This PDF file contains the front matter associated with SPIE Proceedings Volume 6709, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

This paper will address the application of optical free space communication to distributed network architecture. High speed optical links in digital and/or analog forms enable new satellite and land based communication and sensing system architectures with significantly enhanced system performance over conventional single platform architectures. These applications include: RF communications supported by multiple spatially separated satellites or cellular-like base stations performing beam forming and nulling and long baseline interferometry over multiple platforms for sensing. We will examine both the physical transport design of the optical free space links and their architecture implications.

Cone tracking is a well known method to optimize the pointing of a beam, and has been previously reported in the case of direct mode free-space optical communication (FSOC) links, using a beacon or a dedicated wavelength to carry the feed-back. In a retro link, because the beam is reflected back to the transmitter, feedback data is locally available. We present here the results of an evaluation (both simulation and experiments) of the cone tracking technique applied to a retro link and how the implementation can be optimized for this specific case. We show that using only a small number of samples (as low as 16 with 625 Hz modulation frequency) we can retrieve the beam offset. We also show how the technique can be used to estimate the beam wander. We finally demonstrate cone tracking (closed loop maintaining the beam center on the retro-reflector) with modulation amplitude as low as 1% of the beam divergence.

It is necessary, on the ground in a laboratory, to test the technical parameters and to verify the working performance for the optical pointing, acquisition and tracking (PAT) of an inter-satellite lasercom terminal. In this paper, we report a completed PAT test-bed for this aim. The test-bed works in a fully physical way and is an integration of a 2D optical scanner of two rotating prisms, a 2D fine beam deflector of two tilting optical wedges and a double-focus laser collimator, the overall aperture is about Φ440mm. The optical scanner is designed to scan the beam in the range of 30° with an accuracy of 100μrad and used to simulate the mutual movement between two satellites. The fine beam deflector has the maximum beam deviation of about 1mrad with a step of 0.5μrad and is used to measure the tracking error of a terminal. The collimator has the double focal lengths, respectively, of 1.5m and 10m, the former provides a wide view of field for the use in the acquisition process of the terminal and the latter a narrow view of field for the use in the tracking process. In this paper, the design and fabrication considerations of the PAT test-bed as well as the main specifications of the completed integrated test-bad are given.

The potential of lasercom could often be much more attractive to system designers if the terminals could be made very small. In particular, in systems where one end of the link is allowed to be somewhat more capable than the other, the lesser of the two terminals could take advantage of the asymmetry and shrink as much as possible. We have investigated how such asymmetry factors into the requirements for a small terminal and have designed a terminal with a very small aperture (35-75 mm) and an inertial stabilization scheme. The space-worthy terminal has applicability to Moon-to-Earth as well as near-Earth lasercom missions.

Photon-counting is known to be the practically most efficient means for detection of free-space optical communications. Data rates will always be limited, however, by the speed at which such devices can operate. We calculate here the performance one can expect as one demands speeds so fast that device-limiting timing jitter substantially corrupts the measurements.

Traditional coarse pointing, acquisition, and tracking (CPAT) systems are pre-calibrated to have the center pixel of the camera aligned to the laser pointing vector and the center pixel is manually moved to the target of interest to complete the alignment process. Such a system has previously demonstrated its capability in aligning with distant targets and the pointing accuracy is on the order of sensor resolution. However, aligning with targets at medium range where the distance between angular sensor and transceiver is not negligible is its Achilles Heel. This limitation can be resolved by imposing constraints, such as the trifocal tensor (TT), which is deduced from the geometrical dependence between cameras and transceivers. Two autonomous CPAT systems are introduced for FSO transceiver alignment in mid- and long-range scenarios. This work focuses on experimental results that validate the pointing performance for targets at different distances, backed up by the theoretical derivations. A mid-range CPAT system, applying a trifocal tensor as its geometric invariant, includes two perspective cameras as sensors to perceive target distances. The long-range CPAT system, applying linear mapping as the invariant, requires only one camera to determine the pointing angle. Calibration procedures for both systems are robust to measurement noise and the resulting system can autonomously point to a target of interest with a high accuracy, which is also on the order of sensor resolution. The results of this work are not only beneficial to the design of CPAT systems for FSO transceiver alignment, but also in new applications such as surveillance and navigation.

We present a novel high-speed electro-optic beam scanner that provides a significantly improved scanning angle, angular resolution, and response time. Compared to conventional moving mirrors such as servo-controlled mirrors and galvanic mirrors, the demonstrated laser scanning device can improve the response time by 100 times. The presented device has many other unique features such as light weight, small dimension, low power consumption, and no-moving components, which are particularly suitable for airborne and space-borne applications. Electro-optic beam scanners are key components in advanced laser radars, laser communication systems. It also has important commercial applications in various fields such as display, printing, imaging, optical storage, optical communication, and so on.

For Free Space Optical Communication (FSOC) systems, employed on moving platform or communicating with a moving remote terminal, the quality of the communication channel strongly depends upon the tracking performance. In these systems quadrant Position Sensitive Detectors (PSD) are commonly used for beam tracking. This paper presents the results of significantly improved performance in acquisition and tracking of the FSOC system using a custom made 8-segments PSD and minimizing the tracking spot size on the detector.

Optical wireless channels that use modulated retro-reflectors can provide low-data rate communications to self-powered 'smart-dust' motes. The retro-reflectors are illuminated using a base station that incorporates diffractive beamsteering to direct radiation onto the motes, and these return the radiation to an imaging receiver within the base station. The motes consist of a photodiode to provide power, a novel logarithmic receiver to receive data from the base station, and a modulating retro-reflector to send information from the mote to the base station. Several of the components elements of this system have been implemented and tested. In this paper we report a logarithmic receiver that can be self powered by the source communicating with it, and a retro-reflecting LC modulator component that operates at 30b/s when driven at 0.7V. In addition an overall system model, together with the challenges for future work are presented.

Networks of sensors are an emerging technology for real-time data gathering in applications such as pollution monitoring, home security, surveillance, industrial control, etc. Many miniature nodes with sensing, computing and wireless communication capabilities are randomly deployed in an area or volume to be probed. One of the possible communication modalities for sensor networks is optical wireless communication (OWC). Initially, the sensor population must be mapped prior to interrogation by the base station and data communication from the sensor node. In this paper we review some theoretical and experimental work in this area and underline some of the challenges and possible solutions. The specific scenario of wireless sensor networks in a disaster recovery operation is modeled.

Earth-observation (EO) satellite missions produce a large amount of data using high-resolution optical or radar sensors. During the last decades the amount of data has steadily increased due to improved sensor technologies with increased temporal resolution, sensor resolution, and pixel count. As a consequence EO satellite missions have become limited by the downlink data rates of microwave communication systems, which are inhibited by spectrum restrictions, manageable antenna sizes, and available transmit power. Optical downlinks from EO satellites with data rates of several Gbps mitigate the limiting effects of microwave communication systems; however optical links do not provide the necessary link availability through the atmosphere due to cloud blockage above the ground station. Apart from diversity concepts with several ground stations or satellite networks, a stratospheric High Altitude Platform (HAP) could act as a relay station to forward the optical communication beam over the last 20km through the atmosphere to the ground station, where short-range, high data-rate microwave systems are feasible. This paper will discuss the capabilities of HAP and GEO relay stations to increase the downlink capacities of LEO satellites. Environmental aspects for the deployment of HAP relays and regulatory/technology issues for a microwave downlink on the last 20km to the ground will be discussed.

Previous research at Montana State University led to the development of the Infrared Cloud Imager (ICI) for measuring downwelling cloud and sky thermal emission for producing cloud coverage statistics using radiometrically calibrated images of the sky. This technique, that was developed primarily for detection of clouds for studies of arctic climate, provides benefits over commonly used systems by producing localized high resolution data in comparison to satellites images, and, in contrast to visible systems, provides continuous day and night operation. As a continuation of the first effort, in collaboration with the Optical Communications Group at the NASA's Jet Propulsion Laboratory (JPL), here we present a new generation of the ICI that can be used to monitor the cloud coverage of a site that can house a ground telescope dedicated to Earth-space optical communication paths. This new instrument, based around the FLIR Photon camera, expands the field of view (FOV) from 20° to 50° (up to 100° in the latest version), reduces instrument size, reduces instrument cost, and extends the time between calibrations to hours instead of minutes. This has been accomplished by characterizing the changes in the output data for changes in the camera's internal temperature while viewing a constant source. Deployment of this instrument has taken place at JPL's Table Mountain facility, CA, and Bozeman, MT.

In free space optical (FSO) communication networks, pointing, acquisition, and tracking (PAT) techniques are needed to establish and maintain optical links among the static or mobile nodes in the network. First, this paper describes a precise pointing technique to steer the local directional laser beam of an optical transceiver to a target optical transceiver at a remote transceiver node. The pointing technique utilizes Real-Time Kinematic GPS coordinates, local angular sensors, and a reference baseline, to retrieve accurate navigation information (roll, pitch, yaw) of the mobile or static platform that carries an optical transceiver. Through experiments using gimbal pointing stages, we have demonstrated "dead-reckoning" pointing accuracy in the milliradian range in our outdoor testbed. Second, we provide an application example of the pointing method in a bi-connected ring network, in which the pointing technique is combined with heuristic algorithms for dynamic reconfiguration of ring network topology. The heuristic algorithms achieve near optimal solutions in a short amount of time. Lastly, we present a GPS-based autonomous reconfiguration scenario for mobile nodes, which combines the PAT technique and heuristic algorithms.

Recent developments in pointing, acquisition, and tracking have enabled the formation of point-to-point FSO or narrow beam directional wireless networks that are capable of dynamic changes in their topology. Autonomous changes to topology in response to varying available link capacities and load demands of various nodes is called topology control. Topology control consists of computing new topologies to dynamically optimize the network under changing traffic conditions, and then carrying out the reconfiguration process to achieve the target topology. Our current work in this area studies the process of topology reconfiguration by using the packet drops that happen during this process as a cost metric. It is shown that the reconfiguration cost can be minimized when the target topology is reached by implementing the topology reconfiguration as a series of smaller steps (successive approximations). It is also shown that a topology computation algorithm that results in lower overall packet drops can be obtained by including the reconfiguration cost in the objective function along with the typical objective of congestion minimization. Simulations are used to evaluate and compare the performance of topology computation heuristics when the objective function includes reconfiguration cost.

Next generation wireless networks are increasingly complex in terms of their heterogeneity (terminal, edge and backbone nodes; directional and omnidirectional wireless links) and dynamic behavior (node mobility, atmospheric obscuration, fading). Modeling such complex systems is becoming a very challenging and cumbersome mathematical problem. This paper proposes a novel physics-based approach to the modeling, characterization and control of complex wireless networks. Heterogeneous wireless networks are modeled as physical systems where nodes are represented as particles and communication links as attraction forces between them. Forces are defined based on network connectivity and include the effects of link distance, link directivity and atmospheric obscuration. The network energy usage is used as a cost function that is shown to be related to the potential energy of the analogous physical system. We formulate the joint coverage-connectivity optimization problem in backbone-based wireless networks as an energy minimization problem and present a mobility control algorithm that mimics the natural reaction of a physical system to minimize potential energy driven by local forces exerted on network nodes. Our mobility control algorithm is shown to be completely distributed, scalable and self-organized. Initial results show the efficiency of our mobility control approach to autonomously adjust the position of controlled backbone nodes in order to optimize coverage and connectivity in dynamic scenarios.

Turbulence inner scale affects scintillation in laser projection and laser communication systems especially in strong scintillation regime. Analytical and numerical models are used for performance analysis and design of these systems. Turbulence inner scale is critically important to anchor theoretical predictions to an experiment. However, the inner scale is usually not measured in the experiments along extended atmospheric paths. Commercial scintillometer commonly operates over the range of a few hundreds meters and requires an optical transmitter and receiver at different ends of the propagation path. We introduced a concept for turbulence inner scale sensor, which is based on phase related phenomenon and can operate along arbitrary atmospheric paths including the strong scintillation regime both during daytime and nighttime. We evaluated the feasibility of this approach. We developed an analytical model for a tilt-corrected point spread function (PSF) of a distant source that enables turbulence inner scale sensor determination from optical measurements, evaluated the PSF sensitivity to the inner scale variations for ground-to-ground and space-to-ground engagement scenarios, designed and built a sensor breadboard prototype Finally, for the first time we performed turbulence inner scale measurements along space-to-ground propagation paths by imaging stars. We found that the turbulence inner scale on space-to-ground paths is in the range from 1 cm to 3 cm, whereas it is in the range from 0.2 cm to 1.2 cm near the ground. Thus, initial inner scale measurements by imaging stars revealed that turbulence inner scale on extended elevated paths exceeds that value near the ground.

In free-space optical communication links, atmospheric turbulence causes fluctuations in both the intensity and the phase of the received light signal, affecting link performance. Influence of Kolmogorov and non-Kolmogorov turbulence statistics on laser communication links are analyzed for different propagation scenarios.

Intensity fluctuations from a 532nm CW laser source were collected over an outdoor 1km path, 2m above the ground, with three different receiving apertures. The scintillation index was found for each receiving aperture and recently developed theory for all regimes of optical turbulence was used to infer three atmospheric parameters, C_n², l₀, and L₀. Parallel to the three-aperture data collection was a commercial scintillometer unit which reported C_n² and crosswind speed. There was also a weather station positioned at the receiver side which provided point measurements for temperature and wind speed. The C_n² measurement obtained from the commercial scintillometer was used to infer l₀, L₀, and the scintillation index. Those values were then compared to the inferred atmospheric parameters from the experimental data. Finally, the optimal aperture sizes for data collection with the three-aperture receiver were determined.

A new method of reconstructing and predicting an unknown probability density function (PDF) characterizing the statistics of intensity fluctuations of optical beams propagating through atmospheric turbulence is presented in this paper. The method is based on a series expansion of generalized Laguerre polynomials ; the expansion coefficients are expressed in terms of the higher-order intensity moments of intensity statistics. This method generates the PDF from the data moments without any prior knowledge of specific statistics and converges smoothly. The utility of reconstructed PDF relevant to free-space laser communication in terms of calculating the average bit error rate and probability of fading is pointed out. Simulated numerical results are compared with some known non-Gaussian test PDFs: Log-Normal, Rice-Nakagami and Gamma-Gamma distributions and show excellent agreement obtained by the method developed. The accuracy of the reconstructed PDF is also evaluated.

The fades and background noise that are seen by the receiver(s) in clear atmospheric optical communication can be mitigated with diversity and coherent receivers. However, in certain applications, one may need to be concerned with not only fades/outages, but also with an interferer that may be actively trying to interfere with our communication by sending his own optical signal towards our receiver(s). It is important to understand how our optical system performs in the presence of such an interferer. In this paper, we describe two potential interferer strategies when our communication system uses Diversity Direct Detection and two potential strategies when our system uses Diversity Homodyne Detection. We also derive and plot the performance of our systems in these scenarios, both in the absence of and in the presence of clear atmospheric turbulence. We find that if the interferer optimizes the fraction of bits over which it spreads its power, it degrades the performance of our Diversity Direct Detection or Diversity Homodyne Detection significantly more than if it were simply on all the time. Moreover, we find that the performance of Homodyne Detection with no diversity and Direct Detection with no diversity are almost the same in the presence of analogous interferers, and that diversity improves Homodyne Detection's performance while worsening Direct Detection's performance.

A performance metric is proposed as a general measure for optimizing the transverse coherence length l_c of a partial spatially coherent beam for a given communication scenario. The expression is essentially the mean intensity minus the standard deviation of the intensity and we seek to maximize this quantity. It is preliminarily verified by the probability of fade with log-normal distribution model under the weak turbulence condition. We also examine it as a function of l_c using wave optics simulations and compared these results with the relationships predicted by analytic theory under weak to medium-strong turbulence conditions. Our results verify there exists a unique coherence length that can optimize the receiver beam quality. After calculating the probability of fades of the optimal partially coherent beam and the fully coherent beam and comparing them with the wave optics simulation results, good agreement was observed.

In recent experimental research on adaptive control of jitter in laser beams, sufficiently high levels of high frequency sensor noise have produced high frequency spikes in the output errors. The phenomenon has been observed for both adaptive and high bandwidth linear-time-invariant (LTI) control loops. Recently, the source of the problem has been discovered to be saturation associated with the MEMS fast steering mirror used as the control actuator. Results in this paper demonstrate that the spikes in the output error are eliminated by a recently developed frequency-weighting method for the tuning signal used to determine adaptive control gains. The method places more weight on jitter in frequency ranges where large sensor noise otherwise produces the unwanted response. The frequency-weighted adaptive control loop is based on a recursive least squares lattice filter that implicitly identifies the disturbance statistics from real-time sensor data. The adaptive controller achieves both fast adaptation and true minimum variance steady state performance. Results from an experiment with a MEMS fast steering mirror used in current free space optical communications systems illustrate suppression of jitter with simultaneous multiple bandwidths produced by multiple jitter sources.

We present an optical heterodyne receiver for data rates up to 10 Gb/s. Its outer dimensions are 44x44x18 cm³, it weighs 16.5 kg and consumes 70 W of power. This optical receiver is single-mode fiber coupled to distribute the received signal from the outside of the spacecraft to the inside. This approach improves the ruggedness of the receiver system against shocks and vibrations. Under an ESA funded project, the photodiodes of this receiver have passed space qualification tests, such as Particle Impact Noise Detection tests, shock and vibration survivability, as well as proton and gamma radiation exposure. High receiver sensitivities (BER=1·10^-9) of 390 photons/bit and 619 photons/bit were measured at 1550 nm for differential phase-shift keying (DPSK) and on/off keying (OOK), respectively. No low noise optical preamplifier (EDFA) was used in this case. These are one of the highest sensitivities reported for heterodyne detection of 10 Gb/s signals without using optical amplification. Avoiding the use of an EDFA allows to adapt the coherent receiver to other wavelengths such as 1064 nm. We also investigated the receiver sensitivity of the coherent receiver when combined with a low noise optical preamplifier. For 10 Gb/s DPSK and OOK sensitivities of 74 photons/bit and 132 photons/bit were measured, respectively.

The design of optical transceivers for mobile free space optical communication systems is a sophisticated task. A typical transceiver design includes the combination of coarse- and fine-tracking-sensors, a data receiver, and a laser source for sending data. For heavily vibrating environments or for systems that have stringent requirements in terms of pointing accuracy, a fast steering mirror might be used. This results in a design with many parts. The Fiberbundle Receiver, consisting of 7 fibers in conjunction with photodiodes, can ease such an implementation process. It allows the combination of some of the mentioned components, which results in a less complicated design. Standard COTS components for fiber based WDM systems can be used for challenging tasks as e.g. wavelength splitting. Furthermore, a cost-effective and very fast tracking system can be implemented. A technology demonstrator for the application of the Fiberbundle Receiver at a wavelength of 1550 nm has been developed and will be discussed in this paper.

A solar illuminated glinting specular object can serve as an in situ sensor probe that is observable from long distances. Retroreflective objects produce bright glints when illuminated by coaxial illumination sources such as lasers. These glints are modulated in various ways by illumination source variances, the local probe environment, the intervening propagation paths and the remote sensing system. The modulating signals can be recovered by using reflectivity detectors with temporal, spatial, wavelength, directivity and polarization sensitivity. Clustered and moving specular probes provide additional information through geometry extraction, beam forming and multisensor noise reduction. Experimental results are shown for omnidirectional specular imaging, atmospheric wake turbulence measurement, redeye sensing and acoustic sensing.

This paper investigates the approximation of the Bit Error Rate of a coherent optical focal plane array receiver for PPM signals in the presence of atmospheric turbulence. Analysis of the expressions needed to obtain the Bit Error Rate (BER) for the real system under study in the laboratory is shown, specifically an approach using the Saddle-Point approximation of the Marcum-Q function and a procedure is further discussed to approximate the Marcum-Q function using the cumulative distribution function (cdf) of the Nakagami-m distribution. It is shown that by approximation of the Marcum-Q function by means of the cdf of the Nakagami-m distribution, the numerical value of the probability of error can be readily computed.

Motivated by rapid advances in solar blind ultraviolet (UV) light emitting diodes (LEDs), filters and photomultiplier tubes (PMTs), together with unique UV atmospheric propagation characteristics, a non-line-of-sight (NLOS) UV communication test-bed has been recently built and utilized for extensive experimental evaluation of performance of NLOS UV links in outdoor environments. Towards this end, key link components are first characterized and their limitations are identified. The tradeoffs among communication range, received number of photons, and bit-error-rate are revealed via field measurement results. Wavelength diversity is achieved by utilizing combinations of sources and detectors centered at different wavelengths in the solar blind band. It is demonstrated that signals can be reliably transmitted to their destinations of dozens of meters away through an NLOS channel. Although all reported results in this paper are based on open field experiments, it is found that reflections from surrounding objects such as trees and buildings can enhance the received signal strength, up to an order of magnitude increase in the received number of photons in some cases, thus significantly improving link performance.

Superconducting nanowire single photon detectors have recently been demonstrated as viable candidates for photon-counting optical receivers operating at data rates in excess of 100 Mbit/s. In this paper, we discuss techniques for extending these data rates to rates > 1 Gbit/s. We report on a recent demonstration of a 2-element nanowire detector array operating at a source data rate of 1.25 Gbit/s. We also describe techniques for emulating larger arrays of detectors using a single detector. We use these techniques to demonstrate photon-counting receiver operation at data rates from 780-Mbit/s to 2.5 Gbit/s with sensitivities ranging from 1.1 to 7.1 incident photons per bit.

Silicon Geiger-mode avalanche photodiodes (Si GM-APDs) have desirable properties for an optical photon-counting receiver, including high single-photon detection efficiency, low reset time, and low timing jitter; however, they do not detect near-IR photons. In this work, we demonstrated a sensitive photon-counting receiver in the near-IR by combining a wavelength converter consisting of a periodically-poled lithium niobate (PPLN) waveguide and a commercial Si GM-APD detector. We measured a receiver sensitivity from 1.4 to 3.5 incident photons/bit from 5.5 Mb/s to 22 Mb/s for a single detector, and achieved a sensitivity of 4 photons/bit at 78 Mb/s using an emulated array of 25 detectors.

High data-rate atmospheric free-space optical (FSO) lasercom systems typically suffer from relatively long time duration link degradations. These are caused by pointing- and tracking-errors or deep signal-fades produced by the index of refraction fluctuations caused by atmospheric turbulence. Based on measurement results we will present in this paper a channel characterization model for free-space optical links. Further a forward-error-correction (FEC) coding scheme is introduced that is able to overcome link outages. The performance of these codes has been proven by measurements. Code design recommendations and validation test results are discussed in this paper.

Establishing a link between a ground station and a geosynchronous orbiting satellite can be aided greatly with the use of a beacon on the satellite. A tracker, or even an adaptive optics system, can use the beacon during communications activities to correct beam pointing for atmospheric turbulence and mount jitter effects. However, the pointing lead-ahead required to illuminate the moving object and an aperture mismatch between the tracking and pointing apertures can limit the effectiveness of the correction as the sensed tilt will not be the same as the tilt required for optimal transmission to the satellite. In this paper we present an analytical model that addresses the combined impact of these tracking issues in a ground-to-satellite optical link. The analysis considers geosynchronous Earth orbit satellites as well as low Earth orbit satellites.

The mobile free-space optical channel mainly suffers from relatively long link outages, produced by short-term blockings of the line-of-sight (obstacles, clouds), pointing- and tracking-errors or deep signal-fades caused by index of refraction turbulence effects. This paper discusses the applicability of commonly used communication protocols like UDP, TCP, ARQ and the SCPS-TP from the Space Communications Protocol Standards (SCPS) in various scenarios. The performance of the protocols in the selected scenarios is evaluated using the simulation software OMNeT++. The simulations are based on channel measurements from the three FSO demonstrations FASOLT (61 km Ground - Ground link), KIODO (LEO satellite downlink), and ATENAA (land-mobile link) and from ongoing measurements at the German Aerospace Center (DLR) (short-range Ground - Ground) as part of the MINERVAA project. Based on the simulation results, recommendations for protocols in free-space optical communication scenarios are given.

The choice of wavelength is essential for the variety of different communication scenarios in the field of free space optics (FSO). Possibilities are Satellite and HAP (High Altitude Platform) Downlinks, HAP-HAP links, HAP-Satellite links and all kinds of links involving aeronautical vehicles. This paper addresses the influence of the wavelength dependent attenuation of clouds, the atmospheric transmission in the NIR and MIR and a statistical analysis of cloud coverage data for an estimation of link availability. Regarding the calculation of atmospheric transmission the free available simulation tools libRadtran and GENLN2 have been used. To identify advantageous wavelengths to increase link availability, cloud attenuation is determined by Mie scattering calculations of particle size distributions of various cloud types. Here the MIR wavelength interval between 10 μm and 12 μm has been found to give the lowest attenuation in clouds. However in most cases clouds will block the optical link. For that matter a statistical analysis of satellite based data from the European Cloud Climatology (ECC) is done to reveal favorable places with high availability in Europe. The improvement of link availability when a concept of ground station diversity is applied has also been investigated. An availability of almost 99 % is reached with four hypothetical stations in southern Europe. Further the difference between availability values of single years decreases with multiple stations.

There are a number of options for modulation formats in optical fiber and free-space lasercom systems. There are a seemingly even greater number of options for receiver architectures. Although essentially all of these possibilities and combinations have been analyzed in journal publications, conferences, and textbooks, it seems to the author that much of this information has not been compiled in such a way that the community can easily appreciate relationships between performance and related costs of these many options. In this brief overview, then, we will compare the power and bandwidth efficiencies of coded and uncoded systems using some of the most commonly used formats and receivers. We will also examine some of the more recent results in quantum-optimum receivers in the same context.

High definition (HD) quality imagery provides clearer and more detailed information for use in advanced video surveillance systems. The deployment of such surveillance systems where no fixed communications infrastructure exists presents an ideal application for high data rate directional FSO/RF links and networks. Next generation surveillance systems using HD imagery will be able to detect and analyze objects in detail and at large distances. Such flexibly deployable surveillance systems will be very valuable for military and homeland security surveillance. Nevertheless, designing these types of systems is not an easy task. First, HD images require large amounts of bandwidth: compressed high definition television (HDTV 1080i) images require bit rates of approximately 20Mb/s, which rise to above 1Gb/s for uncompressed images at 30 frames/s, and an increase in the number of cameras in one single system can saturate the available bandwidth. Second, advanced surveillance requires significant computational power for real-time object detection, tracking, and discrimination. This paper analyzes these issues and proposes a solution with on demand video compression and real-time object detection algorithms. A system architecture of a HD scalable system with the ability to track and discriminate objects and events within the system's deployed area will be described. Practical examples of autonomous event detection in wirelessly transmitted HDTV images will be given.

The experimental results of an inter-orbit laser communication performed under an atmospheric influence is presented. The demonstration was planned so that the optical link was supposed to graze the earth's rim because of the satellite revolution around the earth. The trial was successfully carried out on 5th April, 2006. The measured experimental data are introduced to show the temporal behavior of the OICETS's optical terminal. The atmospheric influence on the optical link is calculated with a theoretical model to obtain a probability density of normalized intensity as a predictive value. The probability density is also estimated from the experimentally measured data. The comparison shows that the theoretical prediction well describes the experimental results.

The first bi-directional laser communications demonstration between the optical ground station developed by the National Institute of Information and Communications Technology (NICT) located in Koganei, Tokyo and the Optical Inter-orbit Communication Engineering Test Satellite (OICETS) was successfully conducted in March, May, and September, 2006. The Kirari Optical communication Demonstration Experiments with the NICT optical ground station (KODEN) were jointly conducted by the Japan Aerospace Exploration Agency (JAXA) and NICT. Data from the uplink and downlink optical communication links were analyzed. For the downlink, the scintillation index agreed well with the theoretical results calculated based on the strong fluctuation theory. The aperture averaging effect was the dominant factor in reducing the variation of the downlink signals. The probability density functions as a function of elevation angles were measured and compared with the theoretical model, showing good agreement. For the uplink, the scintillation index disagreed with the calculated results based on the strong fluctuation theory. The multiple beam effect of the uplink transmission with large beams will have an additional reduction factor, which will help to establish ground-to-satellite laser communication links in the future. Four laser beams transmitted from the optical ground station to the OICETS satellite also helped to reduce the optical signal's intensity fluctuation due to atmospheric turbulence.

The original scanner of tilting orthogonal double prisms is studied for testing the tracking performance in inter-satellite laser communications. Two prisms respectively rotate around the horizontal axle and the vertical one within the admissible range to determine the corresponding orientation and position of the passing beam, therefore the high accuracy deviation angle of passing beam can be performed. The test experiments performed with autocollimator and interferometer, as well as the theoretical analysis, indicates that the scanner can meet the requirements of the deviation accuracy superior to 0.5 μrad with the deviation range greater than 500 μrad, which accords to our design requirements.

Nearly diffraction-limited laser beam in the laser communications is often to be evaluated by the interferometers. A series of wave fronts sensors are manufactured which are designed as over 500 millimeters apertures and three kinds of thickness of 50, 100 and 200 millimeters according to different need of shearing amount. They are suitable for measurement of diffraction-limited laser wave front or the larger aberrations. The plate and the wedge are the basic structures for the sensors which are shearing interference between the forward surface reflection and the backward. These simple plates will make the mechanical design and adjustment easier. The wedge of aperture-divided is so designed that the precision of measured wave front is higher than the full aperture design for that the error comes from the material and optical manufacture is reduced through compare the up part and the down part each other. The method is explained in detail in this paper. Some experiment interferograms are analyzed and the wave heights are deduced.

Recent successful demonstrations of laser communications have demonstrated the feasibility of some of the key aspects of this technology. The demonstrations can not success without the full-up ground test and validation. So an integrate test-bed was build in build to test the technical parameters and to verify the working performance for the optical pointing, acquisition and tracking (PAT) of various inter-satellite lasercom terminals. In this paper, we detail the test technical scheme (TTS) and the corresponding experiments. The integrate test-bed is a high quality optical system that will measure the key characteristics of lasercom terminals, such as point error, tracking error, acquisition possibility etc.. The test-bed can operate over the relative wavelength range. Through quantitative tests, the terminal could be optimized base on the test results.

We study the behaviors of scintillation of the electromagnetic beam Gaussian Schell-model (GSM) beam propagating in atmospheric turbulence based on the cross-spectral density matrix. It is shown analytically that the scintillation index the beam is influenced by correlation properties of source and the effect of turbulence simultaneously. The competition between the two types of parameters results in a maximum value of scintillation of the GSM beam propagating in turbulence.

In the inter-satellite laser communication, the laser wavefront reaches the diffraction limit. For the test of the diffraction-limited wavefront, we have developed a Jamin double-shearing interferometer of which the detectable wave-front height is in the order of 0.1 wavelengths. Based on this interferometer, a polarization phase-shifting double-shearing interferometer is proposed to improve the performance. The existing Jamin double-shearing interferometer is consists of two Jamin plates to form lateral shearing and four wedge plates to divide the aperture. The polarization phase shifter is composed of three polarizers, a quarter-wave plate and an analyzer. The first polarizer is placed in front of the first Jamin plate. The second and third polarizers are placed behind the wedge plates and their transmission axes are parallel and perpendicular to the incident plane of the Jamin plates, respectively. The fast axis of the quarter-wave plate has an angle of 45 degrees to the transmission axis of the second and third polarizers. By rotating the analyzer, the phase-shift interferogram is obtained. In the interferometer, the polarization phase shifter is simple to be inserted and the phase shifting is easy to realize. The interferometer is kept as an equal optical path system and still suits wavefront testing of the low coherent light. In experiments, phase-shifting interferograms are obtained and the usefulness of the interferometer is verified.

Due to the improved receiver sensitivity and wavelength selectivity, coherent detection became an attractive alternative to direct detection in inter-satellite laser communications. A novel method to coherent detection of position errors information is proposed. Coherent communication system generally consists of receive telescope, local oscillator, optical hybrid, photoelectric detector and optical phase lock loop (OPLL). Based on the system composing, this method adds CCD and computer as position error detector. CCD captures interference pattern while detection of transmission data from the transmitter laser. After processed and analyzed by computer, target position information is obtained from characteristic parameter of the interference pattern. The position errors as the control signal of PAT subsystem drive the receiver telescope to keep tracking to the target. Theoretical deviation and analysis is presented. The application extends to coherent laser rang finder, in which object distance and position information can be obtained simultaneously.

The complex model of terrestrial FSO link is based on two models: the power-budget model of a given link (steady model) and the installation site model (statistical model). The steady model is represented by the power balance equation and the power level diagram. The statistical model consists in the knowledge of cumulative exceedance probability of the random atmospheric attenuation coefficient. The parameters of the statistical model depend only on the atmospheric phenomena on installation site. Data from more than 200 sites in Italy, France and Germany are presented.

We present a compact Extended Cavity Laser (ECL) system based on a high-power laser diode optimized for maximum efficiency of the Rb optical pumping process. The system represents the crucial part of the HpG (hyperpolarized gasses) production process. We concentrated on the ECL system optimization - linewidth matching and frequency stabilization - for the optical pumping process. We show that the intensity of optical feedback in the ECL laser influences linewidth and output power and it is possible to find an optimum value for the highest power spectral density at the absorption line of desire. The emission linewidth was reduced approximately 10 times with only half of the total optical power loss. The ECL system is controlled by electronic servo-loop for laser frequency stabilization.

Atmospheric noise signals are fundamental limitation of free-space optical communications, as the decrease in signal-to-noise ratio reduces the range and/or bandwidth of the link. In this paper we consider the limitations that this imposes, and investigate the use of discrete wavelet transformation (DWT) to overcome them. Simulations are performed to validate the use of the DWT in the demodulation of the analog data in the presence of noise. Results of the experiments are presented.