Three types of electroabsorption modulators (EAMs) based on III-V semiconductor multiple quantum wells (MQW) are presented in this work. One is a novel monolithic integration traveling-wave EAM for an analog optical transmitter/transceiver to achieve integrated photonic mm-wave functions for broadband connectivity. Another one is composed of an integrated EAM 1D array in a photonic beam-former as a Ku-band phased array antenna for seamless aeronautical networking through integration of data links, radios, and antennas. The third one addresses the use of MQW EAMs in free space optical links through biological tissue for transcutaneous communication.
In recent years, laser-induced breakdown spectroscopy (LIBS) has been established as a promising analytical tool for online
chemical analysis. The emitted light spectrum is analyzed for instantaneous determination of the elemental
composition of the sample, enabling on-line classification of materials. Two major strengths of the technique are the
possibilities to perform both fast and remote chemical analysis to determine the elemental composition of the samples
under test. In order to reduce the size of LIBS systems, the use of a compact Q-switched diode-pumped solid-state laser
(DPSSL) in a LIBS system is evaluated for the industrial sorting of aluminium alloys. The DPSSL, which delivers 150μJ
pulses of high beam quality at more than 7KHz repetition rate, provides irradiance on the target that is appropriate for
LIBS measurements.
The experimental results indicate that alloy classification and quantitative analysis are possible on scrap aluminium
samples placed 50 cm apart from the focusing and collecting lenses, without sample preparation. Similar calibration
curves and limits of detection are obtained for traditional high-energy low-frequency flashlamp-pumped and low-energy
high-frequency diode-pumped lasers, showing the applicability of compact diode-pumped lasers for industrial LIBS
applications.
The steady progress in photonic components in terms of cost-to-performance ratio, maturity and robustness opens new avenues for the commercial deployment of photonic sensor systems in a wide range of industrial applications. Advanced sensing can be used to optimize complex processes and thereby enable significant savings in energy consumption. Three cases of robust photonic instrumentation for process optimization and quality control in manufacturing industries are presented: improved metal recycling with laser-induced breakdown spectroscopy, quality control in precision machining by white-light interferometry with optical fiber probes embedded in machining tools, and process optimization in steel foundries by stand-off temperature measurements in blast furnaces with optical fiber lances and spectral analysis techniques. Each of these methods utilizes a low-cost spectrometer, and requires dedicated calibration and signal processing methods to guarantee robust operation in industrial environments with varying conditions. Experimental results are presented, including on-line steel alloy analysis with correct classification rates in excess of 95%, distance measurements with axial resolution of +/- 2nm over a 75μm range, and continuous temperature monitoring of molten steel in oxygen blast furnaces with temperature measurement accuracy better than 1%.
Narrow bandgap semiconductors GaSb, InAs, and InSb are important building blocks for infrared photodetectors based
on type-II InSb quantum dots or an InAs/GaSb strained layer superlattice. Understanding the surface chemical
composition of these materials can provide valuable information that enables optimization of device surface passivation
techniques leading towards surface leakage free IR photodetectors. We report on an investigation into Ga-, In-, Sb-, and
As-oxides and other chemical species on the surface of untreated, dry etched and thermally treated GaSb, InAs and InSb
samples by x-ray photoelectron spectroscopy. The experimental results reveal the presence of Sb- and Ga-oxides on the
surfaces of the untreated and treated GaSb samples. Both Sb- and In-oxides were observed on the surface of all InSb
samples, and especially the dry etched sample had thicker oxide layers. In the case of the InAs samples, not only In- and
As-oxides XPS signals were obtained, but also AsCl species were found on the ICP dry etched sample. These results
helped to analyze the dark current of our fabricated IR detectors.
We report on the optical and structural characterization of InSb QDs in InAs matrix, grown on InAs (100) substrates, for
infrared photodetection. InSb has 7% lattice mismatch with InAs forming strained QDs, which are promising for longwave
IR applications, due to their type-II band alignment. This report contains material development results of InSb QDs
for increasing their emission wavelength towards long-wave IR region. Samples were grown by two techniques of MBE
and MOVPE, with different InSb coverage on InAs (100) substrates. Structures grown by MBE reveal QD related
photoluminescence at 4 μm. AFM investigations of the MBE grown structures showed uncapped dots of ~ 35 nm in size
and ~ 3 nm in height, with a density of about 2 x 1010 cm-2. Cross-section TEM investigations of buried InSb layers
grown by MBE showed coherently strained QDs for nominal InSb coverage in the range of 1.6 - 2 monolayers (MLs).
Layers with InSb coverage more than 2MLs contain relaxed QDs with structural defects due to large amount of strain
between InSb and InAs. Samples with such large dots did not show any InSb related luminescence. The MOVPE grown
InSb samples exhibit a strong QD related emission between 3.8 to 7.5 μm, depending on the amount of InSb coverage
and other growth parameters. We report the longest wavelength observed so far in this material system.
Performance, advantages and drawbacks of GaN- and SiC-based ultraviolet (UV) photodetectors are analyzed and
compared. This includes metal-semiconductor-metal photodetectors, p-i-n photodiodes and avalanche photodiodes.
Design, process and characterization of these devices are described. Fabricated p-i-n InGaN/GaN quantum well
photodetectors are shown to exhibit a quantum efficiency of about 50% at 365 nm with a peak ultraviolet to visible
rejection ratio more than 3 orders of magnitude. A novel SiC avalanche photodiode design is shown to produce stable
avalanche breakdown characteristics for devices up to 2mm in diameter. A significant increase of its photo responsivity
is also demonstrated when the avalanche voltage is applied.
We have demonstrated surface normal detecting/filtering/emitting multiple functional ultraviolet (UV) optoelectronic devices based on InGaN/GaN, InGaN/AlGaN and AlxGa1-xN/AlyGa1-yN multiple quantum well (MQW) structures with operation wavelengths ranging from 270 nm to 450 nm. Utilizing MQW structure as device active layer offers a flexibility to tune its long cut-off wavelength in a wide UV range from solar-blind to visible by adjusting the well width, well composition and barrier height. Similarly, its short cut-off wavelength can be adjusted by using a GaN or AlGaN block layer on a sapphire substrate when the device is illuminated from its backside, which further provides an optical filtering effect. When a current injects into the device under forward bias the device acts as an UV light emitter, whereas the device performs as a typical photodetector under reverse biases. With applying an alternating external bias the device might be used as electroabsorption modulator due to quantum confined Stark effect. In present work fabricated devices have been characterized by transmission/absorption spectra, photoresponsivity, electroluminescence, and photoluminescence measurements under various forward and reverse biases. The piezoelectric effect, alloy broadening and Stokes shift between the emission and absorption spectra in different InGaN- and AlGaN-based QW structures have been investigated and compared. Possibilities of monolithic or hybrid integration using such multiple functional devices for biological warfare agents sensing application have also be discussed.
KEYWORDS: Modulation, Modulators, Eye, Sensors, Signal to noise ratio, Signal detection, Transceivers, Free space optics, Retroreflectors, Optical communications
An optical communication link with a modulated retro-reflector in a cat's eye optics arrangement has been evaluated. A
transmissive multiple quantum well modulator with a modulation bandwidth in excess of 10 MHz was placed in a wide
field-of-view cat's eye optic system. A laser transceiver with one line and one broadband source around 1550 nm was
used to interrogate the modulated retro-reflector and detect the reflected and modulated signal. Multi-level digital
encoding was evaluated as a way to increase the information transfer rate in conditions with relatively high signal-to-noise
ratio. The links were evaluated in indoor as well as outdoor conditions.
In this paper, some devices were reviewed to be used in quantum communications. We presented a low density of
Quantum Dots, which could be used to get single quantum dot as light emitting source for generating single photons. An
analytical model to study the thermal behavior of a solid media in interaction with one, two or three laser beams was
developed using the classical heat equation. Integrated optic micro-ring resonators and its simulated result also are
presented. Development of active micro-ring in silicon is at an early stage, where both vertical and horizontal techniques
are feasible. With the epitaxy growth techniques, a possibility for achieving controllable QD density, size and good
uniformity are proposed. A low density of QDs in range of 108 cm-2 has demonstrated through successive adjustment of
the growth parameters. Details among the devices are presented and discussed.
Retro-communication by the joint use of light modulators and retro-reflecting devices has been proposed and demonstrated as a promising technique to further extend the use of free-space optical communication (FSO) to mobile communication. We have developed and demonstrated high-speed surface-normal GaAs-based and InP-based multiple quantum well electroabsorption modulators and arrays operating at different near-infrared wavelengths for FSO link applications. In this paper we report on the structure and device design issues, and trade-offs when GaAs-based or InP-based modulators are needed in the FSO links. In particular, the modulator structure properties, the device fabrication techniques and the device static and dynamic performance are investigated and compared.
We report on a quantum dot (QD) structure grown on a 4'' GaAs substrate by metal organic vapor phase epitaxy (MOVPE), which consists of five stacked InAs/InGaAs/GaAs QD layers embedded in the center of a typical in-plane waveguide. The density of the QDs is about 2.5 x 1010 cm-2 per QD layer. The photoluminescence (PL) peak wavelength at 1322 nm corresponding to the interband transition of the QD ground states was observed at room temperature with a full width at half-maximum of 49 meV. A good uniformity of the QD structure across the 4'' wafer was verified with a variation of the PL peak wavelength of 0.9 % from the wafer center to the edge. Top p-contacts and a bottom n-contact were processed on the QD structure, and electroluminescence (EL) spectra were measured at different temperatures. An EL peak corresponding to the QD ground states emission was obtained at 1325 nm at room temperature.
We report on the design, fabrication and characterization of 1550 nm electroabsorption modulators based on InGaAs/InAlAs coupled quantum wells grown on InP substrate by MBE. Large and small single modulators and modulator arrays have been fabricated on a wafer scale with an optimized device fabrication technology. The modulator size, shape, contact arrangements, and the array configurations have been varied to achieve suitable device performance for different retro-reflective free-space optical communication links. The device electrical and optical properties have been characterized by I-V, photoluminescence, absorption, transmittance and reflectance measurements. Modulators exhibit contrast ratios of 2:1 at a 3V driving bias and contrast ratios of 2:1 over a 30 nm bandwidth at 6V. A maximum contrast ratio of 4:1 is obtained at a 12 V driving voltage.
A novel retrocommunication link utilizing reflective multiple quantum well (MQW) optical modulators and nonmechanical beam steering and tracking is demonstrated. Large aperture reflective MQW modulators using AlGaAs/GaAs are optimized and manufactured. The modulators exhibit a contrast ratio larger than 4:1 and a modulation bandwidth of 10 MHz. Nonmechanical beam steering and tracking are studied using nematic liquid crystal (NLC) spatial light modulators (SLMs). The communication link is comprised of a retromodulating array with four MQW modulators and a transceiver using a NLC SLM for beam steering and tracking. Transfer of audio, real-time image data and pseudorandom bit sequences over 100-m range while tracking the moving retromodulator is shown. The link is capable of transferring data at approximately 8 Mbps.
Retrocommunication is a new technique for asymmetric free-space optical communication that has attracted interest during recent years. Novel technologies such as multiple quantum well (MQW) optical modulators and non-mechanical laser beam steering and tracking have been studied for implementation in a retrocommunication link. Large and small aperture reflective AlGaAs/GaAs MQW modulators were optimised and fabricated. The modulators exhibit high contrast ratios (from 5 to 100) and high modulation rates (up to 16 Mbit/s). A retroreceiver consisting of four large aperture MQW modulators, associated optics and drive electronics was fabricated. Nematic liquid crystal spatial light modulators have been evaluated, characterised for beam steering and tracking and implemented in a transceiver. Small area MQW modulators, used in focal plane configurations, were studied for static communication links. Results from a novel retrocommunication link utilising a retroreceiver and non-mechanical laser beam steering and tracking will be presented. Bit rates of 8 Mbit/s were observed during non-mechanical tracking of a moving retroreceiver over 100 m range. The demonstrator system was capable of transferring audio-, real-time images or bit streams. The demonstrated principles show promising features for future low weight free-space communication links. Performance calculations including requirements for a retrocommunication link using MQW modulators and non-mechanical beam steering are discussed.
We report on the development of different surface-normal multiple quantum well (MQW) modulator devices. Owing to their unique speed advantage, arrays of surface-normal MQW modulators are very well suited for fast and parallel signal processing, and can be developed for both digital or analogue signals. We present the design and fabrication process for single surface-normal MQW modulators, as well as for high fill factor (80-90%) 1-D arrays of such modulators. Design issues and trade-offs in terms of modulator size, speed and contrast ratios are described. Contrast ratios in excess of 100:1 have been demonstrated. 3-dB frequency modulation bandwidths in excess of 1GHz have been obtained for single modulator devices of 125μm in diameter. Performances of 1-D arrays of 64 and 128 modulators (pixel size: 2mmx80μm and 2mmx40μm, respectively) are also presented, with response time for programmable filtering in the order of 10 ns.
We present design and fabrication methods for surface normal monolithic amplitude modulators with an aperture up to 14 x 14 mm2, a contrast ratio of 6:1 and for low driving voltages (≤8 V). The modulators consist of undoped GaAs/AlGaAs quantum wells embedded in a Fabry-Perot (FP) resonance cavity grown by MOVPE. To improve the device performance the FP cavity, the period and thickness of the quantum well and doping concentration were optimised. Also, the dimension of the modulator were varied from 0.5 x 0.5 to 14 x 14 mm2. The results show that the yield of the modulators increases significantly when decreasing the size of the modulators. To remedy the low yield issue for wide aperture modulator, a pixelated approach was used to divide the mono pixel in a monolithic modulator into several pixels, for example from 4 to 48. The modulation speed of the modulators with different dimensions was characterised by electro-optic (EO) response measurements. The temporal optical response of the large modulators was satisfactory up to the order of MHz modulation frequency where the RC constant limited the performance. A few of the modulators with wide apertures are to be assembled into an optical link system for free-space communication.
A new interesting hybrid digital/optical correlator, which performs the first Fourier transform electronically, hereafter referred to as a 2-f correlator, has been presented by Young. One of the advantages of this architecture compared to the classical 4-f correlator, an example of this architecture is presented by Chao, is that the optical system becomes less complex and does not require the same strict optical tolerances. The signal processing performance of these two architectures is expected to be the same if ideal Spatial Light Modulators (SLMs) with fully complex-valued coding domains are available. This study investigates the influence of a limited coding domain on the signal processing performance provided by current available SLMs. Optimal trade-off filters3 have been used for this investigation since they can be tuned, as the name suggests, to an optimal trade-off between being discriminant against distortions and being robust against noise. This has been used to evaluate the influence of different coding domains on the two optical correlator architectures. The coding domains of some commercially available SLMs have been implemented and their effect on the different correlator architectures have been analysed. These SLMs are: binary phase/amplitude SLM from Displaytech, Colorado, USA; micromirror SLM from Fraunhofer Institute, Dresden, Germany and the bipolar grey-scale FLC SLM from Boulder Nonlinear Systems, Colorado, USA. The coding domain has a strong influence on the signal processing capabilities. Generally, the 2-f architecture shows to be more discriminant and the 4-f architecture more robust to noise. The final choice between the two correlator systems depends however on available SLMs and the required trade-off between robustness and discriminance that is needed for the application.
To be competitive with their electronic counterparts, correlation-based optical processors require very fast spatial light modulators (SLMs) that can perform simultaneously phase and amplitude modulation. Owing to their ultra-high speed, multiple quantum well (MQW) SLMs have been early identified as very good candidates. However, the coding domain of MQW SLMs is not widely known. We present here a study of available coding domains of MQW SLMs. We demonstrate that pure amplitude modulation, ternary { -1, 0, + 1} modulation and quaternary {0, + 1, ei2π/3, ei4π/3}, modulation are examples of coding domains that can be achieved by tuning a few parameters in the design of Fabry-Perot MQW modulators. We show that ternary and quaternary filters provide much better results than binary filters for the recognition of objects embedded in highly cluttered noise. Finally, we present a technique, the time-averaged pseudo-random encoding technique, which enables encoding of any complex filter onto a quaternary modulator. Combined with the time-averaged pseudo-random encoding technique, MQW SLMs may pave the way to the development of new optoelectronic correlator systems with improved speed and accuracy performance.
Multiple quantum well spatial light modulators (MQW SLMs) are promising devices for future high-speed applications. We present results obtained with a single-pixel amplitude modulator. We discuss the status of our work on a 128x128-pixel ternary SLM. This SLM will run at 10 kHz and have one low-reflectance level and two high reflectance levels with a phase difference of pi. We also present a study of the relation between the coding domain and the structural design of modulators.
We propose a brief overview of different techniques to implement complex-valued filters on binary spatial light modulators (SLMs), including encoding techniques at both pixel- and cell- (i.e. group of pixels) levels. We show that group-oriented methods offer extended coding domains, at the expense of a reduced space bandwidth product and spurious noise in the correlation plane. We propose the concept of a time multiplexing technique that combines at the pixel level the pseudo-random encoding method with the minimum Euclidean distance approach. This pixel-oriented method offers the advantage to keep the full space bandwidth of SLMs, and the possibility to very efficiently encode ternary filters (-1, 0, +1) on binary phase (-1, +1) SLMs. Tested on a sequence of images acquired with an IR sensor in a tracking scenario (1 target), encoded ternary filters have offered superior performance in terms of peak-to-clutter ratio than classical BPOF (+45% in the simulation, +35% in the first optical experiments).
We present a compact optical correlator with Internet access. Users can remotely download images and get the optically computed correlation results back on their monitor.
A new encoding technique for Joint Transform correlators is presented, with no loss of information for correlation filters displayed on non-ideal SLMs, and narrower autocorrelation peaks.
We present in this paper a study of the influence of the target resolution and contrast on correlation filter performance. Classical filters and the Optimum filter designed by Javidi et al. for non-overlapping noise were considered and tested on a set of realistic images based on real-world non- overlapping noise realizations. We show that for the images that we have tested, there is a limit of the resolution after which classical linear filters fail to detect properly targets, while the Optimum filter still performs well. Furthermore, it was observed that for some background images and fairly low target resolutions, there are values for the target illumination that prevent the classical filter to detect the target, while again it is possible to tune the Optimum filter to make it tolerant to varying target illumination. Eventually, we propose a new composite filter for in-plane rotation tolerance based on individual optimum filters and built according to the equal correlation principle perform best for our test images.
A compact optical correlator based on binary liquid crystal spatial light modulators has been built. The correlator has a 21 X 28 cm2 footprint and can process 256 X 256 pixel images at a maximum frame rate of 220 Hz. The system is insensitive to transportation and can be used both in VLC and JTC configuration. It can process live images from an external camera as well as images from computer memory. Variations of illumination conditions can change the graylevels and perceptible details of a target considerably. When this is the case, classical filters like phase only and optimal trade-off fail to detect the target. An improvement of the results can be achieved by using the optimum generalized filter. In most cases, the contour of the target remains more or less the same even if the graylevels are fluctuating. We show that by edge-enhancing and binarizing the input images used together with the optimum generalized filter we can achieve comparable results as compared to using grayscale images.
This paper reviews some results of an ongoing research program at the Institute of Optical Research that addresses optical correlators dedicated to pattern recognition. A two- class discrimination problem with in-plane distortion is considered. To cope with the loss of information that may occur when input images are directly displayed on experimental binary SLMs in a Vanderlugt correlator, we have implemented several types of preprocessing operators on the original gray scale images. Performance criteria are presented to assess and grade preprocessors that can be implemented in real-time on an electronic frame grabber board. Optical implementation in a VanderLugt correlator is described. Experimental results are provided and emphasize the benefits of preprocessing prior to binarization. Significant improvements are found, both digitally and optically, with classical derivative and rank operators.
This paper presents experimental and simulation results from ongoing research on the influence of clutter on OTSDF filter performance. The degrading influence of overlapping clutter can be reduced by including colored noise in the filter construction or by tuning a white noise filter. Simulation results together with experimental results from a van derLugt correlator indicate that tuning of a white noise filter is the most effective for low levels of clutter in the input images whereas inclusion of colored noise is more effective for high levels of clutter. The experimental results are well in line with the simulations.
Optical fiber sensors for monitoring of structures (OSMOS) is a European collaborative research project which has, over the past three years, embraced a number of technological issues related to the problem of structural monitoring in the civil engineering and aerospace industries. A key technical objective of the program was the measurement of temperature and strain using a single sensor length. A laboratory prototype using the differential sensitivities of polarimeters based on the fundamental, LP01 mode and the first higher order LP11 mode of polarization maintaining fiber demonstrated parameter recovery to within 2 C and 5 (mu) (epsilon) . A receiver enabling quasi-distributed measurements to be made with a linear spatial resolution of 70 cm using white light polarimetry was assembled. White light polarimetry was also used in conjunction with pressure sensitive fiber to detect impact damage on a composite radome structure. Impacts of 5 Joules in magnitude were detected with a spatial resolution of around 1 cm. Microwave radio frequency subcarrier measurement techniques were used to develop the engineering processes necessary to integrate optical sensors into civil engineering structures for simulated applications trials. This enabled issues such as stress transfer, mechanical bonding and sensor protection to be addressed. For the aerospace industry, embedding of optical fiber sensors remains an important issue. Here we developed techniques for embedding connectorized fibers such that the component could be machine finished after curing, an important feature of the manufacturing process.
This paper reviews the progress of an ongoing research program at the Institute of Optical Research which addresses optical correlators dedicated to pattern recognition. A two-class discrimination problem with in-plane distortion is considered. For this purpose, we provide here a comparison between the performances of several synthetic discriminant function (SDF) filters implemented on a binary ferroelectric liquid crystal spatial light modulator (FLC SLM). While non-constrained optimal trade-off (OT) SDF filters perform very well, binary filters offer lower efficiencies. The relative merits of direct binarization of the complex transmittance of the filter and the Jared-and-Ennis method (relaxation algorithm) for filter construction are discussed. To overcome the poor modulation capability of available SLMs and take full advantage of the tuning ability of OT SDF filters, we propose to encode composite filters as detour-phase holograms. We illustrate this discussion with computer simulations as well as experimental results obtained with our optical VanderLugt correlator. In-plane rotation tolerance over 90 degrees and two-class target discrimination are experimentally achieved with a single composite filter.
This paper reviews the progress of an ongoing research program at Thomson-CSF and Bertin & Cie which addresses an optical fiber system dedicated to the assessment of impact induced damages on airborne multilayered structures. The method is based on the use of embedded high birefringence optical fiber sensors and distributed white light interfero-polarimetry. The first part is devoted to the transduction process efficiency within optical fibers depending on the applied force intensity, direction versus the fiber eigen axes and the interaction length. To understand the behavior of these optical fibers and calibrate the detection system, experiments have been conducted on elliptical core fibers, `bow-tie' fibers and side-hole fibers and showed a wide range of available sensitivities. The second step is related to the inclusion of optical fibers in a sandwich structure representative of an airborne dome, and composed of foam between glass/epoxy composite skins. Different designs of grooves in the foam and tube sheathings have been investigated to support and protect the optical fiber. Impacts have been performed on the structure in the 1 to 10 Joules energy range. Experimental impact location and energy measurements have been achieved for a variety of stress fields.
We describe here a practical implementation of phase stepping moire interferometry for determination of a strain field in areas located close to optical fibers embedded in composite materials. The results show that fibers introduce only a local perturbation without noticeable influence on mechanical properties of materials.
At present, there is an acute need for techniques in monitoring civil engineering structures, and optical fiber sensors are acknowledged to be amongst the best candidates. For more than ten years, interferometric optical fiber sensors have been widely investigated and now provide a rich extended basis for measuring strains experienced by structural elements. However, because of their periodic response, those sensors need extending measuring techniques to fulfill civil engineering requirements. Amongst different methods, Thomson-CSF and the University of Strathclyde have recently employed a microwave subcarrier system [1]. A specific sensor dedication to the arena of large civil engineering structures has been designed and tested.
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