Laser vibrometry based on coherent detection technique allows to measure vibration characteristics of objects, based on its high Doppler resolution. Point targets were measured up to 40 km under medium turbluence conditions. Specifically vibration imaging offers an extensive potential for short-range civil applications and for long-range target classification and identification. For short range applications (up to few meters distance) laser vibrometry is used for investigating and testing of all kind of mechanical structures with respect to their vibration characteristics. Laser-Doppler based acoustic-to-seismic detection of buried mines shows a potential of this attractive technique at short range, mostly based on λ = 632 nm (HeNe laser). At longer ranges, the wavelengths of λ = 10.6 μm (CO2 laser) and λ = 1.5 μm (erbium fiber laser) are of interest, because of laser safety and better beam propagation through the atmosphere. Examples of the vibrometry technique with and without spatial resolution capability are shown here.

Ground-based optical transmitter and receiver systems designed for active imaging, active tracking and laser ranging of satellites in Earth orbit are very sensitive to physical conditions limiting the radiometric returns for achieving these measurements. The initial design of these systems is often based on simple radiometric scaling laws that provide estimates of average radiometric returns and are derived from experimental data or from more complex theoretical calculations. While these laws are quite useful, it is often easy to lose sight of the initial assumptions made in their formulation, and hence, the limits of their accuracy for designing certain systems. The objective of this paper is to review some of the commonly used radiometric scaling laws for active systems and to establish guidelines for their use based on comparisons of their predictions with results from detailed wave-optics simulations for different system design requirements and physical conditions. The combined effects of laser and transmitter beam parameters, wave-front aberrations, atmospheric turbulence, and satellite optical cross-section are considered.

In the frame of the SILEX project, the European Space Agency (ESA) has put into orbit two Laser Communication Terminals, to establish an experimental free space optical communication link between a GEO satellite (ARTEMIS) and a LEO satellite (SPOT IV), to relay earth observation data. In order to perform In Orbit Testing (IOT) of these, and other, optical communications systems, ESA and the Instituto de Astrofisica de Canarias (IAC) reached an agreement for building the Optical Ground Station (OGS), in the Teide Observatory of the IAC. With ARTEMIS placed in a circular parking orbit at about 31000 kilometres, its optical payload has been preliminary tested with the OGS. First results and analysis are presented on the space-to-ground bi-directional link, including pointing acquisition and tracking performance, Bit-Error Rate (BER) and transmitted beam divergence effects related with atmospheric models and predictions. Future plans include deeper optical bi-directional communication tests of OGS, not only with ARTEMIS but also with OSCAR-40 (downlink) and SMART-1 (up-link) satellites, in order to do a full characterisation of the performances of laser beam propagation through atmospheric turbulence and a comparison with theoretical predictions.

ORNL is developing a high-speed, full-duplex all weather communications link for ranges up to 5 kilometers. To accomplish this project, we have constructed an RF-driven waveguide CO₂ laser and a dielectric-waveguide Stark modulator. The 10-micron wavelength was selected for its ability to penetrate smoke, fog, and rain. The modulator is based on the Stark shift of NH₂D (deuterated ammonia). The laser is driven by a 60 MHz RF amplifier at a power level of approximately 50 watts. The resonator cavity of the laser is formed by a 2.4 mm internal diameter ceramic waveguide with external optics. The RF electrodes are formed from aluminum heatsink extrusions that also provide cooling for the discharge. Details of the laser design will be presented.

Wide area (such as 2π steradian) coverage transmit-receive lasercom sub-system is proposed. The lasercom sub-system design utilizes recently proposed new beam scanning approach called Multiplexed Optical Scanner Technology (MOST) that promises high-speed reconfiguration, low power consumption, large aperture, and wide coverage desired for agile, low cost, and easy to install free-space optical link system. The stringent sub-system goals are met by introducing the concept of 'Multiplexing' at both the scanner interconnection level and the light property level. The scanner interconnections can be parallel or serial or both. The light properties that can be multiplexed include polarization, time, wavelength, space, and spatial codes. Manipulating the mentioned parameters of light, various multiplexed optical scanners (MOSs) can be realized. The highly sought after lasercom sub-system design will consist of two or more MOSs combined together in a smart fashion.

Fourier Telescopy is an active laser-based imaging method for high-resolution imaging of dim objects in Geosynchronous (GEO) orbit. The Geo Light Imaging National Testbed will be buit to demonstrate new powerful imaging capability. Several processes including laser speckle, atmospheric turbulence on the downlink and 1-km horizontal path, as well as Poisson shot noise can contribute to the measurement error of the Fourier phase of the object and thus degrade the reconstructed image. We investigated the impact of three processes including laser speckle, turbulence, and Poisson shot noise on the measurement error of the Fourier phase. We introduced the concept of power-in-the-bucket receiver and applied this concept to the receiver in the Fourier telescopy system. We found that the power-in-the-bucket receiver cancels the effects of turbulence on the horizontal path on the Fourier telescopy system. We evaluated variance of the real and imaginary parts of the triple product, as well as variance of the Fourier phase of the object by using a numercal simulation code. The twelfth moment of the optcal filed was calculated in the resenceof laser speckle, atmospheric turbulence, and Poisson shot noise. Simulation results confirmed that Fourier telescopy system is immune to the efects of turbulence on the horizontal path. It also showed that the effect of turbulence on the downlink path on the triple product is small. Laser speckle contributes strongly to the variations of the real part of the triple product and weakly to the imaginary part. Statistical properties of the triple product depend on the noise source. Poisson shot noise and laser speckle poduce the main contribution to the variance of the triple product and measurement error of the object Fourier pase. Phase variance reduces with increasing the number of heliostats, number of pulses, fringe visibility, and fringe signal-to-noise ratio. For 40 heliostat receivers, 100 averaged pulses, and all considered fringe SNRs and fringe visibilities the phase variance caused by Poisson noise, laser speckle, and turbulence on the downlink path is significantly less than 0.36, which corresponds to the phase measurement accuracy of 1/10 of the wave.

In this paper an analytic expression governing the two-frequency mutual coherence function (MCF) for a Gaussian beam wave is developed for moderate to strong fluctuations. Using a temporal moments approach combined with this new expression for the MCF, expressions governing pulse statistics (delay and broadening) are developed. These general analytic results are compared with previous results in special cases.

Recent experiments performed at UNC Charlotte indicate a reduction in the bit-error rate for a laser communication system with the implementaion of low-order adaptive optics in a free-space communication link. With simulated atmospheric tilt injected by a conventional PZT tilt mirror, an adaptive optics system with a Xinetics tilt mirror was used in a closed loop. The laboratory experiments replicated a monostatic propagation with a cooperative wavefront beacon at the receiver. Due to constraints in the speed of the processing hardware, the data is scaled to represent an actual propagation of a few kilometers under moderate scintillation conditions. We compare the experimental data and calculated bit-error rate before correction and after correction and compare it with a rigorous theoretical prediction.

The effects of aperture averagign in a free-space laser communication system with a phase diffuser placed in front of the transmitting aperture are studied. The partially coherent beam created by the use of a phase diffuser is modeled as a Gaussian Schell beam. To evaluate aperture averaging, an analytic expression is used for the spatial covariance of irradiance fluctuations that is valid for Gaussian beams of any degree of coherence, subject to the restriction that the atmospheic turbulence-corrupted laser beam can be described as a Gaussian stochastic process. In the weak fluctuation regime this analytic expression is found to compare reasonably well with published data. It is shown that when a partially coherent source beam is used irradiance fluctuations can be significantly reduced in the weak fluctuation regime.

By using ABCD ray matrix theory and a random phase screen located near the source, analytic expressions are developed for the mutual coherence function and scintillation index of a Gaussian-beam wave propagating through weak atmospheric turbulence in both the pupil plane and image plane of a receiving. The phase screen model that we use is based on a previous double-pass analysis by the authors for analyzing speckle propagation from a rough target in a lidar system. In the present context, it serves as a model for a partially coherent Gaussian-beam wave that is currently used in laser communications. The effect of partial coherence (induced by a diffuser) on the scintillation index of the beam in the presence of weak atmospheric turbulence is investigated as a function of the correlation length of the diffuser and the propagation distance.

With growing interest in terrestrial, inter-building and short distance wireless communication for high data-rate transmissions, solutions are sought for the crippling problems presented by multi-scattering phenomena typified by fog and particulate media. The multiple scattering results in spatial, temporal and angular spread of the light as it propagates through the medium. This both attenuates the total power incident on the receiver and increases the Bit Error Rate (BER) as subsequent pulses are not distinguishable due to Inter-symbol interference (ISI). A model of light transmission through fogs of different optical thicknesses and types is presented at four different wavelengths, using Monte-Carlo simulations. An adaptive field of view (FOV) receiver for optical wireless communication is proposed and the possibility of thus enhancing communication system performances through fog is indicated. Necessarily, the limitations presented by thermal noise in a detector of dimensions affording large fields of view restrict the applicability of the proposed solution. Hence, in this work we investigate optimal FOV settings, taking into consideration thermal noise signal degradation. The essence of the concept is to be configured as a simple design tool whereby environmental data are correlated to optimal FOV settings.

We introduce a wave-optics based simulation code written to model a complete free space laser communications link, including a detailed model of an adaptive optics compensation system. We present the results obtained by this model, where the phase of a communications laser beam is corrected, after it propagates through a turbulent atmosphere. The phase of the received laser beam is measured using a Shack-Hartmann wavefront sensor, and the correction method utilizes a MEMS mirror. Strehl improvement and amount of power coupled to the receiving fiber results for both 1 km horizontal and 28 km slant paths will be presented.

Free space optical communications (FSO) are beginning to provide attractive alternatives to fiber-based solutions in many situations. Currently, a handful of companies provide fiberless alternatives especially aimed at corporate intranet and sporting event video. These solutions are geared toward solving the 'last mile' connectivity issues. There exists a potential need to extend this pathlength to distances much greater than a 1 km, particularly for government and military applications. For cases of long distance optical propagation, atmospheric turbulence will ultimately limit the maximum achievable data rate. In this paper, we propose a method of improved signal quality through the use of adaptive optics. In particular, we show work in progress toward a high-speed, small footprint Adaptive Optics system for horizontal and slant path laser communications. Such a system relies heavily on recent progress in Micro-Electro-Mechanical Systems (MEMS) deformable mirrors as well as improved communication and computational components.

It has been shown that for optical communication receivers with large, signal-dependant noise components (multiplicative noise), the optimum detection threshold can be derived from a Bayes' Likelihood Ratio Test (LRT); however, the mean and variance of the bit levels must be known to obtain the order of magnitude bit-error-rate (BER) improvement over the typical matched filter type detector which assumes equal variances of the bit levels. In free-space communication systems, atmospheric conditions can cause variations in optical transmission and subsequently in the bit level means and variances. The bit level means and variances must be tracked and estimated and the detection threshold updated at a rate greater than the frequency of atmospheric changes, or the BER performance may actually be worse than that of the equal-variance threshold. Adaptive thresholding methods have been proposed and developed which track the bit means and variances and update the detection threshold to maintain near optimum performance. In this paper, simulated data based on actual optical receiver component characteristics and measured average received power data containing atmospheric turbulence induced fluctuations are used to test the tracking and BER performance of adaptive thresholding algorithms. The results of simulations comparing performance of three adaptive methods, maximum likelihood estimation/prediction, Kalman filter predictor/smoother, and a Least-Mean-Square (LMS) adaptive predictor, will be presented.

The worldwide demand for broadband communications is being met in many places through the use of installed single-mode fiber networks. However, there is still a significant 'first-mile' problem, which seriously limits the availability of broadband Internet access. Free-space optical wireless communication has emerged as a technique of choice for bridging gaps in the existing high data rate communication networks, and as a backbone for rapidly deployable mobile wireless communication infrastructure. Because free space laser communication links can be easily and rapidly redirected, optical wireless networks can be autonomously reconfigured in a multiple-connected topology to provide improved network performance. In this paper we describe research designed to improve the performance of such networks. Using topology control algorithms, we have demonstrated that multiply-connected, rapidly reconfigurable optical wireless networks can provide robust performance, and a high quality of service at high data rates (up to and beyond 1 Gbps). These systems are also very cost-effective. We have designed and tested on the University of Maryland campus a prototype four-node optical wireless network, where each node could be connected to the others via steerable optical wireless links. The design and performance of this network and the topology control is discussed.

The Johns Hopkins University Applied Physics Laboratory (JHU/APL) has conducted a feasibility study to determine if a high altitude (20 km) tethered balloon-based space-to-ground optical communication system is a feasible concept. To support this effort, a detailed concept definition was developed and associated issues were identified and analyzed systematically. Of all the adverse atmospheric phenomena, cloud coverage was identified as the most prohibitive obstacle for a space-to-ground optical communication link. However, by placing a receiver on a balloon at a 20 km altitude, the proposed high altitude system avoids virtually all atmospheric effects. A practical notional scenario was developed (i.e. surveillance and/or reconnaissance of a regional conflict) involving end-to-end optical communication architecture to identify system elements, system level requirements, and to quantify realistic data rate requirements. Analysis of the proposed space-to-ground communication elements indicates that while significant development is required, the system is technically feasible and is a very cost effective 24/7solution.

This paper describes a novel laser communications transceiver for use in multi-platform satellite networks or clusters that provides internal pointing and tracking technique allowing static mounting of the transceiver subsystems and minimal use of mechanical stabilization techniques. This eliminates the need for the large, power hungry, mechanical gimbals that are required for laser cross-link pointing, acquisition and tracking. The miniature transceiver is designed for pointing accuracies required for satellite cross-link distances of between 500 meters to 5000 meters. Specifically, the designs are targeting Air Force Research Lab's TechSat21 Program, although alternative transceiver configurations can provide for much greater link distances and other satellite systems. The receiver and transmitter are connected via fiber optic cabling from a separate electronics subsystem containing the optoelectronics PCBs, thereby eliminating active optoelectronic elements from the transceiver's mechanical housing. The internal acquisition and tracking capability is provided by an advanced micro-electro-mechanical system (MEMS) and an optical design that provides a specific field-of-view based on the satellite cluster's interface specifications. The acquisition & tracking control electronics will utilize conventional closed loop tracking techniques. The link optical power budget and optoelectronics designs allow use of transmitter sources with output powers of near 100 mW. The transceiver will provide data rates of up to 2.5 Gbps and operate at either 1310 nm or 1550 nm. In addition to space-based satellite to satellite cross-links, we are planning to develop a broad range of applications including air to air communications between highly mobile airborne platforms and terrestrial fixed point to point communications.

The National Aeronautics and Space Administration (NASA) continues to plan and anticipate the development of high data rate communications for future deep space missions. The Johns Hopkins University Applied Physics Laboratory is responding to this challenge by developing a breadboard laser transceiver package using commercial off-the-shelf components. We plan to demonstrate a breadboard transceiver unit, integrated with a fine pointing and tracking capability by the end of FY-03. A potential mission application is to ultimately demonstrate a live video link from Mars. Our near-term demonstration goals are to achieve a modest 5 Mbps data rate over an equivalent range of 2 AU. To achieve this we are modeling and testing the components for a hybrid analog/digital receiver in conjunctino with semiconductor laser diodes and silicon PIN and avalanche photodiodes. Our efforts leading up to hardware implementation and test have consisted of a trade-of between coherent and direct detection receiver architectures, and a link analysis for deep space applications, which established the laser power requirements for supporting a real-time video link from Mars as well as other missions, where the encoded bit error rate is from 10^-6 to 10^-9. Current efforts include the development of a direct-detection 4-ary pulse position modulation scheme using a FPGA-based modulator/demodulator as well as a separate quadrant photodiode receiver for tracking. We plan to integrate this transceiver with lightweight diffractive optical elements for beam-forming. The design and initial testing of the transceiver components will be discussed.

In this paper, the performance of a compact, eyesafe, all-solid-state, mid-wave IR (MWIR) transceiver for data communication through low visibility conditions is discussed. The transceiver was developed for Multiple Integrated Laser Engagement System (MILES) application. The MWIR wavelengths are derived using a passively q-switched Nd:YAG laser pumped periodically poled lithium niobate based optical parametric oscillator. MILES weapon code transmission for small and heavy weapon platforms have been demonstrated through dense theatrical fog. With 2 μJ/pulse at ~4 mm and a room temperature IR detector, greater than 5 km range has been successfully demonstrated. A bit map image transmission at MWIR wavelengths was also accomplished using this device. Test images consisting of 50x40 pixels and 100x80 pixels were successfully transmitted through free space.

The concept of a realistic interstellar explorer has been addressed by the Johns Hopkins University Applied Physics Laboratory with support from the NASA Institute for Advanced Concepts. This paper discusses the requirements, conceptual design and technology issues associated with the optical and RF communications systems envisioned for this mission, in which the spacecraft has a projected range of 1000 AU. Well before a range of 100 AU interactive control of the spacecraft becomes nearly impossible, necessitating a highly autonomous craft and one-way communications to Earth. An approach is taken in which the role of the optical downlink is emphasized for data transfer and that of the microwave uplink emphasized for commands. The communication system is strongly influenced by the large distances involved, the high velocities as well as the requirements for low-mass, low prime power, reliability, and spacecraft autonomy. An optical terminal concept is described that has low mass and prime power in a highly integrated and novel architecture, but new technologies are needed to meet the range, mass, and power requirements. These include high-power, 'wall-plug' efficient diode-pumped fiber lasers; compact, lightweight, and low-power micro-electromechanical (MEM) beam steering elements; and lightweight diffractive quasi-membrane optics. In addition, a very accurate star tracking mechanism must be fully integrated with the laser downlink to achieve unprecedented pointing accuracy. The essential optical, structural, mechanical, and electronic subsystems are described that meet the mission requirements, and the key features of advanced technologies that need to be developed are discussed. The conclusion from this preliminary effort is that an optical communications downlink out to 1000 astronomical units is within the realm of technical feasibility in the next 5-10 years if the identified technical risks for the new technologies can be retired.

The National Aeronautics and Space Administration is planning high data rate optical communications for future deep space missions. The Johns Hopkins University Applied Physics Laboratory (JHU/APL) is responding by developing concepts for implementing optical communications terminals that are more compact and lightweight than heretofore. An essential requirement for these long-range optical links is a high-precision pointing and tracking system. Focal plane array (FPA)-based star trackers that enable open-loop pointing and tracking are necessary. Spacecraft attitude instabilities, emphemeris errors, tracking sensor noise, clock errors, and mechanical misalignments are among the error sources that must be minimized and compensated for. To achieve this JHU/APL has developed an imaging star tracker concept using redundant multi-aperture FPA's symmetrically disposed about the laser downlink. Centroid estimation and pattern matching techniques account for aberration and motion errors. Robustness, sensitivity to detection thresholds, field-of-view sizing, number of stars per frame, missed detections, false alarms, and position biases, as well as stellar catalog size and star selection, will be described. Finally the conceptual design of a frame-to-frame integration method and sensor fusion algorithm (such as a Kalman filter) will be considered. The goal is to achieve a system pointing and tracking error significantly less than 1 μrad.

This work presents research in the field of laser satellite communication between a cluster of nano-satellites and a ground station. The scenario under consideration is a cluster of nano-satellites, communicating by means of a laser beam with a detector array receiver, which is located on the Earth's surface and equipped with a common optical system for all incoming beams. A critical parameter, determining the successful receipt of a transmitted signal for a given configuration, is the angular separation between the satellites within the cluster. This separation must be retained to prevent critical overlapping of the spots on the detector. The maximum allowable overlapping is calculated in terms of given BER. The spatial spreading of the beams, caused by scattering from aerosols in different layers of the atmosphere, is calculated for the case of single scattering, as appropriate for the stratified model used. Turbulence influences the beam width especially for the case of short exposure. In this research a new approach is adopted to characterize the atmospheric channel using OTF (Optical Transfer Function) concepts from the field of imaging and remote sensing. We evaluate the effectiveness of this new approach inapplications where spatial spread is very important, and detector array feasibility is currently under investigation.

This paper describes experimental results on the injection locking of high-power broad-area semiconductor lasers in a commercially available 19-laser array driven by a common current source. Single-frequency optical spectrum and single lobe far-field pattern are observed as a result of injection locking. We discuss the temporal dynamics, the amplification of the injection light, and the phase coherence between the injection-locked lasers, which are key issues in their applications to free-space laser communication.

The use of a 2-D optical phased array (OPA) steering antenna in microsatellite clusters is very attractive as it is lightweight and compact, and provides agile and inertia-free 2-D beam steering. In this work, we combined OPA theory with optical wireless communication system concepts to achieve a new perspective on potential technologies for microsatellite cluster communication. The optical wireless communication intersatellite link using 2-D integrated OPA steering antenna was modeled. Then we theoretically analyzed the proposed communication system and illustrated the model by a numerical simulation. Taking into account the phasing errors, the antenna's gain distribution statistics were derived from OPA theory by use of the Monte Carlo method. We then applied these results to make a broad analysis of an optical wireless communication system and to investigate the effect of OPA phasing errors on system performance.

The Army's objective is to design, develop and demonstrate its 'ability to distribute information around the battlefield.' Future Army systems will be based on a survivable, adaptable network capable of integrating commercial services and securely utilizing bandwidth for voice, data, and video applications. However, microwave bandwidth allocation has been a serious problem (given crosstalk, interference and frequency management) for a mobile, adaptive communication network. Because of the inherent advantages of the high data rate, crosstalk independence, jam - resistance, covertness and quick system setup time, the Army is looking into optical wireless communication as a means to address this communications requirement. However, development of a fielded laser communication system requires the development of enabling technologies, the understanding of physical limits and performance, and concept of operations (CONOPS).

We report on the performance characterization and issues associated with using Gigabit Ethernet (GigE) over a highly turbulent 1.3 km air-optic lasercom links. Commercial GigE hardware is a cost-effective and scalable physical layer standard that can be applied to air-optic communications. We demonstrate a simple GigE hardware interface to a single-mode fiber-coupled, 1550 nm, WDM air-optic transceiver. TCP/IP serves as a robust and universal foundation protocol that has some tolerance of data loss due to atmospheric fading. Challenges include establishing and maintaining a connection with acceptable throughput under poor propagation conditions. The most useful link performance diagnostic is shown to be scintillation index, where a value of 0.2 is the maximum permissible for adequate GigE throughput. Maximum GigE throughput observed was 49.7% of that obtained with a fiber jumper when scintillation index is 0.1. Shortcomings in conventional measurements such as bit error rate are apparent. Prospects for forward error correction and other link enhancements will be discussed.

An infrared or optical signal propagating along a line-of-sight horizontal path near the earth's surface can encounter substantial perturbations. There are two prominent factors which can generate fluctuations: first, refractive distortions are low-frequency modulations which can amplify or reduce a signal, and second, scintillation is a higher frequency fluctuation in signal intensity. We will discuss models developed to predict these effects, and associated field test efforts to corroborate and correct the model predictions. The field efforts include tests along horizontal near-surface paths over land and over the coastal ocean surface. Previous transmission field tests have revealed that slow-scale refractive effects can create very pronounced changes in the recorded one-minute average intensity of a source. We will show the results of an analysis of this signal based upon wavelet transforms and filtering. We focus on the detection of a detectable signature frequency of the variations in signal intensity that is based upon the Fresnel zone size. This Fresnel frequency is correlated with the location of the Kolmogorov power law scaling.

Two methods of measuring atmospheric turbulence as it affects free space optical communications are presented. Each method yields a value of the structure constant of refractive index fluctuations, C_n². A scintillometer is used as the basis or 'truth' for measurements taken by the first method by fitting data from the other instrument to the simultaneous scintillometer data. The first method utilizes a device conceived at New Mexico Tech using microphones to measure a pressure differential. This device was altered to provide both pressure and temperature measurements at two points separated by a specific distance. A thermocouple was added beside each microphone to provide temperature data, and the data collection method was altered. The device currently measures two pressure and temperature gradients. Also, the Naval Research Laboratory conducted the first efforts to quantify and calibrate the data collected by the device. Second, measurements are made from the angle of arrival of light from a laser transmitted across 16.4 km of the Chesapeake Bay. The variance of the angle of arrival over time is obtained from the variance of the centroid location of the focused light on a position-sensing detector. The same measurement is made over the same path using a halogen spotlight, a CCD camera, and a video tracker. The angle of arrival variance is used to calculate C_n². The microphone/thermocouple apparatus took measurements over land alongside the scintillometer. Results from both methods are provided.

Until recently atmospheric dust was not a major concern for free-space optics communication systems, however the heavy pollution and the recent increase in dust-storm events in Asia and particularly in China, has motivated the studies we present in this paper. In this paper we will discuss the impact of dust on free-space optics system operations for transmission wavelengths up to 10μm. We focus our studies on China as dust is normally present in its lower atmospheric regions, and since dust originating in China is transported over most of Asia during heavy dust storms. In general, heavy dust loadings consists of larger dust particles that are lifted from the ground and whose atmospheric concentrations can reach values that are high enough to interrupt the operation of free-space optics communication links. Similar to the characterization of fog conditions, dust storm studies are usually based on visibility data. However, in the case of mineral dust, visibility data are not sufficient to accurately predict the impact of dust on atmospheric transmissions. Transmission coefficients during dusty days cannot be simply calculated from visibility data for the entire range of useful laser wavelengths due to complex wavelength dependencies on mineral dust refractive indices. In order to test laser operating conditions in China, we ran extensive MODTRAN calculations of atmospheric transmissions for different dust conditions in the visible-IR range. Wavelength-dependent optical properties of atmospheric dust, which are required as input to the MODTRAN dust models, were calculated via Mie theory (spherical dust) using representative dust morphological and mineralogical properties such as refractive indices, size distributions and concentrations. We also tested an effect of particle nonsphericity at several wavelengths. We found that the effect of non-sphericity is small for direct beam transmissions, thought, it may become an important factor for diffused forward-scattered radiation. We found that changes in dust particle concentrations and dust particle size distributions during dust-storms storngly affect the transmission in the entire visible and IR wavelength range.

We present experimental results for the adaptive compensation of atmospheric turbulence effects on a free-space laser communication links at near horizontal propagation paths over 2.5 km and 5 km lengths. A high-resolution micro-machined piston type mirror array (12x12 elements) and a fast beam steering mirror were used in an adaptive optics laser communication system based on the model-free stochastic parallel gradient descent (SPGD) optimization wavefront control technique. Control of the mirror was performed by a VLSI SPGD micro-controller. The experimental results demonstrate the improvement of the receiver performance (fiber coupling efficiency) on a summer day with a refractive index structure constant in the order of C_n²≈10^-14 m^-2/3.

Atmospheric scintillation negatively affects point-to-point laser communication requiring either an increase in power to maintain link throughput rate or decreasing the data rate at a constant power. The effect of atmospheric scintillation on irradiance can be modeled using a gamma-gamma probability distribution. With the model of irradiance known, the Poisson transform of the irradiance is determined. Using the Poisson transform of the gamma-gamma distribution, the probability of word error is calculated for a pulse-position modulation (PPM) receiver. The word error probability for M-ary PPM decoding is then plotted with respect to varying turbulence parameters.

Regularities of appearance and evolution of optical vortices (wave-front dislocations) in the light field of laser beam in the inhomogeneous refractive medium are studied with the use of simple field models. We associate a dynamic system for the intensity and phase transformation with the parabolic equation describing the light propagation. Redistribution of energy fluxes when appearance and annihilation of the dislocations is considered in the phase space of this system. The hydrodynamics approach to the dislocation problem based on an analogy of the phase gradient vector field to the liquid flow vector field is developed in the paper. Novel method to calculate the dislocation statistical characteristics in the turbulent atmosphere is proposed on the base of the before-derived equation connecting the phase vortex density and intensity spatial distribution.

We describe the development of a quantum key distribution (QKD) scheme based on ultrafast laser pumped sources of entangled photon pairs and the engineering of their entanglement properties. Though quantum entanglement has been shown to be a useful resource for quantum key distribution, little work has been carried out in making use of the full range of joint entanglement behavior present in hyper-entangled photon pairs. We consider the principal advantages of our QKD scheme in connection with the way it makes use of ultrafast laser pumped spontaneous parametric down-conversion and hyper-entanglement. In particular, we consider how polarization quantum interference may be modified by manipulating the spatial features of the down-converted light.

The quantum state of the photon pair generated from type-II spontaneous parametric down-conversion pumped by a ultrafast laser pulse exhibits strong decoherence in its polarization entanglement, an effect which can be attributed to the clock effect of the pump pulse or, equivalently, to distinguishing spectral information in the two-photon state. Here, we discuss novel spectral engineering techniques to eliminate these detrimental decoherence effects. In addition, spectral engineering provides a means for generating polarization entangled states with novel spectral characteristics: the frequency-correlated state and the frequency-uncorrelated state. Such states may find usefulness in experimental quantum information science and quantum metrology applications.

Parametric down conversion permits the generation of entangled photon pairs. However, the production rate is unfortunately very low, typically with nine to ten orders of magnitude between input and output power. A combination of approaches is considered to significantly enhance the overall prodcuiton rate. The use of large crystals simply improves the production rate by increasing the interaction length, as does the use of beam-folding optics. Since the produced entangled photon pairs have twice the wavelength of the pump beam, the use of 'hot' and 'cold' mirrors can be used to redirect unused pump power back into the crystal. Analysis of these approaches is used to indicate a potential improvement of six orders of magnitude. Practical design limitations such as the rejection of waste heat, currently available crystal dimensions, and differential walkoff of correlated photons due to birefringence are considered. Application to type I and type II parametric down conversion and walkoff compensation techniques are detailed. Application to an entangled optical communication link and its use over astronomical distances are outlined.

One of the most surprising consequences of quantum mechanics is the entanglement of two or more distance particles. The 'ghost' image experiment demonstrated the astonishing nonlocal behavior of an entangled photon pair. Even though we still have questions in regard to fundamental issues of the entangled quantum systems, quantum entanglement has started to play important roles in practical applications. Quantum lithography is one of the hot topics. We have demonstrated a quantum lithography experiment recently. The experimental results have beaten the classical diffraction limit by a factor of two. This is a quantum mechanical two-photon phenomenon but not a violation of the uncertainty principle.

A new quantum optics tool for simulating quantum probability density functions resulting from the linear and nonlinear interaction of photons with atoms and with other photons is developed and presented. It can be used to design and simulate quantum optics experiments used in quantum communications, quantum computing, and quantum imaging. Examples of a photon interacting with linears systems of mirrors and beamsplitters are simulated. Nonlinear simulations of the interaction of three photons resulting in photon momentum entanglement is presented. The wavefunction is expanded in Fock states. Fock states cannot be represented by classical modeling and therefore, the results of our modeling can in general represent phenomena in both the linear and nonlinear cases which cannot be modeled by classical linear optics. The modeling presented here is more general than the classical linear optics. Models of atmospheric turbulence and their simulations are presented and demonstrate the potential for first principles physics quantum optics simulations through turbulence in realistic environments.

In this paper, we present a proof-of-concept experimental demonstration of the secret key quantum cryptographic scheme. A tabletop communication link was set up in the free-space channel using ordinary lasers as transmitters, which emit coherent states of light, and quantum-limited direct detection was employed in the receivers. In the secret key scheme, one needs a supply of M possible quantum states that are uniformly distributed over some random variable. In the free-space case, we used polarization angle as the variable determining the state. In the proof-of-concept emonstration, we aimed towards sending data messages encrypted with a short secret key from the transmitter to the receiver. The messages could be successfully deciphered by the receiver by its knowledge of the secret key. However, when the secret key was taken away, in order to mimic an eavesdropper, the messages could not be deciphered.

We describe the status of the NIST Quantum Communication Testbed (QCT) facility. QCT is a facility for exploring quantum communication in an environment similar to that projected for early commercial implementations: quantum cryptographic key exchange on a gigabit/second free-space optical (FSO) channel. Its purpose is to provide an open platform for testing and validating performance in the application, network, and physical layers of quantum communications systems. The channel uses modified commercial FSO equipment to link two buildings on the Gaithersburg, MD campus of the National Institute of Standards and Technology (NIST), separated by approximately 600 meters. At the time of writing, QCT is under construction; it will eventually be made available to the research community as a user facility. This paper presents the basic design considerations underlying QCT, and reports the status of the project.

The performance of nondeterministic nonlinear gates in linear optics relies on the photon counting scheme being employed and the efficiencies of the detectors in such schemes. We assess the performance of the nonlinear sign gate, which is a critical component of linear optical quantum computing, for two standard photon counting methods: the double detector array and the visible light photon counter. Our analysis shows that the double detector array is insufficient to provide the photon counting capability for effective nondeterministic nonlinear transformations, and we determine the gate fidelity for both photon counting methods as a function of detector efficiencies.

A synchronized broad-area laser array (SBLA) can produce a highly coherent light source whose output intensity is proportional to the square of the number of lasers in the array. High contrast optical intensity modulation can be achieved by utilizing the nonlinear response of the total output intensity to the injected light, resulting in fast optical switching. SBLA can be applied to phased array antenna and beam steering. Semiconductor laser array capability provides a unique opportunity not only for free-space adaptive optical communication but also for free-space quantum adaptive optical communications through the atmosphere. In this paper, we propose a paradigm that illustrates how quantum communication (which provides quantum ultra-security) can take advantage of ultrashort pules, high repetition rate, high power density (due to coherent beam coupling), and spatial beam control.

We report our studies on the performance of new NbN ultrathin-film superconducting single-photon detectors (SSPDs). Our SSPDs exhibit experimentally measured quantum efficiencies from ~ 5% at wavelength λ = 1550 nm up to ~10% at λ = 405 nm, with exponential, activation-energy-type spectral sensitivity dependence in the 0.4-μm - 3-μm wavelength range. Using a variable optical delay setup, we have shown that our NbN SSPDs can resolve optical photons with a counting rate up to 10 GHz, presently limited by the read-out electronics. The measured device jitter was below 35 ps under optimum biasing conditions. The extremely high photon counting rate, together with relatively high (especially for λ > 1 μm) quantum efficiency, low jitter, and very low dark counts, make NbN SSPDs very promising for free-space communications and quantum cryptography.

As currently implemented, single-photon sources cannot be made to produce single photons with high probability, while simultaneously suppressing the probability of yielding two or more photons. Because of this, single photon sources cannot really produce single photons on demand. We describe a multiplexed system that allows the probabilities of producing one and more photons to be adjusted independently, enabling a much better approximation of a source of single photons on demand. The scheme uses a heralded photon source based on parametric downconversion, but by effectively breaking the trigger detector area into multiple regions, we are able to extract more information about a heralded photon than is possible with a conventional arrangement. This scheme allows photons to be produced along with a quantitative 'certification' that they are single photons. Some of the single-photon certifications can be significantly better than what is possible with conventional downconversion sources, as well as being better than faint laser sources. With such a source of more tightly certified single photons, it should be possible to improve the maximum secure bit rate possible over a quantum cryptographic link. We present an analysis of the relative merits of this method over the conventional arrangement.

We report the realization of the first electrically-driven single photon source. The device is based on a GaAs p-i-n diode containing self-assembled InAs quantum dots embedded within the i-region. Time resolved electroluminescence from the device yields information on the radiative lifetimes of the single quantum dots involved. Applying a continuous current a dip is measured in the second-order correlation function at zero time delay indicating that the luminescence is antibunched. Under pulsed excitation by subnanosecond voltage pulses, strong suppression of multi-photon pulses is achieved with respect to a laser of the same average intensity. The results suggest that conventional semiconductor technology can be used to mass-produce a single photon source for applications in quantum information technology.

The Naval Research Laboratory (NRL) has established a free-space laser communication link across 16.2 km of the Chesapeake Bay between the Chesapeake Bay Detachment of NRL and Tilghman Island. The transmitter consists of a modulated 1550 nm oscillator amplified to 2 watts in an erbium doped fiber amplifier developed at NRL. The beam is fiber coupled to a 4 inch collimating lens on a remotely controllable gimbal mount. The beam is transmitted to a retro-reflector array at Tilghman Island and back to the receiver at CBD (32.4 km round trip). The receiver consists of a 16" Meade telescope either directly or fiber coupled to a variety of fast photo-detectors. Experiments have been conducted to study the stability and quality of the link. These include: bit-error rate measurements, probability density functions, power spectrum densities, and angle of arrival measurements of the received signal. Results of these experiments are presented.

Progress in the last decade has enabled the generation and detection of both single and entangled photons for use in new applications of quantum cryptography, secure key distribution in particular, Nevertheless, fundamental and practical restrictions restrict the implementation to protoype systems. Methods of circumvent certain of those are presented in configurations that retain the features essential to single photon and photon pair signal processing.

Fog and low clouds are the two atmospheric elements with the greatest impact on the performance of a free space optical (FSO) network. Predicting the effects of low clouds and ground based fog on FSO equipment performance is a challenging exercise. Usually, surface visibility records from airports in proximity to the deployment area are used to calculate the link availability. However, very little data are available on visibility within clouds, which have a larger impact on elevated links. To estimate the visibility in low clouds we have deployed visibility sensors at three different heights (33, 119, 188 meters above mean sea level) and a ceilometer in San Francisco from June to October of 2001. The data collected show substantial difference between the visibility reported at San Francisco International Airport (SFO) and the visibility recorded by our sensors in downtown San Francisco. More importantly, the data indicate a greater prevalence of low clouds downtown than at the airport.

This paper is an update in the progress of the development of NRL's Multiple Quantum Well retromodulators for compact, low power communications. We report results for data-in-flight on a small, unmanned aerial vehicle at up to 5 Mbps, in preparation for real-time video transfer using an array of devices. This data was taken at Chesapeake Bay Detachment. We also report transference of color video using wavelet compression at 15 and 30 frames per second, at 4 to 6 Mbps in lab, at eye safe intensity levels. The unit is a cornercube modulator using a 980 nm shutter. A five-element array was used for the data-in-flight. First results of our 1550 nm devices are also presented as is progress in a 'Cats Eye Retromodulator'.

This paper describes a novel concept for optical interrogation, communication, and navigation between spacecraft platforms. The technique uses a gimbaled laser source on a pursuer spacecraft and an array of solid-state, multiple quantum well modulating retroreflectors on a target spacecraft. The sensor system provides high-bandwidth optical communication, centimeter-level relative positioning, and better than arc-minute-level relative attitude of the target platform with minimal sacrifice in target size, weight, and power. To accomplish the relative navigation, each target retroreflector return is modulated with a unique code sequence, allowing for individual discrimination of the detected composite signal at the pursuer location. Experimental results using a dual-platform, multi-degree-of-freedom testbed provide verification and demonstration of the concept, highlighting its potential for applications such as inter-spacecraft rendezvous and capture, long-baseline space interferometry, and formation flying.

Presented is a new revolutionary free-space optical communication sensor mount. Featuring 70 arcseconds average repeatability, this gimbal-like pointing mechanism provides over 180 degrees azimuth and declination singularity-free pointing capability for a wide range of sensors for the entire electromagnetic spectrum. Applications include air, sea, and space as well as land-based vehicles.