Optical acquisition, tracking and communication tests were performed between a Japanese laser communications terminal placed within the ESAs optical ground station at Tenerife, Spain and a European optical payload onboard the ARTEMIS satellite in geostationary earth orbit at 21.5° East. The optical communications tests at Tenerife were to verify the end-to-end optical characteristics such as intensity, sensitivity, wavelength, and polarization, as well as the modulation scheme of optical signals and acquisition sequences of the terminals under fairly good atmospheric conditions. The downlink's bit error rate was on the order of 10^-10 in spite of atmospheric turbulence. Atmospheric turbulence induced signal fading increased the uplink bit error rate, the best value of which was 2.5x10^-5. The Japanese laser communication terminal itself autonomously established and maintained the ground-to-satellite optical link with the ESA's optical payload from the beginning to the end of a 20-minute session. The test results show that the laser communication terminal which is to be launched with the Japanese OICETS satellite is optically compatible with the optical communications payload onboard the European ARTEMIS satellite.

This paper presents outline of the optical terminal for Next-generation LEO System (NeLS) in-orbit demonstration, which will be conducted as part of Phase 2 of NeLS project. Two small satellites are assumed to launch into GTO orbit changing distance between them from 500km to 3000km. Acquisition and tracking experiments with a star or planet and 2.4Gbps data transmission between two SmartSat is also planned. The design of optical terminal is briefly presented.

An optical sensors web in a star pattern is formed and tested utilizing modulating retro-reflectors, where a multitude of sensors was simulated by a retro-modulator on a mini-rover. A CCD camera provides the multi-mega-bit sensor signal. Two-dimensional acquisition of a retro-modulator located away in the field, by a base-station equipped with a laser and a receiver was demonstrated while reading out data at 10 Mbps data-rate.

When designing free-space optics systems, one key issue is to assess the impact of scintillations and to find an appropriate link margin to cope with atmospheric fading. Huge effort is spent to find mathematical models to describe laser beam propagation through the atmosphere. However, these models are quite cumbersome to use for the communications engineer. On the other hand, there are empirical models that try to describe the influence of scintillations and other system parameters in a simple and easy to use manner. Nevertheless, they are empirical and not based on theory. This paper is intended to close the gap between mathematical theory and empirical models. Therefore, a simple yet accurate receiver model is introduced. Based on turbulence theory and using the recently proposed convolution method assuming independent sub-aperture intensities, probability distributions of the received power are derived. Aperture averaging as well as multiple transmitter systems can be described this way. Power penalties are found by numerically calculating the resulting bit error probabilities for varying mean values of received power. Finally a model for appropriate link margins under different atmospheric conditions, taking transmitter diversity and aperture averaging into account, is derived and compared to empirical models.

High-speed free-space optical communication systems have recently utilized components that have been developed for fiber-optic communication systems. The received laser beam in such a system must be coupled into a single-mode fiber at the input of a commercially available receiver module or a wavelength division demultiplexer. However, propagation through atmospheric turbulence destroys the spatial coherence of a laser beam and limits how much of the available power can be coupled into the single-mode fiber. In this paper, we numerically evaluate fiber coupling efficiency for laser light distorted by atmospheric turbulence. This involves the overlap integral of the optical field and the fiber mode and the calculation makes use of the mutual coherence function of the incident light. The results for weak fluctuation conditions indicate what level of coupling efficiency can be expected for a given turbulence strength. In addition, the results provide an estimate of how long the link distance can be before the coupling efficiency degrades to an unacceptable level.

Performance of long-range mobile laser systems operating within Earth's atmosphere is generally limited by several factors. Movement of the communicating platforms, such as aircraft, terrain vehicles, etc., complemented by mechanical vibrations, is the main cause of pointing errors. In addition, atmospheric turbulence causes changes of the refractive index along the propagation path that lead to phase distortions (aberrations), thus creating random redistribution of optical energy in the spatial domain. The combined effect of these factors leads to an increased bit-error probability under adverse operation conditions. While traditional approaches provide separate treatment of these problems, suggesting the development of high-bandwidth beam steering systems to perform tracking and jitter rejection, and wavefront control for the mitigation of atmospheric effects, the two tasks could be integrated. In this paper we present a hybrid laser beam steering/wavefront control system comprising an electrically addressed spatial light modulator (SLM) installed on the Omni-Wrist sensor mount platform. The function of the Omni-Wrist is to provide coarse steering over a wide range of pointing angles, while the purpose of the SLM is twofold: it performs wavefront correction and fine steering. The control law for the Omni-Wrist is synthesized using the decentralized approach that provides independent access to the azimuth and declination channels, while the algorithm for calculating the required phase profile for the SLM is optimization-based. This paper presents the control algorithms, the approach to coordinating the operation of the both systems and the simulation results.

We report a laser link that can correct atmospheric aberrations. We use a fiber collimator array, fed by a master oscillator with multiple fiber amplifiers (MOMA), and accomplish phase adjustment via pump diode current control. Each of seven channels is tagged by a different 1-20 kHz diode current dither. At the receiver, each channel's phase information is extracted from the <50 kHz signal. Our measurements show 5 kHz phase adjustment capability, so even turbulence-induced aberrations, as well as typical atmospheric aberrations (< 200 Hz) can be corrected. Only in >~100 km-range scenarios is the correction bandwidth limited by light's travel time. The low dither frequencies and amplitudes do not interfere with the typically GHz laser communications signal. Importantly, our system reduces transmitter power requirements by correcting small pointing errors and atmospheric-path aberrations. Of course the multiple-fiber amplifier array also enables power scaling. We describe our near- and far-field beam measurements in the laboratory.

The effects of the atmosphere on laser beam propagation are briefly reviewed, with the emphasis on scintillation and fog attenuation. It is shown that even saturated scintillation can be rendered harmless by the combination of receiver aperture averaging and multiple spatially diverse transmitters. Fog attenuation is shown to be the design driver for high availability links, and design techniques to increase the clear air link margin can improve link availability. Based on measured data from fSONA FSO hardware, it is shown that approximately four 9s of availability can be achieved in the most challenging North American cities at 350m range. With some design improvements (such as active pointing), FSO systems can potentially achieve four 9s availability at 1 km range.

A conceptual design is presented to achieve efficient, static, economical and reliable collection of modulated lasercom beam signals propagating from space to ground through turbulent media. Multiple small apertures collect signals that are detected and coherently combined at the video level.

We describe several properties of deep space optical channels that lead to an appropriate selection of modulation format, pulse position modulation (PPM) order, error control code rate, and coding scheme. The selection process is motivated by capacity considerations. We compare the Shannon limit to the performance of Reed-Solomon codes and convolutional codes concatenated with PPM and show that, when iteratively decoded, concatenated convolutional codes operate approximately 0.5 dB from capacity over a wide range of signal levels, about 2.5 dB better than Reed-Solomon codes.

The satisfaction of all communication needs from single households and business companies over a single access infrastructure is probably the most challenging topic in communications technology today. But even though the so-called "Last Mile Access Bottleneck" is well known since more than ten years and many distribution technologies have been tried out, the optimal solution has not yet been found and paying commercial access networks offering all service classes are still rare today. Conventional services like telephone, radio and TV, as well as new and emerging services like email, web browsing, online-gaming, video conferences, business data transfer or external data storage can all be transmitted over the well known and cost effective Ethernet networking protocol standard. Key requirements for the deployment technology driven by the different services are high data rates to the single customer, security, moderate deployment costs and good scalability to number and density of users, quick and flexible deployment without legal impediments and high availability, referring to the properties of optical and wireless communication. We demonstrate all elements of an Ethernet Access Network based on Free Space Optic distribution technology. Main physical parts are Central Office, Distribution Network and Customer Equipment. Transmission of different services, as well as configuration, service upgrades and remote control of the network are handled by networking features over one FSO connection. All parts of the network are proven, the latest commercially available technology. The set up is flexible and can be adapted to any more specific need if required.

The performance of a coherent free-space optical communications system is investigated. Bit Error Rate (BER) performance is analyzed, and laboratory equipment and experimental setup used to carry out these experiments at the Jet Propulsion Laboratory are described. The key components include two lasers operating at 1064 nm wavelength for use with coherent detection, a 16 element (4X4) focal plane detector array, and data acquisition and signal processing assembly needed to sample and collect the data and analyze the results. Combining of the signals is accomplished using the least-mean-square (LMS) algorithm. Convergence of the algorithm for experimentally obtained signal tones is demonstrated in these initial experiments.

An optical array receiver concept is developed and analyzed. It is shown that for ground-based reception, the number of array elements can be increased without any performance degradation, as long as the array telescope diameters exceed the coherence-length of the atmosphere. Maximum likelihood detection of turbulence-degraded signal fields is developed for the case of pulse-position modulated (PPM) signals observed in the presence of background radiation. Performance of optical array receivers is compared to single-aperture receivers with diameters ranging from 4 to 8 meters, both in the presence of turbulence and in a turbulence-free environment such as space. It is shown that in the absence of atmospheric turbulence, single-aperture receivers outperform receiver arrays when significant background radiation is present. However, it is also shown that for ground-based reception of deep-space signals, the number of array elements can be as great as several thousand without incurring any performance degradation relative to a large single-aperture receiver.

Optical links from a spacecraft at planetary distance to a ground-based receiver presume a cloud free line of site (CFLOS). Future ground-based optical receiving networks, should they be implemented, will rely on site diversity of cloud cover to increase link availability. Recent analysis shows that at least 90% and as high as 96% CFLOS availability can be realized from a cluster comprised of 3-4 nodes. During CFLOS availability variations of atmospheric parameters such as attenuation, sky radiance and “seeing” will determine the link performance. However, it is the statistical distributions of these parameters at any given node that will ultimately determine the data volumes that can be realized. This involves a complex interaction of site-specific atmospheric parameters. In the present work a simplified approach toward addressing this problem is presented. The worst-case link conditions for a spacecraft orbiting Mars, namely, maximum range (2.38 AU) and minimum sun-Earth-probe (SEP) angle of 3-10° is considered. A lower bound of ~100 Gbits/day under the most stressing link conditions is estimated possible.

Laser communication systems developed for mobile platforms, such as satellites, aircraft, and terrain vehicles require fast wide-range beam steering devices to establish and maintain a communication link. Conventionally, the low-bandwidth, high steering range part of the beam-positioning task is performed by gimbals that inherently constitutes the system bottleneck in terms of reliability, accuracy and dynamic performance. OmniWrist, a novel robotic sensor mount capable of carrying a payload of 5 lbs and providing a full 180° hemisphere of azimuth/declination motion is known to be free of the most of deficiencies of gimbals. Provided with appropriate controls, it has the potential for becoming a new generation of gimbals systems. The approach demonstrated in this paper describes an adaptive controller enabling OmniWrist to be utilized as a part of a laser beam positioning system. It is based on a Lyapunov function that assures global asymptotic stability of the entire system while achieving high tracking accuracy. The proposed scheme is highly robust, does not require the knowledge of complex system dynamics, and facilitates independent control of each channel by full decoupling of the OmniWrist dynamics. The paper summarizes the basic algorithm and demonstrates implementation results.

For several future imaging and communications spacecraft, a challenging area of technology development is the fine acquisition, tracking, and pointing (ATP) control of the spacecraft and its payload. For example, some spacecraft with large aperture(s) in the range of 10~30 m diameter requires a few arc-seconds accuracy, 10~15 nano-radians jitter, and a fast slewing rate to acquire the target. Furthermore these stringent requirements are at risk of great structure and control interactions. This paper we will focus on the control of optical beam jitter. A Laser Jitter Control (LJC) testbed has been constructed to test jitter algorithms. The testbed consists of two fast steering mirrors (FSM), three position sensing modules (PSM), one diode laser, and several beam splitters and mirrors, all on an isolated Newport optical bench. Jitter is injected with one FSM and the other FSM is used to control it. The jitter spectrum, representing the on-orbit spacecraft and beam jitter environment, contains not only narrow band noise due to rotating devices such as gyroscopes and reaction wheels but also broadband noise. The performance of a Wiener Filter-adaptive algorithm with ideal reference signal is established as the baseline for comparison of adaptive control methods in suppressing both broadband and narrowband disturbances. Specifically, the Least Mean Squares (LMS) approach and the Gradient Adaptive Lattice (GAL) approach are investigated during these experiments.

A multi-fiber network is shown to provide programmable time delays so that an ensemble of pulselets can be temporally synchronized over a large field-of-view, leading to high-bandwidth, multi-aperture beam-steering systems, including MEMS and liquid crystal arrays.

With the advent of efficient fiber laser and amplifiers, low noise photon-counting detectors, turbo-codes, and low-cost ground receiver architectures, it is now feasible to consider very high rate data links from deep space. A set of options leading to a 10-100 Mbps link from Mars to Earth is described.

A systems-level analysis of a high data rate Mars to Earth optical communications link is presented. A feasibility of a minimum 10 Mb/s optical link with the possibility of achieving > 100 Mb/s under certain conditions will be shown. The link design employs a Pulse Position Modulated (PPM) 1.06 μm Mars transmitter with a photon-counting Earth receiver. This study will characterize system performance (link data rate) as a function of orbital position including the complex diurnal and annual variations in the Mars-Earth system. Key system impairments that vary diurnally/annually include loss and turbulence due to the Earth's atmosphere, daytime/nighttime sky background noise, background noise from Mars itself, and space loss due to the relative planetary distances. In addition, transmitter/receiver design parameters and their impact on system performance are discussed.

We develop a coding method called binary shaping that generates a binary channel input of arbitrary duty-cycle with Bernoulli statistics, which is useful for laser communications using generalized on-off keying. We couple binary shaping with a serially concatenated turbo code for forward error correction, and show by simulation that the performance of this scheme with iterative decoding approaches the capacity of generalized on-off keying on the Poisson channel for medium duty cycles, significantly exceeding the performance of pulse-position modulation (PPM) in this regime. Analogous to trellis shaping for the AWGN channel, binary trellis shaping utilizes a convolutional shaping code to which we apply the Viterbi algorithm at the encoder to minimize Hamming weight. We show how low-duty-cycle communications is a special case of information embedding and discuss how binary shaping relates to information embedding methods.

We present a new laser communications receiver architecture. It consists of an array of individually mounted telescopes, each with a Geiger-mode photon counting detector array. The detector outputs are sent to a central processor using standard digital networking hardware. The concept has many benefits including low cost and scalability.

NASA is presently overseeing a project to create the world's first free-space laser communications system that can be operated over a range much larger than the near-earth ranges that have been demonstrated to date. To be flown on the Mars Telecom Orbiter, planned for launch by NASA in 2009, it will demonstrate high-rate laser communications from Mars orbit to one of several planned earth receiver sites. To support 1-10 Mbps over the up to 400 million kilometer link, the system will make use of a high peak-power doped-fiber transmitter, a hybrid pointing and tracking system, high efficiency modulation and coding techniques, photon-counting detectors, and novel optical collector architectures that can point near the sun. The project is being undertaken by the NASA Goddard Space Flight Center (GSFC), MIT Lincoln Laboratory (MIT/LL), and the Jet Propulsion Laboratory (JPL).

NASA Jet Propulsion Laboratory (JPL) is interested in adding optical communications to its deep space communications network. Clouds adversely affect the transmission of optical communications; in order to mitigate the effects of clouds and achieve reliable communications, a geographically diverse set of ground receiver stations is needed. To study cloud effects on optical communications we have developed a high-resolution cloud climatology based on NOAA Geostationary Environmental Operational Satellite (GOES) imager data. The GOES imager includes multi-spectral channels, one visible and four infrared, at 4-km spatial resolution and 15-minute time resolution. Cloud detection is accomplished by modeling the radiance of the ground in the absence of clouds and comparing the actual radiance values from the imagery. A composite cloud decision is formed by objectively combining the results of the tests from the individual channels. Ground site selection studies are accomplished using the Lasercom Network Optimization Tool (LNOT). LNOT applies a discrete optimization algorithm to the cloud climatology dataset to find the optimal number and locations of ground stations for a given concept of operations. Applying LNOT to the JPL problem we find that 90% availability could be achieved with 4-5 ground stations in the continental US and Hawaii. We also present the results of a pilot study that includes 6 months of cloud data over South America.

Atmospheric laser communications using direct-detection systems do suffer from severe degradation caused by scintillation. Because the atmospheric cut-off frequency can be as low as 100 Hz, temporal averaging is not applicable in high-speed communications. The simplest way of reducing fading is to increase the receiver size and to take advantage of aperture averaging. Spatial and temporal variations of the received intensity have to be investigated in order to predict the efficiency of aperture averaging. This paper reviews briefly the theory of spatial averaging that characterizes the direct-detected optical power. For comparison purposes, results of measurements are presented. These measurements consist of recorded pupil intensity patterns for a scenario with known turbulence profile. Statistics derived from measurement data are compared with theoretical second-order statistics.

In this paper we introduce a simulation method for modelling clear-air atmospheric turbulence effects for long horizontal stratospheric paths. Divergence angles of several hundred microradians in combination with link distances up to 800 km require to adapt the appropriate resolution of the transverse optical field along the path. For this purpose, we implemented a propagation method in Cartesian coordinates. We choose two reference scenarios for high-altitude platform crosslinks and discuss the influence of simulation parameters to the derived results. Finally a method for computation of temporal IM/DD-time signals form simulated intensity matrices is presented.

The authors have recently developed analytical expressions governing atmospheric induced frequency fluctuations of an optical signal along a horizontal path. Expressions valid in conditions of weak irradiance fluctuations were derived using the Rytov approximation and extended to conditions of moderate to strong irradiance fluctuations via an effective atmospheric spectral model. However, many optical systems, such as coherent ground to satellite communication, imaging, and astronomical systems, operate in a slant path setting. In this paper, the horizontal path frequency variance results have been extended to slant path scenarios. Integral expressions for one-way slant path, both uplink and downlink, are presented. Additionally graphical results for various operational settings are also provided.

Recently several investigators proposed an extension of the Navy Aerosol Model (NAM) based on analysis of an extensive series of measurements at the Irish Atlantic Coast and at the French Mediterranean Coast. This work extends NAM by use of a similar analysis using data collected at three different Middle East Coastal areas: the Negev Desert (Eilat) Red Sea Coast, the Sea of Galilee (Tiberias) Coast, and the Mediterranean (Haifa) Coast. Results of the aerosol size distribution are compared with those obtained through measurements carried out over the Atlantic Ocean and Mediterranean Coasts. An analysis of these different results allows a better understanding of the similarities and differences between different coastal and open ocean zones. Parameterization is introduced. The aerosol particle concentrations and their dependences on wind speed for these coastal zones are analyzed and discussed.

An imaging LIDAR system for measuring vertical profile of the atmospheric refractive index turbulence has been developed and its performance demonstrated. The turbulence profile retrieval technique is based on image motion analysis. In the present work LIDAR measurements of C_n² vertical profiles are demonstrated. Unlike the existing turbulence models, the experimental results show the various strata and layers in the vertical turbulence profiles.

Most models of optical radiation propagation through turbulent media are based on the assumption that turbulence is of Kolmogorov's type. It is assumed, also, that this type of turbulence corresponds to fluctuations of passive scalar field (temperature, aerosol, etc.). In the present report we demonstrate that in a real atmosphere (above the surface layer) the turbulent field of passive scalar fluctuations can differ from Kolmogorov’s model. From the spectrum of the intensity fluctuation of LIDAR signals scattered by aerosol concentration inhomogeneities, the behavior of atmospheric turbulence spectrum (power law exponent γ) is estimated. As follows from the experimental data the power law exponent of the turbulent spectra should be different from the case of purely Kolmogorov's (-5/3).

Free space optical (FSO) communication systems offer several advantages over conventional radio frequency (RF) systems but, because of shorter wavelength, are subject to various atmospheric effects. Particularly significant in this regard is the signal fading below a prescribed threshold value owing primarily to optical scintillations associated with the received signal. In this paper we utilize some recent advances in the modeling of optical scintillation under weak-to-strong fluctuations associated with terrestrial links and examine fade probability and bit error-rate (BER) for direct detection systems using a single large aperture receiver and the BER for an array of smaller receiver apertures.