Intersatellite free-space optical communications are the backbone of the future highspeed global communication networks. In orbit, thermo-mechanical loads create perturbations that detriment the performance of these links. Among these perturbations, the transmitter pointing jitter and optical aberrations are of special relevance. We present an analysis of the coupled effects of transmitter pointing jitter and optical aberrations on intersatellite free space optical communications. A mathematical model is presented to evaluate the performance of average bit error probability, probability of outage, and reliability on intersatellite free space optical communication links subjected to these perturbations. Furthermore, the optimum non-aberrated truncated Gaussian beams are obtained for each of these performance parameters for different telescope architectures. The results demonstrate that the performance parameters are highly sensitive to the optimal far-field irradiances. These optimum operation points are then perturbed by Seidel aberrations to study the effect of these aberrations in the system. The results show that optical communication terminals are most sensitive to coma aberrations, mainly due to the induced apparent angle of arrival on the beacon beam. Finally, Monte Carlo simulations of combinations of Seidel aberrations show a strong dependency on the telescope architecture of the sensitivity of the communication performance parameter to the magnitude of the optical aberrations.

Laser satellite communications is a growing market for telecommunications services and secure communications. Due to the small footprint of the beam, it provides security, and it can serve as inherently safe communication, using the quantum properties of light, i.e. quantum communications, and quantum key distribution. For quantum communications, the detector sensitivity is of utmost importance. Hence, detector technologies such as Superconducting Nanowire Single Photon Detector (SNSPD), are designated to support optical communications. In this research, we estimate the efficacy of SNSPD arrays for optical downlinks to earth. In this paper, we investigate how spatial resolution of the SNSPD and the spatiotemporal statistics of the incoming turbulence can be matched, in order to effectively use SNSPD arrays. We simulated the downlink using a split-step approach, using a receiver aperture of 1 m and a Fried parameter of 50 and 12.5 cm (D/r₀ is 2 and 8). We obtained the average intensity, the scintillation index, the probability density function (PDF) and the power spectral density (PSD) using 4000 samples. We project instantaneous images on the SNSPD, to estimate how the PSD is distributed. We conclude that stronger turbulence conditions could improve detection performance, because the point spread function (PSD) is wider, and the incident light is distributed over more pixels.

To support the development of free-space-optical (FSO) communication technologies, an end-to-end physical layer model of a satellite communication service was developed. This service involves physical processes spanning multiple time scales: hours (relative platform dynamics), minutes (link selection, atmospheric attenuation), milliseconds (atmospheric turbulence, platform disturbances), and nanoseconds (photon and bit transportation). The modified multi-scale method (MMM) was used to combine the physics of these processes and to model an end-to-end global FSO communication service between an airborne platform and a satellite constellation. The method provides a better understanding of physical interdependencies, allows performance analysis on multiple time scales, and enables valuable insight into where to optimize such a service. The results show realistic performance metrics when compared to other smaller-scale models and demonstrations. The MMM can be used as a mission performance indicator of an end-to-end satellite communication service.

In this study, we examine the propagation characteristics of vortex-Gaussian beams in atmospheric turbulence for uplink free-space optical (FSO) links. Using the extended Huygens–Fresnel principles, we analytically derive key parameters, including the received intensity, second-order and fourth-order intensity moments, the Strehl ratio, the kurtosis parameter, and the effective beam spot radius. Then, we used phase-screen simulations to analyze the behavior of the scintillation index. Our findings demonstrate that vortex beams offer superior Strehl ratio performance compared to the Gaussian beam and exhibit greater resilience to atmospheric turbulence variations. Furthermore, the vortex beams exhibit kurtosis values below 3 and have a larger effective beam spot radius compared to Gaussian beams. Additionally, vortex beams exhibit a lower scintillation index compared to the Gaussian beam at longer propagation distances.

Laser satellite communication technologies are promising next-generation communications systems, offering higher data rates, more secure links, and cost-effective operations. One of the remaining challenges to tackle for ensuring sufficient link availability is atmospheric turbulence. While turbulence effects on laser links can be partly compensated for with corrective technologies and algorithms, these methods would benefit from better knowledge of turbulence profiles on the communications channel, both for system design or real-time assistance during operations. As state-of-the-art turbulence profilers are not designed to measure profiles from a laser beam on a moving satellite, this paper proposes a line-of-sight turbulence profiling tool tailored for laser satellite communications. Speckle-based turbulence observation and reconstruction via machine learning uses surrogate learning to build a model that can reconstruct optical turbulence profiles (C_n²(h)) from a single shot measurement of a speckle pattern. In this paper, the first modeling results of this flexible approach demonstrate that eight-layer turbulence profiles can be reconstructed from simulated single speckle images of a star with less than 10% error on the Fried parameter.

One of the most ambitious goals of modern astronomy is to uncover signs of extraterrestrial biological activity, primarily achieved through spectroscopic analysis of light emitted by exoplanets to identify specific atmospheric molecules. Most exoplanets are indirectly identified through techniques like transit or Doppler shift of the host star's flux. Long-term surveys have yielded statistical insights into the occurrence rates of different planet types based on factors such as radius/mass, orbital period, and the spectral type of the host star, initial estimates of terrestrial planets within the habitable zone have also emerged. However, the difficulty of detecting light from these exoplanets leaves much unknown about their nature, formation, and evolution. As the number of rocky exoplanets around nearby stars rises, questions about their atmospheric composition, evolutionary trajectory, and habitability increase. Direct measurement of an cxoplanct's atmospheric composition through its spectral signature in the infrared can provide answers. Measuring the infrared spectrum of these planets poses significant challenges due to the star/planet contrast and very small angular separation from their host stars. Previous research showed that space-based telescopes are mandatory, and unless large primary mirrors (>30m in diameter) can be sent into space, intcrfcrometric techniques become essential. Combining light from distant telescopes with interferometric techniques allows access to information at minimal angular separation, operating within the diffraction limit of individual telescopes. Successful demonstrations of on-ground nulling interferometry open a new era for such space-based missions. They are vital to sidestep and tackle these scientific questions. We recently initiated a new study with the European Space Agency to explore the design parameters and the performances related to an interferometric concept based on a single spacecraft and sparse multiple sub-apertures. Launch constraints are linked to the use of an Ariane 6 launch vehicle. Our parametric study covers a range of 1-4 m for the diameter of the telescope and a 10-60 m baseline. The most promising concept working in the infrared range (3-20μm) will be highlighted. This study is conducted by TUDelft in cooperation with KULeuven, CSL/ULiege, and Amos with the support of the European Space Agency.

Traditional laser communication terminals are limited to point-to-point links, which constrains their scalability and flexibility for global networks that require simultaneous connections with multiple targets. While multiple single-beam terminals can expand capacity, this approach multiplies Size, Weight, Power, and Cost (SWaPC), limiting scalability. Multi-beam laser communication terminals offer a promising alternative, though the design of an effective beam steering system remains a key challenge. This paper explores the design process of such a system, providing an overview of multi-beam steering literature as well as an comparison and trade-off of existing space-borne multi-beam steering technologies. It also analyzes insights from related fields such as terrestrial laser communications, LiFi, single-beam laser communication, optical cross-connects, radio links, and multiple target tracking. Key system functions are identified and visualized in a function flow diagram, and various design options are evaluated, culminating in a design options tree which serves as a design recipe. Two application scenarios, involving high and low target densities, demonstrate that steering systems based on micro-mirror arrays and spatial light modulators present significant advantages over alternatives. This study offers a comprehensive framework for designing multi-beam steering systems for space-based laser communication terminals.

One of the most ambitious goals of modern astronomy is to uncover signs of extraterrestrial biological activity, primarily achieved through spectroscopic analysis of light emitted by exoplanets to identify specific atmospheric molecules. Most exoplanets are indirectly identified through techniques like transit or Doppler shift of the host star's flux. Long-term surveys have yielded statistical insights into the occurrence rates of different planet types based on factors such as radius/mass, orbital period, and the spectral type of the host star. Initial estimates of terrestrial planets within the habitable zone have also emerged. However, the difficulty of detecting light from these exoplanets leaves much unknown about their nature, formation, and evolution. As the number of rocky exoplanets around nearby stars rises, questions about their atmospheric composition, evolutionary trajectory, and habitability increase. Direct measurement of an exoplanet's atmospheric composition through its spectral signature in the infrared can provide answers. Measuring the infrared spectrum of these planets poses significant challenges due to the star/planet contrast and very small angular separation from their host stars. Previous research showed that space-based telescopes are mandatory, and unless large primary mirrors (>30m in diameter) can be sent into space, interferometric techniques become essential. Combining light from distant telescopes with interferometric techniques allows access to information at minimal angular separation, operating within the diffraction limit of individual telescopes. Successful demonstrations of on-ground nulling interferometry open a new era for such space-based missions. They are vital to sidestep and tackle these scientific questions. We recently initiated a new study with the European Space Agency to explore the design parameters and the performances related to an interferometric concept based on a single spacecraft and sparse multiple sub-apertures. Launch constraints are linked to the use of an Ariane 6 launch vehicle. Our parametric study covers a range of 1-4 m for the diameter of the telescope and a 10-60 m baseline. The most promising concept working in the infrared range (3-20μm) will be highlighted. This study is conducted by TUDelft in cooperation with KULeuven, CSL/ULiège, and Amos with the support of the European Space Agency.

We present a machine learning-based measure-correlate-predict approach that predicts a multi-year time-series of optical turbulence strength (Cn2) with high accuracy (r = 0.78 at 16 locations) based on a single year of in-situ Cn2 measurements and reanalysis data.

Steering multiple laser beams using spatial light modulators (SLMs) creates unwanted diffraction and reflections that are not modulated by the SLM, which can make beam tracking difficult. A novel, to the best of our knowledge, and simple beam steering methodology is proposed, which aims at reducing the influence of this clutter while maintaining tracking performance. The beam(s) are deliberately defocused before steering with a superposition of a phase ramp and Fresnel lens (PRFL) phase screen on the SLM. As a result, the non-modulated reflections and diffracted light are decreased in relative intensity to the steered beam, in turn allowing simple and standard peak intensity and center of gravity (CG) algorithms for tracking. Hardware demonstration shows tracking performance using the PRFL remained on-par with more complex filtering approaches while adding no additional hardware. This method has potential to improve the communication performance of multi-beam laser communication terminals.

For free-space optical communication or ground-based optical astronomy, ample data of optical turbulence strength (C 2 n) are imperative but typically scarce. Turbulence conditions are strongly site dependent, so their accurate quantification requires in situ measurements or numerical weather simulations. If C 2 n is not measured directly (e.g., with a scintillometer), C 2 n parameterizations must be utilized to estimate it from meteorological observations or model output. Even though various parameterizations exist in the literature, their relative performance is unknown. We fill this knowledge gap by performing a systematic three-way comparison of a flux-, gradient-, and variance-based parameterization. Each parameterization is applied to both observed and simulated meteorological variables, and the resulting C 2 n estimates are compared against observed C 2 n from two scintillometers. The variance-based parameterization yields the overall best performance, and unlike other approaches, its application is not limited to the lowest part of the atmospheric boundary layer (i.e. the surface layer). We also show that C 2 n estimated from the output of the Weather Research and Forecasting model aligns well with observations, highlighting the value of mesoscale models for optical turbulence modeling.

A 10 km ground-To-ground field test of a Terabit/s Optical Feeder Link demonstrator has been carried out. The demonstrator aimed for end-To-end communication performance with the use of Adaptive Optics pre-correction to mitigate the atmospheric turbulence disturbances of the FSO channel. It further includes the multiplexing of multiple uplink channels and RF end-To-end modems to prove the technical feasibility of supporting a Terabit/s communication link. The field test-performed in July 2022-demonstrates error free communication for over 10 minutes. This achievement proves the feasibility of a digital transparent communication architecture for the Terabit/s Feeder link application. The field test results are presented jointly with a companion paper [1] which focusses on the communication aspects. The Adaptive Optics (AO) pre-correction performance is analyzed against the atmospheric turbulence strength and the number of corrected AO modes. The encountered turbulence conditions during the field test caused irradiance fluctuations in the strong fluctuation regime. The field test results show a significant AO wavefront error correction on the downlink. For the uplink irradiance levels, a modest advantage of pre-correcting higher wavefront modes is found against the pre-correction of only the tip-Tilt modes. Finally, with respect to the uplink fading characteristics, the AO pre-correction leads to a reduction of the mean-fade time of approximately 20-30%, compared to AO tip-Tilt pre-correction only.

Optical feeder links (OFLs) benefit from the vast amount of bandwidth available in the THz-regime of the electromagnetic spectrum, and can be considered as enablers for future terabit-per-second satellite systems. A particular challenge for OFLs is to mitigate the effects of fading, caused by a combination of turbulence-induced scintillation, beam wander and pointing errors. The conventional solution is to exploit temporal diversity by a combination of interleaving and forward error correction (FEC). In this study we present an overview of fading mitigation techniques for latency-constrained coherent ground-to-satellite OFL and contribute a generic model which combines various diversity schemes including temporal, spatial, frequency and site diversity. To unlock spatial diversity, multi-beam space-time block coding and multi-beam, multi-λ are proposed and simulated. Though space-time block coding (STBC) provides more diversity gain, it requires accurate timing synchronization at the transmitter and channel state information at the receiver. Temporal, frequency and site diversity all rely on some form of interleaving and the potential diversity, pros and cons of each of these diversity techniques are covered in the presented study. In general, with a strict latency constraint and a tight link budget, frequency diversity, spatial diversity - either by STBC or multi-beam multi-λ - and site diversity can be effective methods to mitigate the effects of fading and close the link budget.

Turbulent fluctuations of the atmospheric refraction index, so-called optical turbulence, can significantly distort propagating laser beams. Therefore, modeling the strength of these fluctuations (𝐶2𝑛) is highly relevant for the successful development and deployment of future free-space optical communication links. In this Letter, we propose a physics-informed machine learning (ML) methodology, Π-ML, based on dimensional analysis and gradient boosting to estimate 𝐶2𝑛. Through a systematic feature importance analysis, we identify the normalized variance of potential temperature as the dominating feature for predicting 𝐶2𝑛. For statistical robustness, we train an ensemble of models which yields high performance on the out-of-sample data of R2 = 0.958 ± 0.001.

For the next generation of very high throughput communication satellites, free-space optical (FSO) communication between ground stations and geostationary telecommunication satellites is a potential solution to overcome the limitations of RF links. To mitigate atmospheric turbulence effects, TNO proposes Adaptive Optics (AO) to apply uplink pre-correction. As a successor of Optics Feeder Link Adaptive Optics (OFELIA) breadboard [1], [2], the Terabit Optical Communication Adaptive Terminal (TOmCAT) project phase 2 aims to demonstrate the AO precorrection technology for a terabit Optical Ground Station (OGT) in a ground-to-ground link field test over 10 km. Within this demonstrator an upgraded version of the OFELIA breadboard is used as optical bench for the AO (pre-)correction, but moreover the demonstrator enables the (future) integration of equipment for the final OGT configuration including the Beam Multiplexer (BMUX) and communication equipment. Apart from the OGT demonstrator, the overall layout of the field test has been upgraded, including the test site and the Ground Support Equipment (GSE), with the goal to create a better understanding of the encountered link phenomena and the instant (turbulence) conditions at which it was measured. New additions to the GSE are several weather stations placed along the link path to quantify the local turbulence and to relate the measured link performance to the instant turbulence conditions. The test campaign is split in two successive field tests: First the AO Demonstrator evaluates the upgraded AO pre-correction performance with a single non-modulated link, followed by the OGT Demonstrator which will include the multiplexing of multiple uplink channels and RF end-to-end modems to prove the technical feasibility of supporting a terabit communication link. This paper covers the design of the AO demonstrator and GSE, the field test layout and the preliminary results for the AO demonstrator field test. For the downlink correction the residual Wave Front Error (WFE) is presented. The pre-correction performance is depicted in the uplink transmission loss and scintillation, all as function of the encountered turbulence conditions.

Free-Space Optical Communication (FSOC) links are considered a key technology to support the increasing needs of our connected, data-heavy world, but they are prone to disturbance through atmospheric processes such as optical turbulence. Since turbulence is highly dependent on local topographic and meteorological conditions, modeling optical turbulence strength (Cn 2) is challenging during the design phase of an optical link or network. Over the past 25 years, (see manuscript PDF for symbol) parameterizations of varying complexities have been combined with various numerical weather prediction models for the spatio-temporal estimation of (Cn 2). However, the outputs of these models can exhibit substantial variability based on the user-defined configuration that determines how atmospheric processes are represented. To address this concern, we propose to run not a single model configuration but multiple diverse ones to generate an ensemble estimate of (Cn 2). We employ the Weather Research and Forecasting model (WRF) with ten different Planetary Boundary Layer (PBL) physics schemes forming a diverse ensemble yielding a probabilistic (Cn 2) estimate. We demonstrate that this ensemble outperforms the individual runs when compared to scintillometer field measurements and show it to be robust against outliers. We believe that FSOC downstream tasks such as link budget estimations should also become more robust if based on a (Cn 2) ensemble estimate compared to single model runs.

Laser satellite communications provides an attractive application for optical communications. It provides more bandwidth with improved security at potentially a lower cost due to reduced size weight and power (SWAP) and reduced license cost compared to radio frequencies (RF) communications. These advantages have been recognized for years and lead to the successful European data relay service (EDRS) [1]. This service provides the proof that optical communications can be used as a reliable means of satellite communications. In addition, the inherently save quantum key distribution (QKD) technology is demonstrated in a satellite to ground link in [2]...

Due to the distance limitation of quantum communication via ground-based fibre networks, space-based quantum key distribution (QKD) is a viable solution to extend such networks over continental and, ultimately, over global distances. Compared to Low Earth Orbits (LEO), QKD from a Geostationary Orbit (GEO) offers substantial advantages, such as large coverage, continuous link to ground stations (cloud cover limited), 24/7 operation (background limited), and no tracking required. As a downside, QKD from GEO comes with large link losses due to the space-ground distance, lowering the achievable key rates. From our feasibility and conceptual design study it is concluded that although link losses are high, QKD from GEO is technically feasible, and a favourable solution if the satellite needs to act as an untrusted node (that is, no security assumptions required for the space segment). However, the optimal solution, generating a higher value-for-money, is to have the possibility to operate it in trusted mode as well, as higher key rates can be obtained. But this will be at the cost of security as key material needs to be (temporarily) stored on board of the satellite. In order to arrive at a minimum required secure bit rate of ~1 bit/s in untrusted mode, two ~0.5m diameter telescopes in the space segment are required with <0.65μrad pointing accuracy each, a >1GHz entangled photon pair generation rate, in combination with ~2.5m diameter telescopes on ground, operating at 810nm wavelength. In trusted mode, with the same optical system but only using one telescope in the space segment, a factor of ~300 to ~10000 more key can be obtained. Details on our assumptions and results and drawings of the high level system design are presented, as well as a description of the required technology improvements and building blocks needed, which is applicable to non-GEO applications as well.

Free space optical (FSO) satellite communications has very attractive properties for Quantum Key Distribution (QKD). The quantum channel loss and noise are major factors in setting the maximum achievable secure key rate of QKD systems. Primarily for a LEO satellite node and the BB84 protocol a number of technologies are proposed to reduce the quantum channel loss and noise. These technologies apply to the receiver side of the QKD link, the Optical Ground Terminal. It is shown that an optical beam shaper to optimize fiber mode matching, dedicated thermo-mechanical design to reduce misalignment induced by temperature gradients and an Adaptive Optics (AO) system to counteract optical turbulence effects can result in a significant reduction of loss and background noise. A breadboard verification experiment shows that using an medium-size AO system can maintain a fiber coupling efficiency up to 40%. The developed technologies have a general applicability with respect to satellite orbit and QKD protocol used.

Optical satellite communications is a maturing technology to enable word-wide access to high throughput internet. In the past years a lot of effort has been taken to increase the applicability and the TRL of this technology. In collaboration with industry, TNO initiated several developments for space and ground technologies. Many of these technologies have already passed critical design review (CDR) and are in an advanced state. A missing piece of the puzzle is an in orbit demonstration (IOD), which proves the technologies to be working. This paper presents the plans for an IOD with CubeCAT on the NorSat-TD. As ground segment the TNO optical communications lab is equipped with an 80 cm diameter telescope. By an successful IOD, worldwide available internet at high throughput is yet one step closer.