The use of lasers in Army communications has many advantages, such as a broad bandwidth, covertness, reduced weight and size, freedom from allocating frequencies, and difficulty of intercept or jamming. The major shortfalls of using lasers are the need for line-of-sight and the potential for atmospheric degradation. The line-of-sight issue becomes a problem when either the transmitter or receiver are moving. In these instances, there is a need to develop a means of automatically acquiring and locking onto other laser devices to assure intra-beam communications. The Steered Agile Beams (STAB) program is designed to develop this technology. This presentation highlights how the STAB technology is being prepared for transition to the services.
The Army is in the midst of a transformation to design a force to meet the needs of the 21st Century. The force will have the capability to rapidly deploy in a bridge size unit. The transformation will require significant changes in the force structure that will imply the need for robust secure communications with much increased bandwidth. The Army Research Laboratory has installed an Atmospheric Laser Optics Testbed to meet the challenges of the Objective Force communications needs. The testbed has a 1.7-mile free space path to test and evaluate laser communications systems and imaging systems. The testbed became operational in June 2001. Both traditional radio frequency and optical communications will be required in the future force. The importance technology challenge is to find right proportion and architecture for integration of new optical communications technology within the existing and future radio frequency networks.
Laser communication systems offer several advantages over conventional radio frequency (RF) systems but, because of shorter wavelength, are subject to various atmospheric effects. Particularly significant in this regard is the signal fading below a prescribed threshold value owing primarily to optical scintillations associated with the received signal. Over terrestrial paths of 1 - 3 km, or at large zenith angles between the transmitter and receiver in an uplink/downlink channel, the intensity fluctuations can easily exceed the limitations imposed by weak fluctuation theory. Under strong conditions the intensity fluctuations can no longer be modeled by a lognormal distribution - instead, we find the gamma-gamma distribution to be an excellent model over virtually all conditions of irradiance fluctuations. In this paper we discuss some recent advances in the modeling of optical scintillation under weak-to- strong fluctuations associated with both terrestrial links and satellite/ground links. The analysis presented here specifically addresses scintillation effects on detector signal-to-noise ratio (SNR) and on related fading probability and error probability or bit error rate (BER). We also discuss the use of multiple aperture receivers to mitigate the effects of optical turbulence.
Significant progress has been made over the last 15 years in our understanding and development of laser imaging systems for observing space objects from the ground. We review theoretical and experimental work on three techniques that have received much of the attention: imaging correlography, sheared-beam imaging and Fourier telescopy. We summarize signal-to-noise analyses that account for low-light levels and speckle noise, we discuss atmospheric turbulence compensation attributes, and we reference work on issues specific to each technique including measurement noise, effects of partial coherence and wave front reconstruction. Laboratory results are summarized and their impact on our understanding of the techniques is discussed. A brief discussion of field experiment programs is presented.
We investigate a speckle sensitivity of the receiver in the Fourier telescopy imaging system that has been proposed for high-resolution imaging of Geostationary (GEO) satellites using laser illumination. A theory and numerical results are presented for a speckle bias of the triple-product, which is used in a phase-closure-procedure to remove turbulence- induced low frequency phase distortions on the uplink propagation path. We show that in the far zone of the object and far zone of the turbulence coherence scale, atmospheric turbulence degrades spatial coherence of a reflected field and increases speckle averaging by the receiver. This reduces speckle bias of the phase-closure triple-product by several orders of magnitude and thus reduces speckle sensitivity of the receiver. The Van Cittert-Zernike theorem generalized to randomly inhomogeneous medium is presented and applied to interpretation of the results obtained. Two competing processes, diffraction of the illuminated beam on the satellite and turbulence-induced wavefront distortions of a speckle field in the atmosphere determine the size of the spatial coherence scale at the receiver. On one hand, the coherence scale of a speckle field increases with the distance due to diffraction on the satellite. On the other hand, it decreases due to turbulence-induced wavefront distortions. For the propagation scenario corresponding to a GEO satellite, the second process predominates. Coherence degradation of a speckle field caused by turbulence reduces speckle sensitivity of the receiver in the Fourier telescopy system. Experimental data that validate the generalized Van Cittert Zernike theorem in the turbulent atmosphere are reviewed.
Various schemes have been investigated that facilitate the imaging of objects viewed through time-varying scattering elements such as the atmosphere. Analyses we have performed in connection with enhanced-backscatter imaging and a particular class of superresolving imaging systems have led to a general principle that describes the transfer of the coherence properties of a wavefield that passes twice (double-passage) through a dynamic scatterer. Restoration of the coherence properties of the wave allow the formation of images with light that has been doubly scattered. Analyses are presented for the enhanced-backscatter, superresolution, and general case, and brief remarks presented regarding the possible application of the principle to remote image formation through the turbulent atmospheric.
We investigate the effects of turbulence on pupil-plane speckle imaging system that has been proposed to provide high-resolution images of a low Earth orbit (LEO) satellite using laser illumination. We present a theory and numerical results for the normalized speckle covariance for one- dimensional object in the turbulent atmosphere and demonstrate that turbulence-induced anisoplanatism and scintillation degrades this characteristic. The effect of anisoplanatism is associated with the finite size of a satellite. Due to a finite satellite dimension, optical waves arrive at the receiver from different directions determined by the angular size of the satellite and sample different turbulence. The resulting phase difference degrades the normalized speckle covariance. This effect is similar to the degradation of the performance of adaptive optics system caused by turbulence-induced anisoplanatism when a reference beacon is separated from the target at some distance. Scintillation on the downlink path also degrades the normalized speckle covariance. It causes the dc- and ac- components of the speckle correlation to exceed their values in a free space when the separation between the observation points is smaller than the spatial correlation scale of the scintillation. At the same time, both components are decreased for large separations when scintillation is uncorrelated. This reduces the normalized speckle covariance and can degrade the performance of a pupil-plane speckle imaging system. The effect of scintillation on the dc- and ac-components of the speckle correlation is similar to the enhanced backscatter phenomenon, in which the average power of the laser return in the strictly backward direction in the turbulent atmosphere exceeds its value in free space. For a small object, the effect of scintillation on the speckle covariance predominates. The speckle statistics for the space-to-ground scenario differ from that for the horizontal path.
The GEO Light Imaging National Testbed (GLINT) will use three laser beams producing simultaneous interference fringes to illuminate satellites in geosynchronous earth orbit (GEO). The reflected returns will be recorded using a large 4,000 m2 'light bucket' receiver. This imaging methodology is termed Fourier Telescopy. A major component of the 'light bucket' will be an array of 40 - 80 heliostats. Each heliostat will have a mirrored surface area of 100 m2 mounted on a rigid truss structure which is supported by an A-frame. The truss structure attaches to the torque tube elevation drive and the A-frame structure rests on an azimuth ring that could provide nearly full coverage of the sky. The heliostat is designed to operate in 15 mph winds with jitter of less than 500 microradians peak-to- peak. One objective of the design was to minimize receiver cost to the maximum extent possible while maintaining GLINT system performance specifications. The mechanical structure weights approximately seven tons and is a simple fabricated steel framework. A prototype heliostat has been assembled at Stallion Range Center, White Sands Missile Range, New Mexico and is being tested under a variety of weather and operational conditions. The preliminary results of that testing will be presented as well as some finite element model analyses that were performed to predict the performance of the structure.
The use of segmented mirrors for the primary elements of large telescope systems is becoming increasingly popular owing to the cost effectiveness of the design. Likewise, the use of non-optical flat glass in large collectors can further reduce cost. Unfortunately, both non-optical flatness and segment misalignment do result in phase errors. We have used a computer simulation to study the effects of these phase errors on the polarization state of an optical beam. We report herein on the effects of surface warping and of segment piston and tilt on the Stokes parameters of an optical beam reflected from a segmented mirror.
Experiments were conducted during four days in January and February 2001 at Tucson AZ, to measure the performance of an optical wireless link between two telescopes 8.7 km apart. The transmission rate was OC-3 (155 Mbps) and the maximum total radiated power was 29 dBm. Extreme fluctuations in the received 1550-nm beam were observed, and these occasionally caused error bursts. The error bursts had frequencies (a maximum of 12 per hour) and durations (mostly < 1 sec.) That were low enough to permit data transmission, e.g. via Ethernet TCP/IP. Fluctuations in the power of the 1550-nm received beam were positively correlated with variations in the elevation centroid. No correlations with meteorological measurements were found.
In case of fog ground to ground free space link with a laser is almost impossible because of the strong attenuation. In principle this event is relatively rare and the usual solution is to provide an alternate radio link in such a situation. However, in several practical cases, optical links for the 'last mile' are required in order to avoid to obtain licensing for a radio channel from local authorities. Usually, however, the fog has a vertical thickness that is very limited. It is not uncommon, for instance, that in foggy nights one can easily see 'stars' while a direct vision over the ground is completely forbidden. In this case one can send a modulated beam 'onto the sky' and recover it from its side by means of Rayleigh scattering. In order to achieve competitive data rate, however, one is forced to make on-ship fast tracking of the pulse train, in order to accumulate signal for a significant range of the Rayleigh beacon. A preliminary photon budget shows that such an approach can give interesting data rate with reasonable laser power and affordable optics. A preliminary discussion of potential drawbacks and circumventing possibilities is also reported.
Laser satellite communication is one of the most promising methods of communication outside the earth's atmosphere. In the continuing quest to optimize atmospheric optical wireless communication, arrays of photodetectors are replacing solitary photodetectors in receivers, affording the advantages of the small fast photodiode while effectively increasing the receiver aperture. Thus, power dispersed by atmospheric turbulence and scattering may be collected by the enlarged receiver area, and high BER, caused by low received power, can be decreased. We propose a mathematical model, which can be used to improve the data processing from detector photocurrent by incorporating thoroughly researched concepts from optical imaging theory such as atmospheric turbulence and aerosol optical transfer functions. This model forms the basis of an analytical tool, which will help in the implementation of smart detector arrays for WDM communication systems.
In free-space optical links using intensity modulation and direct detection, atmospheric turbulence can cause signal fading. Pilot-symbol (PS) assisted modulation (PSAM) can help to mitigate this fading, improving system performance. We periodically insert an On-state PS in front of M-1 information bits and form a M-bit frame. We derive the PS assisted maximum-likelihood (PSA-ML) decision rule under the assumption that the temporal coherence of fading is known, but the instantaneous fading state is not known. We also propose a simpler PS assisted detection scheme with variable threshold (PSA-VT), Although these two techniques will introduce some delay to the detection system, they can help to mitigate the atmospheric turbulence induced fading. We have performed numerical simulations to show this improvement. We also describe how to choose the frame size M to optimize the performance of PSAM systems.
It is well known that laser beams spread as they propagate through free space due to natural diffraction, and that there is additional spreading when optical waves propagate through atmospheric turbulence. Previous studies on Gaussian beams have mainly involved the lowest order mode (zero- order). The study of higher order mode Gaussian beams has involved Hermite-Gaussian and Laguerre-Gaussian beams for rectangular and cylindrical geometry respectively. These studies have developed expressions for the field and intensity in free space in addition to developing new definitions of beam size in the receiver plane for the higher order modes. In this paper we calculate the mean intensity of higher order mode Gaussian beams propagating through atmospheric turbulence, and, based on previously developed definitions for beam radius, we calculate the additional beam spreading due to random media. It is shown that higher order mode Gaussian beams experience less percentage of additional broadening due to atmospheric fluctuations than the zero order mode beams.
Coherent light launched into an atmospheric channel experiences substantial degradation in spatial coherence as it traverses the channel, resulting in beam wander and intensity scintillation at the receiver. Launching partially coherent light with well-defined 'beam-like' properties into the channel may exploit the spatial and frequency diversity properties of the atmospheric path so that acceptably low levels of system bit error rate performance can be obtained at fiber-optic data rates. To better understand the properties of partially coherent light in an atmospheric channel, here we develop an analytic expression for the second moment of a partially coherent lowest-order Gaussian beam propagating in the presence of isotropic, homogenous atmospheric turbulence. From the second moment equation expressions for the average intensity, beam size, complex degree of coherence, and lateral coherence length are derived. These results are valid for any beam type: focused, collimated, divergent, and the limiting cases of the plane and spherical wave. Expressions obtained here exactly reduce to the diffractive equivalents for a fully coherent beam when full coherence and the absence of turbulence are assumed. In addition, results correspond to expressions obtained previously for a partially coherent collimated beam in the absence of turbulence.
The Mesoscopic MEMS (MicroElectroMechanical Systems) technology developed at UIC allows the fabrication of structures not possible with conventional planar thin film patterning methods. These techniques enable the fabrication of an agile micro-mirror that can rapidly tip and tilt by large angles in two independent directions with a small footprint on the substrate. The mirrors can be electrostatically deflected, and rotate around a spherical pivot that is a drop of a conducting liquid. The drop can be forced to spread by applying a small voltage to an electrode surrounding the drop and this provides piston motion for the third degree of freedom. The drop is confined to a lithographically defined wetting area on the mirror and the substrate surfaces. The drop provides a surface tension restoring force to balance the electrostatic torque, as well as electrical and thermal conduction between the mirror and the substrate. The fabrication method uses aligned shadow masks to deposit electrodes on a non-planar substrate. The fabrication requires precision dispensing of approximately 10 pL liquid drops using inkjet printing technology.
We describe an adaptive optical fiber coupling system for free- space optical communication comprising a micro-electromechanical deformable mirror and a VLSI gradient descent controller for model-free performance optimization. A comparison of Strehl ratio maximization with direct model-free coupling efficiency optimization revealed an advantage of the latter method.
Modulating retro-reflectors provide means for free space optical communication without the need for a laser, telescope or pointer tracker on one end of the link. These systems work by coupling a retro-reflector with an electro- optic shutter. The modulating retro-reflector is then interrogated by a cw laser beam from a conventional optical communications system and returns a modulated signal beam to the interrogator. Over the last few years the Naval Research Laboratory has developed modulating retro-reflector based on corner cubes and large area Transmissive InGaAs multiple quantum well modulators. These devices can allow optical links at speeds up to about 10 Mbps. We will discuss the critical performance characteristics of such systems including modulating rate, power consumption, optical contrast ratio and operating wavelength. In addition a new modulating retro-reflector architecture based upon cat s eye retroreflectors will be discussed. This architecture has the possibility for data rates of hundreds of megabits per second at power consumptions below 100 mW.
With the increasing interest in laser satellite communications, new methods are sought to solve the existing problems of accurate and rapid laser beam deflection. Current solutions in the form of galvanometers or piezo fast steering mirrors with one or two degrees of freedom are bulky, power-consuming and slow. The Multi-Quantum Well (MQW) is a semiconductor device with unique potential to steer laser beams without any moving parts. We have conducted a preliminary evaluation of the potential application of the MQW as a laser beam-steering device for laser satellite communication, examining the performance of critical parameters for this type of communications.
Free space laser communication between satellites networked together can facilitate high-speed communication between different places on earth. The advantages of an optical communication system by comparison with a microwave communication system in free space are: a) smaller size and weight, b) less transmitter power, c) larger bandwidth, d) higher immunity to interference, and e) smaller transmitter beam divergence. The use of optical radiation as a carrier between the satellites engenders very narrow beam divergence angles. Due to the narrow beam divergence angle and the large distance between the satellites, the pointing from one satellite to another is complicated. The problem is further complicated due to vibrations of the pointing system caused by two fundamental mechanisms, stochastic in nature; 1) tracking nose created by the electro-optic tracker and 2) vibrations created by internal and external mechanical mechanisms. The vibrations displace the transmitted beam and the receiver field of view with respect to one another. Such movement decreases the average received signal, and increases the bit error rate (BER). In this paper we will review five methods to mitigate the effect of vibrations on laser satellite communication system. The methods are a) receiver with adaptive detector arrays, b) Bandwidth/data rate/coding rate adaptation, c) Power minimization using adaptive beam-width, d) Communication diversity within the satellite network, and e) Power control.
Applied Technology Associates' ARS-12 series sensor is the most sensitive inertial angular vibration sensor currently available. The sensing mechanism is based on magnetohydrodynamic (MHD) principles. This sensor series has a nominal bandwidth from 1 - 1000 Hz and a noise-equivalent angle of less than 50 nanoradians from 2 - 1000 Hz. The ARS- 12 can measure inertial angular motions of less than 10 nanoradians at discrete frequencies. Their solid-state design makes these sensors small and extremely rugged. In addition, the ARS-12 is essentially impervious to linear acceleration and angular cross-axis sensitivity is limited to incorrect physical alignment. The ARS-12 has recently undergone several design changes in order to perform in the space environment and improve performance. The most recent sensor package, the AADS-002, contains three orthogonal ARS- 12 sensors that produce a signal voltage proportional to displacement directly instead of the inherent velocity domain. The AADS-002 design, testing, and performance will be reviewed in this paper.
Laser satellite communication is one of the most promising systems of communication outside the earth's atmosphere. The communication line of sight is achieved by steering a very narrow transmitter laser beam in the direction of the receiver satellite, while simultaneously directing the narrow field of view (FOV) receiver telescope in the transmitter satellite direction. In order to communicate efficiently, steering direction estimation methods, using a priori procedures such as acquisition and identification, are required. The acquisition procedure estimates the direction of a satellite while the identification procedure identifies the specific satellite within the flight formation cluster. We use an electro optic detector matrix, a bank of tunable band pass filters, envelope detectors and a decision unit in our proposed system. Each of the satellites transmits an optical beacon modulated with a different RF frequency. The mathematical model takes into consideration the noise, the signal, the Doppler shift and the field of view of the system of each satellite in the cluster. In this paper we develop an acquisition algorithm scheme of the system and a mathematical model of the probability of missing the satellite and of erroneous identification.
We present a new concept to link two ground-based optical stations through a satellite. In our approach the optical relay in orbit is a completely passive one, with no requirements in terms of active control attitude and with a more simple opto-mechanical design. The relay acts as a reference source for the ground stations, equipped with an adaptive optics system to enlarge the photon return and the bandwidth of the transmitted signal. The technological requirements are mainly on the ground side, then technological up-grades are easily obtainable and less cost- demanding.
A free space Gigabit Ethernet optical communication system is proposed in this paper. We experimentally transmit medical image though the Gigabit Ethernet optical-fiber communication system with a multiple quantum well semiconductor optical applier (MQW-SOA). The beam divergence of the light free space transmission is also analyzed. The bit error rate (BER) formula as derived with considering the free space transmission beam divergence. To increase the transmission distance of Gigabit Ethernet optical-fiber communication system, we use the optical amplifier as a booster amplifier. We study the divergence angle of beam below 0.05 rad. And the relation between SNR and BER is also considered.
Atmospheric turbulence produces scintillation at an optical receiver, which leads to fading of the received signal. This fading affects the bit-error-rate (BER) of a digital signal in a way that depends on the depth of the fade, the decision threshold at the receiver, and the average signal-to-noise ratio. The degree of fading can be dramatically reduced by the use of a time-delayed diversity technique, which involves retransmission of the data stream after a short delay, and resynchronization of the received data streams.
Free space optical wireless communication is an attractive way of connecting vast numbers of urban area customers to the fiber optic communication network. We have designed and tested a prototype 2 km long 1.2 Gb/s optical wireless link operating at 1550 nm. An EDFA amplified signal from a standard fiber optic transmitter unit was sent via a small telescope to a 5 inch corner cube mounted on the roof of a building located over 1 km from the transmitter. An estimated 10 mWatt incident on the corner cube was reflected back to the transmitter/receiver unit, where the signal was successfully recovered. Using this test range we have tested the two-fold time-delayed diversity scheme. Diversity delays of 5 ms, and 10 ms show significant reductions in the probability of a joint fade at a particular level. Delays beyond about 10 ms do not significantly improve link performance. The system we have developed allows straightforward DWDM and polarization diversity extensions. Design issues for such optical wireless systems are discussed. We believe that such optical wireless transmitter/receiver units, which operate as an extension of the fiber network, offer a reliable and inexpensive solution for the 'last mile' problem in optical communications.
This paper presents the numerical analysis about the thermal effects, produced by the high-energy laser in a beam control system, on the laser beam propagation. The propagation of laser is described by the paraxial wave equations, solved by the phase-screen technique and the FFT method. The thermal turbulence motion and the air density variation are governed by the complete Navier-Stokes equations, so that the variety factors could be in consideration. The Navier-Stokes equations are solved by using the LU-SGS factorization technique. The methods could be used for other kinds of aero-optical problems. The numerical results show that, with the additional initial thermal-phase, the energy concentration and the quality of the laser beam at far field would be prominently degraded for the typical situations.
We have previously shown that amplitude weighting can improve the accuracy of measurements of the frequency offset of a signal contaminated by multiplicative Gaussian noise. We have investigated the more general non-Gaussian case through study of the statistics of a simple phase-screen scattering model and derived formulae for the low-order moments of the intensity-weighted phase-derivative. In this paper we extend numerical simulation of the problem to the case of a phase screen with Kolmogorov spectrum. We also report the results of some preliminary experimental measurements.