Channel measurements were performed by the German Aerospace Center in various near ground optical channels including a 1.5 km horizontal path as well as a 61km path. These measurements clearly showed that the atmosphere causes very slow fading (compared to the high data rates usually used in optical communication systems), which significantly degrades the transmission quality. As transmitter power and receiver sensitivity are limited by the transmission technology, fading depicts a severe problem, that can be reduced by the use of forward error correction schemes (FEC) in order to improve system performance. Therefore FEC was subject of investigations by means of simulations. To figure out which FEC methods are useful for applications in the atmospheric optical channels simulations of standard block codes and interleavers have been done. They were based on data sets taken in the various channel measurements. The simulations point out that only very long interleaving can increase performance significantly.
Free-space optical communications systems in the atmosphere, based on intensity modulation and direct detection, are heavily affected by fading caused by turbulence cells of varying scale and motion. Several data sets of fading measurements under different scenarios have been recorded demonstrating this effect. In this paper we introduce a form of free-space laser communications involving a source operating on several wavelengths. The goal is to overcome atmospheric interference on a communications link. We have performed simulations using the DLR PILab Matlab toolbox. These indicate the extent to which the turbulence and beam properties interact. Experimental investigations are planned. Further properties are also taken into account, including the choice of appropriate laser bandwidth and wavelengths, the effect of atmospheric absorption from aerosols and molecular absorption lines, as well as effects of atmospheric structure on beam propagation. Possible scenarios for application of this scheme will be presented as well.
We present experimental results for an optical free-space high-speed link using direct modulated mid-infrared (8.1 μm) quantum cascade lasers. A stable link was realized over a distance of 200m and QPSK encoded multimedia data were transmitted and received error-free, incorporating several hundred digitally encoded multimedia channels. The reliability of the system against weather influence (fog) was experimentally compared to that of a near-infrared (0.85 μm) link. Under clear weather conditions comparable results were obtained in both links. However, a higher stability of the MIR link was clearly observed under a dense fog situation.
Development of long-range laser communication devices capable of operating within Earth's atmosphere is a challenging task. Turbulence effects cause changes of the refractive index, thus creating random redistribution of optical energy in the spatial domain and imposing performance limitations on the laser systems. A mathematical model describing effects of turbulence on the optical wave propagation, based on the Rytov approximation, is presented. A common method of mititaging these effects suggest dynamic wavefront control of the laser beam. In our approach this goal is achieved by placing an adjustable electrically addressed spatial light modulator in the transmitter output. A novel spatially distributed gradient minimization scheme is developed for calculating the required phase profiles. The resultant system is implemented in a simulation setup and tested for convergence, speed, robustness and the ability to reduce the scintillation effect. The paper presents the basic algorithm and dmeonstrates the results of its application.
For high rate communications such as optical communications, tracking loss can result in substantial reduction of average data rate and the total data volume of the transmitted data. For optical communications, which transmits laser beam through atmosphere, atmospheric induced fades of the beacon signal can vary significantly as observed in ground-to-ground optical experiments. In this paper, we propose a new scheme of compensating the atmospheric induced fading effects using inertial sensors. By measuring the platform vibrations, the beacon movements on the Focal Plane Array can be deduced even if the beacon is lost due to fading. By avoiding the new cycle of reacquisition and tracking, high rate communication can be maintained. The allowable period of beacon fade depends on the inertial sensor noise characteristics and acquisition and tracking Field-Of-View. We will present the results of our analysis for the planned Altair UAV-to-Ground optical communications demonstrations using an accelerometer.
In this paper we studied about optical and millimeter radio wave cross-link for formation flight communication satellite system in a geostationary orbit. In formation flight system, since the distance among the satellites becomes short compared with the conventional inter satellite comunication link, the size and the weight of the communication system can be reduced. If the high data rate cross-link among the satellites which is equal to the whole transponder bandwidth can be established, the functionally distributed communications satellite system can be constructed. Then, by exchanging a part of the satellite system without the physical contact, in other words, by exchange some old satellites and new ones, the fully reconfigurable and long lifetime (from the point of the function) satellite communication system which can follow the paradigm shift in the terrestrial communications technology can be realize. On the other hand, however, since the maximum of relative angle error among two satellites is enlarged, the tracking becomes difficult. In this study, it turns the electric power which is gotten from the shortening the distance to making beam width large. Here, we examine communication among two satellites (10Km distance) where the data rate is 1Gbps.
Free Space Optics (FSO) is an excellent supplement to conventional radio links and fiber optics. It is the broadband wireless solution for closing the "Last Mile" connectivity gap throughout metropolitan networks. FSO is useful in dense urban and urban applications. The realiabilty and availability of the FSO-link is mainly determined by the local atmospheric condition. In this paper the results of the investigations of the research group "OptiKon" on different installed common network-architectures (ring, mesh and star) are presented. An "Optical Repeater" was used, if there was no line of sight between transmitter and receiver. The usage of optical free space propagation wthi Light Emitting Diodes are source instead of conventionally broadband transmission systems like optical fibers or radio links was also investigated at the Department of Communications and Wave Propagation. Using extended light sources like LEDs and/or multiple transmitter optics due to new laser safety regulations allows multiple output power within the same laser class, increasing the power link budget especially for short range FSO systems. In a first step two families of cost-effective systems were developed and realized. One is based on a modular concept using available standard components, allowing demonstrates at 2×10 Mbps data rae. Another is used for the larger distnaces of about 300 m at a specific power margin of 25 dB/km. Systems for higher data rate are under development. Additionally some ideas and their possible realization for point-to-multipoint configuration are considered. In the last chapter a short overivew about the main results of the work of "OptiKom" is discussed.
Coherent lightwave receivers are experimentally demonstrated with a laser diode as local light source. The laser diode local oscillator is available for not only reducing weight and power consumption of a coherent receiver, but expanding the wavelength tuning range of receiver. In our study, a narrow line-width (<50 kHz) DFB laser diode is introduced into coherent receivers as a local oscillator light source. And two kinds of heterodyne coherent lightwave receiver (ASK-envelope detection and PSK-synchronous detection) are designed and evaluated. In the experiment, the signal light was generated in a system consist of a DFB laser diode operated at the wavelength of 1.55 μm, an electro-optic extra-cavity wideband modulator and an Erbium doped fiber amplifier (EDFA). After attenuating power, the signal light was introduced into the receiver. An optical attenuator was inserted between the EDFA and the receiver instead of space loss in actual communication link. Coherent lightwave detection using a laser-diode local light source was successfully performed at data rate of 2.5 Gbps. Sensitivity under 174 photons/bit at Bit Error Rate (BER) of 10-9 is experimentally achieved in the PSK coherent system. The PSK coherent receiver also shows good wavelength tracking characteristics for the variation of signal beam wavelength. It is demonstrated that the combination of tunable local optical sources and an intermediate frequency-phase lock loop (PLL) is effective to compensate Doppler effect, which is indispensable in inter-orbit communication link. In addition, a trade-off between the intensity modulation direct detection (IM-DD) system and coherent system oriented to the inter-orbit optical communication link is discussed based on the experimental results.
A new technique for evaluating the performance of quantum signals observed in the presence noise is described and evaluated. The quantum theory for detecting coherent-state signals has been developed previously, however the quantum "signal plus noise" problem has received little attention due to its complexity. Here we develop a discrete approximation to the coherent-state representation of signal-plus-noise density operators, and present solutions to optimum receiver performance in terms of quantum "measurement states" whose performance is optimized via generalized rotations in Hilbert space. An efficient algorithm for carrying out the required numerical optimization is described and applied to binary signals observed in the presence of noise, for which exact results are available for comparison. The algorithm is then applied to the detection of ternary signals observed in the presence of noise, a previously unsolved problem, and the performance of the optimum receiver characterized.
Modulating retro-reflectors (MRR) couple passive optical retro-reflectors with electro-optic modulators to allow free-space optical communication with a laser and pointing/acquisition/tracking system required on only one end of the link. In operation a conventional free space optical communications terminal, the interrogator, is used on one end of the link to illuminate the MRR on the other end of the link with a cw beam. The MRR imposes a modulation on the interrogating beam and passively retro-reflects it back to the interrogator. These types of systems are attractive for asymmetric communication links for which one end of the link cannot afford the weight, power and expense of a conventional free-space optical communication terminal. Recently, MRR using multiple quantum well (MQW) modulators have been demonstrated using a large area MQW placed in front of the aperture of a corner-cube. For the MQW MRR, the maximum modulation can range into the gigahertz, limited only by the RC time constant of the device. This limitation, however, is a serious one. The optical aperture of an MRR cannot be too small or the amount of light retro-reflected will be insufficient to close the link. For typical corner-cube MQW MRR devices the modulator has a diameter between 0.5-1 cm and maximum modulation rates less than 10 Mbps. In this paper we describe a new kind of MQW MRR that uses a cat's eye retro-reflector with the MQW in the focal plane of the cat's eye. This system decouples the size of the modulator from the size of the optical aperture and allows much higher data rates. A 50 Mbps device has been demonstrated.
A robust acquisition, tracking and pointing (ATP) subsystem is being developed for the 2.5 Gigabit per second (Gbps) Unmanned-Aerial-Vehicle (UAV) to ground free-space optical communications link project. The demonstration will gather HDTV images of regions of geological interest (e.g. volcanic) and then downlink those images to ground receivers at a range of 50 km while the UAV is at an altitude of 18 km. With a 200 mW downlink laser at 1550 nm for a BER of 1E-9, the pointing requirements on the flight terminal are a jitter error of 19.5 urad and a bias error of 14.5 urad with a probability of pointing induced fades of 0.1 %. In order to mitigate the effect of atmospheric fades and deal with UAV flight and vibration uncertainties (relatively new craft) the ATP subsystem requirements have been set to a stringent level in order to assure success of the communication link. The design, analysis and development of this robust ATP subsystem will be described in this paper. The key innovations that have been developed to make the ATP subsystem robust are i) the application of inertial sensors to make the acquisition and tracking functions tolerant to atmospheric fades, ii) the usage of active exposure control to provide a 16 dB dynamic range on the Focal Plane Array (FPA) tracking window, and iii) the introduction of a second ultra wide field of view camera to assure acquisition of the ground beacon.
Commercially available cameras are not designed for the combination of single frame and high-speed streaming digital video with real-time control of size and location of multiple regions-of-interest (ROI). A new control paradigm is defined to achieve low-level camera control with high-level system operation. This functionality is achieved by defining the indivisible pixel read out operation on a per ROI basis with in-camera time keeping capability. This methodology provides a Random Access, Real-time, Event-driven (RARE) camera for adaptive camera control and is well suited for target tracking applications requiring autonomous control of multiple ROIs. This methodology additionally provides for reduced ROI read out time and higher frame rates compared to a predecessor architecture by avoiding external control intervention during the ROI read out process.
Within the frame of the FASOLT project the German Aerospace Center (DLR) performed channel measurements and optical data transmission tests on a 61 km near ground horizontal path. The transmitter was situated on a mountain top in the German Alps with the receiver placed on top of a building at the DLR site in Oberpfaffenhofen. Partners in the FASOLT project were Contraves Space, Switzerland and EADS Military Aircraft, Germany. During a period of several months various data sets of scintillation data were recorded in one and two laterally separated transmitter configurations and under different environmental conditions. A significant decrease of number and depth of fades was observed for the two transmitter setup. This paper presents an overview on the scintillation statistics of this particular optical channel. Also beam offsets due to refraction have been measured and results are presented here. As well as these measurements, data transmission tests at bit rates of 100 Mbps have been performed. A two transmitter configuration with a transmit power of 1 W per laser and a sensitive APD receiver front-end plugged in to a 75mm Rx telescope have been used. Despite severe scintillations, bit error rates (BER) below 1e-4 could be observed, though synchronization losses of the data and clock recovery affected the results. Tests at 155 Mbps (OC-3) and 270 Mbps (SMPTE 259M) were not successful due to high atmospheric attenuation. This paper gives an overview of the entire experimental setup, sums up the results of this long-haul data transmission experiment, and gives an outlook to further DLR activities in the field of free-space optics.
An underwater-airborne laser communication system is presented as a practical alternative to VLF and ELF radiowave technology for submerged vessels. The system is compact, self-contained, and employs frequency doubled Nd:YAG laser sources. This system has applications in secure high-bandwidth communication for semiautonomous and autonomous submerged vessels operating at depths of less than 100m. A propagation analysis is provided and theoretical and experimental data are given for the transmission channel. A CCD camera system is also described, which provides feedback for laser pointing and surface-state beam compensation.
Fixed-pointed Free Space Optics (FSO) systems must use beamwidths that are broadened sufficiently to account for building motion, wind loading, and other pointing instabilities. Actively pointed systems can potentially use beamwidths more than an order of magnitude narrower for 20 dB or more additional link margin. Pointing requirements and general tradeoffs with regard to active pointing are discussed, and the specific approach selected for fSONA's
actively pointed FSO design is described.
NeLS project is a Japanese effort aiming to develop key technologies for global multimedia mobile satellite communications service. In the NeLS System, all satellites are connected to four neighboring satellites by using optical inter-satellite link (ISL) considering four-wave WDM technology. Optical ISL requires very high optical output power. And to meet such requirements, a high power optical amplifier such as EDFA will be used. In such case, nonlinear effects of an EDFA will casue degradation of communication performance of OISL. We develoepd an OISL-WDM Simulator to eavluate such degradation experimentally. A Yb co-doped EDFA is used as OHPA. In the experiment, BER characteristics were measured and found that degradation is 0.4dB up to 2W output and 0.9dB at 4.5W output. From these results we concluded this technology could be applicable to WDM optical intersatellite link applications.
Free Space Optical communications pointing problems will be described and a solution presented. Conventional systems used in sea, land, and space are illustrated. Our integrated approach investigates three major problems areas: kinematics, structure, and dexterity. Their interrelationship will be discussed.