Complex Optical Components for Laser Communications

Free Space Optical (FSO) communications is the only viable solution for creating a three-dimensional global communications grid of inter-connected ground and airborne nodes. The high amount of data exchange between satellites and ground stations demands enormous capacity that can not be provided by strictly regulated, scarce resources of the Radio Frequency (RF) spectrum. Free Space Optical (FSO) communications, on the other hand, has the potential for providing virtually unlimited bandwidth. Furthermore, due to the spatial confinement of laser beams, such links are very secure. In other words, security is guaranteed at the physical layer. However, the promised enormous data rates are only available under clear weather conditions, and atmospheric phenomena such as clouds, fog, and even turbulence can degrade the performance, dramatically. While scattering media such as clouds and aerosols cause pulse broadening in space and time, turbulence presents itself as scintillation and fading. Hence, to exploit the great potential of FSO at its best under all weather conditions, prudent measures must be taken in the design of transmitter and receiver. More specifically, multiple transmitters and receivers can be used to de-correlate the turbulence induced fading and to compensate for pulse attenuation and broadening caused by scattering. In this paper, Multiple-Input Multiple-Output (MIMO) transmitter and receiver designs in FSO communications are investigated and the achievable performance improvements are discussed.

Free-space laser communications are subject of current research and development in many research and industrial bodies. Long distance air-ground and space-ground can be implemented in future communication networks as feeder, backbone and backhaul links for various air- and space-based scenarios. The Institute of Communications and Navigation of the German Aerospace Center (DLR) operates two ground stations to investigate the communication channel and system: the Optical Ground Station Oberpfaffenhofen and the Transportable Optical Ground Station. The first one is a fixed installation and operated as experimental station with focus on channel measurements and tests of new developments. Various measurement devices, communication receivers and optical setups may easily be installed for different objectives. The facility is described with its dome installation, telescope setup and infrastructure. Past and current deployment in several projects is described and selected measurement achievements presented. The second ground station is developed for semi-operational use and demonstration purposes. Based on experience with the experimental ground station, this one is developed with higher level of integration and system robustness. The focus application is the space-ground and air-ground downlink of payload data from Earth observation missions. Therefore, it is also designed to be easily transportable for worldwide deployment. The system is explained and main components are discussed. The characteristics of both ground stations are presented and discussed. Further advancements in the equipment and capability are also presented.

Some current and future airborne payloads like high resolution cameras and radar systems need high channel capacity to transmit their data from air to ground in near real-time. Especially in reconnaissance and surveillance missions, it is important to downlink huge amount of data in very short contact times to a ground station during a flyby. Aeronautical laser communications can supply the necessary high data-rates for this purpose. Within the project DODfast (Demonstration of Optical Data link fast) a laser link from a fast flying platform was demonstrated. The flight platform was a Panavia Tornado with the laser communication terminal installed in an attached avionic demonstrator pod. The air interface was a small glass dome protecting the beam steering assembly. All other elements were integrated in a small box inside the Pod’s fuselage. The receiver station was DLR’s Transportable Optical Ground Station equipped with a free-space receiver front-end. Downlink wavelength for communication and uplink wavelength for beacon laser were chosen from the optical C-band DWDM grid. The test flights were carried out at the end of November 2013 near the Airbus Defence and Space location in Manching, Germany. The campaign successfully demonstrated the maturity and readiness of laser communication with a data-rate of 1.25 Gbit/s for aircraft downlinks. Pointing, acquisition and tracking performance of the airborne terminal and the ground station could be measured at aircraft speed up to 0.7 Mach and video data from an onboard camera has been transmitted. Link distances with stable tracking were up to 79 km and distance with data transmission over 50 km. In this paper, we describe the system architecture, the flight campaign and the results.

The European Space Agency (ESA) has built an optical ground station (OGS) for commissioning and checkout of its laser communication payloads in orbit. The first such payload is the laser communication terminal (LCT) onboard ARTEMIS, ESA's latest data-relay and telecommunication satellite in geostationary orbit. ARTEMIS is now routinely relaying Earth observation data sent via a similar LCT from the low-earth orbiting satellite SPOT-4 to a ground station in Toulouse. This paper focuses on bidirectional space to ground laser communication experiments, which have been performed between the OGS and ARTEMIS. ESA's interest in laser communication is first briefly explained, then the design of the ground and space terminals is introduced, the pointing, acquisition and tracking strategies are explained and a summary of all laser links performed so far is given. Experimental uplink and downlink results are presented in terms of temporal irradiance behavior and link statistics. The uplink irradiance behavior is investigated with changing number of transmit beams. Finally, ESA's future activities and upgrades planned for the OGS are discussed.

The amount of scientific data to be transmitted from deep-space probes is very limited due to RF-communications constraints. Free-space optical communication can alleviate this bottleneck, increasing data rate while reducing weight, mass and power of communication onboard equipment. Nevertheless, optimizing the power delivery from spacecraft to Earth is needed. In RF communications, the strategy has been to increase the aperture of ground terminals. Free-space optical communications can also follow it, as they share the limitation of low power received on Earth. As the cost of big telescopes increases exponentially with aperture, new ideas are required to maximize the aperture-to-cost ratio. This work explores the feasibility of using telescopes of the future Cherenkov Telescope Array as optical-communication ground stations. Ground-based gamma-ray astronomy has the same power limitation, hence Cherenkov telescopes are designed to maximize receiver's aperture with minimum cost and some relaxed requirements. Both critical issues of the reutilization and possible adaptations of the telescopes to optimize them for communications, and telescopes simulations and numerical computations of several link budgets applied to worst-case scenarios are discussed, concluding that the proposal is technically feasible and would bring important cost reductions and performance improvements compared to current designs for deepspace optical ground stations.

Free space optical (FSO) communications links is a promising solution for the provision of high data rate point to point communications. The FSO links due to their huge bandwidth and license free spectrum provide a viable communication solution between Unmanned Aerial Vehicles (UAVs) and fixed Ground Stations (GS). It is of great importance to characterize the quality of the optical channel with a proper model, taking into consideration the interesting challenges of the mobility and the slanted path of the optical beam in order to design a high performance communication link. In this paper we investigate the performance of a mobile link between a fixed GS and an UAV in terms of Bit Error Rate (BER) and average Signal to Noise Ratio (SNR). It deals with modelling the influence of the scintillation index and the received optical power with noise on link performance, separating the slanted path into small intervals assuming a Gaussian-beam wave. An extensive comparative analysis among different FSO configurations links considering the altitude of the UAV, the wavelength and the atmospheric conditions is provided. The results show that there is degradation at the BER over a slanted path compared to a horizontal path at the same conditions.

This paper addresses the concept of optical transport network based on high altitude platforms (HAPs) flying in lower stratosphere and equipped with optical communications payload. The stratospheric transport network is formed of optical links between HAPs and optical backhaul uplinks and downlinks between HAPs and ground stations (GSs) hosting gateways to the backbone network. In order to consider limitations of free space optics (FSO) for the dimensioning of stratospheric optical transport network (OTN), we investigate the physical layer aspects for a DWDM based optical interplatform link (IPL). Taking into account the physical constraints imposed by FSO, we estimate the number of wavelengths required for full interconnectivity without wavelength conversions. We are using an analytical approach for bus and full mesh regular topologies, and a numerical approach for circumcircled star, ring and star regular topologies. We also evaluate the performance of a representative network with irregular topology using different routing and wavelength assignment algorithms. We show that the number of wavelengths needed to achieve full interconnectivity strongly depends on the physical topology of the network, and that adaptive routing yields better overall performance of the network compared to fixed and fixed alternate routing. The analysis also shows that resources in realistic network topologies tend to be used very inefficiently, which could be improved by traffic engineering solutions or wavelength conversions.

Space-to-ground laser communication (SGLC) offers a paradigm-shifting solution to overcome the bandwidth constraints of radio frequency systems by leveraging laser beams for ultra-high data throughput, although its link availability probability is significantly affected by atmospheric conditions such as cloud cover. Existing ground station (GS) placement methods decouple site selection from downlink scheduling, failing to effectively quantify the data throughput of candidate sites. This study proposes a data throughput-driven joint optimization framework that integrates downlink scheduling into the site selection model for the first time. Additionally, the site selection model also incorporates equipment cost constraints and service capacity limitations by introducing an integer variable Q to characterize the deployment scale of laser communication terminals (LCTs) at each GS. Through auxiliary variable linearization techniques, the site selection problem is transformed into a tractable integer linear programming (ILP) formulation. A branch-and-bound algorithm is proposed to achieve global optimal solution search. Numerical results demonstrate that the proposed approach improves data throughput compared to the existing method.

The Communications Research Laboratory is planning a laser optical communication project to establish the basic technology of optical communications in space. The most important part of the experiment is maintaining the laser link between the satellite and the ground station by means of on-board laser communication equipment. The satellite will be launched in the summer of 1993. In this study of optical communication in space, several experiments are planned. The first is the acquisition and tracking of the satellite by using the point-ahead technique, which is the key to keeping the laser link. The second experiment is to measure the propagation characteristics of the laser beam through the atmosphere and to measure the beam pattern of the on-board laser diode. The third experiment is the high-accuracy attitude determination of the satellite by a controlled, polarized laser beam. The fourth and primary goal of the study is the dual-link optical communication experiment. The ground station includes a transmitting telescope with a fine pointing mechanism, a main telescope for receiving the signal from the satellite, and optics to modulate the Ar laser located in the Coude room under the telescope. In this paper, we will present the specifications of the ground equipment and some preliminary results of experiments that used the ground station.

Future space-borne synthetic aperture radar, hyper-spectral imaging, and high spatial and temporal resolution imaging will place increasing demands on space network data throughput. It is expected that space data relay systems will require capacities of multiple Terabits per second (Tbps). Free-space optical communication is likely the only technology capable of supporting these data demands. A previous study1 has shown that implementing these high-capacity optical feeder links through a traditional geostationary constellation will pose significant development challenges. Meshed satellite constellations in low earth orbit (LEO), operating at significantly shorter ranges, were shown to have the potential to support Tbps feeder links using the technical capabilities of current free-space optical communication systems. Meshed LEO constellations, however, provide unique challenges, including complex constellation maintenance, dynamic meshing and data routing, and short contact periods with ground stations. We review the design requirements for high capacity LEO relay optical terminals and satellites. The global coverage and ground station downlink opportunities of constellation options are evaluated to develop constellation designs utilizing the minimum number of satellites while providing required feeder link performance. The constellations are designed to provide mesh links with Tbps feeder link capabilities using optical terminals comprised of components within current state-of-the-art and with demonstrated capabilities. These meshed feeder link constellations were then modelled to demonstrate the ability of a LEO constellation to gather Tbps of data over a global network, relay this information around the constellation, and downlink this data to a limited number of ground stations.

Ground stations networks for Free-Space Optical communications: maximizing the data transfer

Recent advances in pointing and tracking capabilities of small satellite platforms have enabled adoption of capabilities such as high-resolution Earth Observation (EO), inter-satellite laser communications and, more recently, quantum communications. Quantum communications requires unusually narrow optical beams and tight pointing performance (on the order of ten microradians) to close an inherently brightness-limited quantum link. This limit is due to quantum communication protocols such as quantum key distribution and teleportation requiring individual quantum states to be transmitted with photon number restrictions. We examine an opportunity to combine quantum communications with laser communications in sharing an optical link. We discuss a combined quantum and laser communication terminal capable of performing space-to-ground entanglement-distribution and high data rate communications on a 12U CubeSat with a 95mm beam expander and an 60 cm aperture optical ground telescope. Photon pairs produced by the quantum terminal are entangled in polarization so the polarization must be maintained throughout the optical link. We discuss active and passive compensation methods in space and polarization reference frame correction using a polarized reference beacon at the ground station. The combined quantum and laser communication terminal approach enables secure communications over an optical channel with rates of 100 Mbps and sub-nanosecond time transfer.

In this paper, we investigate the secrecy performance of radio frequency (RF) eavesdropping for a high altitude platform station (HAPS) aided satellite communication (SatCom) system. More precisely, we propose a new SatCom scheme where a HAPS node is used as an intermediate relay to transmit the satellite's signal to the ground station (GS). In this network, free-space optical (FSO) communication is adopted between HAPS and satellite, whereas RF communication is used between HAPS and GS as the line-of-sight (LoS) communication cannot be established. To quantify the overall secrecy performance of the proposed scheme, closed-form secrecy outage probability (SOP) and the probability of positive secrecy capacity (PPSC) expressions are derived. Moreover, we investigate the effect of pointing error and shadowing severity parameters. Finally, design guidelines that can be useful in the design of practical SatCom networks are presented.

General Atomics Electromagnetics (GA-EMS) has developed a free space optical laser communication terminal (LCT) for space applications. The system operates at 1550 nm and utilizes on-off keying to support a data rate of up to 5 Gbps. The system architecture is expandable regarding total output power and can support links from various orbits up to and including GEO-GEO as predicted from the amplifier testing and link budget analysis. The amplifier is based off of a TRL9 system originally used by GA-EMS for airborne applications that has been redesigned for space applications and is currently TRL6 based on a TVAC test conducted in 2018. The system utilizes a software defined modulation scheme that can change between non-return to zero (NRZ) and return to zero (RZ) to support various cross link distances by transitioning between RZ and NRZ. While the current LCT uses OOK, the architecture can support multiple modulation schemes including DPSK, which was tested as part of the 2018 T-VAC test. The LCT uses a novel acquisition scheme which is introduced here that enables rapid acquisition for systems even when the bus level pointing accuracy is in excess of 350urad. This results in a bus agnostic LCT architecture that can be used on multiple mission without necessitating extensive redesign and qualification. GA-EMS plans to launch two of these terminals in cubesats to host an on orbit demonstration of crosslinks between the two terminals and downlinks to a ground station.

Because of its high directivity, free-space optical (FSO) communication offers a number of advantages. It can, however, give rise to major system difficulties concerning alignment between two terminals. During the link-acquisition step (a.k.a. coarse pointing), a ground station can be prevented from acquiring optical links due to pointing errors and insufficient information about unmanned aerial vehicle locations. We propose, in this letter, a radio-frequency (RF) lens antenna array to increase the performance of coarse pointing in hybrid RF/FSO communications. The proposed algorithm using a novel closed-form angle estimator, compared to conventional methods, reduces the minimum outage probability by over a thousand times.