Optical space communications is now a reality. It will be a key building block for wide-area space data networks of the future. This new technology will provide quantum-leap improvements to satellite network performance and cost and have profound transforming effects on space system architectures and applications.

In traditional Risley prism beam pointers, two wedge prisms are used to steer the optical path. This type of beam director is attractive for free space laser communication and beam scanning systems because the beam director is compact and conformal. Furthermore, moderately large apertures can be accommodated without significant weight or power consumption. However, this approach is not well suited to tracking systems, since while tracing a continuous, constant velocity path, control singularities can occur that require infinite rotational speeds of the prisms. This phenomenon is particularly evident at pointing angles near the system boresight. A third prism can be used to eliminate these singularities, but the system is then under-constrained and infinite solutions exist for the prism orientations. In this paper, a method of uniquely determining the proper orientations of three prisms in an optical beam pointer is presented. It is shown how the method can be used to optimize a system for optical tracking by minimizing the angular velocities required of the prisms. Also, a specific implementation of the method has been demonstrated in the laboratory. Smooth tracking of arbitrary target trajectories is demonstrated across the field of view of the system.

Coherent optical communication systems are an attractive choice for future high-speed optical satellite networks due to the excellent receiver sensitivities that can be achieved by coherent detection techniques. Recently, the interest in coherent optical systems has increased with advances in photonic devices and high-speed digital signal processing (DSP). The best coherent receiver sensitivity (9 photons per bit) is obtained with phase-shift keying (PSK) modulation in combination with homodyne detection. In this paper, we propose coherent duobinary systems for high-speed optical satellite communications. It is shown that duobinary modulation combined with homodyne detection comes very close to the ultimate receiver sensitivity limit set by PSK. Moreover, duobinary modulation benefits from having a relatively narrow signal spectrum. Thus, duobinary systems generally require half the bandwidth of PSK, and are less susceptible to inter-symbol interference. These are advantageous attributes for overcoming the technological difficulties in high-speed systems. The duobinary homodyne receiver is also compatible with high-speed DSP post-processing for ameliorating the atmospheric channel impairments. Computer simulation studies show that the theoretical performance limits of coherent duobinary systems can be achieved with practical devices.

Atmospheric scintillation noise is a fundamental limitation of free space optical communications, as the decrease in signal-to-noise ratio reduces the range and/or bandwidth of the link. A technique employing dual wavelengths has previously been demonstrated to be effective in mitigating scintillation noise by using common mode rejection to remove co-channel noise. However, any practical implementation of this technique will include uncorrelated noise, e.g. amplifier and photodetector noise, which will not be removed. 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.

Ball Aerospace & Technologies Corp. (Ball Aerospace) has developed a Risley Beam Pointer (RBP) mechanism capable of high pointing accuracy and operational bandwidth. The prisms offer a wide field of regard (FOR) and can be manufactured and operated with diffraction-limited optical quality. The unit is capable of steering a 4-in. diameter beam over a 72° half angle cone with better than 100 μrad precision. Absolute accuracy of the beamsteering is in the range of 100 μrad to 1 mrad, depending on the thermal environment of the system. The system has demonstrated a control bandwidth of 23 Hz and better than 10 deg/sec of smooth target tracking anywhere within the FOR.

In this paper, we present receiver architectures for post-detection combining of multiple transmitted packets in direct-detection (DD) photon channels. It is assumed that the optical signal is intensity modulated (IM) and that the receiver operates under a shot-noise limit condition. Furthermore, it is assumed that the channel is free of atmospheric turbulence or turbulence is negligible enough so that it can be assumed away. An automatic-repeat-request (ARQ) mechanism is utilized with a time-out feature of M retransmissions. Maximum a posteriori (MAP) rule is considered here to arrive at optimal combining mechanism for the above scenarios. Performance is established in terms of average delay and probability of packet loss. Furthermore, the probability of successful transmission of a packet in q (<M) attempts is obtained. It is shown that the proposed technique offers a significant reduction in the number of attempts for successful transmission of a packet as compared with the conventional method.

As free-space laser communications systems proliferate due to improved technology and transmission techniques, optical communication networks comprised of ground stations, aircraft, high altitude platforms, and satellites become an attainable goal. An important consideration for optical networks is the ability of optical communication terminals (OCT) to quickly locate one another and align their laser beams to initiate the acquisition sequence. This paper investigates promising low-cost technologies and novel approaches that will facilitate the targeting and acquisition tasks between counter terminals. Specifically, two critical technology areas are investigated: position determination (which includes location and attitude determination) and inter-terminal communications. A feasibility study identified multiple-antenna global navigation satellite system (GNSS) systems and GNSS-aided inertial systems as possible position determination solutions. Personal satellite communication systems (e.g. Iridium or Inmarsat), third generation cellular technology (IMT-2000/UMTS), and a relatively new air traffic surveillance technology called Autonomous Dependent Surveillance-Broadcast (ADS-B) were identified as possible inter-terminal communication solutions. A GNSS-aided inertial system and an ADS-B system were integrated into an OCT to demonstrate their utility in a typical optical communication scenario. Testing showed that these technologies have high potential in future OCTs, although improvements can be made to both to increase tracking accuracy.

Ground-to-satellite laser communication experiments between the optical ground station located in Koganei of downtown Tokyo and a low earth orbit (LEO) satellite, the Optical Inter-orbit Communications Engineering Test Satellite (OICETS) called "Kirari", were successfully performed in March and May, 2006. The optical communication demonstration experiment at the optical ground station was conducted in cooperation between the Japan Aerospace Exploration Agency (JAXA) and the National Institute of Information and Communications Technology (NICT). Ten trials over the course of two months were conducted during the test campaign. Acquisition and tracking of the satellite were successful on seven days out of the assigned test days. The tests were unsuccessful during three out of the four days when it was cloudy or rainy, but they were successful on all six of the days when partly clear skies were predominant. For the uplink, the fluctuation of the received signal power was well minimized by using multi-beam laser transmissions. The bit error ratio (BER) on the downlink was measured to be as low as 10^-5. The applicability of the onboard optical terminal was demonstrated, aiming not only for geostationary earth orbit (GEO)-LEO links but also for ground-to-LEO optical links.

The propagation of laser beam in inter-satellite laser communications belongs to the far-field diffraction, but in the optical test and verification of pointing, acquisition and tracking (PAT) function on the laboratory the beam from a terminal propagates within the near-field. In this paper, in terms of the Fresnel diffraction theory the inherent difference is found that in the far-field diffraction the optical tracking position error is resulted from both the mutual movement between two laser communication terminals and the tilting of the receiver terminal, but the tilting of the transmitter has no effect on the error position; and that in the near-field diffraction the position error is caused by the tilting of the transmitter or the receiver, but the mutual movement has no effect. It is furthermore found that the use of a beam scanner in the test in the near-field can simulate exactly the mutual movement of satellites in the far-field, and the trajectory formula for the beam scanning is deduced that is the same as the mutual angular trajectory from one satellite to another. Therefore a practical PAT test bed of a double-focus laser collimator, a beam scanner and a fine beam steering device is developed by us to test and verify the PAT function of inter-satellite laser communication terminals. The optical aperture is about φ440mm for this use. And a test bed for concurrent test and verification of both PAT function and communication performance is also demonstrated. The test bed consists of a conventional laser collimator, an optical scanner and a far-field beam transmission simulator, which is a combination of a Fourier-transform lens an a followed multiple-stage imaging amplifier. The details of configurations are given. It is clear that these test beds can be also used to test and verify the functions of laser radar, passive optical tracker, and so on.

Alexander Graham Bell's photophone of 1880 was a simple free space optical communication device that used the sun to illuminate a reflective acoustic diaphragm. A selenium photocell located 213 m (700 ft) away converted the acoustically modulated light beam back into sound. A variation of the photophone is presented here that uses naturally formed free space acousto-optic communications links to provide passive multichannel long range acoustic sensing. This system, called RAS (remote acoustic sensor), functions as a long range microphone with a demonstrated range in excess of 40 km (25 miles).

Fiber laser pumped Er:YOS laser action near 1600 nm was achieved at room temperature. Etalons and a diffraction grating were used to generate and control broad-band laser action near the 1600 nm emission center. Efficient operation was achieved in a non-optimized laser test bed.

NASA anticipates a significant demand for long-haul communications service from deep-space to Earth in the near future. To address this need, a substantial effort has been invested in developing a free-space laser communications system that can be operated at data rates that are 10-1000 times higher than current RF systems. We have built an end-to-end free-space photon counting testbed to demonstrate many of the key technologies required for a deep space optical receiver. The testbed consists of two independent receivers, each using a Geiger-mode avalanche photodiode detector array. A hardware aggregator combines the photon arrivals from the two receivers and the aggregated photon stream is decoded in real time with a hardware turbo decoder. We have demonstrated signal acquisition, clock synchronization, and error free communications at data rates up to 14 million bits per second while operating within 1 dB of the channel capacity with an efficiency of greater than 1 bit per incident photon.

The performance of a coherent optical focal plane array receiver for PPM signals under atmospheric turbulence is investigated and applications of this system are addressed. The experimental demonstration of this project has already been explained in previous publications. This article shows a more exhaustive analysis of the expressions needed to obtain the Bit Error Rate (BER) for the real system under study in the laboratory, specifically an approach using the Saddle-Point approximation of the Marcum-Q function. Selected experimental results of this system are described and compared with theoretical BER expressions, and array combining gains are presented. Receiver sensitivity in terms of photons per bit (PPB) is examined; BER results are shown as a function of signal to noise ratios (SNR) as well as a function of photons per bit.

Laboratory experiments, computations, and physical modeling of laser wavefronts propagating through variable-refractive-index separated shear layers at large Reynolds numbers are conducted in order to examine the relation between the flow behavior and the laser wavefront behavior for airborne laser communications. The new element of this work is the focus on the dependence on scale of the optical behavior as well as of the flow behavior, using multiresolution analysis of the measured and computed data. The experiments are conducted using the UC Irvine variable-pressure turbulent flow facility. Direct non-intrusive imaging of the refractive index field is accomplished with laser-induced fluorescence and a high-resolution digital camera that resolves three decades of scales. Simultaneously, direct imaging of the propagated laser wavefront phase profile is conducted using a Shack-Hartmann array sensor that also has a resolution of three decades of scales. The computational component consists of near-field wavefront propagation through the measured refractive index field, validated by the direct wavefront measurements. We have conducted multiresolution analysis of the flow data and optical data, by a posteriori reducing the resolution of the refractive-index field and phase field. We present evidence of strong scale dependence at large scales, i.e. in the energy-containing range of scales. Physical modeling of this behavior is developed based on the structure of the coarse-grained refractive turbulent interfaces. This approach is useful in order to relate the root-mean-squared optical path difference and Strehl ratio, at variable resolutions, to the refractive-index variations along the laser wavefront propagation path. This facilitates the identification of the dominant refractive interfaces and serves as a guide to developing aero-optical optimization methods for airborne laser communication applications.

It is known that partially (spatial) coherent beams can reduce the effects of scintillation in a free space optical link. However, practical implementations of this type of beam are still under investigation. In this work we apply a wave optics model to analyze the propagation through turbulence of a coherent beam and a particular version of a pseudo-partially coherent beam. The beam we study is created with a sequence of five Gaussian random phase screens for each atmospheric realization. We examine the average intensity profiles, the scintillation index and aperture averaging factor for a horizontal propagation scenario. We find this pseudo-partially coherent beam behaves much like a Gaussian Schell-model beam.

We present an analysis of Free Space Optical (FSO) signal attenuation experienced in a marine environment. This work is in support of the Communication Link Assessment in Marine Environments program (CLAIME), for the Navy's investment in a network infrastructure for high altitude tactical layer connectivity to the Global Information Grid. The expanded bandwidth requirement can be realized using FSO networking capabilities. The performance of the link needs to be evaluated for different platforms such as ship-to-ship, airborne-to-ship, as well as airborne-to-airborne links. Near surface horizontal links required for ship-to-ship communications will be described in detail. The challenges faced in this environment include determining the attenuation due to aerosol scattering as well as optical turbulence. Determining the attenuation due to fog, haze, rain and snow will be addressed as well.

Modulating retro-reflectors (MRR) allow free space optical links with no need for pointing, tracking or a laser on one end of the link. They work by coupling a passive optical retro-reflector with an optical modulator. The most common kind of MRR uses a corner cube retro-reflector. These devices must have a modulator whose active area is as large as the area of the corner cube. This limits the ability to close longer range high speed links because the large aperture need to return sufficient light implies a large modulator capacitance. To overcome this limitation we developed the concept of a cat's eye MRR. Cat's eye MRRs place the modulator in the focal plane of a lens system designed to passively retro-reflect light. Because the light focuses onto the modulator, a small, low capacitance, modulator can be used with a large optical aperture. However, the position of the focal spot varies with the angle of incidence so an array of modulators must be placed in the focal plane, In addition, to avoid having to drive all the modulator pixels, an angle of arrival sensor must be used. We discuss several cat's eye MRR systems with near diffraction limited performance and bandwidths of 45 Mbps. We also discuss a link to a cat's eye MRR over a 7 Km range.

Low-cost, large-aperture optical receivers are required to form an affordable optical ground receiver network for laser communications. Among the ground receiver station's multiple subsystems, here, we only discuss the ongoing research activities aimed at reducing the cost of the large-size optics on the receiver. Experimental results of two different approaches for fabricating low-cost mirrors of wavefront quality on the order of 100-200X the diffraction limit are described. Laboratory-level effort are underway to improve the surface figure to better than 20X the diffraction limit.

The first theory for two novel coherent beam combination architectures that are the first electronic beam combination architectures that completely eliminate the need for a separate reference beam are presented. Detailed theoretical models are developed and presented for the first time.

Commercially available Liquid Crystal on Silicon (LCoS) Optical Phase Arrays (OPA) are capable of non-mechanically beamsteering up to ±3 degrees at 1550 nm. While the existing technology is useful for many applications such as laser communications and pulse-shaping, it is desirable to increase the steer angle and decrease the response time of the OPA. This was accomplished through a research effort funded by Langley Research Center at NASA. Under this research effort Boulder Nonlinear Systems (BNS) designed a new 1x12288 pixel OPA. In the new backplane design the pixel pitch was decreased from 1.8 um to 1.6 um, the backplane voltage was increased from 5 volts to 13 volts, and the aperture was increased from 7.4 x 6.0 mm to 19.66 x 19.66 mm. The OPA, when built with new liquid crystals and calibrated with new automated calibration procedures demonstrated a greater than 2x improvement in steer angle. The OPA that was tested, which was built for operation at 1550 nm, demonstrated the ability to steer to ±6.95 degrees. Additionally the relaxation time of the OPA was improved to 24.8 ms. This paper discusses the benefits of the new backplane design, the liquid crystal (LC) properties that are most desirable for beamsteering, the implementation of the automated calibration procedures, and the results.

We investigated the spatial structure of atmospheric turbulence at Maui Space Surveillance Site (MSSS) using a 3.6 m telescope and a spatial filtering receiver. This receiver simultaneously records four star images on one camera frame. The star images are formed through pupil masks representing aperture diameters of 0.1 m, 0.5m, 1.5 m, and 3.6 m. We determined the camera orientation for each data set by moving the telescope at a given angle in azimuth and elevation. We calculated the horizontal and vertical components of the image centroid and evaluated the statistics of the horizontal and vertical wavefront tilt as a function of the aperture diameter and seeing conditions. We found several evidences of anisotropy of turbulence at MSSS. On four nights we observed that the variance of on-axis horizontal tilt exceeded the variance of the vertical tilt by a factor of 1.3-3.3. We believe that this is due to anisotropy of large-scale turbulence, where the horizontal scale of the turbulent inhomogeneities exceeds their vertical scale. The estimates of the horizontal and vertical turbulence outer scale confirmed this conclusion. In addition, in several data sets the horizontal image spot diameter of the long-exposure star image exceeded the vertical image spot diameter. We also found that large apertures are more likely to have higher anisotropy coefficient values than small apertures. This is because the contribution of small-scale isotropic turbulence to the image centroid reduces with increasing telescope diameter. In the case of isotropic turbulence, the power spectral densities (PSDs) of wavefront tilt are consistent with theoretical models. The telescope vibration modes were observed at 20 Hz. In the case of anisotropic turbulence, the PSDs of the horizontal tilt component have lower slope in the high frequency range, and difference between PSDs for large and small apertures is reduced. The anisotropy of turbulence and atmospheric tilt may affect the design and performance analysis of both active and passive optical systems.

Modulation of the polarization state provides a means for information transfer in an optical communication channel. Polarization shift keying (POLSK) has been proposed as a candidate for modulation in optical fiber communications and shown to have 3dB better sensitivity than an on-off keying (OOK). Unfortunately, the utility of POLSK in standard fibers is reduced because of birefringence of the fibers. The polarization state changes unpredictably and becomes difficult to track at the receiver. On the other hand, POLSK does not suffer from these problems in free space optical (FSO) communication systems. In theory, the atmospheric turbulence channel causes minimal depolarization, and negligible crosstalk occurs between orthogonal polarization states along the transmission path. The polarization state of a propagating optical wave is well preserved over link lengths up to several kilometers, which makes polarization state reliably detectable at the receiver. In this paper, the experimental performance of a POLSK modulation scheme used in our 1km FSO communication system test-bed is described and its theoretical analysis is also presented.

A novel high accuracy all electronic technique for phase locking arrays of optical fibers is demonstrated. We report the first demonstration of the only electronic phase locking technique that doesn't require a reference beam. The measured phase error is λ/20. Excellent phase locking has been demonstrated for fiber amplifier arrays.

New results of the (temperature) refractive index structure parameter (C_T²), C_n² are presented from fast response sensor observations near the ground and also using a kite/tethered blimp platform and an aircraft, at the Edward Air Force Base in Mojave Desert, California. Additional optical measurements include near-ground scintillation observations over horizontal paths. Atmospheric turbidity were also calculated from direct beam solar radiation measurements using pyrheliometer. Comparisons were made of the observed profiles of refractive index structure parameters (C_n²) with theoretical modeled profiles, and two derived quantities such as transverse coherence length (r₀) and isoplanatic angle (θ₀) for a slant path are discussed. All of these parameters are the major indicators of turbulence and are important to design an aircraft or space-craft-based free-space laser communication and high resolution optical synthetic-aperture imaging systems. Non-isotropic turbulence observations from some of the data will be pointed out. Probability density functions (PDF) of the distribution of C_n² will be described using histograms. Fundamental limits imposed by atmospheric effects in high data rate communication and optical synthetic-aperture imaging systems will be discussed.

Wireless networks are currently replacing connection cables via radio, visible or infrared waves. Modules and base systems are installed to cover zones in relation to a quality of service and availability. There are technological radio solutions: Bluetooth, WiFi, UWB and optics constituted by infrared or visible systems. Optic technology has important advantages: Transmitted data security, radio and medical immunity, etc. Nevertheless, optical systems seem to present a limit because this is basically a line of sight solution and the network management is based on only one wavelength with several users. The solution suggested, in the scope of this document, is to transmit various wavelengths in free space, using optical Multiplexer/Demultiplexer and optical modules, which are compatible in wavelength. Each Emission/reception module could have a defined and personal wavelength, with a link with the terminal identification (MAC address for instance). This approach can improve and give a full duplex data rate with a minimum of a dozen Mbps per user for broadcasting. The application field for the suggested system is potentially included in the following network types: Optic WLAN and Optic WDAN.

In the optical wireless communication systems proposed by Gibson, et al, the information is encoded as states of orbital angular momentum (OAM) of light and the transmitter unit can produce laser beam with single OAM-state in a time-slot. Recently we have proved that it is possible to generate multiple OAM-states simultaneously by single spatial light modulator. This method is adopted in our free-space optical wireless communication system and these OAM-states can be detected in the receiving unit by a computer-generated hologram. Hence, the transmission capacity is enhanced significantly without increasing the complexity of system.

A free-space optical (FSO) communication demonstration was conducted with JHU/APL and AOptix at the TCOM Test Facility in Elizabeth City, NC in May 2006. The primary test objective was to evaluate the performance of an FSO link from a fiber-tethered aerostat to a ground platform at effective data rates approaching 100 Gigabits/sec using wavelength division multiplexing (WDM) techniques. (Multiple optical channels operating near 1550 nm were modulated at data rates of 1, 10 and 40 Gbps). The test was conducted with a 38 meter aerostat raised to an altitude of 1 km and a ground platform located 1.2 km from the aerostat (limited by property boundary). Error free data transfers of 1.2 Terabits in 30 seconds at 40 Gbps were demonstrated. The total data transferred during the test was greater than 30 Terabits with an average BER of 10^-6 without any forward error correction (FEC) coding.

Using imaging sensors in a pointing, acquisition and tracking (PAT) system provides a high degree of accuracy and the capability of controlling multiple transceivers simultaneously. However, this kind of system can suffer from a narrow field-of-view (FOV). Also, for a single image sensor, the resulting PAT system can only accurately acquire a target located far away or moving on a plane. In this paper, we describe an optical PAT system, which has a FOV of almost 180 degrees and is also capable of locating targets at any distance. It is well known that two regular cameras are sufficient to reconstruct three dimensional coordinates by triangulating with two incident rays. The uniqueness of our system is that one of the cameras is an omnidirectional fisheye camera and the other is a regular camera (with a FOV of 30 degrees). Their geometric relationship is encoded by a radial trifocal tensor, which is further discussed in this paper. This scheme leads to a hierarchical structure in the design of PAT systems. A regular camera and an omnidirectional camera form a camera pair. Each of the transceivers on a rotary gimbal and the camera pair is first calibrated in order to retrieve their individual radial trifocal tensor. After the calibration, the camera pair monitors the target of interest and further computes the rotation angles for each of transceivers. A transceiver is then selected and rotates toward the target. The selection process is based on network objectives. The resulting system is fully optical and has three distinct advantages: 1) the radial trifocal tensor is invariant with the motion of the entire platform, 2) a wide field-of-view (close to 180 degrees); 3) three dimensional acquisition capability. However, there is a small penalty to be paid because an additional geometric limit exists compared with a PAT system using two regular cameras.

Multiple Input Multiple Output (MIMO) Radio Frequency (RF) systems can offer substantial increases in capacity over their single antenna wireless link counterparts. In the case of optical wireless links the nature of the channel makes such benefits less clear however. In this paper the likely benefits and applications are discussed, and results from a simple line of sight demonstration are presented.

Free-space Quantum key distribution (QKD) has shown the potential for the practical production of cryptographic key for ultra-secure communications. The performance of any QKD system is ultimately limited by the signal to noise ratio on the single-photon channel, and over most useful communications links the resulting key rates are impractical for performing continuous one-time-pad encryption of today's broadband communications. We have adapted clock and data recovery techniques from modern telecommunications practice, combined with a synchronous classical free-space optical communications link operating in parallel, to increase the repetition rate of a free-space QKD system by roughly 2 orders of magnitude over previous demonstrations. We have also designed the system to operate in the H-alpha Fraunhofer window at 656.28 nm, where the solar background is reduced by roughly 7 dB. This system takes advantage of high efficiency silicon single-photon avalanche photodiodes with <50ps timing resolution that are expected to enable operation at a repetition rate of 2.5 GHz. We have identified scalable solutions for delivering sustained one-time-pad encryption at 10 Mbps, thus making it possible to integrate quantum cryptography into first-generation Ethernet protocols.

In the Capanina project, it has been shown that it is possible to use solely GPS positions for coarse-pointing of a stratospheric free-space optical communication terminal. Although this system design has been shown to be functional, it has to deal with acquisition and reacquisition times of up to 20 seconds. Whenever the line of sight is blocked, the Capanina terminal has to do a reacquisition of the partner terminal. In land-mobile systems, e.g. transmission between a moving vehicle and a fixed station, where objects like trees or buildings can frequently block the line of sight, this type of acquisition/reacquisition is not acceptable. In this paper a revised system design will be shown, which reduces the initial acquisition time to below a second by using information from a GPS-Aided Inertial System (GPS-INS). It will also be shown how reacquisition times and coarse-pointing errors can be reduced to a minimum by using feedback from the optical fine-pointing device. Finally a demonstrator setup and some test results will be presented.

In an on/off-keying atmospheric optical communication system, index of refraction turbulence and PAT (pointing, acquisition and tracking) instabilities create strong and very slow fading compared to the usually used high data-rates. With this channel behavior, forward error correction can only work effectively if the codeword-length is longer than the mean duration of fade. Therefore packet-layer coding is a favorable way to implement a code with codewords longer than typical fading events. A code validation platform based on Ethernet transmission technology and packet-layer coding using UDP packets was built up to prove the performance of packet-layer coding. In this paper a demonstrator setup as well as results from a free-space transmission experiment are presented.

A key challenge for wireless sensor networks is minimizing the energy required for network nodes to communicate with each other, and this becomes acute for self-powered devices such as 'smart dust'. Optical communications is a potentially attractive solution for such devices. The University of Oxford is currently involved in a project to build optical wireless links to smart dust. Retro-reflectors combined with liquid crystal modulators can be integrated with the micro-machine to create a low power transceiver. When illuminated from a base station a modulated beam is returned, transmitting data. Data from the base station can be transmitted using modulation of the illuminating beam and a receiver at the micro-machine. In this paper we outline the energy consumption and link budget considerations in the design of such micro-machines, and report preliminary experimental results.

We consider a terrestrial free-space optical communication link affected by atmospheric turbulence. By means of numerical simulations we estimate the reduction in scintillation achieved by the combined effect of aperture averaging and four transmitters when mutually-incoherent sources and a realistic detector size are used. We evaluate two space-time coding schemes: the first scheme uses a repetition code operating on four transmitters, and the second scheme uses a size-four rate-one space-time block code, originally proposed for wireless radio-frequency links. Both schemes deliver large signal-to-noise ratio gains compare to a single-beam system. The first scheme gives the best performance in all cases because of the natural incoherence found among optical sources.

Receiver architectures are suggested for avalanche photodiode (APD) array detection of binary pulse-position modulated (PPM) in direct-detection free-space optical (FSO) communication systems. It is assumed that the received signal intensity is large enough so that the integrated currents at the outputs of APD detectors may be modelled accurately as Gaussian random variables. Receivers motivated by the maximum a posteriori (MAP) rule are suggested for APDs operating in Gaussian regime. Due to the complexity of the suggested receivers, suboptimum combining schemes are presented. In one scheme, a simple summation of the output of APDs is utilized to render decision. In alternate schemes, a weighting mechanism that combines the output of detectors in an optimum manner is suggested. The proposed weighting mechanism maximized signal-to-noise ratio (SNR). It is assumed that the signal intensity is impaired by optical scintillation and that the signal intensity is estimated at the receiver. Simulation and analytical results are presented to underscore the superiority of the weighted combining methods as compared to the simple summation algorithm, and to compare the performance of the optimum receivers with the linear receivers suggested here.

Establishing and initially configuring a Free Space Optical (FSO) backbone is a challenging problem, especially when nodes only have local connectivity information and a limited number of transceivers. The problem of configuring an initial connected topology or bootstrapping a directional FSO network can be formulated as a Minimum Degree Spanning Tree (MDST) problem, which is known to be NP-Complete. Recently, we developed a distributed approximation algorithm, which constructs a spanning tree with maximal node degree at most one larger than that in the optimal solution [1]. In such a distributed approach, nodes need to coordinate their local decisions to collectively set up a connected topology. For that purpose, algorithms and protocols for local information exchange/dissemination and synchronization are required. This paper presents the design, implementation and evaluation of a complete bootstrapping process model for FSO networks. Our model integrates the algorithm presented in [1] with the required communication and synchronization mechanisms to guarantee the efficient emergence of overall network connectivity from local interactions between individual nodes. Time performance results for the overall bootstrapping process are presented. Our scheme allows the network to form a connected topology whenever one exists and shows linear time complexity.

Intensity scintillation and beam wander caused by atmospheric turbulence are two significant phenomena that affect free space optical (FSO) communication links. 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 index parameter, 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. This paper will present the most accurate empirical data to date for the weak and intermediate turbulence regime. Such results can help build upon existing empirical data and lead to the development of new theories. Aperture averaging of the received irradiance is also shown to be independent of the shape of the receiver aperture, and depends only on its area. This finding allows the use of refractive or catadioptric receivers, whichever is convenient, and the same amount of aperture averaging will be achieved for equal unobscured aperture areas. This can make the telescope design for an FSO receiver more compact.

This paper presents new results on adaptive control of jitter in laser beams. Experimental results illustrate the capability of a recently developed method for variable-order adaptive control reduce jitter in bandwidths well beyond the bandwidth of linear time invariant control systems. The adaptive control loop is based on 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. The main innovation in this paper is frequency weighting in the adaptive control loop to emphasize the relative importance of jitter in particular bandwidths and mitigate the effects of high-frequency sensor noise. 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.

Mitigation of index of refraction turbulence (IRT) effects is crucial in long-range atmospheric communication links. Diversity-transmission is one favorable way for fading compensation. One of several different diversity concepts is the exploitation of the wavelength-dependent index of refraction of the atmosphere, which leads to ideal stochastically independent fading at different wavelengths, depending on the scenario. This concept is here named wavelength-diversity (WLD). Theoretical analysis and numerical simulation for the strength of this effect are given and verified by experimental tests.

In this paper, the performance of quad-APD spatial tracking loop (STL)in the presence of scintillation is studied for a frozen atmosphere turbulent model for free-space communications (FSO). The atmospheric turbulence responsible for the beam scintillation is assumed to follow the weak turbulence model, described by Rytov approximation, which in turn suggests log-normal statistics for the received optical signal intensity. It is assumed that pointing error in large part is due to atmospheric wander and that the correlation time of the beam wander is comparable to the correlation time of the amplitude variations due to amplitude scintillation. Provided recently reported estimation strategies, a new model for spatial tracking is proposed where estimates of channel coefficient are used to adjust the gain of the tracking loop, enabling an adaptive bandwidth adjustment in the presence of correlated fading. The performance of the proposed loop as well as that of the standard tracking loop are assessed and compared via simulation in terms of the mean square tracking error (MSTE).

Laser beam propagating through the dynamically changing atmosphere is subjected to severe wavefront distortions caused by the optical turbulence. The resulting spatial and temporal fields of the refractive index lead to performance degradation in the form of reduced signal power and increased BER, even for short link ranges. An electrically addressed liquid crystal spatial light modulator (SLM) is proposed to perform correction of the optical path difference (OPD) pattern resulting from the atmospheric distortions. Controlling every individual pixel of the SLM is a rigorous and time-consuming task that calls for a stable and simple procedure that could be performed in real-time. This could be addressed by approximating the phase profile of the distorted beam using Zernike formalism, which provides efficient mapping between large number of SLM pixels and smaller number of coefficients of Zernike polynomials. A possible solution to the dynamic correction problem is the application of Simplex optimization by Nelder and Mead, which is well known for fast improvement of an optimization metric. As has been shown before, this approach presents a problem of locking up in local minima while correcting dynamic changes. This paper presents experimental results of different approaches to resolve this problem by modifying simplex procedure as well as modification in a previously presented experimental setup.

Free-space laser communication has been demonstrated with application potential in many areas such as line-of-sight communications, satellite communications and the last mile solution in a fiber optics networking. Both 0.8 and 1.5 micron wavelengths are currently used in state-of-the-art free space laser communication systems; unfortunately the system performance is imposed by atmospheric turbulence. To reduce the atmospheric effect in free-space laser communication systems, several techniques have been used, such as adaptive optics, aperture averaging and multiple transmitters; however, significant improvement has not been achieved. Theoretically, the seeing effect may be released using a longer wavelength. In this paper, we present a 3.5 micron free-space laser communication system model and its system performance evaluation. A 3.5 micron propagation model based on MODTRAN simulation results in different weather patterns is presented first, and a propagation link budget system model is described after that. The propagation channel performance evaluation results are presented by means of bit error rate versus various propagation distances.

Establishing a free-space optical communications link between a ground station and an 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. Several models have recently been developed to study the effects of turbulence on a ground-to-geostationary satellite uplink, but published results from these models have not incorporated the effects of tracking. Two effects that need to be considered with a tracking system include a pointing lead-ahead issue and an aperture mismatch issue. The lead ahead problem involves the fact that the uplink beam needs to be pointed at the position the satellite will be when the pulse arrives. This problem exists even for a geostationary satellite. The aperture mismatch involves the sensing of the beacon through an aperture that is smaller, or larger, or even displaced from the uplink transmit aperture. In both cases, the sensed tilt might not be the same as the tilt required for optimal transmission to the satellite. These two issues have been studied separately, to some extent, by researchers in the past, although their combined effects in an optical communications uplink have never been investigated. The present paper applies previously published theories in an initial assessment of the impact of these tracking issues in an optical communications uplink. The analysis considers geosynchronous Earth orbit satellites as well as low Earth orbit satellites.

A high bitrate optical downlink was performed by the stratospheric optical payload experiment (STROPEX), a part of the EU CAPANINA project. The STROPEX objectives were to design and build the necessary hardware to demonstrate an optical backhaul downlink from a stratospheric platform to the ground and to carry out channel measurements on the link. A successful measurement campaign at ESRANGE near Kiruna, Sweden achieved all of these objectives. The transportable optical ground station received an almost error free 1.25 Gbit/s data signal from the payload over a distance of 64.3 km with a bit error rate of better than 10^-9. This paper gives an overview of the stratospheric optical payload experiment, focusing on the airborne free-space experimental laser terminal (FELT). Additionally, the successful measurement campaign is described and the operation of the experiment is outlined.

An optical link has been established between the Canary Islands La Palma and Tenerife. A 1064-nm transmitting laser was located on La Palma whereas a BPSK communication receiver and measurement instruments were installed in ESA's OGS on Tenerife. Beside the demonstration of a high-data-rate coherent signal transmission, the goal of the experiment was to measure the effects of the atmosphere on the beam propagation in order to estimate its impact on optical links. In particular, wavefront distortions have been investigated by means of a DIMM instrument and scintillation was observed by imaging the pupil of the OGS telescope on a CCD camera. Strong scintillation was observed during all the experiment with scintillation peaks at sunsets and sunrises, and saturation at about noon. Because of the narrowness of the beam (15-μrad divergence), beam wander has been a serious issue. Statistical results are compared with theory. Recommendations regarding the specifications of optical coherent systems in such detrimental conditions are given.

We report the concept and preliminary test results of development of a three-stage beam tracking system for demonstration of a Gbps free-space laser communicator. The development goal has been high-speed optical inter-satellite communications. The tracking system consists of three sub-systems: the coarse tracker (a 10.5 cm-diameter motorized Maksutov telescope with acquisition sensor); the intermediate electro-optical tracker (a voice-coil and a Si-based position detector), and the nonlinear optical fine tracker (a nonlinear cell with a liquid crystal). Three-stage tracking allows for efficient compensation of jitter of up to kHz while maintaining a sub-microradian pointing precision. A combination of the tracking system with a signal feed / modulation unit has been designed to demonstrate a 2.5-10 Gbps-level performance at distances of about 5-7 km.

The scanner of orthogonally tilting double prisms is researched for testing the performance of tracking performance in inter-satellite laser communications for the first time. With the reduction ratio of more than a hundred times from the change rate of deviation angle of beam to that of tilting angle of each prism, the scanner can reach the scanning accuracy of sub-microradian order but facilitates the mechanical structure design. The theoretical analysis performed, as well as the validation experiment, indicates the scanner can meet the requirements of the scanning accuracy superior to 0.5 μrad with the scanning range greater than 500 μrad.

The paper comprehensively analyzes and evaluates the mechanical property and thermo-structure distortions of the assembly of circular wedge prism in the device by method of finite element analysis. Regarding the prism assembly as a finite element model, the optical-mechanical-thermal integrating analysis is done. In terms of the principle of structure statics, the structure design and intensity of the prism assembly is verified and checked, and the analysis of surface deformation of the prism is correspondingly provided under static loading; then the thermal elasticity distortions of the prism are analyzed and the estimation of optics performance of the circular wedge prism is given. The analysis results show: the maximal distortion of the prism assembly is 10nm magnitude and the maximal stress is 0.403Mpa, which has much tolerance to the admissible stress of material and the precision requirement of structure; By comparing thermal-structure coupling analysis with statics analysis, the influence of heat effect on the prism surface deformations is proved far greater than the influence of static loading, so the strict temperature-controlled measures must be taken when the device is used.

A laser collimator is necessary for the testing and verification of the PAT performance of inter-satellite laser communication terminals. However, the terminals mostly have a large field of view for the acquisition and a high angular accuracy for the fine tracking needed to be examined. A single collimator has the conflict to reach at both a large field of view and a fine resolution. To compromise, a double-focus laser collimator is proposed. The collimator is mainly composed of a primary lens, a beam splitter, a secondary lens and some reflectors. The primary lens with a 9.9m focal length directly forms the long focal length arm of the collimator. The combination of the primary lens and the secondary lens has a new focal length of 1.3m and constructs the short focal length arm of the collimator. With two CMOS imaging sensors, the collimator can realize a 1.1mrad field of view with a <1μrad resolution in the focal plane of the long focal length arm and a 8.3mrad field of view with a 8.2μrad resolution in the focal plane of the short focal length arm. In combination with a coarse beam scanner (±15°) and a fine beam scanner (1mrad) to simulate the mutual angular movement between two satellites, the united system is capable to test the full PAT performance of inter-satellite laser communication terminals. The optical layouts of the collimator and two detecting units are illustrated. The optical design of the collimator is detailed. The mechanical design of the collimator is given.

Optical communication at 1.25Gbps was successfully demonstrated in a downlink from a stratospheric balloon platform at 22km altitude to a Transportable Optical Ground Station. The experiments took place at ESRANGE, Kiruna, Sweden in August 2005. In addition to optical communications, several atmospheric measurement instruments (Differential Image Motion Monitor, Turbulence Profiler) were used to study the influence of atmospheric turbulence on the optical link. A description of the measurement instruments is given and results of the turbulence instruments (Fried parameter r₀, C_n² profile) are presented.

Inter-satellite laser communications attracted more and more attentions due to its excellent performances compared with the RF communications. But the test and verification of the communication terminals are very difficult because of the accuracy and aperture requirement. Large-aperture laser beam scanner was introduced to simulate the relative movement between satellites in the process of ground test for terminals. The scanner includes two same circular wedge prisms, which can scan an arbitrary position in a cone zone. The motivation of the assembly is to guarantee the accuracy of the scanner. Since the large-aperture and high-precision of the scanner, the assembly and test become a very troublesome problem. The test includes: (1) confirm the principal section of the wedge prisms; (2) test the optical quality of the wedge prisms; (3) confirm the scan range of the scanner; (5) test the scan accuracy of the scanner. The test results indicate the assembly process is reasonable. The scan range and accuracy can satisfy the requirement of the ground test of the inter-satellite laser communications terminals.

A simple channel model of wireless optical links based spatial coding is described and two basic channel constraints are outlined. For representing the spatial patterns as signal points, a signal space based 2-D orthonormal basis functions is defined. After that, critical measures of performance are also defined as metrics for comparing candidate modulation schemes. Then three candidate modulation schemes exploiting spatial dimensions are proposed, which are spatial on-off keying, spatial pulse position modulation and spatial quadrature amplitude modulation schemes. At last, the three modulation schemes are compared by power efficiency and bandwidth efficiency. The appropriateness of each scheme is also discussed.

An experimental study has been made on the contribution to the effective scintillation index due to two retroreflectors, as a function of retroreflector spacing. For closely spaced retroreflectors the effect of coherent interference at the receiver is seen to increase the effective variance of the received signal, whereas spatial averaging is apparent for more widely spaced retroreflectors. The scintillation index, probability density functions, power spectral densities and fade rates are all affected by the interference. The range, under which these experiments were conducted, was typically 500 - 800m over mixed water/land terrains. The interrogator used a monostatic, 1550nm laser probe beam with a divergence of 0.4 mrad and had a 50mm diameter receiver aperture. Data sets of received power were recorded for durations of 10s each, using a photodiode with a bandwidth of 100kHz. For comparison, the received power from a single retroreflector at various radial positions in the probe beam was recorded. Knowledge of the fade rates and fade durations is of practical importance in considerations regarding the optimal transmission of data packets.

For the test of communication-performances of intersatellite laser communication terminals (LCT's) in lab, we had designed an optical simulator for long-distance propagation of laser beams based on optical Fourier transformer and the cascaded imaging systems. Its maximum equivalent propagation distance reaches several hundred thousands kilometers and it can be used to evaluate the communication performances of LCT's under exact transmission distance. In this paper, effects of wavefront aberrations of lens on the simulator are studied in detail. Firstly, we estimate the influence of wavefront aberrations of Fourier transform lens and the cascaded imaging lens according to the 'Strehl intensity'. We found that the wavefront aberrations of Fourier transform lens caused important measurement error. Secondly, using the aberration theory, the influence of typical aberrations such as spherical aberrations, Coma aberrations, Tip/tilt aberrations of Fourier transform lens is analyzed. Finely, the measurement error caused by these aberrations is provided.