This study investigates the deployment of drone swarms as sensor platforms for the real-time characterization of atmospheric properties, including turbulence, humidity, and aerosols, particularly along slant paths or in challenging environments. Utilizing swarms of sensor-equipped drones allows for the measurement of environmental parameters along specific paths, enabling the acquisition of real-time, localized atmospheric data at various points along these paths. To measure turbulence, the Differential Temperature Method is used, complemented by small, lightweight, off-the-shelf sensors for assessing other atmospheric properties. This approach facilitates a detailed understanding of atmospheric conditions, leveraging the mobility and versatility of drone swarms to gather data in real-time.

This paper aims to analyze the characteristics of laser beam propagation from ground to space through atmospheric turbulence at the Gochang SLR Observatory in South Korea. To address issues such as energy loss and pointing errors caused by atmospheric turbulence in various applications like laser optical communication, satellite laser ranging (SLR), adaptive optics systems, and laser energy transmission, we systematically analyzed how laser beam parameters are affected by atmospheric conditions. The study utilizes a 100-watt-class laser to simulate beam propagation up to 100 km altitude, focusing on parameters such as laser wavelength, beam size (diameter), beam jitter, and beam quality (M²) and their impact on long-exposure and short-exposure beam radius. Key findings reveal that beam jitter is the most influential parameter for the long-exposure radius, while beam quality (M²) significantly affects the short-exposure radius. These findings provide critical guidance for optimizing laser system performance under varying atmospheric conditions, potentially contributing to advancements in laser communication and weapon systems.

In the hypersonic environment, the highly compressed and rapidly dynamic flow field significantly impacts optical transmission performance. The aero-optical numerical simulation method, such as ray tracing algorithm, can simulate the optical transmission path of light in hypersonic environments. Different from previous study that treated the flow field simulation as discrete data nodes and calculated optical path using the empirical fixed step sizes, this study introduces a Pixel Reconstruction based Flow Field Image Layers(PR-FFIL) simulation method. It reconstructs the discrete flow field data nodes into pixel, and interpolation is carried out individually in each pixel area. The PR-FFIL encapsulates data from flow field nodes as information within pixel regions. This approach not only aligns more closely with the physical characteristics of the flow field but also reduces the repeated calls to interpolation algorithms within the same pixel, thereby enhancing efficiency. Based on reconstructed pixels, the PR-FFIL iteratively solves the ray tracing step sizes and provides the range of adaptive step sizes. It more flexibly handles simulated flow fields with different accuracies and enables precision up to 0.0001mm. After processing the pixelated flow field image, the PR-FFIL layers flow field image to achieve adaptive step size for ray tracing. By employing PR-FFIL to perform ray tracing in the standard and more refined simulated flow field, the computational efficiency increases to 2.1859 and 2.7082 times compared to using traditional methods respectively, without sacrificing computational accuracy. Therefore, the algorithm demonstrates further advantages when applied to finer flow field simulation and smaller step sizes

In adaptive optics and wave propagation processes, understanding the Kolmogorov turbulence spectrum is essential. It serves as a fundamental framework for assessing the impact of atmospheric turbulence on various electro-optical applications, including imaging, laser power transmission, and optical communication. At the heart of this investigation lies the characterization of the atmospheric turbulence parameter C²_n, which quantifies the strength of the optical turbulence. The Kolmogorov turbulence spectrum delineates the distribution of energy across different length scales within turbulent flows, from low to high frequencies. It encompasses the energy-containing eddies down to their eventual dissipation, governed by characteristic length scales such as the integral scale, outer scale, inner scale, and the Kolmogorov micro-scale. In this study, ultrasonic anemometers measurements at different heights on a 64m-high tower provide crucial data on temperature and wind speed, enabling the characterization of characteristic length scales. Additionally, a laser scintillometer operated close to the ground supplies turbulence measurements. Variance spectra of temperature and wind velocity measurements are investigated to retrieve information about atmospheric turbulence. By scrutinizing the height and stability dependencies of these spectra, deeper insights into the intricate interplay between atmospheric turbulence and environmental conditions are gained.

Satellite-to-ground communications and laser applications often suffer from turbulence, causing loss of signal quality. Key parameters include the coherence length of turbulence (𝑟₀), the isoplanatic angle, and the Greenwood frequency. Specifically, this study focuses on the effects of different assumptions regarding mixing length scales on the vertical profile of optical turbulence. We report a comparison of various parametrizations, based on the ratio of mechanical and thermal heat exchange, alongside existing parametrizations found in literature. The range of mixing scale variations, spanning from centimeters to thousands of meters, poses a significant challenge and raises questions regarding the utilization of standard radiosonde data for turbulence estimation.

Variations in aerosol particle size distributions introduce a time-varying wavelength-dependent scattering probability for optical radiation. Mie calculations on experimental particle data are used to generate extinction coefficients as a function of time and wavelength. Long-term aerosol and meteorological data were collected from the atmospheric and ecosystem research station Norunda, located north of Uppsala in the southern part of the boreal forest zone, in Sweden. For particle size distribution measurements subjected to an aerosol dryer, a growth factor was introduced to account for the hygroscopic effects in particles as a function of relative humidity. Scattering cross-sections were calculated based on Mie theory and translated to extinction coefficients for different wavelengths from the visible to the long-wavelength infrared regions. Visibility and infrared to visual-ratios based on the extinction coefficients have been calculated. Statistics of the variation with close to a year of data is analyzed.

We have modeled, synthesized, and performed preliminary testing of a synthetic aerosol particle with the intent to affect an asymmetric (“one-way”) vision environment when deployed as an airborne plume and aligned in real time via an applied acoustic field. The first aerosol particle iteration under test, the microclub, features asymmetry in particle geometry and material composition to cause asymmetric scattering behavior, dependent on the propagation direction of incident mid-infrared light. Despite this asymmetric scattering behavior, the microclub has been shown to maintain electromagnetic reciprocity in computational simulations, exhibiting a consistent extinction cross section with respect to forward and backward propagation directions of incident light. We expect this asymmetric scattering behavior will ultimately cause vision asymmetry when deployed as an airborne plume incorporated into an imaging path. Before proceeding to in-air testing of the microclub, we have performed an intermediate investigation of the microclub while suspended in a solution of water and polyvinyl alcohol (PVA) to test both the particle’s rotation response to an applied acoustic field and the particle rotation’s impact on optical transmission. Here we present the results of this preliminary investigation and we discuss the impact on visibility and next steps.

LTAO (Laser Tomography Adaptive Optics) encounters errors arising from both adaptive optics and laser tomography processes. This paper presents a comprehensive analysis of these errors under various operational conditions in laser tomography. Specifically, we examine the error variations with changes in atmospheric altitude, providing an in-depth error analysis associated with altitude fluctuations. Additionally, we analyze errors using single conjugate adaptive optics (SCAO) and the learn and apply (L&A) algorithm, highlighting the specific challenges and error metrics encountered in these methodologies.

As an alternative to traditional adaptive optics systems, wavefront shaping techniques show promise in controlling light propagation through turbulent channels. This study explores the feasibility of measuring the transmission matrix and using it to enhance communication in turbulent conditions.

The modal holographic wavefront sensor offers a promising alternative to established wavefront sensing techniques. Since the strengths of individual aberration modes are directly measured, there is no need for time-consuming signal processing and wavefront reconstruction. Bandwidths up to three orders of magnitude higher than those of commercial wavefront sensors can in principle be achieved. However, in practice the accuracy of measurements is compromised by intermodal crosstalk, which arises when the wavefront exhibits additional aberrations to those encoded in the modal wavefront sensor. This issue is particularly prominent when measuring wavefronts disturbed by atmospheric turbulence. To mitigate the effects of intermodal crosstalk, a procedure to optimize the sensor design for prevailing atmospheric turbulence conditions has been proposed. In this paper, we experimentally investigate the effectiveness of this method. We describe the fabrication of a holographic wavefront sensor consisting of a thin phase transmission holographic grating. In an optical testbed, defined wavefront deformations are generated using a spatial light modulator, and the wavefront sensor is used to measure these disturbances. The measurement error is determined for different sensor designs.

Optical turbulence distorts beam amplitude and phase, causing spreading, wandering, and irradiance fluctuations. Reconstructing perturbed beams’ complex fields is experimentally challenging due to these dynamic effects. Our complex phase retrieval technique, using binary amplitude modulation with a DMD and high-speed camera, characterizes collimated beams through turbulence and overcomes interferometric limitations. Conventionally, phase retrieval modulates optical fields via random coded apertures (RCA) to recover amplitude and phase without prior knowledge, solving ill-posed problems with phase-lift algorithms. Our previous approach required ≥20 apertures, increasing acquisition time and complexity. We designed a new coded aperture, reducing time and enhancing quality over traditional RCA. Then we apply a novel deep-learning phase unwrapping algorithm enabling efficient unwrapping of phases with turbulence-induced branch point singularities manifesting as vortices. This is the first experimental observation of turbulence complex wavefronts reconstructed with high spatial resolution and sampling rate. We discuss observed statistical properties and compare with current models.

In atmospheric optics it has become common practice to model the energy input or outer scale using a von Karman model. In terms of the turbulence power spectral density this model assumes that the slope of contributions outside the inertial range are uniform from the outer scale to zero spatial frequency. Recently, it has been suggested that energy outside in the buoyancy range may instead have a slope of -15/3 or -17/5 compared to a 3D index of refraction model in the inertial range of -11/3. The possible existence of these ranges has been suggested as one source for observations of non-Kolmogorov turbulence. In this work, I use wave optics simulation to evaluate the effect of these varied models on the beam wander of a collimated Gaussian beam and compare the results to the von Karman model.

In this paper, we investigate how atmospheric turbulence impacts Symbol Error Probability (SEP) of free-space optical communication systems based on the phase-shift keying heterodyne coherent receiver scheme. The derivation assumes the validity of the Maréchal approximation and requires a high number of “coherence cells” at the receiver pupil (ergodic hypothesis). Results show that such conditions can both be satisfied at the same time when tip and tilt correction is applied. We also found that better performances in terms of SEP are possible when the receiver is located in the far field (supposing a collimated beam at the transmitter). Our analysis is valid for Gaussian beams affected by Kolmogorov turbulence.

Time-variant Free-Space Optical channel behaviour is analysed for temporal variations of C²_n values, satellite motion and satellite passes. These variations lead to non-stationary statistical properties of the irradiance at the receiver. The analyses are based on representative link models and complemented by measured data from recent test campaigns. These are the 10km’s near-horizontal field test with the TOMCAT demonstrator terminal and the in-orbit test between the SmallCAT terminal onboard the NorSat-TD LEO microsatellite and the TNO optical ground station GOCAT in The Hague. The study shows substantial variations of the statistical properties of the irradiance for LEO satellite motion and predominantly for time-variant C²_n behaviour. For variations of satellite passes–with respect to the ground station–the sensitivity of irradiance properties over the full pass is less pronounced. An analysis of the probability distribution of elevation angles over all satellite passes reveals a high probability of low elevation links.

Free-space optical (FSO) communication has recently attracted more attention as an enabler for several 6G applications, such as backhaul and fronthaul connectivity, non-terrestrial networks (NTNs) or satellite communications, data centre interconnects, and quantum-secured communications. In scenarios where the geometry of FSO system lies in the near-field regime (e.g., backhaul/fronthaul), spatial mode multiplexing (SMM) can be implemented. However, atmospheric turbulence distorts the amplitude and phase of propagating spatial modes of light, leading to power leakage among orthogonal modes. In this paper, the impact of atmospheric turbulence on signal-carrying spatial modes of light and the resulting crosstalk among originally orthogonal spatial modes are investigated. Different implementations of SMM FSO systems are studied considering cases with mutually coherent and mutually incoherent channels. The performance of different turbulence mitigation techniques such as spatial mode diversity, zero-forcing beamforming, and mode selection optimization is reviewed and compared with benchmark FSO systems.

Free-Space Optical (FSO) communication is on the verge of becoming an important backbone of the global network infrastructure, and the de facto standard for high-rate data links on satellites, in the near future. In an attempt to overcome link misalignment issues associated with low resolution Four Quadrant Detectors (4QDs) and to widen the scope of feasible optical link scenarios, a detection approach for optical position sensing based on Short-Wave Infrared (SWIR) image sensors is proposed. Thanks to the capability of resolving fine details over a large Field-of-View (FOV), image recognition algorithms can be employed to distinguish different beam objects within the focal plane, making it possible to separate the useful signal from unwanted stray light sources. This classification approach potentially enables Direct-to-Earth (DTE) links during daylight, as these rely on the suppression of the Earth’s albedo superimposing the user signal. Using image acquisition, multivariate tracking algorithms such as Kalman or Particle Filter can be set up to improve the stability of beam tracking needed for more challenging link topologies, such as fast fly-by maneuvers or Intersatellite Links (ISLs) between different orbits. Furthermore, an image-based beam detection system is a useful diagnostic tool for in-orbit calibration or validation of different atmospheric conditions.

The use of free-space optical (FSO) links has the potential to significantly increase the data transmission rates compared to radio links and they enable the possibility of performing quantum communication. For the development of FSO components, including transmitters and receivers, and of novel communications protocols, it is essential to have the ability to comprehensively test the performance across several different communication scenarios – e.g., aircraft-to-ground, aircraft-to-aircraft, satellite-to-ground, and ground-to-satellite. To this end, we have built and deployed two fibre-based FSO channel simulators, having identical design, for testing optical communication in the C-band. These devices are capable of emulating in-the-lab the effects that the transmission through Earth’s atmosphere would have on the received optical signal. Three parameters can be controlled and monitored in real-time: the average attenuation, the signal amplitude fluctuations and the signal phase fluctuations. The (constant) average attenuation emulates the end-to-end transmission loss, from signal source to sink, including the combined effect of geometric loss, due to long communication distances, of atmospheric absorption and fibre-coupling loss. The (time varying) amplitude fluctuations emulate the fast changes in signal intensity due to atmospheric turbulence and implementation specific disturbances like pointing errors. Finally, turbulence can also cause the phase of the signal to fluctuate in time and this effect can also be physically emulated. The amplitude and phase fluctuations time series can be loaded from previously measured data, or from synthetic data. These features allow testing the impact that the transmission through an FSO channel has on different communication schemes, including but not limited to telecommunication applications, like on-off keying and phase shift keying, and quantum communication applications, like quantum key distribution.

Optical ground-to-space links (OGSL) are characterised by a highly dynamic channel, due to optical beam propagation through the atmospheric turbulence, leading to link power outages (also known as fades). The study of such stochastic fading process is an essential task to estimate the performance of OGSLs. The most common simulation methods used require an excessive computation time, hence are incompatible for network-wide simulations with a high number of possible links to simulate. A time-efficient alternative is provided by a random power vector generation approach which leverages on the knowledge of the power outages statistics and power spectral density (PSD). In this work, a modelling approach based on a Butterworth filter construction is presented as an effective solution to define the power outages PSD. The advantages of such method are displayed and the results are compared to OGSL simulation data obtained via a validated, alternative approach.

Quantum key distribution (QKD) enables secure communication even in presence of quantum computers. Satellite based systems enable worldwide key distribution and connection of terrestrial quantum networks. In future, QKD satellite constellations can be operated to service ground users that are connected using an optical ground station. These QKD ground stations are assumed to be deployed in two scenarios. One is the option as end-user ground station, in which the ground user is directly connected to the satellite via the ground station. The other option is the use as a provider ground station, in which the ground user is connected via an intermediary fiber line. The performance of the satellite to ground QKD link depends on, amongst other, the environmental and infrastructural impacts of the site where it is located. Unlike in astronomical sites, the QKD Optical Ground Station (OGS) are not positioned in selected optimal environment, especially considering atmospheric turbulence and extinction. The QKD OGS mainly has to cope with the given environment instead. Knowledge of the site’s meteorological and infrastructural environment is important to assess performance of a QKD OGS when deployed. This paper discusses the most relevant aspects of a QKD OGS site characterization. These are atmospheric turbulence, atmospheric extinction, and background light. As a concrete example, the DLR Optical Ground Station Oberpfaffenhofen nearby Munich is used. Existing data is summarized, reviewed and needs for further data acquisition defined.

To establish a free-space underwater optical connection between a transmitter and a receiver suitable for operation in the single photon regime, techniques must be developed to compensate for: a) rotational and translational motions of both the transmitter and the receiver, b) their vibrations, and c) turbulence-induced deformations of the wavefront leading to beam wander, scintillation and beam spreading. The purpose of this work is to present and discuss experimental results obtained using a pseudo phase-conjugated mirror placed at the end of an underwater optical path disturbed by artificially induced turbulence. This configuration emulates a scenario whereby a modulated retroreflector is used to transmit the data back to the laser transmitter. The measurements obtained with the proposed retracing optical connection are found to be promising and pave the way for the use of this technique for quantum communications.

Quantum Key Distribution (QKD) allows sharing encryption keys with information theoretic security. Satellitebased QKD can establish long distance links due to the quadratic transmission loss in free-space instead of the exponential transmission loss in optical fibers. Atmospheric background light plays an important role in the QKD scheme as it may significantly contribute to the system Quantum Bit Error Rate (QBER). Therefore, background light needs to be examined closely. Due to the high variability of atmospheric conditions, direct measurements of the background light under different meteorological conditions are the best option to properly characterize the effect. Current considerations are mainly limited to the analysis of cloud-free scenarios by simulation and by experiment. Links can also take place when the environment differs from this ideal condition. Measurement data was recorded in C-band at the campus of the University of Waterloo, Canada, during the day with clear sky and during sunset with clear sky and partly-clouded sky conditions. The measurement data is shown and compared to simulation results and to the measurement data taken in Oberpfaffenhofen, Germany. The impact of background light is discussed on a chosen reference scenario outlining the importance of detector gating time and end-to-end transmission loss when wanting to realize daylight QKD.

Wired and fibre-based communication are the backbone of the current global communication network, thanks to their large bandwidth, reliability and stability, capability to serve many users and low cost over their lifetime. However, they show several important limitations when applied in the defence and military domain, mostly due to the possibility of physical attacks and disruption, the requirement of a pre-installed infrastructure and limited flexibility. Free-space and wireless communication work perfectly as a complementary technology and become enabler of many fundamental applications in the military domain. Free-space quantum communication is a growing field that promises to empower and enable new cryptographic applications and distributed sensing and computing use cases. In this paper, we review the most prominent applications of free-space quantum communication in the military domain, with a focus on Quantum Key Distribution. Then we critically analyse important results in the field in the last 15 years, focusing first on ground-to-ground implementations and then on mobile platforms, concluding with satellite-based quantum communication.