Since the European Space Agency (ESA) geostationary data-relay satellite ARTEMIS started its operation in February 2003, ESA and the Instituto de Astrofisica de Canarias (IAC) have carried out routinely bidirectional optical link experiments between ARTEMIS and the Optical Ground Station (OGS), installed in the Teide Observatory of the IAC in the Canary Islands, Spain. The experiments aimed at characterizing statically and dynamically the performance of the optical downlinks and uplinks in different atmospheric turbulence conditions, together with the development and testing of appropriate theoretical models to predict the link performance. An overview of the OGS and additional facilities on top the IAC Teide Observatory is given, as well as a summary of the statistical results on propagation and communication for different experimental configurations, including different number of transmitting subapertures and divergence in the uplink. Key parameters, like scintillation and fade and surge statistics, are correlated with theoretical predictions and an analysis of the far field pattern is included. The results of the deep space uplink experiments between the OGS and ESA satellite SMART-1 are also presented. Finally ESA free space optical communication programs are summarised, including optical payloads on board different satellites.

Future deep-space communications will require the collection and transmission of data from high-bandwidth links. NASA's Jet Propulsion Laboratory (JPL) is investigating the utility of laser communications for future missions to Mars and for future communication stations on the moon. Cloud cover impacts the availability of space to ground optical communications. Mitigating these impacts requires a geographically diverse network of ground communication. Selecting the number and location of stations for a network requires an optimization algorithm that can distinguish and rank site availability based on multi-year cloud climatologies for many locations around the globe. The optimization algorithm must also consider the movement and location of a space-borne probe. In this JPL-funded study, the TASC Lasercom Network Optimization Tool (LNOT) is used to determine optimal networks of receiving stations by analyzing cloud mask data from the continental United States, Hawaii, South America, Europe, northern and southern Africa, the Middle East, central and eastern Asia, and Australia. To generate cloud masks, raw visible and infrared radiance data from GOES (Geostationary Operational Environmental Satellite) and Meteosat satellites are compared to predicted clear sky background values. Several threshold tests in the Cloud Mask Generator (CMG) involving radiance-derived cloud identification tools (e.g., fog product, albedo product) are used to estimate the probability of cloud cover for a given pixel of a satellite image. When stations are chosen from a list of sites of interest, six stations are needed to achieve a network availability of 90 % or better.

Free-space optical (FSO) communication links constitute an alternative option to radio relay links and to optical cables facing growth needs in high-speed telecommunications (abundance of unregulated bandwidth, rapid installation, availability of low-cost optical components offering a high data rate, etc). Their operationalisation requires a good knowledge of the atmospheric effects which can negatively affect role propagation and the availability of the link, and thus to the quality of service (QoS). Better control of these phenomena will allow for the evaluation of system performance and thus assist with improving reliability. The aim of this paper is to compare the behavior of a FSO link located in south of France (Toulouse: with the following parameters: around 270 meters (0.2 mile) long, 34 Mbps data rate, 850 nm wavelength and PDH frame) with airport meteorological data. The second aim of the paper is to assess in-house FSO quality of service prediction software, through comparing simulations with the optical link data and the weather data. The analysis uses in-house software FSO quality of service prediction software ("FSO Prediction") developed by France Telecom Research & Development, which integrates news fog fading equations (compare to Kim & al.) and includes multiple effects (geometrical attenuation, atmospheric fading, rain, snow, scintillation and refraction attenuation due to atmospheric turbulence, optical mispointing attenuation). The FSO link field trial, intended to enable the demonstration and evaluation of these different effects, is described; and preliminary results of the field trial, from December 2004 to May 2005, are then presented.

NRL has established a 20 mile round trip laser communication test facility across the Chesapeake Bay for investigating lasercomm performance in a maritime environment. Experiments at this facility have successfully demonstrated links at data rates up to 2.5 Gbps and at lower rates in light rain and fog. This facility is currently being upgraded to allow long term monitoring of a one-way 10 mile link across the Bay. Parameters monitored will include BER, turbulence conditions, atmospheric transmission, and meteorological conditions. A summary of past results, the design/status of the upgrade to the test facility, and recent results will be presented.

Hybrid Free Space Optical (FSO) and Radio Frequency (RF) networks promise highly available wireless broadband connectivity and quality of service (QoS), particularly suitable for emerging network applications involving extremely high data rate transmissions such as high quality video-on-demand and real-time surveillance. FSO links are prone to atmospheric obscuration (fog, clouds, snow, etc) and are difficult to align over long distances due the use of narrow laser beams and the effect of atmospheric turbulence. These problems can be mitigated by using adjunct directional RF links, which provide backup connectivity. In this paper, methodologies for modeling and simulation of hybrid FSO/RF networks are described. Individual link propagation models are derived using scattering theory, as well as experimental measurements. MATLAB is used to generate realistic atmospheric obscuration scenarios, including moving cloud layers at different altitudes. These scenarios are then imported into a network simulator (OPNET) to emulate mobile hybrid FSO/RF networks. This framework allows accurate analysis of the effects of node mobility, atmospheric obscuration and traffic demands on network performance, and precise evaluation of topology reconfiguration algorithms as they react to dynamic changes in the network. Results show how topology reconfiguration algorithms, together with enhancements to TCP/IP protocols which reduce the network response time, enable the network to rapidly detect and act upon link state changes in highly dynamic environments, ensuring optimized network performance and availability.

Optical wireless communication performance through multi-scattering channels has been researched widely in recent years due to the increasing interest in laser satellite-ground links and urban optical wireless communication. The predominant sources of performance degradation have been identified as the spatial, angular and temporal spread of the propagating beam in the presence of a multi-scattering medium, which cause reduced power reception and inter-signal-interference (ISI), and noise due to background illumination and to receiver circuitry. In consequence, the signal-to-noise ratio (SNR) and the bit-error-rate (BER) are degraded. However, coherence effects due to multipath interference caused by a scattering propagation channel do not appear to have been treated in detail in the scientific literature. In this paper we attempt a theoretical analysis of coherence interference in optical wireless communication through scattering channels, and try to quantify the resultant performance degradation.

The wavelength, availability, range and power budget of an infrared free-space laser communication system critically depend on the atmospheric channel, which in turn is closely related to local weather conditions. As a result, the atmospheric propagation characteristics of the transmission medium must be taken into account from the beginning in the design of a free-space laser communication link. The most important linear effects that affect the attenuation of laser beam propagation through the atmosphere are absorption, scattering and turbulence. Weather parameters such as humidity, temperature and visibility are essential in determining the performance of a free-space laser communication system. Based on weather data recorded in Buenos Aires city (Argentina) at every hour during two years and made available to us by the Servicio Meteorologico Nacional (National Meteorological Service of the Argentinean Air Force), we calculate attenuation of laser radiation for an horizontal transmission path of 1 km for a near infrared direct detection optical communication system. Then, with these results, we estimate link availability and draw conclusions about when it is more convenient to transfer information.

Airborne platforms have historically been used as a pathway for the development of space applications of free-space laser communications. However, they also are an important element of the evolving network centric battlefield and surveillance architecture. This paper provides an historical perspective of previous programs and a look to the future for airborne Lasercom terminals and applications.

Recent advances in optical modulators and folded-path channel simulation bring an extra dimension to ongoing hybrid Free-Space Optics (FSO) and wireless RF networks. Typical FSO transceiver system design and network integration differ from emerging interrogators targeting Modulating Retro Reflector (MRR). At long ranges and in hostile environments, MRR edge nodes provide vital information such as relative GPS coordinates, unit identification, and integrated sensors data. To optimize network scalability, dynamic, instantaneous critical data flow, and real-time response, FSO transceivers and interrogators should overlap in physical layer performance and network layers interface. In this paper, we review system specification, requirement and channel impact on one-pass FSO and RF links as well as double-pass MRR links. Then, we state the underlying challenges on their network integration and ways to optimize performance by re-designing the system physical layer and identifying the corresponding network requirements.

There is a critical need for high bandwidth, high availability free-space optical communication links between the battlefield and the global information grid. Compact large aperture transceivers with low size, weight and power (SWaP) are needed to initiate and maintain communication links involving airborne platforms. The transceiver optical beam director typically contains fine and coarse steering stages. Existing beam director technology is based on electro-mechanical gimbaled mirrors with large SWaP that hinders deployment on many airborne platforms. To address the need for compact beam directors, we designed, fabricated, and tested an optical phased array (OPA) based on electro-optic dual frequency liquid crystal technology. This OPA has a transmissive architecture that enables a lower system SWaP, as compared to conventional reflective OPA. It has an 8 μm pixel pitch and steers over a 2.5° field of regard in one dimension at 1.55 μm. Two such OPAs can be stacked to steer in two dimensions. It has four independently addressable 1 cm x 4 cm regions arranged in a linear array to produce a continuous 4 cm x 4 cm aperture. The device incorporates novel addressing schemes to reduce the number of control channels by over an order of magnitude compared to conventional OPA addressing methods. It also utilizes proprietary low-loss transparent conductive Transcon^TM film for low optical absorption in the infrared. The OPA uses a custom multi-channel controller circuit operating at a 500 Hz frame rate. We present results on OPA design, fabrication, and optical performance on steering.

An optical communications link is used to transmit important navigation and range information between two spacecraft platforms to aid in the spacecraft docking process. This experiment uses NRL's multiple quantum well modulating retroreflector (MRR) array as the light payload optical communications transmitter on the target spacecraft. Spacecraft navigation information is determined by using the MRR array to impose five distinct modulation frequencies on the optical transmit beam, and analyzing the received signal on the pursuer spacecraft to extract target attitude and range information. Design, architecture, and status of the experiment at NRL are discussed.

Receiver performance is calculated for two types of free-space atmospheric optical communications systems − a direct detection system using a p-i-n photodiode preceded by an optical amplifier, and an ideal coherent receiver operated in its shot-noise limited regime without an optical pre-amplifier. The signal format for both is amplitude shift keying (on-off modulation) with an assumed rectangular light pulse of duration T seconds. The more accurate Beckman probability density function (pdf) is used to describe the intensity fading rather than the more commonly used log-normal pdf which overestimates receiver performance. A coherent receiver using asynchronous detection is shown to outperform the direct detection system with an optical pre-amplifier gain of 30 dB by anywhere from 7 to about 10 dB depending on Rytov variance intensity fading levels. In addition, the effect of inner scale turbulence parameter, l₀, on system bit error rate is demonstrated.

In this paper, a prototype of intersatellite laser communication terminal for a principle demonstration is reported, and the corresponding ground support equipments are described, too. The terminal has two main subsystems, the first is one for the laser communication and the second for the pointing, acquisition and tracking. Laser diodes are used for the communication link, and with the average laser power of more than 200mW and the data-rate up to 600Mbps. The PAT unit consists of a fine pointing mechanism and a coarse pointing assemble, which reaches a tracking accuracy of ~5μrad. The on-ground test equipments are included in a communication test bed with a long-distance beam propagation simulator, a PAT test bed with an optical satellite trajectory simulator, and a wavefront test bed with deferent lateral-shearing interferometers.

A portable protocol independent free space optical communication terminal was developed, which enables customer to quickly deploy optical bandwidth services for applications such as fiber extension, wild field point to point communication and wireless backhaul while avoiding costly and time-consuming fiber installation. By using specially designed optical components and optical-mechanical structure, the system is very compact and effective, can establish optical link within a few minutes, with total weight 4kg, size 160 x 360 x 155 mm, effective transmitting/receiving aperture 40mm, data rate 100Mbps, maximum communication distance 1500m. The system and experiments are presented in the paper.

In this paper we provide an overview of how the presence of atmospheric turbulence and scattering/absorbing media in the atmospheric channel degrade high-data-rate free-space laser communication performance. The impact of the atmospheric channel on overall link budget performance is discussed. Fog, rain, dust, snow, smoke, molecular absorption, and aerosol particulate matter all attenuate the signal-carrying laser beam, and to a certain extent can be compensated for by increasing the signal gain or by appropriate selection of the optical wavelength. In contrast, random fluctuations in the atmospheres refractive index severely degrade the wave-front quality of a signal-carrying laser beam, causing intensity fading and random signal losses at the receiver. This results in increased system bit error rates, especially along horizontal propagation paths. Atmospheric turbulence-induced signal losses increase as the distance between the transmitter and receiver is increased, and there is no wavelength window where these effects can be avoided, although longer wavelengths are less affected. With atmospheric turbulence, increasing the signal gain will not necessarily improve laser beam quality. For many cases of practical interest, the limiting factor in robust free-space optical communication link performance can be the presence of clear-air atmospheric turbulence in the optical channel. Various proposed probability density functions of laser intensity fluctuations through atmospheric turbulence will be discussed as they relate to laser communications performance and reliability under different weather conditions. Results from numerical simulations are presented for analyzing communications performance for various scenarios: Downlink, Uplink and Terrestrial (Horizontal) link.

In this paper we consider the optimal coherence for beam propagation through random media. First, we demonstrate that a beam that maximizes the average receiver intensity is fully coherent, and that the upper bounds on received intensity are nearly attained by a beam that is focused for clear air. Second, we demonstrate that a beam that maximizes the scintillation index (along with other criteria that trade-off the mean and standard deviation for the received intensity) is, in general, partially coherent. We conclude with an example in which modal intensities are optimized for a beam that is constructed from Hermite-Gaussian modes.

A combined experimental/computational approach is conducted using a specially-designed laboratory facility and a physical framework based on refractive fluid interfaces in order to enable the direct examination of the turbulent refractive-index field along laser beam propagation paths, as well as of the corresponding optical-wavefront distortions of the propagated beam. The experimental facility utilized is the variable-pressure flow facility at UC Irvine which enables direct imaging of the turbulent refractive-index field at large Reynolds numbers in controlled laboratory flows. Laser-induced fluorescence of acetone vapor seeded in air is utilized to directly image the turbulent refractive-index spatial fluctuations along the beam propagation paths. A custom-built high-resolution Shack-Hartmann wavefront sensor is utilized that is useful to measure the optical-wavefront distortions of the propagated optical beam. The flow-imaging measurements, combined with the beam-wavefront data, enable a direct study of the correspondence between the turbulent refractive interfacial-fluid thickness variations and the beam-propagation parameters of interest in free-space laser communications such as the Strehl ratio. Combined with the experiments, computations are also conducted on the flow-imaging data in order to study the structure of the optical beam as it propagates through the measured refractive-index variations. This enables a one-to-one correspondence to be established between the refractive interfacial-fluid thickness variations encountered by the beam and their effect on the distortions of the optical wavefronts, quantified in terms of a cumulative Strehl ratio, along the entire propagation path examined through the flow.

Spatially partially coherent laser beams have shown good transmission properties in free space laser communications, and a variety of methods have been used to generate spatially partially coherent beams including rotating diffusers and modulated acousto-optic cells or liquid crystals. With respect to high data rate laser communications, however, those methods exhibit serious drawbacks, the most serious being their dependence on mechanically or electrically-induced changes in the medium that are relatively very slow when compared to the gigabit data rates. To solve this problem, a nonlinear effect in a single mode fiber is used to reduce the temporal coherence of the transmitted beam, and its high spatial coherence is then reduced by a multimode fiber bundle made of fibers having very small length differences on the order of the beam's coherence length. Using this method, both the temporal and spatial coherence of the fiber output beam are greatly reduced. A comparison of the propagation properties of the beam transmitted through this combination of optical fibers with those of the original coherent beam reveals a significant performance improvement. To our knowledge, producing a spatially partially coherent beam in this way has not been reported previously.

As a support for the experimental activities related to the operation of bi-directional optical links between the European Space Agency (ESA) ARTEMIS geostationary satellite and the Optical Ground Station (OGS) in Teide Observatory (Tenerife island, Canary Islands, Spain), carried out by ESA and Instituto de Astrofisica de Canarias (IAC), calculations on the behavior of a multi-beam ground-to-satellite link have been performed. The goal is to assess the impact of refractive turbulence on the uplink (deemed to be more critical than the downlink because of beam-wander effects) and the mitigation effect on the power fluctuations in the satellite receiver achievable with such a space diversity approach, that involves several mutually time-incoherent beams in the uplink. Results from the multiple-beam uplink model and comparison with experiments are presented.

Even after several decades of study, inconsistencies remain in the application of atmospheric turbulence theories to experimental systems, and the demonstration of acceptable correlations with experimental results. We have developed a flexible empirical approach for improving link performance through image analysis of intensity scintillation patterns coupled with frame aperture averaging on a free space optical (FSO) communication link. Aperture averaging calculations are invaluable in receiver design. A receiver must be large enough to collect sufficient power and reduce scintillation effects at a given range, but must also be of practical size. We have constructed an imaging system for measuring the effects of atmospheric turbulence and obscuration on FSO links. A He-Ne laser beam propagates over a range of 863 meters in atmospheric turbulence conditions that vary diurnally and seasonally from weak to strong. A high performance digital camera with a frame-grabbing computer interface is used to capture received laser intensity distributions at rates up to 30 frames per second and various short shutter speeds, down to 1/16,000s per frame. The captured image frames are analyzed in Labview to evaluate the turbulence parameter C_n², temporal and spatial intensity variances, and aperture averaging. The aperture averaging results demonstrate the expected reduction in intensity fluctuations with increasing aperture diameter, and show quantitatively the differences in behavior between various strengths of turbulence. The reduction in scintillation with aperture size guides the selection of optimum receiver aperture. Spatial and temporal variance analyses within single frames and between frames are compared and show good agreement.

We describe a numerical, wave optics approach for simulating the propagation of a partially coherent beam. The approach requires the application of a succession of phase screens at the transmit end of the link and the summation of the resulting intensity profiles at the receiving end. To test the approach, a phase screen formulation for a Gaussian Schell-model beam was derived using a Gaussian random phase screen where its parameters are related to a form of the spatial coherence function for the transmittance. The numerical model was applied to several cases of propagation through vacuum and atmospheric turbulence and the resulting average beam intensity profile is shown to be nearly identical to the profile predicted by analytical results for the Gaussian Schell-model beam. Finally, the scintillation index of a coherent beam was investigated and the numerical result was found to generally follow the theoretical result, especially for source sizes that are relatively small or relatively larger. The model developed in this paper can be applied to analysis and design of free space optical laser links that utilize various partially coherent beams.

In this paper, we present the impact of optical turbulence on the performance of coherent optical communications which employs multi-wavelength plane wave optical radiation. Performance is assessed in terms of the achievable signal-to-noise (SNR) ratio as a function of the receiver aperture size. It is demonstrated that the use of multi-wavelength radiation improves the effective SNR. Furthermore, it is shown that the onset of SNR saturation fue to an increase in the aperture size is impacted by the use of multi-wavelength radiation.

The bit error rate of direct-detection, avalanche photodiode (APD) based free-space optical (FSO) communication systems is studied. The system of interest utilizes pulse-position modulation (PPM) and is subjected to scintillation due to optical turbulence. A weak turbulence (clear-air) scenario is considered for which the received signal intensity may be modelled as a log-normal random process. To arrive at the desired results, it is assumed that the system utilizes binary PPM (BPPM) modulation scheme. Furthermore, it is assumed that the receiver thermal noise is non-negligible and that the average signal intensity is large enough to justify a Gaussian approximation at the receiver. Numerical results are presented for the BPPM case to shed light on the impact of turbulence on the overall performance.

Based on the wavelet transformation and adaptive Wiener Filtering, a new method was presented by the authors to perform the synchronization and detection of the binary data from the Free-Space Optical (FSO) signal. It was shown that the Haar wavelet with a fixed scale is an excellent choice for this purpose. The output of the filter is zero-mean and is closely related to the derivative of the binary data. In this effort an analysis of the work in is presented to obtain the probability of bit error using a Bayesian ternary hypotheses testing. The analysis also results in determining optimum thresholds for the detection of binary data.

Our free-space optical (FSO) communication experiments show that, compared to non-return-to-zero (NRZ) encoding, return-to-zero (RZ) data format is more robust to atmospheric turbulence. A forward error correction coding gain up to 5 dB is obtained at a bit error-rate of 1x10^-15 for FSO link with Reed-Solomon (255,239) generic FEC (GFEC) which is specified in ITU-T G.709 standards. The coding gain increases further up to 1.6 dB with enhanced FEC (EFEC) that adds no additional overheads. Still, the link-margin with both GFEC and EFEC decreases with atmospheric turbulence.

We evaluate two error correction systems based on low-density parity-check (LDPC) codes for free-space optical (FSO) communication channels subject to atmospheric turbulence. We simulate the effect of turbulence on the received signal by modeling the channel with a gamma-gamma distribution. We compare the bit-error rate performance of these codes with the performance of Reed-Solomon codes of similar rate and obtain coding gains from 3 to 14 dB depending on the turbulence conditions.

The optimum detection threshold for optical communication receivers with large signal-dependant noise components can be derived from a Bayes' Likelihood Ratio Test; however, the bit level statistics must be known a priori. In free-space communication systems, atmospheric conditions cause variations in optical transmission and subsequently in the bit level means and variances. These bit parameters must be tracked, estimated, and predicted, in order to update the detection threshold at a rate greater than the frequency of atmospheric changes. A laboratory implementation of an adaptive thresholding system is being implemented at the U.S. Naval Research Laboratory's Chesapeake Bay Free-space Lasercom Testbed. Early results of experiments underway and initial design of the system will be presented.

Optical wireless systems, both indoor and outdoor, can offer data rates in excess of those available with RF systems. However, this is usually over a small area in the context of an indoor optical wireless, or over a small range of angular alignments for outdoor Free Space Optics (FSO) systems. Constant radiance considerations play a major part in these limitations, as do the receiver sensitivity for direct detection optical systems. In comparison radio channels operate close to the theoretical limits of channel capacity by using complex modulation, coding and high speed signal processing. In this paper an analysis of the performance of a cellular optical wireless system is undertaken, and opportunities to increase performance examined. More complex modulation schemes appear to offer significant benefits, mainly due to the reduction in channel bandwidth, and the attendant increase in allowable detector area.

Networked sensors are an emerging technology with the purpose of gathering data, profiled in time and space, for monitoring and surveillance in medical, environmental, home security and other applications. Typically, a large number of miniature sensing and communicating nodes are distributed ad hoc at the location of interest, where they establish a network and wirelessly communicate sensed data to one another, or to a base station. The optical modality is a potential solution for the links, due to the small and lightweight hardware and low power consumption, despite the drawback of alignment limitations. We propose an experimental concept to investigate the multi user interference effects arising when a number of nodes operate in a multi-scattering environment simultaneously. The laboratory experiment simulates a single internode link and the multi-scattering environment is modeled by a fog chamber. Linear and angular perturbations of transmitter-receiver line-of-sight (LOS) simulate geometries wherein multiply scattered light from the transmitted beam may interfere with the desired signal at the receiver causing multi-access interference. The potential multi-access interference is evaluated at differing optical densities and wavelengths. Conversely, the possibility for communication in the absence of LOS is inferred. Lastly, interference due to side-scattering from a link in the proximity of a receiver is modeled and assessed. In conclusion, guidelines are outlined for future experimental validation.

In this article a novel system architecture that uses a combination of wavelength and spatial diversity for indoor infrared wireless communications is presented. This configuration promises to fully exploit the available bandwidth of optics and demonstrate all-optical networking. Electronic processing is restricted to mobile terminals, with base stations potentially remaining passive, without any conversion between optics and electronics. For the downlink, multiple transmitter beams with different wavelengths are steered from the fiber infrastructure through the base station to mobile terminals located in different positions. An optimum combination of diffractive optics and reflective optics (a diffraction grating and an array of mirrors) can flexibly steer each transmitter beam and enable full control over the required coverage pattern. For the uplink, in the transmitter, another grating and an array of mirrors can direct multiple beams upward from different mobile users toward the base station. System simulation shows that the downlink has the potential to approach 10 Gbit/s, while maintaining wide-area coverage (such as in a room of 3m×4m×4m) with the help of fine optical tracking. System modeling indicates that the uplink is more susceptible to power losses than the downlink, but the utilization of dynamic beam steering in the uplink can suppress power losses to a tolerable level (e.g. below 30dB). An array of 16 mirrors has been designed to implement point-to-point beam steering in a room of 3m×1m×1m. Two-dimensional coverage patterns measured at a distance of 0.5 m and 1.5 m coincide with simulation results. Operation at 1 Gbit/s has been demonstrated successfully for tracking in two dimensions.

This paper presents an adaptive control scheme for laser-beam steering by a two-axis MEMS tilt mirror used in current free-space optical communications systems. In the control scheme presented here, disturbances in the laser beam are rejected by a high-performance linear time-invariant feedback controller augmented by the adaptive control loop, which determines control gains that are optimal for the current disturbance acting on the laser beam. The variable-order adaptive control loop is based on an adaptive lattice filter that implicitly identifies the disturbance statistics from real-time sensor data. Experimental results are presented to demonstrate the effectiveness of the adaptive controller for rejecting multi-bandwidth jitter. These results demonstrate that the adaptive loop significantly extends the jitter-rejection bandwidth achieved by the feedback controller alone.

An adaptive laser communication system capable of minimizing the impact of atmospherically induced tip-tilt and defocus distortions and tracking an optical antenna mounted on a mobile platform is presented. The laser communication system consists of two independently operating adaptive optical transceivers mounted on wide angle pan-tilt gimbals that each transmit and receive optical data propagating on a single bidirectional laser beam. Each optical antenna incorporates a 3-channel tip-tilt and defocus adaptive mirror constructed at the Army Research Laboratory. The adaptive mirror control system of each optical transceiver is based on phase control of the outgoing wavefront by stochastic parallel gradient descent optimization of the optical power received by the other optical transceiver. A tracking capability of an optical transceiver mounted on a mobile platform is implemnted by using the pan-tilt control signals of each adaptive mirror to define signals that drive the pan-tilt axes of the gimbal on which the adaptive mirror is mounted. System performance in the presence of laboratory generated turbulence is characterized and reported. Results demonstrate that the adaptive laser communication system can reduce signal fading induced by atmospheric tip-tilt distortions and improve the concentration of laser beam energy delivered to each optical transceiver and therefore reduce system bit error rates.

We describe the current performance of an adaptive optics testbed for free space optical communication. This adaptive optics system allows for simulation of night and day-time observing on a 1 meter telescope with a 97 actuator deformable mirror. In lab-generated seeing of 2.1 arcseconds (at 0.5μm) the system achieves a Strehl of 21% at 1.064μm (210nm RMS wavefront). Predictions of the system's performance based on real-time wavefront sensor telemetry data and analytical equations are shown to agree with the observed image performance. We present experimentally measured gains in communications performance of 2-4dB in the received signal power when AO correction is applied in the presence of high background and turbulence at an uncoded bit error rate of 0.1. The data source was a 100Mbps on-offkeyed signal detected with an IR-enhanced avalanche photodiode detector as the receiver.

Laser satellite communications (LCS) appear to have potential applications in the future global communication network. However, bidirectional ground-to-space link performance is degraded by atmospheric turbulence. This paper presents the simulations results of laser uplink and downlink propagation through atmospheric turbulence based on turbulence strength and link specification data obtained from the European Space Agency (ESA) ARTEMIS satellite and its associated optical ground station (OGS). The results obtained using this NMSU bidirectional link model indicate that the uplink performance can be improved significantly using tip-tilt error compensation alone while the downlink performance can be improved with higher order adaptive optics compensation.

We discussed the energy flux conservation law and noted that for all turbulence realizations atmospheric turbulence causes exclusively internal redistribution of laser irradiance inside the laser beam, whereas total energy flux stays the same. We proposed to use an energy flux preserving system architecture in laser communication systems. This architecture reduces the effect of turbulence-induced scintillation on the bit error-rate (BER) to zero and allows us to achieve free-space performance in the presence of strong turbulence using a low power laser. We performed two field demonstrations of the scintillation-free laser communication (SFLC) system using a static retro-reflector. In these demonstrations a BER<10^-10 was routinely measured at 0.5 km range in daytime sea-level turbulence using a 10 mW laser. We discussed an implementation of the SFLC system and benefits provided by this system. We concluded that for any practical application, the benefit provided by SFLC, namely, its ability to achieve an exceptionally low bit-error-rate (BER) using a low power laser, should be compared with the system cost and complexity.

Atmospheric turbulence is caused by inhomogeneities in the temperature and pressure of the atmosphere, resulting in random variations of the refractive index. A laser beam propagating through such turbulences experiences random amplitude and phase fluctuations, which can severely degrade the performance of free space optical (FSO) communication systems. In our time delayed diversity (TDD) technique, we transmit twice and take advantage of the fact that propagation along an atmospheric path is statistically uncorrelated with an earlier-time path for a time interval greater than the atmospheric turbulence correlation time. Communications performance is improved because the joint probability of error is less than the probability of error from individual channels. In this paper, we describe the theoretical and experimental analyses of FSO systems implementing this novel scheme in various performance scenarios. Theoretical models and performance of TDD systems are derived and characterized. The experimental performance results obtained under weak turbulence conditions are shown to be in good agreement with the theory. Related system design and implementation issues, such as: atmospheric turbulence statistics, laser beam depolarization, and diversity receiver architecture are also discussed.

Laser technology plays an ever-increasing role in aerospace and communication systems and is often viewed as a technology that has the potential for providing the material base for high-bandwidth applications. Laser provides the most logical connectivity channel for mobile systems requiring high data rates, low power consumption, covert operation, and high resistance to jamming. While advancements in modern opto-electronics have resulted in small size, reliable and power efficient lasers and modulators, successful operation of any communication technology hinges upon the ability to develop an equally advanced beam steering/positioning system. In many aerospace applications, when the transmitting optical platform is placed on board of an airplane, the ability to track the target is affected by the complex high-speed maneuvers performed by the aircraft and the resident vibration of the airframe. The tracking system must assure that in spite of the relative motion of both the transmitting and receiving stations and adverse environments, such as vibration, mutual alignment of two systems will be maintained to minimize communication errors. The work presented in this paper concentrates on the development of agile beam steering systems for laser communication terminals. Acousto-optic Bragg cells are used as deflectors while feedback information is generated by a quadrant detector. The control system is synthesized using a relatively simple constant-gain controller augmented with an adaptive Kalman filter to mitigate the effects of measurement noise in the tracking system. Laboratory experiments are conducted to investigate communication performance as a function of the sampling rate in the beam position feedback.

In all traditional techniques tracking the solution of the nonlinear Schroedinger (NLS) equation in (3+1) dimensions with spherical symmetry is time consuming because the problem in hand is (3+1) dimensional. In this work we study the propagation of chirped (3+1)-dimensional optical pulses in bulk media with periodic dispersion or alternating nonlinearity, analytically by using the variational approach, and numerically by using a new numerical technique relying on the adaptive spherical Fourier Bessel split-step (ASFBSS) method using spherical symmetry for 3 dimensions respectively. Using fast algorithms for spherical Fourier Bessel transforms along with adaptive longitudinal stepping and transverse grid management in a symmetrized split-step technique, it is possible to accurately study many nonlinear effects, including the possibility of spatio-temporal collapse, or the collapse-arresting mechanism due to saturable nonlinearity dispersion management or alternating nonlinearity. Stability criteria for (3+1)- dimensional solitons are identified, and the long term dynamics of the solitons are studied by the averaged equations obtained using the Kapitza approach. Also, the slow dynamics of the solitons around the fixed points for the width and the chirp are studied. The importance of this work is in generating dispersion managed or nonlinearly managed optical solitons in optical communication. Also, this work is applied to the stabilization of the Bose-Einstein condensate in (3+1)- dimensional optical lattice. We compare results of the new numerical technique with those obtained using fast Fourier split step (FFSS) technique. We feel that the ASFBSS is a fast and more accurate method for tracking spatiotemporal pulse propagation in a saturated and un saturated nonlinear Kerr medium in which the spatiotemporal collapse is expected for the paraxial unsaturated medium approximation, and to cyclic focusing and defocusing in saturable, dispersion managed or alternating non-linearity media.

A free space optical (FSO) network consists of many reconfigurable, directional, high data-rate links. Its performance can be optimized by using topology control algorithms, which involve: (1) potential neighbor information collection, (2) an optimization algorithm with given constraints, and (3) a precise pointing procedure. In general, if a sensor at each node can observe a large field of view (FOV), then more potential link targets can be detected. With more possible link choices, the optimization algorithm will have greater degrees of freedom in determining the optimum topology. The intuitive way to acquire a wide spatial acquisition range is to use a camera with a wide FOV. However, for such a wide angle lens/mirror, there are inevitable large aberrations, which cause errors in a pointing procedure based on image analysis. To mitigate these aberrations, a possible solution is to build a correction procedure from the wide FOV lens imaging model to a pinhole imaging model. In this context, a mapping model is proposed, based on analyses of several wide angle lens sets using CodeV. The proposed model also compensates for the effect of deviations between the center lines of the lens and a CCD imaging array. To obtain the optimum parameters of the model, an off-line calibration procedure based on geometrical constraints is introduced. A sensor system consisting of a widely available fisheye converter (Nikon FC-E8) and a high-resolution CCD camera (1392x1040 pixels) has been built for evaluating the model's performance, as part of our pointing, acquisition and tracking (PAT) system.

This paper addresses the use of liquid crystal devices for electro-optic infrared laser beam steering, such as liquid crystal optical phased arrays (OPAs) and digital beam deflectors (DBDs). In these devices, voltages are synchronously applied to different liquid crystal pixels to steer light, either by diffraction and/or refraction using birefringent prisms. Dual frequency liquid crystals provide an order of magnitude higher speed as compared to conventional nematic liquid crystals, at the cost of more complex addressing algorithms and control circuits. In order to optimize the optical performance of a liquid crystal device, the control voltages must be calibrated. This procedure involves adjustment of the control voltages while monitoring the optical efficiency, and must be done for both steady-state phase levels as well as transitions between phase levels. Manual voltage calibration is unrealistically time consuming for multi-channel beam steering devices. Computer based calibration algorithms for dual frequency liquid crystal devices are discussed, and results are presented for both steady state and dynamic voltage calibration procedures.

A 3-electrode LC beam deflector, consisting of a glass substrate with two transparent lateral electrodes and a third transparent grounded electrode, placed at the opposite glass substrate was designed and fabricated. A differential voltage, corresponding to the required phase step change, was applied to the lateral electrodes of this beam deflector. The gap between the glass substrates was filled with a nematic liquid crystal mixture. It was shown, that the large refractive index gradient generated by the field between the two lateral electrodes, resulted in the deflection of the incident optical beam with deflection angles as large as 25^o, at a high deflection efficiency of approximately 98%. A theoretical analysis based on both ray tracing and far-field diffraction calculations in such a 3-electrode LC cell configuration, confirmed the above experimental results. The advantages of this type of LC deflector are the large deflection angle attainable as well as the continuous deflection operation. The main drawbacks are the difficulties in focusing the beam into the narrow region of the refractive index gradient at the electrode gap, as well as the significant divergence of the deflected optical beam.

Implementation of long-range laser communication systems holds great promise for high-bandwidth applications. They are viewed as a technology that in the nearest future will handle most of the "last mile" communication traffic for the individual subscribers, corporate offices, military, and possibly deep space probes. Indeed, lasers allow for concentration of energy within tightly focused beams and narrow spectral interval, thus offering high throughput, information security, weight and size of components and power requirements that could not be matched by RF systems. However, the advantages of optical communication systems come in the same package with several major challenges. In particular, high data rates should be complemented by high-precision wide-bandwidth position control of a laser beam. In many applications the ability to maintain a link is affected by the complex maneuvers performed by mobile communication platforms, resident vibrations, and atmospheric effects. The search for the most effective and reliable way to shape and steer the laser beam is an on-going effort. This paper is focused on the application of acousto-optic technology as an alternative to electro-mechanical devices. With realization that an acousto-optic Bragg cell is only a component of the entire communication system, which should perform complex tasks of acquisition, pointing, and tracking of the remote terminal, we present an attempt to consider this problem from the "systems" point of view.

Free Space Optical (FSO) communication has evolved to be applied to the mobile network, because it can provide up to 2.5Gbps or higher data rate wireless communication. One of the key challenges with FSO systems is to maintain the Line of Sight (LOS) between transmitter and receiver. In this paper, the feasibility and performance of applying the FSO technology to the mobile network is explored, and the design plan of the attitude positioning and tracking control system of the FSO transceiver is investigated. First, the system architecture is introduced, the requirements for the control system are analyzed, the involved reference frames and frame transformation are presented. Second, the control system bandwidth is used to evaluate the system performance in controlling a positioning system consisting of a gimbal and a steering mirror, some definitions to describe the positioning accuracy and tracking capacity are given. The attitude control of a FSO transceiver is split into 2 similar channels: pitch and yaw. Using an equivalent linear control system model, the simulations are carried out, with and without the presence of uncertainties that includes GPS data errors and sensor measurement errors. Finally, based on the simulation results in the pitch channel, the quantitative evaluation on the performance of the control system is given, including positioning accuracy, tracking capability and uncertainty tolerance.

High resolution, wide range optical beam scanning methods are sought to meet the ever-increasing demands of optical wireless communication, printers, scanners and laser radar. In this paper we present a precise steering method with high angular resolution and a wide scanning range. The proposed beam-steering device is also, lightweight and compact, which are significant advantages in many applications. The scanning is implemented by means of a pair of diffraction gratings, which can be translated relative to one another, and a tunable-wavelength laser. Thus the beam is steered by the combined effect of two independent mechanisms. The agility of wavelength tuning enhances the performance of a grating-based device, while the sensitivity to chromatic aberrations is minimized by performing the coarse steering with the diffraction grating. The system performance is demonstrated experimentally and the results are reported.

Optical oscillators and amplifiers with local electro-optical feedback control loops are very attractive for different applications concerned with coherent optical communication systems, coherent combining and beam steering problems. A mathematical model of a phase-controlled optical amplifier is proposed. Dynamics of the model is studied, different stationary regimes, regular and chaotic modulation regimes are presented. For solution of problems of coherent combining and beam steering it is proposed to use the nonlinear dynamics effects that control arrays of coupled optical amplifiers. The considered model is universal for description the dynamics of high, ultra-high frequency and optical amplifiers with local phase-control loops.

In this paper, an adaptive transmission system with a special multiplex core for the optical wireless communication (OWC) is proposed, which can adjust the transmission mode automatically according to the channel status information (CSI). The atmosphere channel is a time-variable channel in which link performance could be affected by weather and scintillation. Adaptive transmission (AT) technique is introduced to solve the problem. Certain CSI that can evaluate the quality of atmosphere channel should be chosen to vary the transmission mode of OWC terminals in order to confront bad weather condition and maximize average throughput of transceiver. Considering that the system can choose suitable communication bit-rate, transmission power and with or without channel coding and some other flexible features, the special multiplex core has been used to guarantee the data to be multiplexed correctly. The data can be packaged into fixed length frame with the channel ID. Data stream is buffered by Dual-port FIFO with multi-clock feature and multiplexed by a smart controller with a certain algorithm. The special system structure makes it possible to replace the data interface without any change of the rest parts.

In the field of intersatellite communication, laser beam transmitted from the optical terminal is required to be highly collimated with diffraction-limit divergence. Then, the maximal wavefront aberration at the edge of exit pupil is about 0.3λ. In order to measure the diffraction-limit wavefront, we present a whole-field double-shearing interferometer based on double-shearing theory. In this paper, firstly, the principle and configuration of the interferometer is introduced. Then the theory of double shearing is discussed. At the same time, we extend the applications of the interferometer. Lastly, we present the simulations of interference fringes with different primary aberrations and give the experimental results for the measurement of diffraction-limit wavefront.

In this paper, we give the far-field diffraction theory for calculating the divergence by using the superposition of Gaussian beams instead of the aperture function and obtain the explicit express for calculating the beam divergence. We analyze in detail the influence of the diffraction adding defocus aberrations upon the transmitter's divergence and get the simulated result. These researches are useful to system design and give us some suggestions of avoiding the influence of the aberrations and the diffractions on the transmitter.

The characteristics of the terminals for the inter-satellite laser communication must be tested and verified on ground before flight test. Satellite relative-movement trajectory optical simulator is one of the most important devices of the ground test and verification system. It is used for simulation of the relative-movement between arbitrary satellites. Cooperating with the standard satellite laser communication terminal, the dynamical performance such as acquisition time and probability, pointing accuracy and tracking accuracy etc, of the terminal to be tested can be obtained. To keep the base of the terminal to be tested fixed, the clear aperture must be relatively large compare to the terminal's effect aperture. And for the practical application as well as in considering the size of optical glass commercially available, the optical aperture of the scanner was determined as φ420mm, the random accuracy for the simulation is 50-200μrad, Scanning angular coverage: azimuth ±180°, elevation ±15°. So the satellite relative-movement trajectory simulator belongs to the large-scale high-precision opto-mechanic and electrical equipment. In this paper, the design and fabrication of the simulator are introduced in detail. The simulator has the features as compact in construction, high accuracy in measurement, simple realization of the far-field condition in the near-field use, and direct simulation of 2D trajectory and consists of eight main elements: two wedge prisms, two pairs of worm and gear, damping gears, two actuating motors, two damping motors, rotating optical encoders, control computer and relevant electronics and mechanisms. The motivation of the design is to improve the accuracy as high as possible. The fabrication of the element of the simulator is stringent because of large-scale and high accuracy. For example, to solve the gap problem of the transmission mechanics, the damping method is introduced to the system. During the progress of the design and fabrication, many new and creative ideas are proposed. Finally, the calibration and the validation are discussed.

Cooperating with the free-space laser communication terminals, the satellite trajectory simulator is used to test the acquisition, pointing, tracking and communicating performances of the terminals. So the satellite trajectory simulator plays an important role in terminal ground test and verification. Using the double-prism, Sun etc in our group designed a satellite trajectory simulator. In this paper, a high precision dual feedback discrete control system designed for the simulator is given and a digital fabrication of the simulator is made correspondingly. In the dual feedback discrete control system, Proportional- Integral controller is used in velocity feedback loop and Proportional- Integral- Derivative controller is used in position feedback loop. In the controller design, simplex method is introduced and an improvement to the method is made. According to the transfer function of the control system in Z domain, the digital fabrication of the simulator is given when it is exposed to mechanism error and moment disturbance. Typically, when the mechanism error is 100urad, the residual standard error of pitching angle, azimuth angle, x-coordinate position and y-coordinate position are 0.49urad, 6.12urad, 4.56urad, 4.09urad respectively. When the moment disturbance is 0.1rad, the residual standard error of pitching angle, azimuth angle, x-coordinate position and y-coordinate position are 0.26urad, 0.22urad, 0.16urad, 0.15urad respectively. The digital fabrication results demonstrate that the dual feedback discrete control system designed for the simulator can achieve the anticipated high precision performance.

A temporal-spatial continuous Gaussian communication channel model is built by information theory. And the channel capacity is analyzed. Then a send-receive system scheme for imaging communication is proposed and the basic constrain conditions for system design are derived. The system performance is also analyzed and evaluated with spatial bandwidth product. After that an experiment is carried out for transmission 13X15 2-D pattern image through 30m distance. The expected performance is examined by the experiment results. At last a signal detection technique for imaging communication and corresponding pattern code and decode scheme are proposed.

A scanning system of a high accuracy double-wedge is presented, which allows us to perform very small angle deviation of a passing beam in a simple way. The first wedge's principal section is perpendicular to the horizontal axis, and the second is to the vertical axis correspondingly. They respectively rotate around the horizontal axis and the vertical axis as they work. So different small rotation angles of two wedges determine the corresponding orientation and position of the passing beam, and then high accuracy and very small-angle beam deviation can be performed. According to the design result: when the wedge angle is 5°, the refraction light beam will change about 1μrad if the wedge is rotated 1arcmin; the scanning range of light beam in the horizontal direction and the vertical direction can be not less than 600μrad, and the scanning precision of the device can be superior to 0.2 micro μrad

For on-ground performances testing and verification for intersatellite laser communication systems, a compact and multi-function optical testbed and a 2-D sub-micro-radian beamsteerer were designed. The testbed consists mainly of an optical simulator for long-distance propagation of laser beams, two CCD detectors, an autocollimation system, an interferometer, and a noise light source, for the uses is to measure far-field beam characteristics, transmitting powers, the wavefront errors and to evaluate the communication performances of any lasercom terminals with a aperture less than 280 mm in link level. The range of beam propagation distance of the optical simulator is from 2.8km to 3,5000km. An angular deviation yielded by the beamsteerer is to simulate the pointing error caused by all error sources. And a light source is to provide the noise-like illumination of changeable levels. Combined the testbed with the beamsteerer, it can measure the pointing errors and the tracking errors of the lasercom terminals, and estimates dynamic communication performances in the presence of random angular jitter. So the testbed has the potential to become all-purpose and tests the optical, communication and track performances for intersatellite lasercom terminals (ISLTs).

Various wavelength-division-multiplexed (WDM) passive optical network (PON's) have been developed by using spectrum-sliced light source and multiwavelength fiber ring laser source. We design a spectrum-sliced light source for free space WDM broadcasting laser communication system. Fiber ring spectrum-sliced light source can be essentially used to develop a low-cost WDM test bed for free space light source, so it attracts lots of attentions in the wavelength-division-multiplexed (WDM) communication system. Such a wavelength-division-multiplexed (WDM) broadcasting optical networks (BON's) can have a reduced cost of using a simple structure. The spectrum-sliced amplifier spontaneous emission (ASE) light from an erbium-doped fiber amplifier (EDFA) or semiconductor optical amplifier (SOA) were proposed as a suitable light source for multi-Gb/s speed WDM applications recently. The fiber ring light source structure with spectrum slicing has been developed and it can be applied to the dense wavelength-division multiplexing (DWDM) system. And we design some squamosal ball mirrors for free space optical communication. The system is simulated by ray tracing for analyzing the refracted and scattered optical signal. When we consider the effect of Gaussian beam of the incident signal being refracted or scattered by the squamosal mirror, based on the ray tracing study, we investigate the beam uniformity for the covering area. Four cases of the ball mirror are investigated and the fractional energy of the covering range are obtained.

We propose a new pumping method for optical fiber amplifiers at 1480nm and 980nm that is able to provide high gain and low noise-figure. The gain and noise figure obtained was 38 dB and 6 dB with an improvement of 4 dB and 1.4 dB, respectively, at 1550nm, for -30dBm input power when compared to its counter-pumped design. This design also delivers higher gain and lower noise figure when compared to the common bi-directional dual-pumped designs. A power conversion efficiency of 45% was achieved in this design.

It is general that with the distance increasing the communication quality of Free Space Optical (FSO) drops seriously, and the maximum transmitting distance is usually about 2 kilometers. In order to increase the covering range and transmission quality of the FSO system, a relaying device which is between the nodes is researched in the paper. In the device, the optical signal is transformed into electric signal by phototube. The device analyzes the receiving signal, acquires the effective signals and normalizes the signal waveform. Then, the reshaped signal is sent to the objective node. It can reshape, retime and regenerate signals, and the covering range can be extended. At the same time the transmission quality can also be improved. The approach is able to permit maximum transmitting distance more than 2km without deterioration of signal quality. At present, the experiment in our lab has already realized the 200m and 8M communication with the relaying technology successfully. The details are presented in the paper.

As the effects of various weather conditions on laser beams propagating in the atmosphere are hard to mitigate, in order to obtain high quality of the laser propagation, a new method is put forward in this paper. The theory uses feedback compensation technology to mitigate atmospheric scintillation effect, namely collecting the atmospheric turbulence factor in real time, and feedback it to the transmitter of laser real-timely. An experiment and simulation tests to verify the effect of feedback compensation have been taken to analyze the feedback effects and feasibility. The obtained results show the method can mitigate atmospheric scintillation in the atmospheric transmission.

Free-space laser communication is attractive technology regardless of in intersatellites applications or in atmosphere application. However, free space laser transmission environment is very terrible bit error rate will increase because of various interferences. In the paper, the characteristic of free-space laser transmitting is analyzed, and channel models used in the free-space laser communications system are retrospected and summarized. It leads to the suggestion that forward error correction codes should be applied to free space optical communication systems to improve the transmission reliability, and some useful conclusions are drawn.

It is important to be able to characterize and compensate for refraction effects in free-space optical laser communication (FSO lasercom). The refractive index depends on various properties of the propagation medium such as temperature, pressure, and moisture, with temperature having the largest affect. Very strong but slow-varying thermal gradients have been observed at the NRL Chesapeake Bay lasercom testbed, which offers a 16 km one-way (32 km round-trip) FSO lasercom link over water. Thermal gradients affect the elevation-pointing angle, and their magnitudes are a function of the time of day and year and also the weather conditions. These vertical refraction errors are corrected by the use of a fiber positioner controlled by a position-sensing detector (PSD). This system is implemented into the receiver at the NRL Chesapeake Bay lasercom testbed. System test results will be presented.

In this paper, we summarize progress in free space laser propagation research at the U.S. Naval Research Laboratory, specifically in the context of propagating and detecting signals through the atmosphere in a maritime environment. Transmission through the atmosphere over large bodies of water presents different challenges than transmission through the atmosphere over land. Our paper reports some of these findings as well as progress in our collaborative efforts to mitigate turbulence to enhance our data links.

In this work, we describe a hybrid free-space infrared communications link that supports audio transmission. The technique combines conventional frequency modulation (FM) techniques with optical amplitude modulation (AM) with a Multiple Quantum Well (MQW) Modulating Retroreflector (MRR) technology. The method has produced a robust, low power system capable of transmitting high quality audio information over a free space infrared link extending to multiple kilometers, depending on the characteristics of the Transmit/Receiver (interrogator) and the sensor/ MRR unit at the data source.

This paper studies the effect of turbulence on the orbital angular momentum (OAM) of the Laguerre-Gaussian beam propagating through the atmosphere. An integral representation of laser beam OAM through the beam intensity distribution and the gradient of the medium permittivity is derived. The equations are obtained for the first OAM statistical moments: the mean and the mean square. It is shown that the OAM value, averaged over medium realizations, coincides with the OAM value in a homogeneous medium. Asymptotic equations are derived, which allow OAM fluctuations to be estimated under the extreme conditions of weak and strong turbulence, depending on the diffraction parameters of the beam. It is found that the rate of growth of OAM fluctuations increases, as the propagating beam passes from the conditions of weak turbulence to the conditions of strong turbulence.

An optical testbed has been developed for the comparative analysis of wavefront sensors based on a modified Mach Zender interferometer design. This system provides simultaneous measurements of the wavefront sensors on the same camera by using a common aberrator. The initial application for this testbed was to evaluate a Shack-Hartmann and Phase Diversity wavefront sensors referenced to a Mach-Zender interferometer. In the current configuration of the testbed, aberrations are controlled using a liquid crystal spatial light modulator, and corrected using a deformable mirror. This testbed has the added benefit of being able to train the deformable mirror against the spatial light modulator and evaluate its ability to compensate the spatial light modulator. In the paper we present results from the wavefront sensors in the optical testbed.

Modern microprocessors are rapidly reaching performance levels that make them suitable for wavefront sensing and correction. This has the combined advantage of reducing system costs and a smaller footprint for the control system. These reductions have provided adaptive optics system designers with the capability of producing small form factor and low power system compared to just a few years ago. In recent work on the NRL portable adaptive optics system, our group has investigated implementing alternative computer peripherals for deformable and tip/tilt mirror control to support the low power low cost approach. This paper reports on characterization of several computer peripherals for use within adaptive optics systems with a focus on minimizing size, weight, cost and power consumption.