This PDF file contains the front matter associated with SPIE Proceedings Volume 13147, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

This paper describes research conducted by the U.S. Army Combat Capabilities Development Command (DEVCOM) Analysis Center (DAC) to characterize the supercontinuum (SC) effect generated from interactions between ultrashort pulsed lasers (USPL) and commonly used optical materials. The well-known supercontinuum effect occurs when an optical material is irradiated by a laser with pulses of temporal width typically in the femtosecond regime. Interactions between the laser pulses and the material will induce a time dependent self-phase modulation, which leads to an apparent frequency modulation resulting in radiation emitted from the material as a “white light” laser.

Supercontinuum can be generated in many commonly used dielectric materials, otherwise known as bulk materials. The generation of SC in these bulk materials has been well documented and is easily achieved in an indoor laboratory setting. However, inducing SC generation (SCG) in an outdoor setting through atmosphere has not been reported as frequently. We conducted an outdoor experiment at DAC’s Electro-Optical Vulnerability Analysis Facility (EOVAF) laser range in July of 2023. An USPL was used to irradiate an optical collimating system several hundred meters from the USPL with several different bulk materials placed at its exit aperture using a spectrometer system to measure the SCG induced in the materials. After the outdoor experiment was completed, the spectrometer was calibrated using a quartz tungsten halogen (QTH) lamp. The calibration data were used to create a spectral response function which was applied to the uncalibrated spectral data of the SCG resulting in calibrated measurements of spectral fluence.

The propagation of laser pulse trains (LPT) through the atmosphere is analyzed and simulated. The effects of group velocity dispersion, focusing, chirping, and atmospheric turbulence are included in the analysis. The model is general and is solved for pulses longer than several laser wavelengths. We present examples of the propagation of LPTs with parameters chosen to replicate recent experiments which use LPTs to generate rf radiation in the atmosphere. Propagation of LPTs over extended distances is of particular interest. The pulse length of the individual micro-pulses used in the simulation ranges from ~100 fs -100 ps.

For various applications involving the propagation of light through the atmosphere, anisotropy of optical turbulence must be accounted for. At this point, however, there is no consensus about how to realistically model anisotropic turbulence. It is well established that at length scales small compared to a certain outer length scale, L₀, fully developed, optical turbulence is locally homogeneous and isotropic and is well described by the Obukhov- Corrsin similarity theory. At scales large compared to L₀, however, the turbulence is usually anisotropic, and the Obukhov-Corrsin similarity theory is no longer valid.

In this paper, we discuss two questions: first, how to define and predict L₀; and second, how to model the 3D refractive-index spectrum, Φ_n(κ_x, κ_y, κ_z), for wavenumbers comparable to and smaller than 1/L₀. We will address both questions on the basis of classical theoretical concepts and by means of field observations. The classical concepts include Tatarskii’s original definition of L₀, the Richardson criterion, and the Monin- Obukhov similarity theory. Field observations include in-situ measurements by means of ultrasonic anemometer-thermometers and fine-wire turbulence sensors.

An analysis of the coherent mode decomposition of an optical field after propagation through atmospheric turbulence is presented. The coherent modes represent an ideal basis by which to decompose the field for design of mode-limited optical systems. Due to rotational symmetry of the Fredholm integral operator for Kolmogorov optical turbulence, the decomposition yields separable solutions classified by a radial and azimuthal mode index. The study of the coherent modes is then reduced to that of the radial functions determined by the coherence ratio D/r₀ and obscuration ratio ϵ. Analysis of the spectrum of eigenvalues yields sharp bounds on the efficiency of receivers using incoherent or coherent combining with mode-limited photonic devices in the presence of Kolmogorov turbulence. The effective number of modes needed to represent Kolmogorov optical turbulence is studied via the von Neumann entropy, purity, and largest eigenvalue, and the differences in the different definitions is discussed. The similarity of the coherent modes to linearly polarized (LP) fiber modes is quantified yielding a precise characterization of the maximum gain that can be achieved in mode-limited systems via mode shaping techniques. As a final application, a mode sorting technique is presented for optimally splitting power from atmospherically degraded light into a finite number of modes simultaneously maximizing total coupling efficiency and minimizing the difference in average power between channels.

We emphasize the requirement for a validated simulation environment to assess the impact of changes in atmospheric turbulence strength throughout the day and year on the performance of atmospheric electro-optics (EO) systems such as directed energy, power beaming, and laser communications. In the proposed approach, site-specific data from persistent measurements of atmospheric parameters, for instance, turbulence strength, represent the variability of atmospheric propagation conditions. Together with atmospheric models and numerical weather predictions, the measured data are utilized to build atmospheric propagation models for wave-optics simulations of atmospheric EO systems.

We demonstrate this approach with a database of continuous (24/7) C²_n and meteorological measurements on a 7 km atmospheric propagation path at the University of Dayton test range for predictive modeling of laser beam propagation parameters and their fluctuations during diurnal and monthly cycles. We analyze a specific laser beam director system configuration, both with and without the implementation of adaptive optics beam control. The proposed technique provides a pathway to a comprehensive predictive performance evaluation of diverse atmospheric EO systems, allowing for statistical and interactive parametric analysis.

Recent work has shown that calculations of scintillation using wave optic simulation provides a much closer match to experiment than analytic approaches, such as extended Rytov theory. However, link budgets using analytic theories can be calculated much faster than those using wave optic simulation. Although there has been progress in speeding up these calculations using graphics processing units, it would still be convenient to have a low complexity solution for quick calculations with limited computing resources. In this work we pre-calculate wave optic simulations for uniform horizontal links using dimensionless parameters that allow application to a wide variety of cases. We then develop approximations to the pre-calculated values that allow quick computation.

The use of free space optical communication from deep space promises to revolutionize the capabilities of human and robotic exploration, enabling modern-internet-like data rates throughout the inner solar system. Such an improvement will enhance the potential of space science and exploration by enabling higher rate instruments and imagers, lower latency operational capabilities, and less data lost due to limited on-board storage. The adoption of optical communication in National Aeronautic and Space Administration programs, however, has faced numerous budgetary and technical challenges. Building up the Earth-based telescope infrastructure to support high-rate and energy-efficient optical communication from deep space requires significant capital investment that may take years to become operationally significant. The use of many small telescopes in an array is one solution to this problem. We propose a scalable and cost-efficient telescope array architecture based on the Deep Space Optical Communication project, and analyze the primary communication impairments and performance capabilities for a standards-compliant downlink waveform.

In the context of OAM-based signal modulation free-space optical communications affected by atmospheric turbulence, we demonstrate a strategy for designing OAM signal constellations using the Wasserstein distance from the Optimal Transport theory. By recording propagated OAM modes with a spatial detector (complex conjugation, mode sorter, and Shack-Hartmann) and delivering OAM spectra from the images, we create a similarity matrix based on the distorted histograms using the Wasserstein distance. From this, we show that optimal sets of superpositions –in the sense of maximizing the mutual distance of the components– can be found, and be used for improving the quality of detection in the presence of turbulence. We present classification results for three optimal OAM constellations and back these results with our first experimental trial, recorded with a 1-km propagation testbed.

Laser-based applications leveraging coherent laser sources are subject to optical distortions created by random media containing random refractive index variations of various scale sizes. Refractive index fluctuations sizes comparable to the optical wavelength, due to small particles, create complex light-material interactions. Studying this particle interaction can be approached using statistical optics that model the coherence properties of the medium by the implementation of random phase screens. A phase screen represents a statistical realization of the optical phase shifts accumulated during propagation whose statistics are derived from a medium’s optical phase power spectrum. In this work, the phase power spectrum of a turbid medium is connected to the medium’s particle distribution by the medium’s volume scattering function by radiative transfer theory. This model is limited to small-angle forward scattering and small propagation geometries to prevent aliasing by high angle scattered light. Using this relationship, the statistics of the generated phase screens are studied and applied to the propagation of optical vortex beams through turbid water. Experimental orbital angular momentum mode spectrum measurements and intensity attenuation data are presented showing good agreement with the model. Furthermore, the validated phase screen model is shown to predict, as particle sizes increases, the effects of forward peaked scattering leading to vortex beam distortion.

The explosive growth of satellites in low Earth orbit (LEO) demands advanced surveillance and communication capabilities. However, atmospheric turbulence hinders high-resolution imaging and high-speed communication through optical wavelengths, which remains the only viable option. SEETRUE (Sharp wavefront sEnsing for adaptivE opTics in gRound-based satellite commUnications and spacE surveillance), proposes a game-changing solution: cost-effective, AI-driven wavefront sensing for Adaptive Optics (AO) in optical ground stations. It features a unique ground station equipped with a 50 cm robotic telescope with AO capability, a multi-purpose 38 cm binocular telescope, and an atmospheric profiling system. AI-powered wavefront sensors (WFS) within the system leverage novel turbulence models and a revolutionary ”end-to-end” design approach to maximize information extraction. This enables compact and low-cost AO solutions, overcoming a major barrier to widespread adoption. Paving the way for a future with accessible and affordable space communication and surveillance for all.

The effects of dynamic weather on the transmission of a pulsed 2.09-μm laser beam across a 1-km coastal channel are presented. Tests were conducted at the University of Central Florida's Townes Institute Science & Technology Experimentation Facility (TISTEF). By collecting data before, during, and after the morning quiescent period, diverse atmospheric conditions are explored, including periods of clear and cloudy skies and calm and turbulent optical paths. Spatial variations of the laser beam in response to temperature fluctuations and atmospheric turbulence were captured by an infrared camera positioned at the target site. Recorded data were analyzed to assess changes in beam diameter and beam wander relative to range conditions. A suite of weather sensors provided air temperature, wind speed, and solar irradiance. A line-of-sight scintillometer quantified optical turbulence by providing estimates of the refractive-index structure parameter (C_n²), ranging from 2×10^–15 m^–2/3 to 4×10^–13 m^–2/3. Data obtained from range sensors provided inputs for new split-step wave-optics simulations. This work presents the first known comparison of 2-μm laser propagation with the simulation software “High Energy Laser - Performance Estimation Test Technology (HEL-PETT)” developed by Coherent Aerospace & Defense. Experimental results agree well with simulations. Data also revealed the quantitative effects of small, dense cloud cover on range conditions: one-minute cloud cover had no effect on range temperature, optical turbulence, or laser beam properties, whereas four-minute cloud cover generated distinct changes in these properties.

A multiple-input single-output configuration is used to study the effectiveness of spatial diversity in reducing single-photon losses caused by atmospheric turbulence in quantum communications over free-space. The system consists of two transmitters and a single receiver, where two parallel beams are transmitted, each comprising a 660-nm beacon and a 515-nm collinear single-photon signal. The beacon is intended to compare the turbulence conditions on each path to determine the most suitable transmitter for the single-photon signal. An outdoor experimental campaign with a propagation distance of 500 meters was conducted to obtain results under real turbulence conditions. First, the correlation between the 515-nm and 660-nm collinear signals was measured at 0.82, showing that the beacon signal accurately represents the state of turbulence affecting the single-photon signal most of the time. The spatial correlation was measured at separation distances between the transmitters of 12 and 20 centimeters, yielding correlation coefficients of 0.35 and 0.17, respectively. As expected, the spatial correlation decreases with increasing separation distance. Finally, a slight improvement in the detection of the single-photon signal was observed when using spatial diversity, but further improvements to the system and longer propagation distances are needed to obtain more promising results.

This publication will depict recent results from a co-located multi-wavelength transmissometer. The system uses a single telescope to transmit and receive a temporally modulated cascading pulse train from multiple super-luminous LEDs (SLEDs). The signal return is facilitated by retroreflector(s) mounted in a static ground-based array or mounted to a dynamic Un-crewed Aerial System (UAS). The system measures electro-optical (EO) transmission along chosen propagation vectors to develop ground truth timeseries datasets of wavelength dependent transmission and attenuation parameters that will improve the accuracy and speed of test and evaluation efforts for EO atmospheric propagation models. Recent results of ground-based and aerial-based testing will be compared to the performance to physics-based propagation models, and a fog monitor providing localized liquid water content.

This paper explores the fundamental phenomenology of weather-driven diurnal and nocturnal optical turbulence trends. Examining long duration persistent atmospheric measurements at Townes Institute Science and Technology Experimentation Facility (TISTEF), an outdoor laser range operated by the University of Central Florida (UCF), reveals key correlations between observed meteorological quantities and optical turbulence strength. A distributed set of meteorological instruments provide information on local conditions via temperature, pressure, relative humidity, net radiation, wind anemometers, cloud ceilometer, and a sky imager. The strength of optical turbulence is captured via a boundary layer scintillometer (BLS) and the delayed tilt anisoplanatism (DELTA) sensors. The paper compares the turbulence measurements against the performance of a physical weather-driven turbulence model and a deductive machine learning (ML) based turbulence model. These models attempt to accurately capture the relationship and phenomenology between meteorological conditions and optical turbulence. Additionally, the paper discusses an instrument concept that could augment current turbulence forecasting techniques to have improved short term forecasts.

Laser communication link performance in free space depends heavily on atmospheric conditions present on the propagation path. Distortions due to atmospheric turbulence, such as scintillation and beam wander, greatly diminish signal detection and performance. With the final goal of improving the communication link, experimental measurements and analysis of turbulence strength are presented as metrics for determining the system’s detection limits. Experimental optical trials were recorded over a 1-km horizontal path in order to study the intensity fluctuations, beam dance, and the spatial spectrum properties under different regimes. By taking into account wind speed and vibrations of the building where the measurements were carried on, correlations between variables are shown with the use of a photodetector, and a 2D lateral effect position sensor.

Free space optical communication (FSOC) provides high capacity and data security without the spectral allocation challenges of RF systems. These advantages have led to the proliferation of FSOC technology in terrestrial, air-to-ground, and space-based telecommunication network architectures. Propagation through the atmosphere introduces turbulence impacts on FSOC systems which include beam wander and optical scintillation. Optical scintillation causes both spatial and temporal distortions on the received beam, spanning many milliseconds and causing signal fading that can result in significant data loss. In-situ monitoring of scintillation experienced by a given FSOC system in operation can not only provide a better understanding of the underlying distribution functions of various scintillation conditions for link availability planning but also inform more efficient error correction protocol techniques that can better adapt to the changing atmospheric channel conditions. In previous work, we demonstrated a tone-based irradiance variance characterization approach that can provide real-time measurements of the scintillation index, power spectral density, and distribution function of scintillation using an FSOC system’s data modulation envelope. Here, we expand this capability to operate on modulation envelopes with data rates in excess of 1Gbps while simultaneous supporting high bandwidth data reception.

With increasing congestion and demand for more bandwidth within the radio spectrum, alternatives are required for future communication capabilities. Free space optical communications (FSOC) has the potential to achieve secure, reliable, and high-performance wireless connectivity. However, due to the perceived lack of resilience to natural weather conditions and suspended particulate matter such as fog/smog, FSOC has yet to reach mass market adoption.

In this work we demonstrate a communication link using commercial-off-the-shelf components and equipment, for resilient FSOC in the presence of an artificial fog. Using the 10 μm wavelength a successful link is shown to be viable at distances of up to 290mm. Insignificant changes are observed to the communication performance with varying levels of artificial fog, where data rates of 200 kbit s⁻¹ are achieved over three modulation schemes (on-off keying, 4-level pulse position modulation, 4-level pulse amplitude modulation).

This research summarizes the events of a laser propagation test at the TISTEF laser range during January 2024. A 1064 nanometer (nm) continuous-wave (cw) fiber laser was focused at 1 kilometer and propagated in a variety of conditions over a week-long period. Meteorological instruments including a Scintec BLS900, MZA DELTA, and sonic anemometers were deployed along the optical path. The propagated beam spot was recorded at 100 Hz from both transmit and receive site locations. The processed imagery from both cameras generated beam profile data such as short-term spot size, long-term spot size, and beam wander. These statistics were explored as a function of measured atmospheric parameters such as visibility, refractive index structure parameter, wind speed, and more.

Interleaving and repeat coding are techniques that provide immunity to channel effects for optical communications links. Interleaving provides robustness at the physical layer of the network stack, at the expense of additional latency. The temporal diversity ratio, L, is defined as the number of statistically-independent fade events experienced by an individual code-word in an interleaved framing structure. L characterizes an interleaver's ability to whiten a channel so forward-error correction codes for mitigating additive white Gaussian noise function effectively. For large L's, an interleaver can tolerate a wide duration of fade dropouts with high latency, while smaller L's improve latency yet suffer more fade-induced power penalties. Repeat coding is another technique that can enhance a channel's resilience to atmospheric fading by sending identical copies of frames. Repeat-coding techniques allow a link to operate at lower signal-to-noise ratios, while still maintaining a fixed slot clock to simplify receiver design. We define Q as the frame replication factor for a repeat-coded waveform.

We investigate interleaver performance in different regimes of both L and Q. We develop a model to supplement interleaver dynamics with repeat coding under atmospheric fading, and investigate regimes with different values of fading strength, interleaver diversity L, and repeat-coding Q. An experimental optical modem testbed with a flexible configuration is used to validate the model. We show good agreement between the model and experimental laboratory tests over a wide range of cases studied.

This presentation explores the possibilities of using AO, which is widely applied to the correction of turbulent phases in astronomy but is now used in free-space communications (FSO). Optimal controllers have been demonstrated to be a helpful way of improving the error budget on extremely demanding AO systems in astronomy. Still, no recent study shows the full benefits of implementing an optimal control AO in a working FSO communication system. Here, we present our first results after implementing a closed-loop AO system in the Universidad de los Andes laboratory that allows us to test real-time controllers and implement the AO correction directly to FSO communication systems.

The effectiveness of free-space laser communications is limited due to wavefront deformations caused by atmospheric optical turbulence. To determine these deformations, we propose a wavefront sensor that utilizes the angular selectivity of an optical transmission filter to measure the first derivative of the wavefront.

The transmission filter converts the gradients of the incident wavefront into an intensity distribution. For each direction (x and y) this distribution is captured twice, with different angles between filter and optical axis for each measurement. The contrast of both measurements (calculated for each pixel of the used detector) and the local gradients of the wavefront (averaged across each pixel) has a nearly linear relation. To reconstruct the wavefront from the obtained local gradients, algorithms developed for the Shack-Hartmann wavefront sensor are used. Simulations demonstrate the applicability of the sensor in atmospheric turbulence. For the experimental proof of concept, we have designed and fabricated volume Bragg gratings (VBG) as angular selective filters. The VBGs were implemented in an optical testbed to evaluate the sensor response to wavefront tilts.

In problems involving optical wave propagation in the atmosphere it is common to assume turbulence statistics obey a Kolmogorov power spectral density model. It is well known that this model has a key weakness in that it does not model the outer scale. Also, some observations indicate a deviation from the Kolmogorv -11/3 power law often referred to as non-Kolmogorov turbulence. Understanding the effect of a change in power law has been a difficulty for some researchers and most assume the turbulence volume is homogeneous, which is unlikely. In this work, I describe how anisoplanatic error can be described for inhomogeneous turbulence using the mean turbulence height and the generalized Fried parameter. I also consider a two scale outer scale model and describe its effect compared to the single parameter von Karman model.

Coherent light propagation in the atmosphere, as occurs with lasers, is significantly impacted by fluctuations in the refractive index of air, which are a function of temperature and pressure. The fluctuations cause beam degradations, including spatial incoherence, power fades, and surges. Conventionally, numerical wave propagation methods with phase screens are used for modeling imaging and optical transmission. Phase screens assume turbulence isotropy and thin turbulence regions to simplify complex turbulence behaviors in the atmosphere. However, these assumptions may result in large deviations due to shear and inhomogeneous regions in the atmospheric boundary layer. An alternate optical turbulence model is proposed using a spherical bubble packing scheme. The broad spectrum of turbulent length scales is represented by the bubbles with radius based on power law distribution based on a linear-eddy model approach. The refractive index of each bubble is prescribed based on the Tatarskii spectrum. Imaging is accomplished by a ray tracing algorithm through Snell’s law. The effects of length scales, path, and refractive-index structure function coefficient,C²_n are investigated by imaging analysis. We take several steps to assess model uncertainty and inadequacy for validation and verification. Images generated from ray tracing are used for verification of the bubble model. A numerical wave propagation approach with phase screens is used to validate imaging techniques used on the bubble model with prescribed C²_n values and profiles.

Atmospheric turbulence can significantly impact the quality of an image by causing distortions during image acquisition. The turbulence can affect the image sharpness and contrast. We will quantify atmospheric impacts on image quality by using standard image quality metrics for measuring image blur (such as the variance of the Laplacian). In this paper we will utilize meteorological and contrast targetboard observations to train machinelearning models. We utilize approximately two years of optical turbulence data and meteorological data along a coastal path to train and test the models. A comparison between various ML algorithms, such as XGBoost (Extreme Gradient Boosting) and LightGBM (Light Gradient Boosting Machine), is performed to determine which best predicts the image quality metrics.

Commercial and military free space communication needs are growing at a remarkable pace. The requirements for free space data transfer have increased as the technology increased, and now engineers are considering higher frequency bands for free space, including W/V, millimeter wave and optical bands. Laser communications can offer much higher data rates than traditional radio frequency (RF) systems and have the added advantage of not being regulated by the International Telecommunication Union (ITU). This along with typical hardware being much smaller in size, weight and power (SWaP) make optical communications a desirable solution for high-rate communication systems. W/V band systems also have the advantage of higher data rates and less frequency congestion than traditional RF systems, as well as the advantages of being able to link through clouds. As the need for “data on demand” increases, the likelihood of users moving to systems that can switch between multiple radios (hybrid systems) is very high. Today, multiple users in both commercial and government sectors are looking to integrate both laser communication (lasercom) and RF solutions onto their platforms. The goal of this work is to utilize a pre-trained Alexnet Convolution Neural Net (CNN) on Doppler radar and GOES satellite imagery to make decisions on which a hybrid system will provide the highest performance differing atmospheric conditions. This method can be scaled for any terrestrial or space-based system.

This research presents the analysis of Resistance Temperature Detectors (RTDs) at the Townes Institute Science and Technology Experimentation Facility (TISTEF) one kilometer laser range. The RTDs produce nodal turbulence measurements that leverage the precision of PT100 RTDs. TISTEF aims to complement its current fleet of turbulence detection instruments with an additional precise nodal measurement of turbulence strength. The system consists of four, four-wire RTDs yielding differential temperature measurements used to estimate the Refractive Index Structure Parameter (C_n²). These data are then validated against a co-located sonic anemometer and a BLS900 Large Aperture Scintillometer to confirm accuracy.

The strength of a machine learning algorithm is that given enough data and computation, any trend or pattern appearing within that data can be approximated without explicitly involving underlying understanding. The refractive index structure parameter, C²_n, is a measure of the strength of fluctuations in the refractive index along a path and one of the most significant contributors to beam quality. Weather data including C²_n is collected over a period of several months at the TISTEF (Townes Institute Science and Technology Experimentation Facility) laser range. Once data is aggregated, important features are selected and the data is processed. The AutoGluon framework is used for exploratory analysis of the effectiveness of neural network structures and Keras is used in final model building. The model chosen as a result of this process indicates that machine learning is able to generalize trends in the fluctuations of C²_n over time.

Low-noise, high bandwidth avalanche photodiode (APD) detectors are instrumental in free space optical communication systems as communications receivers. APDs with large active areas help mitigate the challenge of focusing received optical power onto a detector after being distorted due to atmospheric turbulence. However, larger active areas increase the capacitance of the detector and reduce its bandwidth performance. Therefore it is necessary to weigh the increased field of view provided by larger active areas against the bandwidth performance of such devices. Details of an APD characterization testbed, and initial performance results, will be presented.

We propose using polarization-independent liquid crystal on silicon (PI-LCoS) phase modulators in all-optical free space optical (FSO) transceivers to mitigate the impact of atmospheric turbulence and beam wander. Fiberto- fiber optical signal transmission minimizes signal latency and loss, but direct light coupling from free space into an optical fiber necessitates a sophisticated beam pointing and tracking system. Conventional LCoS phase modulators, while capable of high-resolution beam steering and wavefront correction, face the limitation of singlepolarization modulation. The introduction of a PI-LCoS phase modulator into the FSO transceiver eliminates the need for complex polarization management optics, significantly reducing system complexity and improving overall performance. Such phase modulators perform atmospheric turbulence correction, beam divergence control, and beam steering to allow for efficient optical power coupling into the output fiber.