A low Earth orbit (LEO) satellite laser communication (lasercom) terminal, built under funding by the Ballistic Missile Defense Organization (BMDO), was flown as part of the Space Technology Research Vehicle 2 (STRV-2) experiment module. The STRV-2 experiment module was housed on the Air Force Tri-Service Experiments 5 (TSX-5) spacecraft, which was launched on June 7, 2000. The lasercom flight hardware weighs 31.5 pounds, with a volume less than a cubic foot, and is capable of communication at data rates up to 1 Gbps. The LEO satellite-to-ground lasercom experiment is designed for slant ranges up to 2000 km and elevation angles above 15 degrees over the horizon. This experiment's goal was to demonstrate a satellite lasercom link that would validate the capability and readiness of lasercom for inter-satellite crosslinks, and data downlinks from LEO Earth sensing satellites. The mechanical and electrical systems of the satellite terminal survived launch and were functioning correctly. Unfortunately, no lasercom link from space was achieved because the satellite terminal had troubles acquiring and tracking the ground terminal beacons. The acquisition sequence was designed based on a spacecraft interface specification. The primary reason for the lack of acquisition was that the ephemeris and spacecraft attitude control was out of specification. The acquisition sequence required that the ground terminal beacons illuminate the satellite. The divergence of the ground terminal beacons was designed to the ephemeris specification, which stated that ephemeris be within +/- 100 m cross-track and +/- 500 m in-track error would be provided prior to a satellite pass.

The SATEX I project, is a Mexican effort with the purpose of design, construct and operate an experimental microsatellite in low-earth orbit, in a university and multi-institutional environment. The scientific mission is focused on electronic telecommunications research with a Ka band experiment and the optical payload; also, a CCD camera is included for remote image acquisition. The SATEX Optical Payload (SOP) is an experimental system aimed to perform BER and attenuation measurements. The SOP consists in a laser transmitter in the 830 nm and a quad-photo receiver in the 530 nm. The experiment is divided in two features: the downlink where the measurements will be performed and the uplink that will be used to perform the pointing of the experiment. The SOP has a control system to establish and keep a link by opto-electro-mechanical means, which tracks and acquires the optical beacon. This beacon is a non-modulated light source generated by the earth station. When the satellite receives it, it is then used to acknowledge the location of the earth station; therefore, the modulated laser beam can be transmitted to Earth. The technical looks of design of all the subsystems and the prototype obtained are presented and the link calculation is discussed.

In the first stage of laser communications experiment between the International Space Station (ISS) and several ground stations using onboard laser communications demonstration equipment (LCDE), an experiment to track and point geodesic satellites with corner cube reflectors is planned in order to evaluate the pointing/tracking performance and optical/mechanical misalignments of LCDE. As a visibility analysis of existing geodesic satellites from LCDE, we counted the number of the satellite passes which continue more than five minutes in the year 2004 and found that there are 20 passes for EGS and five passes for TOPEX. In order to detect the sunlight reflected from the satellite body and to start the tracking, we need a sensitive tracking sensor whose sensitivity is better than -95 dBm. This sensitivity will be achieved by using a quadrant photo- detector with a GaAsP photo-cathode and optimizing the sensor field-of-view. To evaluate LCDE's pointing performance, a pulsed laser at 1552 nm wavelength will be transmitted to the geodesic satellite, and the light reflected from the corner cube reflector on the satellite will be received by LCDE. The current power budget shows the signal-to-noise ratio of the detection of the returned laser pulse will be about 5 dB.

A free space optical communication testbed using multi-channel optical code division multiplexing was demonstrated using a multiplexed optical transmitter utilizing SONET OC-12 signals, a pair of matched telescopes, and a single mode fiber coupled receiver and CDMA decoding system. An active beam alignment system was used at the receiver to maintain alignment on the receiver input fiber, with bit error rates under transmitter jitter of better than 2x10^-12 for a transmitter beam perturbation of 50 Hz and 45 (mu) rad peak to peak amplitude.

Clusters of small satellites which orbit in tight constellations and cooperatively perform radar, surveillance, data collection, and bent pipe communications, are being explored for potentially replacing the use of a single, larger, more complex satellite. Two basic requirements of these clusters are: communications must be available between any satellite pair, and range and position between any satellite pair must be continually monitored for computing the antenna configuration. While up/down links will most likely be RF, compact, lightweight, low power, lasercom direct detection optical packages can perform the crosslinking and position location operations. In this paper, a lasercom architecture is presented that utilizes pairs of relatively wide beam divergence optical beams to minimize pointing and tracking requirements and a point-to- point network topology. Data transfer is accomplished by the use of pulsed optical Code-Division-Multiplexing signaling formats, with address codes assigned to each satellite. A given satellite can establish a link with any other satellite by encoding data onto the proper address code. Some basic link budgets will be presented to scope the optical design in terms of power, data rate, and constellation size. Range and position can be obtained from the same lasercom architecture, using the address codes as ranging markers.

High data rate of laser communication systems should be matched by wide operational bandwidth of beam positioning. This could be achieved by the application of non-mechanical beam steering technologies utilizing acousto-optic phenomena. Major components of acousto-optic beam steering systems, a Bragg cell and a quadrant detector, are subjected to comprehensive laboratory testing and characterization as components of a control system. Mathematical and computer simulation model of the entire steering system, addressing system dynamics, cross-coupling of azimuth and elevation channels, and nonlinearity, and facilitating synthesis of advanced control laws, is developed.

In an intersatellite optical communication system, as the vibration of a satellite's platform it will influence bit error rate of the system. In this paper, the influence of vibration is researched in theoretical and experimental. The experimental results are given and the theoretical results are accorded with experimental results very well. This researching work will be important and useful for research of intersatellite optical communications.

High data rates of laser communication systems must be complemented by high tracking bandwidth of acousto-optic beam steering technology utilizing Bragg cells. This research investigates applications of advanced control techniques to enhance characteristics of a laser beam steering system containing a Bragg cell - quadrant detector combination. Laboratory characterization of both devices resulting in detailed mathematical description and simulation models is presented. An adaptive model reference controller is designed and validated by computer simulation.

We analyze the properties of erbium-doped fiber amplifiers (EDFAs) with respect to the gain provided for RZ input pulses of various duty cycles, but at a constant average input power. For representative data rates the EDFA is average power limited, i.e. the gain of the amplifier is determined by the average input power only, regardless of the RZ duty cycle. The time constants associated with the gain dynamics depend on the peak input power of the pulses, which justifies a detailed comparison between NRZ and RZ operation. We derive a dynamic model of the fiber amplifier, treating the erbium doped fiber as a basic three level laser system and neglecting the influence of amplified spontaneous emission (ASE). The results from our model are valid whenever saturation of the EDFA due to ASE can be neglected, i.e. for sufficiently large input signal levels. The numerical calculations show that average power limitation can be assumed for data rates above a certain threshold rate; depending on the pump power, threshold values for the data rate are found to be on the order of several hundred kbit/s. Further, the predictions of the theoretical model are confirmed by experimental verifications.

In this paper we investigated potassium Faraday filter at 770 nm theoretically and experimentally. With a potassium cell of 0.01 m in length in an axial magnetic field of 0.0815T, line-center operation has been achieved. The calculated peak transmission has reached 93% with a FWHM bandwidth of only 1.6 GHz, the measured FWHM bandwidth of the filter is 2.2 GHz, which is in detail agreement with the theoretical result.

We discuss several potential methods of generating optical RZ data signals, distinguishing between direct RZ modulation and modulation of a primary pulse train which is either generated by using a modelocked laser, by sinusoidally driving of an external modulator, or by gainswitching of a laser diode. We analyze the properties of each method with regard to the most critical aspects for space-borne laser communication systems such as repetition rate, duty cycle, extinction ratio, frequency chirp, timing jitter, robustness, complexity, commercial availability, and lifetime. Most modelocked lasers are highly sensitive to ambient perturbations, necessitating accurate temperature control and mechanical stabilization. Also, they typically provide pulses with less than 10% duty cycle, which can result in a decreased sensitivity of optically preamplified receivers. Directly modulated semiconductor lasers are compact and robust but suffer from large frequency chirp, which deteriorates the receiver sensitivity. One reliable RZ source is a conventional DFB semiconductor laser with two intensity modulators, one for pulse generation and one for data modulation. Both Mach-Zehnder modulators co-packaged with a laser diode or monolithically integrated electroabsorption modulators should be considered. These modulators can provide almost transform-limited pulses at high repetition rates and with duty cycles of about 30%. Robustness and lifetime are highly promising.

The laser diode is widely used in optical fiber communications and intersatellite optical communication. And there are some differences between the modulator of laser diode in these two communications. This paper has mainly analyzed the modulation in these two communications. And because the high power AlGaAS semiconductor lasers operating around 800 nm have been successfully qualified for deployment in many intersatellite optical communication programs, in this paper we have given the experimental simulation results of some modulation methods by using the model of laser diodes. From the simulation results, we have compared the methods of laser diode modulation. And we can know which method is suitable for optical space communications, which demand both high power and high bandwidth.

We give optimum values for the bandwidths of realistic optical and electrical filters (optical Fabry-Perot filters and fiber Bragg gratings, electrical Bessel filters and first order RC low pass filters), as well as for realistic NRZ and (33% duty cycle) RZ input pulses for a free space laser communication system employing an optical booster and a direct detection receiver with optical preamplification. Different extinction ratios, the presence of background radiation, and the influence of the booster amplifier's amplified spontaneous emission (ASE) are emphasized. Our results show that the optimum optical filter bandwidth (both for NRZ and RZ) has to be sought in the range of 1.5 to 3 times the data rate. Using the optimum filter bandwidths, RZ coding yields a sensitivity improvement of up to 1.5 dB compared to NRZ transmission. For typical system parameters and link distances higher than several thousand kilometers the booster ASE becomes less important than strong background radiation, while it causes severe sensitivity degradations for shorter distances.

A unique active pointing system for point-to-point optical communications capitalizes on the limited angular travel requirements to achieve a high angular precision, frictionless, lubrication-free design, at a price point consistent with commercial production. Gimbal components and three design configurations were traded for performance, cost, reliability, and technical risk. A gimbal ring configuration with voice coil actuators, flexure bearings, and LVDT position sensors was selected as the baseline, and prototypes were fabricated and assembled. Servo integration and testing is ongoing, which will be followed by commercial production.

The use of sun-illuminated Earth images has been explored to aid in meeting the stringent pointing requirements for deep space optical downlink telemetry. It has been shown that fully illuminated Earth images provide a symmetry that easily lends to the determination of the geocenter. Since more often than not, the Earth is only partially illuminated by the sun, additional work is required to meet the pointing requirements under all illumination conditions. This paper develops an algorithm for determining the geocenter of the Earth regardless of the illumination by the sun using sub-pixel scanning and a simple thresholding technique. To complete this approach the acquisition algorithm is paired with a tracking technique based on maximum likelihood estimation. Results have indicated that the algorithm works well to locate and track the position of the Earth and, thereby, a receiving station. Results of the experiments are presented and compared with other techniques.

A successful controller design is crucial for establishing and maintaining an optical link between free-space communication stations engaged in a laser communication session. This task is quite difficult due to nonlinear behavior, cross-coupled dynamics, and time-varying characteristics of all known beam steering technologies. A novel adaptive control technique utilizing Lyapunov function to ensure global asymptotic stability of the system, thus resulting in a highly robust system performance, is developed. The technique applied to a piezo-electric mirror setup results in a highly efficient controller design that does not require prior knowledge of system dynamics, while providing independent access to azimuth and elevation positions of the laser beam. The paper presents the basic algorithm and demonstrates the results of its application.

An optical downlink is a method for high data-rate communication for downloading data acquired by a satellite to terrestrial networks. However, directing a laser beam at the ground may not be safe for the eyes of human observers when such irradiance is higher than the maximum permissible exposure (MPE). The MPE is an index of acceptable irradiance for eye safety as a function of the duration that human eyes are exposed to a laser beam. The hazard level of free-space laser communication systems should be examined with regard to eye safety. The probability that irradiance exceeds the MPE can be considered statistically based on the characteristics of the laser tracking and transmission system in the acquisition and tracking phases. As an example, the probabilities calculated from the results of a thermal vacuum test with the laser communications terminal onboard the OICETS satellite were used to assess the risk of impairing eye safety. As a result, the irradiance of the laser beam directed from the satellite to the ground can be evaluated relative to the permissible irradiance level for the eyes of general human observers.

We investigate the influence of chirp on free-space laser communication systems employing optically preamplified direct-detection receivers. We analyze the changes of receiver sensitivities in the presence of chirp and optimize both the optical and electrical receiver bandwidth with regard to sensitivity. For return-to-zero (RZ) coding with 33% duty cycle the receiver sensitivity decreases due to spectral broadening of the received signal by typically 3 dB when the chirp parameter amounts to alphaequals5. For optimized bandwidths, the penalty can be kept as low as 1 dB. In contrast, in the case of non-return-to-zero coding, chirp can reduce inter-symbol interference (ISI), thus improving receiver performance due to a pulse compression effect. The gain in sensitivity can be as large as to 1.8 dB when the magnitude of the chirp is alphaequals4. A larger chirp decreases the receiver sensitivity. In the case of NRZ, the optimum receiver bandwidths are quite insensitive with regard to chirp.

In the article the project of a laser channel of remote power transmission in space is shown. In details is circumscribed elements and system for implementation of experiment on power transmission, within the frame of the possible scientific and applied projects of experiments on the International Space Station (ISS), on a distance more than 1000 m with a level of power more than 1000 W. The capability of increase of power of radiation is shown. The capability of reaching efficiency of a channel of power transmission up to 30% is designed. Also in the article a number of possible additional applications of the given system-data transfer with high speed and measurement of geometric parameters of interposition satellite-satellite is shown. The article contains analysis of using a channel power transmission for information-measuring whole.

Progress has recently been made in both the collection and modeling of fluid-optic disturbances imparted by compressible, shear flows. This field of research, termed Aero-Optics, has been motivated primarily by the development of directed-energy weapons; however, it is equally applicable to a general class of applications involving laser transmission and reception from aircraft. Examples of these-type applications include free-space, laser transmission from ground to air, from air to air and from air to space. In the present paper, we examine the effect of laser transmission through high-Mach-number, subsonic, compressible free shear layers on the ability to focus the beam on distant targets. Time-resolved time series of distorted wavefronts due to propagation through a Mach-0.8 free shear layer collected at the Aero-Optics facility at Arnold Engineering and Development Center, are used as the input to a Fourier-Optics routine that computes time series of far-field irradiance patterns. These patterns are then used to compute the time-averaged Strehl ratio directly, and these are compared to time-averaged Strehl ratios computed using the wavefront's rms Optical Path Differences and the large-aperture approximation. Conclusions are drawn about the appropriateness of using the large-aperture approximation for Aero-Optic-type wavefront aberrations.

For a Gaussian-beam wave incident on a point target, we use a recently developed scintillation theory and the gamma- gamma probability density function to calculate the fade statistics of the echo wave as a function of threshold below the mean irradiance. We also compare the results with experimental data collected by a coherent receiver array over a 1-km propagation path to the target using from one to eight apertures of the array. The propagation channel was determined to be a bistatic configuration for the lidar system. Values of inner scale and refractive index structure constant were simultaneously measured during the experiment by use of a scintillometer instrument. The scintillation theory employed in this work utilizes a spatial filter function that results from adapting a modulation scheme in which it is assumed that the intensity of the propagating laser beam is a product of two quantities: one related to the large scale turbulent eddies of the atmosphere and the other related to the small scale eddies. The theoretical curves arising from this analysis provide a good fit with experimental data up to four active apertures of the array, but deviate somewhat from the data for a greater number of apertures.

Infrared satellite communication system performance is estimated from Barbaliscia's worldwide millimeter wave attenuation maps. The attenuation maps are used to derive new results for total liquid water content in clouds, which in turn is used to estimate 10 micron and 1 micron attenuation. The liquid water content of severe cloud cover is found to be fatal for most laser satcom, but cloud cover at the 80 percentile level would allow attractive 10 micron satellite communication throughout most of the Rocky Mountain states and the state of Maine. Site diversity, with sites separated by 100 km, would allow the infrared system to approach normal satcom reliability standards.

Observations made during a mountain-top-to-mountain-top horizontal optical link demonstration are described. The optical link spans a range of 46 Km at an average altitude of 2 Km above sea level. A multibeam beacon comprised of eight laser beams emerging from four multimode fiber coupled lasers (780 nm) is launched through a 0.6 m diameter telescope located at the JPL Table Mountain Facility (TMF) in Wrightwood, California. The multibeam beacon is received at Strawberry Peak located in the San Bernardino Mountains of California. The NASA, JPL developed optical communications demonstrator (OCD) receives the beacon, senses the atmospheric turbulence induced motion and using an upgraded fine steering loop actively points a communications laser beam (852 nm, 400 Mbps on-off key modulated, PN7 pseudo random bit sequence) to TMF. The eight-beam beacon allowed a four-fold reduction in normalized irradiance or scintillation index. This in turn was sufficient to eliminate beacon fades sensed by the OCD and enable performance evaluation of the fine steering loop. The residual tracking error was determined to be +/- 1.1 to +/- 1.7 (mu) rad compared to a model prediction of +/- 3.4 (mu) rad. The best link performance observed showed average bit error rates (BER) of 1E-5 over long durations (30 seconds); however, instantaneous BERs of at least 0.8E-6 over durations of 2 ms were observed. The paper also discusses results pertaining to atmospheric effects, link analysis, and overall performance.

Two commercially available large area silicon avalanche photodiodes (APD) were characterized in the laboratory. The response of the APD's to a sequence of 8-bit pulse position modulated (256-PPM) laser pulses, with and without additive background noise, was recorded and stored for post analysis. Empirical probability density functions (pdf's) were constructed from the signal and noise slot data and compared to pdf's predicted by an analytical model based on Webb+Gaussian statistics. The pulse sequence was used to generate bit-error rate (BER) versus signal photons per pulse plots, albeit with large error bars due to the limited number of signal pulses stored. These BER measurements were also compared with analytical results obtained by using the Gaussian and Webb+Gaussian models for APD channel statistics. While the measurements qualitatively reflect features predicted by theory, significant quantitative deviations were displayed between the measurements and theory. The source of these discrepancies is not currently well understood, but it is surmised that inaccurate knowledge of detector parameters such as gain and noise equivalent temperature models may explain the discrepancies.

In this paper we derive the capacity of Pulse Position Modulation (PPM) on a general soft output, memoryless channel, and evaluate the capacity formula for a variety of optical channel models, including AWGN, Webb, and Webb plus Gaussian distributions. Unlike a typical RF link, the optical channel has correlated signal and noise, complicating the statistical model to the point that capacity and code performance cannot be summarized by a single SNR parameter. Nevertheless, we are able to define a small set of fundamental parameters (two for AWGN and three for Webb) which are sufficient to determine the capacity. Numerical results indicate that over a wide range of operating points, a single fundamental parameter dominates the capacity calculation. A second contribution of the paper is the description of the relationship between the fundamental parameters and a multitude of physical parameters that describe the laser, channel, and detector. Using this relationship and the gradient of capacity, the sensitivity of capacity with respect to each fundamental and physical parameter is derived. This enables engineers to focus laser and detector development efforts in areas that will result in the largest capacity increases.

Ground-based reception of optical signals from space suffers from degradation of the optical phase-front caused by atmospheric turbulence, leading to a reduction in the effective diameter of the receiving aperture and to random fluctuations of the point spread function in the focal plane. A proportional increase in the receiver's field of view, required to collect all of the signal, also causes a corresponding increase in the amount of interfering background radiation, resulting in degraded communications performance. These problems may be mitigated through the use of an optical detector array assembly in the focal plane that can adaptively select areas of higher signal density while ignoring areas predominated by background noise. This concept is investigated for both Poisson photon counting detector arrays and avalanche photodiode arrays. Kolmogorov phase screen simulations are used to model the sample functions of the focal-plane signal distribution due to turbulence and to generate realistic spatial distributions of the received optical field. The optimum photon counting array detector is derived and approximated by a simpler suboptimum structure that replaces the continuous weighting function of the optimal receiver by a hard decision on the selection of the signal detector elements. It is shown that for photon counting receivers observing Poisson distributed signals, performance improvements of up to 5 dB can be obtained over conventional single detector photon counting receivers, when observing turbulent optical fields in high background environments. For the avalanche photodiode detector case, it is shown that gains of up to 4 dB may be achieved by using the array receiver rather than a single APD, but that a photon-counting array still performs about 5.5 dB better than an APD array.

This paper summarizes NASA/JPL progress on sub-microradian pointing system design. Sub-microradian pointing has been found to be critical for the deep space optical communications from earlier studies. The objective of current effort is to develop the needed technologies and demonstrate a sub-microradian pointing capability under simulated spacecraft vibrations. This is expected to establish the foundation for future deep space optical communication missions. The point system, once built, should be able to support optical communications anywhere within solar system for non-orbiting spacecraft. The proposed pointing system is based on high precision inertial sensors and large format focal plane arrays, which can operate under low intensity beacon sources such as stars. This design concept drastically deviates from the conventional design limited for short range, which assumes high signal level and quadrant detectors or small format focal plane arrays. We will present the architecture of the pointing system, pointing error analysis, and the progresses on the laboratory validations.

NASA/JPL has been developing technologies to accurately point a laser beam from deep space with sub-micro-radian precision for data transmission systems. A novel approach to achieve this goal is based on using high bandwidth inertial sensors to compensate for jitter caused by spacecraft vibrations. The use of high bandwidth inertial sensors promises to enable the implementation of laser communication links anywhere within the solar system and beyond. A functional demonstration of closed-loop accelerometer- assisted beacon tracking under simulated spacecraft vibration was undertaken, in order to validate innovative concepts, technologies, sub-systems and algorithms that achieve the sub-micro-radian pointing accuracy necessary for optical communication systems from deep space. The laboratory demonstration included integration of the complete acquisition, tracking, and pointing system with inertial sensors (e.g. accelerometers). Double integration, bias and initial velocity estimation algorithms were developed, verified and implemented. Accelerometer performance was characterized and integrated to the system. A laser beacon was mounted on a platform that simulates spacecraft vibrations. Vibrations were introduced into the beacon and were simultaneously sampled by the accelerometer. These signals were used to close the pointing loop. Closed loop tracking of the vibrating beacon was achieved using the accelerometer information interlaced with a slow-rate reference update (laser beacon centroids). This presentation will describe the details of the functional demonstration of accelerometer-assisted beacon tracking and pointing in a laboratory environment under simulated spacecraft vibration.

Precise Acquisition, Tracking and Pointing (ATP) remain a key issue in the use of free-space optical communication systems for deep space missions. The Optical Communications Group at the Jet Propulsion Laboratory is developing a vibration platform to assist with the development and characterization of an ATP system to be used for deep space optical communications. The vibration platform will provide a means for subjecting ATP systems to the vibration spectrum likely to be experienced while onboard a spacecraft. The platform consists of a 61 cm x 61 cm optical breadboard mounted on a ball bearing pivot that is driven by a single piezo-electric actuator (PZT). The PZT provides motion in a single axis, giving the platform approximately 200 mrad of angular motion with a bandwidth in excess of 100 Hz. When placed on the platform, the performance of ATP systems can be tested under several cases of vibration. This paper will discuss the physical properties of the vibration platform. A model for the system will be discussed and experimental performance data will be presented.

A correlation between near-IR atmospheric attenuation measurements made by the Atmospheric Visibility Monitor (AVM) at the Table Mountain Facility and airport surface weather observations at Edwards Air Force Base has been performed. High correlations (over 0.93) exist between the Edwards observed sky cover and the average AVM measured attenuations over the course of the 10 months analyzed. The statistical relationship between the data-sets allows the determination of coarse attenuation statistics from the surface observations, suggesting that such statistics may be extrapolated from any surface weather observation site. Furthermore, a superior technique for converting AVM images to attenuation values by way of MODTRAN predictions has been demonstrated.