters for processing lightwave signals. Models of error sources in the fiber optical realization and their effects on both time and frequency response characteristics are explained. Cascading of lower order subsystems to obtain higher order filters is discussed. A possible way of improving the insertion loss using Erbium doped fiber optic amplifiers is derived.
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