We report on the demonstration of a bidirectional free-space link over 18 km as part of the European-Union project “VERTIGO” that investigates technologies for the optical GEO-satellite feeder link. Two different terminals were deployed: a single-aperture “satellite” terminal and a 4-aperture “ground” terminal. Using SFP+ transceivers with OOK modulation at 10 Gbit/s, real-time bit error rates (BER) were measured for each aperture in both directions using an FPGA platform. In both directions, diversity signals at the receiver were processed digitally for combining. We report on the achieved performance improvement compared to a single aperture.
With recent advances in quantum technologies for applications such as communication, cryptography, computing, metrology and sensing, the performance and scalability of single-photon detection as a vital key component is becoming increasingly important. At the same time, ongoing efforts in the development of high-performance photonic integrated circuits (PIC) benefit the miniaturization and scalability of these quantum technologies. Waveguide-integrated superconducting nanowire single-photon detectors (WI-SNSPDs) allow to combine excellent performance metrics, such as high detection efficiency, low dark-count rates and low timing jitter below 20 ps with the scalability and functionality that PIC platforms such as Si3N4 provide. We have previously demonstrated broadband efficient single-photon detection with a single device over a range from visible to mid-infrared wavelengths and ultra-fast detector recovery times allowing for up to GHz count rates. Here, we present the utilization of WI-SNSPDs for discrete-variable quantum cryptography receivers with the complete photonic circuitry embedded together with the single-photon detectors on a single silicon chip, where the secret-key rates greatly benefit from the short recovery times of the detectors especially for metropolitan distances. We further realize a fully packaged 64 channel WI-SNSPD matrix for use in a wavelengthdivision multiplexed QKD setup.
Upcoming quantum technologies require scalable and cost-efficient technical solutions for widespread functionality. In order to exploit the quantum states of light, single-photon detectors are essential for application. Here, we present a low-footprint plug-and-play multi-channel single-photon detector system featuring integrated photonics that allows for ultra-fast quantum key distribution (QKD). Each channel comprises a superconducting nanowire single-photon detector (SNSPD) patterned from a niobium-titanium nitride (NbTiN) superconducting film atop silicon nitride waveguide structures. Subsequently, the on-chip photonics are interfaced by broadband 3D polymeric fiber-to-chip couplers to the ports of an 8x8 fiber array. The readout electronics allow for individual evaluation of up to 64 channels simultaneously. Integrated to a QKD experiment, a pair of the system's detection channels achieves secret key rates of up to 2.5 Mbit/s employing a coherent one-way protocol.
To concurrently cope with the scarcity of RF frequency bands, the growing capacity demand and the required lower cost of the ground segment, Very High Throughput Satellites systems must rely on new technical solutions. Optical feeder links are considered as a promising alternative to surpass classical RF technology, offering assets inherent to optical technologies (large bandwidth, no frequency regulation, low beam divergence, components availability). Nevertheless the potential of this technology shall not conceal the remaining challenges to be overcome to make it relevant for operational missions : clouds, turbulence, power generation and high efficiency modulations. VERTIGO (Very High Throughput Satellite Ground Optical Link) is a 3-year H2020 project funded by the European commission and started mid-2019 focusing on the optical link itself regardless of site diversity aspect and aiming at demonstrating in a ground demonstration required technologies to implement very high capacity optical feeder links. In particular, VERTIGO is built on 3 pillars each addressing a key issue for the implementation of optical feerder links: 1) Throughput increase through the use of advanced schemes with high spectral and power efficiency compared to current modulations used in space, as well as RF-over-Fiber approach. 2) High optical power generation to close the demanding link budgets by developing on-board and ground means to raise the transmitted optical power, not only based on amplifier power increase, but also on incoherent/coherent power combining. 3) Opto-mechanical and digital techniques for the mitigation of atmospheric propagation impairments, to make full use of throughput and power increases. Several demonstrations in-flight or on-ground already demonstrated separately key aspects (atmospheric propagation and impairments mitigation techniques, modulation format, high power…), for the implementation of optical (feeder) links. These aspects are closely linked since the solutions to each of them are necessary but not sufficient to allow for high throughput transmissions. VERTIGO concept is to address each key issue with at least one solution and to combine them in an unprecedented manner. To reach these objectives, VERTIGO will lean on a highly skilled consortium composed of : CREONIC, ETH Zürich, Fraunhofer HHI, Gooch and Housego, Leo Space Photonics RD, ONERA, Thales Research and Technology, Thales Alenia Space in France and Switzerland. This paper will present the VERTIGO project and its status.
Existing radio-based satellite up- and downlinks are reaching their limits in terms of transmission rate. In order to enable higher data rates, optical links are being researched and developed. On ground, spatially displaced multi-aperture systems using different wavelengths can mitigate atmospheric scintillation and increase the uplink transmit power. A four-aperture free-space optical communication system was demonstrated at Fraunhofer HHI over a bidirectional link. For the representative uplink, each array aperture sent a beam carrying the same data but over a different wavelength to avoid inter-beam interferences. At the receiver, the combined signals are optically amplified, optically filtered, incoherently photodetected and electrically filtered. Signal path delays are pre-compensated on the ground-station site. This system shall be scaled up for a long-range demonstration within the EU-funded H2020-project VERTIGO. In this paper, we investigate the performance and scalability of this uplink configuration. When targeting a capacity of Terabits/s, spectral efficiency must be carefully addressed. Direct detection with optical pre-amplification of OOK or DPSK signals is a conventional optical fiber receiver technique, for which the bandwidth of the optical filter can be kept larger than the signal bandwidth only to a certain extent. Above a certain optical filter bandwidth relative to the signal bandwidth, degradation of the signal-to-noise ratio (SNR) becomes significant because of a stronger ASE×ASE noise term. We focus on optimizing power and spectral efficiencies as these are impacted by the introduction of diversity channels. Because these optimizations are essentially independent of the fading channel, we do not assess the reduction of scintillation through channel diversity. We formulate the SNR in terms of number of uplink beams, filter bandwidths (optical and electrical), wavelength channel spacing and received power (in photons per bit). Although the SNR is ideally proportional to the received power, we show how increasing the number of beams while keeping the total received power constant will result in a significant SNR degradation at some point. An optimum for the optical filter bandwidth and for the spacing between the wavelength-division diversity channels is investigated. Polarization-division spatial diversity is kept as an option.
We investigate a quantum key distribution (QKD) system with relay satellite for communication between two distant Earth-based parties, Alice and Bob. The satellite acts as a relay station which simply redirects the QKD signal. It has several advantages that can be decisive. The optical relay provides a high transparency to protocols and wavelengths. The relay node does not have to be trusted. Like entanglement-based or measurement-device-independent satellite QKD, relayassisted QKD suffers from a higher propagation loss than trusted-node scenarios. Challenges are expected when pointing the relay mirror precisely between Alice and Bob and dealing with significant point-ahead angles that result from the satellite velocity. We start our analysis by evaluating the point ahead angles (PAA) of Alice and Bob for scenarios of interest. The difference between the two PAA vectors tells us whether the relay mirror will be able to maintain a bidirectional transmission with beams sent from the ground. Considering a satellite altitude of 500 km and an Alice-Bob separation of 1000 km, observation of large PAA vector difference enjoins us to place additional beacon sources on the satellite. A conceptual design of the relay-tracking system is drafted. Onboard measurements of the beacons from Alice and Bob provide feedback to the relay-mirror positioning control loop. With a typical relay-mirror size of 0.2 m, propagation losses are calculated for different ground antennas. We conclude on the attractiveness and feasibility of satellite relay QKD.
Quantum key distribution (QKD), the first applicable quantum technology, is able to distribute a secret key to two parties. This key can then be used as a one-time-pad for absolutely secure communication. The first QKD protocol was the polarization based BB84 protocol proposed in [1]. Since then many QKD protocols have been proposed and investigated [2, 3].
Optical satellite communications play an increasingly important role in a number of space applications. However, if the system concept includes optical links to the surface of the Earth, the limited availability due to clouds and other atmospheric impacts need to be considered to give a reliable estimate of the system performance. An OGS network is required for increasing the availability to acceptable figures. In order to realistically estimate the performance and achievable throughput in various scenarios, a simulation tool has been developed under ESA contract. The tool is based on a database of 5 years of cloud data with global coverage and can thus easily simulate different optical ground station network topologies for LEO- and GEO-to-ground links. Further parameters, like e.g. limited availability due to sun blinding and atmospheric turbulence, are considered as well. This paper gives an overview about the simulation tool, the cloud database, as well as the modelling behind the simulation scheme. Several scenarios have been investigated: LEO-to-ground links, GEO feeder links, and GEO relay links. The key results of the optical ground station network optimization and throughput estimations will be presented. The implications of key technical parameters, as e.g. memory size aboard the satellite, will be discussed. Finally, potential system designs for LEO- and GEO-systems will be presented.
An optical link based on a multiplex of wavelengths at 1.55μm is foreseen to be a valuable solution for the feeder link of the next generation of high-throughput geostationary satellite. The main satellite operator specifications for such link are an availability of 99.9% over the year, a capacity around 500Gbit/s and to be bent-pipe. Optical ground station networks connected to Terabit/s terrestrial fibers are proposed. The availability of the optical feeder link is simulated over 5 years based on a state-of-the-art cloud mask data bank and an atmospheric turbulence strength model. Yearly and seasonal optical feeder link availabilities are derived and discussed. On-ground and on-board terminals are designed to be compliant with 10Gbit/s per optical channel data rate taking into account adaptive optic systems to mitigate the impact of atmospheric turbulences on single-mode optical fiber receivers. The forward and return transmission chains, concept and implementation, are described. These are based on a digital transparent on-off keying optical link with digitalization of the DVB-S2 and DVB-RCS signals prior to the transmission, and a forward error correcting code. In addition, the satellite architecture is described taking into account optical and radiofrequency payloads as well as their interfaces.
Future Very High Throughput Satellite Systems (VHTS) will perform at several Tbit/s throughput and thus face the
challenge of limited feeder-link spectrum. Whereas with conventional RF feeder links several tens of ground gateway
stations would be required, the total capacity can alternatively be linked through a single optical ground station using
Dense Wavelength Division Multiplexing (DWDM) techniques as known from terrestrial fiber communications. While
intermittent link blockage by clouds can be compensated by ground station diversity, the optical uplink signal is directly
affected by scintillation and beam wander induced by the atmospheric index-of-refraction turbulence. The transmission
system must be capable to mitigate these distortions by according high-speed tracking and fading compensation
techniques. We report on the design of a near-ground long-range (10km) atmospheric transmission test-bed which is,
with its relatively low elevation of 1.8 degrees, exemplary for a worst case GEO uplink scenario. The transmitting side
of the test-bed consists of a single telescope with a a fine pointing assembly in order to track the atmospheric angle-ofarrival
and precisely aim towards the beacon of the receiver. On the other side of the test-bed, the receiver telescope is
also capable of fine pointing by tracking the transmitted signal. The GEO uplink scenario is modelled by a precise
scaling of the beam divergence and the receiver’s field of view as well as by the beacon offset to model the point-ahead
angle. In order to make the experimental test-bed correspond to an actual feeder link scenario, the link budget as well as
the turbulence profile of the experimental scenario are modelled and compared to the GEO uplink. Several DWDM
channels are multiplexed to reach the total link capacity of above one Tbit/s.
We consider n independent disruptive channels with random availability periods and make use of channel diversity to maintain a link as often as possible. This situation arises when an optical link is to be maintained between a spacecraft and n ground stations affected by cloud coverage. Statistical independence between the stations and equal (un)availability duration distributions are assumed. Based on a given single-station (un)availability duration distribution, we derive analytically the network (un)availability duration distributions. Also derived is the distribution of the station operation duration within a network. Derived expressions allow evaluating the increase of the continuous operation duration of a station and the decrease of the network unavailability durations.
The long-term throughput of optical links between a circular-LEO satellite and a ground station is assessed for systems
with fixed and variable data rates. For a fixed data rate, we evaluate the minimum elevation angle above which the link
should be established in order to maximize the throughput. The ratio of this optimized constant-data-rate throughput to
the throughput of an adaptive transmission with a data rate proportional to the received power is found to depend mainly
on the satellite altitude.
KEYWORDS: Clouds, Signal attenuation, Data modeling, Binary data, Atmospheric modeling, Space operations, Multiplexing, Space telescopes, Transceivers, Multiplexers
The impact of clouds on optical Earth-space links is assessed by means of theoretical analyses. The spacecraft is
assumed geostationary so that a stationary model can be considered for the link availability. Binary and continuous
attenuations are analysed separately. With a network of ground stations, both spatial diversity and multiplexing are
considered. The impact of the ground-station network size and spatial correlation on the availability and throughput is
studied. Using a continuous attenuation model, it is shown that an adaptive data rate essentially mitigates thin clouds (on
the order of 10-dB attenuation).
The tracking and pointing performance of the Laser Utilizing Communications Equipment (LUCE) equipped on the
Optical Inter-orbit Communications Engineering Test Satellite (OICETS) is presented. The operation characteristics of
LUCE observed in the ground-satellite communication demonstrations are focused on. Since the angular movement of LUCE's optical antenna required in the ground-satellite trials exceeds the specification demanded in the inter-satellite communications, the marginal performance for proper tracking and pointing can be observed. During the ground-satellite communication trials, the ground stations found periodical discontinuances in the optical link. By looking at the
LUCE's telemetries, the cause of the repeated breaks is revealed.
Optical LEO downlinks from the Japanese OICETS to the optical ground station built by the German Aerospace Center
(DLR) near Munich have been performed. This was the first optical LEO downlink on European grounds. The ground
station received a 50-Mbit/s OOK signal at 847 nm on its 40-cm Cassegrain telescope and sent two spatially displaced
beacon beams towards OICETS. Five out of eight trials could be performed successfully while the other three were
hindered by cloud blockage. A BER of 10-6 has been reached. The elevation angle above the horizon ranged between 2°
and 45°. The Fried parameter and the scintillation were measured with instruments inside the ground station. The beacon
power received by the LUCE Terminal onboard OICETS has also been recorded. This paper describes the setup of the
experiment and highlights the results of the measurement trials.
In optical communications through the atmosphere, the evaluation of a link feasibility often requires the quantification of the scintillation penalty in terms of power loss. To find how much additional optical power is needed to reach the bit-error-rate (BER) requirements, the optical-power fluctuations must be characterized as well as the response of the receiver to those fluctuations. In the present analysis, the direct-detected optical power is assumed to be either lognormal or gamma-gamma distributed. To account for the dynamics of the atmospheric channel, a distinction is made between short-term and long-term BERs. For a simple On-Off Keying (OOK) modulation, expressions of scintillation losses are given for different system requirements. Specifically, an upper bound is set to any of the three following quantities: the long-term BER, the probability of having a too-high short-term BER, or the mean time during which the short-term BER is too high. Results show that, without any fade mitigation, losses under moderate scintillation are considerable. Finally, a simple code-word approach shows how scintillation losses can be reduced by channel coding.
An optical link has been established between the Canary Islands La Palma and Tenerife. A 1064-nm transmitting laser was located on La Palma whereas a BPSK communication receiver and measurement instruments were installed in ESA's OGS on Tenerife. Beside the demonstration of a high-data-rate coherent signal transmission, the goal of the experiment was to measure the effects of the atmosphere on the beam propagation in order to estimate its impact on optical links. In particular, wavefront distortions have been investigated by means of a DIMM instrument and scintillation was observed by imaging the pupil of the OGS telescope on a CCD camera. Strong scintillation was observed during all the experiment with scintillation peaks at sunsets and sunrises, and saturation at about noon. Because of the narrowness of the beam (15-μrad divergence), beam wander has been a serious issue. Statistical results are compared with theory. Recommendations regarding the specifications of optical coherent systems in such detrimental conditions are given.
Optical communication at 1.25Gbps was successfully demonstrated in a downlink from a stratospheric balloon platform at 22km altitude to a Transportable Optical Ground Station. The experiments took place at ESRANGE, Kiruna, Sweden in August 2005. In addition to optical communications, several atmospheric measurement instruments (Differential Image Motion Monitor, Turbulence Profiler) were used to study the influence of atmospheric turbulence on the optical link. A description of the measurement instruments is given and results of the turbulence instruments (Fried parameter r0, Cn2 profile) are presented.
Mitigation of index of refraction turbulence (IRT) effects is crucial in long-range atmospheric communication links. Diversity-transmission is one favorable way for fading compensation. One of several different diversity concepts is the exploitation of the wavelength-dependent index of refraction of the atmosphere, which leads to ideal stochastically independent fading at different wavelengths, depending on the scenario. This concept is here named wavelength-diversity (WLD). Theoretical analysis and numerical simulation for the strength of this effect are given and verified by experimental tests.
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.
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.
Atmospheric laser communications using direct-detection systems do suffer from severe degradation caused by scintillation. Because the atmospheric cut-off frequency can be as low as 100 Hz, temporal averaging is not applicable in high-speed communications. The simplest way of reducing fading is to increase the receiver size and to take advantage of aperture averaging. Spatial and temporal variations of the received intensity have to be investigated in order to predict the efficiency of aperture averaging. This paper reviews briefly the theory of spatial averaging that characterizes the direct-detected optical power. For comparison purposes, results of measurements are presented. These measurements consist of recorded pupil intensity patterns for a scenario with known turbulence profile. Statistics derived from measurement data are compared with theoretical second-order statistics.
In this paper we introduce a simulation method for modelling clear-air atmospheric turbulence effects for long horizontal stratospheric paths. Divergence angles of several hundred microradians in combination with link distances up to 800 km require to adapt the appropriate resolution of the transverse optical field along the path. For this purpose, we implemented a propagation method in Cartesian coordinates. We choose two reference scenarios for high-altitude platform crosslinks and discuss the influence of simulation parameters to the derived results. Finally a method for computation of temporal IM/DD-time signals form simulated intensity matrices is presented.
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.
The intensity of a laser beam after propagation through turbulent media such as the atmosphere may follow different probability density functions (PDFs) depending on the fluctuation regime. For non-coherent receivers the aperture averaging effect reduces the power scintillation leading to a different PDF. Since the analytical approach of deriving the received power PDF knowing the joint-PDF of the intensity at more than just a few points becomes rapidly complex, we review here a much more simplified approach as well as a simulative approach. Both approaches are based on the results of scintillation theory. First, starting from the PDF of the intensity and its spatial correlation, aperture sub-areas can be defined over which the intensity is assumed equal and independent from other sub-areas' intensity. Under those conditions the power PDF is easily worked out. The validity of this method is evaluated according to the level of spatial correlation of the intensity. In a second method, intensity variables are sampled from the Rx-aperture and an approximation of the power PDF is obtained by generating multivariate correlated intensity values. Weak and strong fluctuation regimes are treated separately and the effects of different resolution of the input-intensity-field are discussed. In addition, this paper compares the predicted power characteristics to those deduced from experimental data where the intensity characteristics (PDF, spatial correlation) have been evaluated.
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