For a number of reasons homodyne BPSK (binary phase shift keying) is superior to all other optical modulation schemes. Since BPSK has been verified as a reliable technique for space applications, laser communication terminals based on this modulation scheme are ready for in-orbit verification, which is the goal of the running LCTSX program. With this, an optical satellite-to-ground link shall be established to verify the impact the atmosphere has on a homodyne BPSK based communication link as well as the pointing and tracking performance of a laser communication terminal. In a follow-on program, an optical inter-satellite link will be established to verify the communication performance.
This paper summarizes the results of a coherent transmission feasibility study which has been carried out for the coherent optical downlink from the German TerraSAR-X satellite. The receiver is located at an optical ground station. To evaluate the quality of the downlink experiment, effects of atmospheric refractive-index turbulence are investigated.
By means of numerical simulations, the influence of turbulence on the communication system is analyzed. The simulated distorted complex field at the receiver is focused by means of a Fourier transform and superimposed on the local oscillator. The impact on the performance of a shot-noise-limited receiver is studied. This investigation showed that a mean bit error rate of 10-9 is achieved easily for all cases and turbulence conditions.
The impact of phase-piston temporal fluctuations on the optical phase locked loop is investigated analytically. Beam motion with respect to the turbulence cells is taken into account. The calculated values for the additional residual phase noise due to the atmosphere are too small to cause significant deterioration of the receiver performance.
The advantages of optical links like small, light and power efficient terminals are practical for high data rate services over high altitude platforms (HAPs). However, atmospheric effects can disturb the optical links and must be considered in link design. In this paper we evaluate clear sky and non clear sky attenuation effects and their impact on the link-quality of up- and downlinks from HAPs. As vertical links could be restricted by very large cloud and fog attenuation, investigations of the scattering effects in cloud media has been done. The Mie-theory shows that cloud transmittance is not depending on the wavelength, whereas the attenuation of fog and dust is smaller for longer wavelengths. Satellite cloud data has been used to predict the link availability for a ground station in Germany. A ground station diversity concept is introduced to achieve higher link availability. As high receiver sensitivity helps to reduce terminal mass, power and size, evaluation of receiver sensitivity is shown. Also, a receiver model is developed which enables to calculate for the background light loss in direct detection systems.
Performance of laser communication links between ground terminals, both fixed and mobile, and satellites is generally limited by several factors. Continuous movement of the communicating platforms, 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, thus creating wavefront distortions of the optical beam resulting in spatio-temporal redistribution of the received energy. The total effect of these phenomena leads to an increased bit-error probability under adverse operation conditions. This paper presents a combined approach to the analysis of a laser link in the presence of pointing errors and turbulence effects, and their contribution to the increased bit-error rates (BER). Analysis of both uplink and downlink communication is performed in the simulation environment. Two distinct approaches to wavefront distortion modeling are used for these scenarios. In uplink propagation the beam is distorted in the initial transition through the atmosphere, and then it travels over a long distance in free space, where even more self-interference occurs. In downlink communication the effects of distortion are only observed during the final transition through the atmosphere, and; therefore, are less severe. Communication performance under different conditions is assessed in terms of the bit-error rate as a function of the pointing error variance and the scintillation index.
Free space optics (FSO) is a technology that uses modulated optical beams to transmit information line-of-sight through the atmosphere. To date, the primary focus of FSO research and development has been toward the transmission of digital signals, primarily for "last mile" applications. This paper reports the use of FSO to transport modulated radio frequency (RF) analog signals, together with an investigation of key performance measures. Results indicate minimal RF signal distortion when transmitted over FSO. The advantages of using FSO include increased security and insusceptibility to electromagnetic interference (EMI).
The information capacity of dense wavelength division multiplexing (DWDM) optical systems is reviewed. The effect of optical beat interference of closely spaced optical carriers sets the limit on the allowed channel spacing even in the absence of nonlinear effects. Present DWDM networks operating at 10.7 Gb/s in the C and L bands region have channel spacing no less than 25 GHz. We show that this limit is a constraint and smaller spacing may not be allowed without further optical and electrical pulse shaping.
NASA’s upcoming Mars Laser Communication Demonstration (MLCD) scheduled for the 2010-2011 time-frame is planning to use the Hale telescope at Palomar Mountain, California to receive the downlink. The optical links will be demonstrated in the presence of daytime sky backgrounds with the characteristic faint laser signal associated with transmission from deep space. A system level description for acquiring and tracking the laser downlink signal in order to achieve the desired communications performance is presented.
The paper presents the results of experimental study of an optical-beam tracking concept involving two systems based on different principles. One is all-optical tracking, which utilizes a nonlinear optical material providing automatic fine-tracking feature. Another is traditional opto-mechanical technology using a quadrant avalanche detector, a voice coil-mirror actuator, control electronics, and computer interface. The possibility of establishing automatic mutual tracking between two communicating parties without involving computer-aided beam addressing has been experimentally proven. Capabilities and limitations of both systems are described. The all-optical system performs better than the traditional one when it tracking laser beam angular disturbances of magnitude of a few mrad and the jitter frequency is high (≥100 Hz). The traditional opto-mechanical system shows higher efficiency at lower jitter frequencies. A combination of an all-optical fine-tracking module and an opto-mechanical coarse tracking module is suggested for applications where using our originally proposed all-optical approach for both coarse and fine beam steering / tracking would be less efficient.
A free space optical (FSO) link requires precise pointing, acquisition and tracking (PAT) for reliable communication. Among the various tracking schemes, optical tracking with an imaging system offers great precision. For a point-to-point FSO link, the tracking problem has six degree of freedom (DOF), including position coordinates and orientation angles. By using imaging systems at both end of the link this can be reduced to a problem with three DOF. By converting from Cartesian to spherical coordinates, we can further reduce the problem to two DOFs. Thus, a single camera at each end of the link is sufficient. In this paper, we propose an outside-in, real-time tracking system, which is based on blob extraction and two-dimensional prediction. The system is inspired by the stereo vision algorithm from the computer vision community. It can be divided into two parts: (1) the tracker, and (2) the pointer. They are operated in a closed-loop, which stabilizes performance and accuracy. The tracker is used for extracting target information. Its accuracy has to be in the milliradian range, which provides a first constraint. To operate in real-time, an optical beacon is placed at each end of the link, which is imaged as a "blob". The size of this "blob" imposes a second constraint on the tracker. To work within these constraints, a 1.4Mpixel digital camera is used. The pointing system consists of two
stepping motors with resolution of 0.125 mrads and slew speed up to 800,000 steps per second. The overall system covers a whole sphere in less then 1 second.
The requirements and design concepts for a ground-based laser assembly for transmitting an uplink beacon to a Mars
bound spacecraft, carrying a laser communications terminal, are reported. The effects of the atmosphere are analyzed
and drive the multi-beam design.
This paper presents results from an ongoing effort at the University of Oklahoma to develop a real-time active alignment system for free-space optical communication system. An initial prototype of a FSO active alignment system using Global Positioning System (GPS) sensors, two gimbals, and point-to-point spread spectrum RF communication is described. The positions of both FSO transceivers are exchanged over the radio frequency (RF) communication link. A controller uses the exchanged information to calculate azimuth and elevation bearings to achieve initial alignment between the transceivers. The gimbals are used to steer the beams. The paper also presents a binary scan algorithm developed to expedite the initial alignment process. The algorithm incorporates power measurements as feed back to the original transceiver for comparison. In minimizing convergence time, simulation results confirm that the algorithm performs better than raster scan, spiral scan, and raster spiral scan algorithms, all of which are used in laser satellite communications. The results also show that the initial design is not able to achieve real-time alignment. For real-time alignment, different augmenting technologies (for example, steering mirrors) should be considered.
Free-space optical communication systems are adversely affected by weather conditions, especially fog. The objective of this paper is to examine the use of wavelength diversity in free-space optics to mitigate the effect of fog on the received optical signal strength. The source information was encoded and transmitted onto three carrier wavelengths obtained from different parts of the infrared spectrum: 0.85 μm, 1.55 μm, and 10 μm. The transmitted carriers traveled through two different simulated fog conditions, radiation and advection, before being detected and decoded by the receiver. Then, the multiple carriers were combined and processed using two diversity schemes: equal gain and selective diversity. The study was conducted using simulation software PcModWin by onTar Corporation. The results show an average power reception improvement in tens of percent, by comparison to the use of a single carrier. Hence, the increase of the received power translates into a distance improvement of at least fifteen percent.
Hybrid free space optical/radio frequency (FSO/RF) networks promise broadband connectivity, high availability and quality of service (QoS), together with the capability of autonomous reconfigurability to deal with changing atmospheric and traffic conditions in dynamic environments. Nodes with n-connectedness (multiple transceivers) offer great flexibility in constructing new network topologies. Moreover, topologies using hybrid links are more effective in changing atmospheric conditions than those, using either communication modality alone. While FSO links can be expected to be available >99% of the time on links up to 1km in length, high performance RF provides backup connectivity in heavily obscured conditions. We have designed and implemented gimbal-mounted, hybrid FSO/RF nodes with combined apertures for joint pointing, acquisition, and tracking (PAT) operation. These nodes incorporate directional RF antennas for PAT network setup and management, and FSO links for very high data rate transmission. We describe these hybrid nodes and their performance, our hybrid network simulations, and our re-configurable network testbed for high data rate video transmission. Our simulations include realistic modeling of obscuration, traffic management, and topology control to deal with link non-availability and optimization of network performance. Hybrid, directional networks are scalable and provide low probability of intercept/detection (LPI/LPD) operation, especially in FSO mode.
Laser communication systems operating in the atmosphere require certain power and beam quality to establish and maintain a reliable communication link. Although such systems utilize the most advanced materials and technologies, their performance is adversely affected by optical turbulence, often posing a serious problem, even for short-range links. Atmospheric effects change optical properties of the propagation channel, causing signal fades, beam wander and scintillations. A common method of mitigating turbulence effects suggests dynamic wavefront control. In this paper the proposed technique is based on correction of the distorted beam using an electrically addressed programmable spatial light modulator (SLM). The phase profile that we impose on the distorted laser beam is described using Zernike formalism to calculate the wavefront OPD function. The Nelder-Mead simplex optimization algorithm is used as a correction procedure that provides fast results, required for real-time operation. In general, calculation of the required phase profile for an SLM with large number of pixels could be highly computationally intensive. Coupling modulator inputs to the first several Zernike coefficients allows significant reduction of the dimension of the optimization problem. The algorithm is tested in the simulation environment and its ability to compensate dynamic distortions is assessed. The results show that both dimension of the input space and the initial conditions affect the speed and convergence to a particular minimum. Recommendations for improving the system performance are also presented.
In our attempt to better characterize optical turbulence effects, we generalized the common split-step approach of propagation simulations to spatio-temporal simulations. The time dimension is introduced by making a "local frozen turbulence assumption" which states that the local changes of the medium are dominated by wind transport and can be fully characterized by a (local) mean wind vector. We make a second simplifying assumption by neglecting wind components in the direction of propagation. The shifting theorem of Fourier theory is used in our implementation of the relative movement of the medium. As an example application, an optical downlink from a LEO satellite is illustrated and the fluctuations of the received optical field are estimated.
Among the atmospheric properties that adversely affect laser propagation is air turbulence. One common optical parameter of air turbulence is the refractive index structure constant that quantifies the fluctuations in the refractive index caused by temperature fluctuations and hence air density fluctuations. There is a reason to believe, from theory and from sparse data that, when propagation occurs under widespread cloudy conditions, the refractive index structure constant is significantly reduced. Therefore the intensity of a propagating laser beam will not be degraded nearly as much as would be expected under clear or lightly scattered cloud conditions. New experimental data will be presented that support this hypothesis. The refractive index structure constant was measured for various cloud-cover conditions during daytime with additional factors present, such as changing crosswinds and precipitation. It was possible to observe the apparent pattern of the decrease of the refractive index structure constant by two orders of magnitude during the periods of increasing cloud-cover evaluated by the measurement of solar irradiance. The statistical correlation coefficient between the log of solar irradiance and the log of the refractive index structure constant was found to be around 0.9 (the closer it is to the maximum of 1.0, the stronger the correlation). The measurements were conducted with a commercial scintillometer/anemometer (1 m above ground, 500-m optical path length) in Northern Alabama in late spring and summer. The effect is believed to be due to the reduction of solar radiation, which caused the temperature gradient that initiated convection in the air. The results of this work can find their application in designing free space laser communication systems and military laser systems.
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In free-space optical communications, atmospheric turbulence causes fluctuations in both the intensity and the phase of the received signal. We propose to use focal-plane arrays to collect optical signals from different spatial modes simultaneously, and then recombine them optimally. Experimental setup for proof-of-concept coherent adaptive array detection experiment using 32-Pulse Position Modulated (PPM) signals under atmospheric turbulence has been completed. Adaptive combining of experimentally obtained heterodyned PPM signals with pulse-to-pulse coherence, in the presence of simulated atmospheric turbulence is demonstrated. The adaptively combined PPM signals are phased up via a Least-Mean-Square (LMS) algorithm suitably optimized to operate with PPM in the presence of additive shot-noise, and detected via a maximum likelihood software receiver. Convergence study of the algorithm is presented and results with simulated PPM signals and real PPM signals experimentally obtained at the laboratory are presented.
We present a decoding architecture for high-speed free-space laser communications. This system will be used by NASA's Mars Laser Communication Demonstration (MLCD) project, the first use of high-speed laser communication from deep space. The Error Correction Code (ECC) and modulation techniques for this project have been motivated by an analysis of capacity, and existing designs have been shown to operate within 0.9 dB of the Shannon limit on the nominal operating point. In this paper, we give the algorithmic description and FPGA implementation details that led to the development of a 50 Mbps hardware decoder.
Feasibility of utilizing large diameter custom designed and fabricated Fresnel lenses as the front optical aperture for Earth-based reception of optical communication signals from remote spacecraft is investigated. This includes preliminary optical designs, investigation of stray-light effects for a particular optical design, effect of temperature variations and mechanical sag on the performance of the photon bucket, and effect of temporal dispersion on the link performance. Experimental results for several commercial off-the-shelf Fresnel lenses with diameters exceeding 1-meter are presented as well as plans for custom diamond turning fabrication of two-meter diameter Fresnel lenses.
An optical receiver concept for binary signals with performance approaching the quantum limit at low average signal energies is developed and analyzed. A conditionally nulling receiver that reaches the quantum limit in the absence of background photons has been devised by Dolinar, however this receiver requires ideal optical combining and complicated real-time shaping of the local field, hence tends to be difficult to implement at high data rates. A simpler nulling receiver that approaches the quantum limit without complex optical processing, suitable for high-rate operation has been suggested earlier by Kennedy. Here we formulate a vector receiver concept that incorporates the Kennedy receiver together with a physical beamsplitter, but also utilizes the reflected signal component to improve signal detection. It is found that augmenting the Kennedy receiver with classical coherent detection at the auxiliary beamsplitter output, and optimally processing the vector observations, always improves on the performance of the Kennedy receiver alone, significantly so at low average photon rates. This is precisely the region of operation where modern codes approach channel capacity. It is also shown that the addition of background radiation has little effect on the performance of the coherent receiver component, suggesting a viable approach for near quantum-limited performance in high background environments.
A second generation optical communications demonstrator (OCD-2) intended for airborne applications like air-to-ground and air-to-air optical links is under development at JPL. This development provides the capability for unidirectional high data rate (2.5-Gbps) transmission at 1550-nm, with the ability to receive an 810-nm beacon to aid acquisition, pointing and tracking. The transmitted beam width is nominally 200-μrad. A 3x3 degree coarse field-of-view (FOV) acquisition sensor with a much smaller ~3-mrad FOV tracking sensor is incorporated. The OCD-2 optical head will be integrated to a high performance gimbal turret assembly capable of providing pointing stability of 5-microradians from an airborne platform. Other parts of OCD-2 include a cable harness, connecting the optical head in the gimbal turret assembly to a rugged electronics box. The electronics box will house: command and control processors, laser transmitter, data-generation-electronics, power conversion/distribution hardware and state-of-health monitors. The entire assembly will be integrated and laboratory tested prior to a planned flight demonstration.
Next-Generation LEO System (NeLS) Research Center is now conducting continuous effort to demonstrate feasibility of key technologies for optical inter-satellite links in space. Evaluation of critical components for the NeLS optical terminal, such as Wide-range FPM, RX-collimator combined with a fine tracking sensor and devices for optical receiver, were carried out using trial models. In this paper, performance evaluation results are presented including mechanical environmental test and radiation test.
The conceptual design, theoretical performance, and experimental verification of a two-telescope optical array receiver currently under development at the Jet Propulsion Laboratory, is described in this paper. A brief summary of optical communications theory for array reception of pulsed laser signals is developed, and the impact of coding discussed. The development of the optical detection, array processing, and data-acquisition assemblies required for experimental demonstration is described, and preliminary results obtained in a field environment are presented and evaluated.
Data transmission between rotating and stationary systems, e.g. required for radar antennas or for undersea cable installation ships can be realized with so called rotary joints. For the transmission of several high bit rate optical data channels a micro optical rotary joint is now available which guarantees a dead reliable, low loss transmission for up to 21 parallel single mode channels. The free space transmission in the rotary joint implicates a highly precise collimation of the parallel channels. For this purpose compact two dimensional fiber collimator arrays based on micro lens arrays have been developed. These arrays and the complete opto-mechanical system are designed with the help of tolerance analysis using Monte Carlo simulations. Besides these results also some more information on the behavior and the characteristics of the micro optical rotary joint under real conditions which demonstrate the excellent characteristics of this novel system will be given.
A laser beam with a pseudo random bit stream pattern amplitude modulation is retro reflected off a target to produce a real time ranging signal using a cross correlation technique. The measured resolution of the system was 0.2 mm with an absolute accuracy of ± 2 mm over 2 m. The use of a modulated retro reflector allows a communications signal to be added to the ranging capability.
The pointing knowledge for the deep space optical communications should be accurate and the estimate update rate needs to be sufficiently higher to compensate the spacecraft vibration. Our objective is to meet these two requirements, high accuracy and update rate, using the combinations of star trackers and inertial sensors. Star trackers are very accurate and provide absolute pointing knowledge with low update rate depending on the star magnitude. On the other hand, inertial sensors provide relative pointing knowledge with high update rates. In this paper, we describe how the star tracker and inertial sensor measurements are combined to reduce the pointing knowledge jitter. This method is based on the 'iterative averaging' of the star tracker and gyro measurements. Angle sensor measurements are to fill in between the two gyro measurements for higher update rate and the total RMS error (or jitter) increases in RSS (Root-Sum-Squared) sense. The estimated pointing jitter is on the order of 150 nrad which is well below the typical requirements of the deep space optical communications. This 150 nrad jitter can be achieved with 8 cm diameter of telescope aperture. Additional expectations include 1/25 pixel accuracy per star, SIRTF class gyros (ARW = 0.0001 deg/root-hr), 5 Hz star trackers with ~5.0 degree FOV, detector of 1000 by 1000 pixels, and stars of roughly 9 to 9.5 magnitudes.