This PDF file contains the front matter associated with SPIE Proceedings Volume 12691, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

The objective of this study is to explore the feasibility and accuracy of image subtraction for estimating optical turbulence. The proposed approach involves creating a differential image by subtracting consecutive recorded frames. Post processing techniques are applied to the differential image, allowing temporal changes caused directly by turbulence to be identified. Image subtraction was implemented in python and evaluated against traditional turbulence instruments such as a Scintec BLS2000 and a MZA DELTA.

This publication will depict ongoing efforts in development and ground based validation of an aerial transmissometer utilizing a ground control station composed of a collocated altazimuth mounted transmitter and receiver along with a gimbal mounted retroreflector operating on an Un-crewed Aerial System (UAS). The transmitter is composed of multiple super luminous LEDs of different wavelengths. The system measures bulk point-to-point transmission through the atmosphere and enables an investigation into atmospheric species due to wavelength dependent absorption. The measurements will be along dynamic propagation paths and enable the development of hemispherical ground truth datasets.

In 2010, we [Cota et al., Proc. SPIE 7013, 1-14 (2010)] described how The Aerospace Corporation’s Parameterized Image Chain Analysis & Simulation SOftware (PICASSO) may be used with “with a limited number of runs of AFRL’s MODTRAN4 radiative transfer code, to quickly predict the top-of-atmosphere (TOA) radiance received by an earth viewing sensor, for any arbitrary combination of solar and sensor elevation angles.” More specifically, the 2010 paper detailed the extension of “the method to the short and midwave IR, where reflected solar and emitted thermal radiation both contribute to the TOA radiance received by a downlooking sensor.” In the decade since publication, we have developed a series of updates, to improve both fidelity and robustness, whilst maintaining consistency. First, the functional form of the TOA radiance equation has been modified, in order to provide users with more degrees of freedom in modeling solar contributions, without significantly increasing the overall number of terms. Secondly, we’ve provided an alternate formulation of terms, which utilizes an additional iteration of MODTRAN to produce higher fidelity results, in some opaque bandpasses and for some collection geometries. Finally, our methodology is now applicable with the latest version of MODTRAN (6), including all of the syntax changes involved therein.

In order to improve laser communication link performance, turbulence strength is an important parameter to characterize the system’s correcting limits and detection availability. By setting an experimental optical link through free space horizontal propagation in a 1-km path, we study the strength of different turbulence scenarios through the Rytov approximation and scintillation of the beam, and compare methods of experimental detection of the refractive-index structure constant of the turbulence, C²_n . Results show that, under low and medium turbulence regimes, both methods behave similiarly as a way to predict C²_n ; however, with larger turbulence strength, the beam’s displacements in the focal plane are more sensitive than the intensity fluctuactions.

Autonomous Vehicles (AVs) operating outside of fair-weather conditions will inevitably encounter winter weather. In populous cities like Boston, Chicago, New York, Detroit and many others, this weather may sometimes be severe. Severe winter weather conditions are characterized by low temperatures, freezing rain, and snow. Over the span of three years we have collected over forty terabytes of severe winter weather driving data in the Upper Peninsula of Michigan featuring seven different AV or automotive focused lidars. In this work we summarize the effects of falling and blowing snow and other precipitation on lidar and its implications for AVs as well as some approaches for mitigating these effects.

This research paper discusses the application of several image-based techniques for measuring optical turbulence. University of Central Florida researchers have previously prototyped and fielded a differential disturbance tracker at the TISTEF 1 kilometer range. This effort has evolved into the development of a software suite that implements image processing techniques such as blob detection, centroid tracking, and optical flow for estimating the refractive index structure parameter. To validate each method, imagery was collected over the 1 kilometer path. The processed results were compared against measurements from an MZA DELTA system.

The integrity of a free-space optical communications link can be directly correlated to the atmospheric conditions through which the link is made. The link may become degraded due to optical turbulence and absorptive/ scattering losses resulting in reduced signal strength and may result in failure to close a link. It is important to analyze atmospheric conditions in order to predict and mitigate these performance losses. The goal of this work is to characterize local atmospheric conditions in a coastal environment over a 560 m terrestrial optical range. Local atmospheric and weather data were gathered utilizing several data collection instruments including an anemometer, a scintillometer, and a weather station hub. The data analyzed in this work were temperature and path averaged scintillation index (C_n²). Experimental data collections and analysis of monthly weather conditions are presented with C_n² values ranging from 10^-15 to 10^-12 (m^-2/3) and temperature values ranging from 8 to 31 (℃).

Free space optical communication (FSOC) links in the maritime environment have been demonstrated in ship-to-ship and ship-to-shore tests. FSOC over water is attractive because of long lines of sight and more stable turbulence conditions. Link performance, range, and availability depend on terminal design, visibility, and turbulence conditions. However, the ability to predict turbulence conditions and their effect on link performance is still an open question. Turbulence introduces an effective scintillation loss into the link that depends on turbulence, the desired quality of service and the design of the communication protocol. In this work we show that this loss can be calculated if the distribution function for scintillation is known. We use experimental measurements from the Naval Research Laboratory’s Chesapeake Bay Lasercom Testbed and both extended Rytov theory, and wave optics simulation to find predictive models for scintillation loss that use only environmental and system parameters as inputs.

Any beam that propagates through optical turbulence will experience distortions in both its amplitude and phase, leading to various effects such as beam wandering, beam spreading, and irradiance fluctuations. Reconstructing the complete field of a perturbed beam is a challenging task due to the dynamic nature of these effects. Interferometric wavefront reconstruction techniques—such as those based on holography—are commonly used but are hindered by their sensitivity to environmental disturbances and alignment errors. However, new complex phase retrieval methods based on propagation equations have emerged, which do not require prior knowledge of the beam to be reconstructed and are suitable for amplitude or phase objects, or both. We propose an experimental implementation of a complex phase retrieval technique for characterizing Gaussian beams propagating through optical turbulence, using binary amplitude modulation with a digital micro-mirror device (DMD). This approach is ideal for dynamic applications and has enabled us to achieve experimental high-speed complex wavefront reconstruction of optical beams through controlled real turbulence. This experiment corresponds to the initial step in our research focused on gaining a deeper understanding of optical turbulence from an experimental perspective.

As the RF spectrum becomes increasingly congested, the development of terrestrial free-space optical communication (FSOC) systems has taken on greater importance. The performance of these systems is strongly impacted by optical scintillation which causes both spatial and temporal disturbances that can have a significant impact on link availability. The scintillation index, σ_Ι², is a measure of the normalized irradiance variance observed across a given receiver aperture and represents a key parameter in the modelling of link availability. Conventional measurement techniques of σ_Ι² are based on a CW laser transmitter illuminating a DC-coupled, wide dynamic range photodetector over a propagation path of interest. As such, the measurement setup is typically solely dedicated to scintillation index collection and solar background contributions need to be closely monitored to avoid measurement bias. Here, we describe a versatile, tone-based characterization approach that provides a real-time measurement of σ_Ι² using either a FSO system’s tracking tone or its data modulation envelope. Furthermore, since the approach operates exclusively on the AC component of the tone, background correction is no longer required. Initial results using tone-based σ_Ι² monitoring are presented and compared directly with σ_Ι² evaluated using the conventional DC-based approach.

Current Free Space Optical Communications (FSOC) technologies require Direct Line of Sight (DLoS) between two users. In a dynamic environment where users are distributed across the battlespace, reestablishing DLoS links between multiple users becomes challenging; hence, easing the harsh Line of Sight (LoS) requirement may make FSOC more accessible to these types of environments. Our on-going FSOC research has identified Indirect Line-of-Sight (ILoS) optical physical layer approaches that enable a novel Multi Access Tactical Optical Communications (MATOC) network layer while maintaining many of the benefits offered by a LoS system. In this paper the authors will summarize successful experiments in one physical layer approach, diffuse reflection. Further experimentation beyond the timeline of this conference is underway and will be included in future publication(s).

Free space optical communication (FSOC) has gained importance during the last few decades, among others, due to its higher data rate, which can go up to 2.5 Gbps¹ in commercial systems, and its secure transmission. There are several challenges an FSOC channel encounters, one of which is atmospheric turbulence. Atmospheric turbulence can degrade the optical signal due to effects such as intensity scintillation and beam wandering. In this work, a machine learning algorithm has been optimized to forecast the scintillation index for the next 512 time steps. Meteorological data, such as air temperature, relative humidity, and wind speed, is obtained together with the scintillation index during an experiment along a 7 km propagation path in Dayton, OH. The data is divided into four parts corresponding to the four seasons and the data in each season is divided into training and validation data. Bi-directional long short-term memory (Bi-LSTM) models have been optimized and tuned to forecast the scintillation index. The mean squared error (MSE) is used to compare the predicted scintillation index with the measured scintillation index, and the adaptive moment estimation (Adam) optimizer is used to update the trainable parameters to minimize the MSE. The root mean squared error (RMSE) is used to validate the model predictions in the validation data. The training process is performed with different Bi-LSTM models on the training data for each season and the performance of the model is measured using the validation data for the corresponding season. The Bi-LSTM model predicts the scintillation index with a weighted average of the RMSE around 0.03 for all seasons.

The interest of beams carrying orbital angular momentum (OAM) for free-space optical communication (FSOC) has increased over the years, due to the large number of orthogonal modes which can be used to transmit data at high data rates, and the secure transmission it offers. One of the main concerns in FSOC applications is atmospheric turbulence through which all beams propagate. The atmospheric turbulence is degrading the quality of the laser beam, due to the creation of intensity variances, beam wander, loss of spatial coherence, and OAM cross talk. A wave optics simulation framework is used to model OAM beam propagation through atmospheric turbulence. In these simulations Laguerre-Gaussian (LG) beams are used as beams carrying OAM. The atmospheric turbulence is modeled as phase screens and a split-step algorithm is used to model the beam propagation. LG beams with different mode order are propagated through atmospheric turbulence of different strengths. The results will be compared against experiments performed in the laboratory.

An experimental campaign for the propagation of laser modes carrying orbital angular momentum (OAM) over 1 kilometer has been established at UANDES university campus. We describe our method for estimating OAM spectra and the average topological charge values from the images delivered by Shack-Hartmann sensor. For OAM beams transmitted with a single topological charge we analyze the average departure of the measured charge with respect to the intended one and the spread of these values as a function of turbulence strength.

Free-space optical (FSO) communication has gained in interest for a long time due to its ability to have secure transmission and high data rates. Interest has increased in using beams carrying orbital angular momentum (OAM) for FSO communication, due to their (theoretically) infinite number of orthogonal modes and potential high resistance to atmospheric turbulence. In this experimental study, Laguerre-Gaussian (LG) beams are used as OAM beams and LG beam of different orders are generated using a spatial light modulator (SLM). A wave-optics simulation method is used to generate phase screens containing simulated atmospheric turbulence, which in turn are used on two SLMs to generate atmospheric turbulence in our experimental setup. In this study, beams with different orders of OAM are propagated through (simulated) atmospheric turbulence, ranging in strength from weak to strong. The distorted beam is recorded using a CCD camera and the images are processed to determine their spot size, intensity, and (spatial) scintillation index. The effect of the strength of atmospheric turbulence on different orders of LG beams is analyzed using these beam parameters.

Vector vortex beams (VVB) combine a nontrivial phase structure and a transverse polarization pattern that can be used for optical free-space or optical fiber communication links. In this presentation we will show, using numerical propagation, the effects of atmospheric turbulence on the estimation of the pixel-by-pixel Stokes parameters, and how the concepts of Optimal Transport allow an optimal selection of basis symbols and a more accurate detection of the VVBs in moderate turbulence.

In this presentation we outline the results of the analysis and numerical simulations of the physical phenomena associated with the propagation of laser pulse trains (LPT) in air. In the performed studies the intensity of the micro-pulses in the LPT is far below that of tunneling ionization. The ionization process relies on the background level of radioactivity, which plays an important role in initiating a collisional ionization process. The focused LPT ionizes the air forming a plasma filament. The ponderomotive forces associated with the LPT drive the plasma oscillations predominantly in the radial direction. As the plasma density builds up on axis, the latter portion of the LPT is defocused, resulting in scattering of the incoming laser radiation and shortening of the laser’s interaction length. In our model, a low intensity LPT photo-ionizes background negative ions (produced by ambient ionizing radiation) and provides the seed electrons necessary to initiate collisional ionization. The driven radial electron currents in turn generate directed rf radiation. The frequency of the rf radiation is given by 1/Tp where Tp is the separation time of micro-pulses in LPT.

This paper discusses the results of experimental studies of low intensity laser pulse train (LI-LPT) propagation in air. The train of ultra-short laser pulses of adjustable repetition rate became possible with the intra-cavity longitudinal mode selector that improves the efficiency of the mode locking mechanism. This technique enabled the generation of a LPT with an envelope duration T_PL ≈ 40 ns (FWHM). The envelope is filled with a train of micro-pulses that form a temporal comb with an individual micro-pulse duration τ_L ≥ 150 ps. The micro-pulse separation time, T_P, can be tuned from ~10 ns to 0.45 ns. Depending on the pump level, the total energy E_LPT of the LPT is in the range of 150 mJ and 1.2 J. When focused in air, the LPT with peak micro-pulse intensity ranging from 5×10¹⁴ W/cm² to 10¹⁶ W/cm² generates a plasma. The laser induced plasma leads to laser light scattering, broadband luminescence, and generation of rf radiation. We report the first results of the experimental studies of the interaction of the LI-LPT with air. Theoretical analysis and simulations of the filamentation and rf radiation have been carried out. The results of the experiment are in agreement with the theoretical and simulation results.

Optical applications, such as imaging, communications, and sensing, can be severely limited by the effects of oceanic turbulence when the water is free of particulate matter. To study this phenomenon in a controlled environment, a Rayleigh B`enard tank, housed at the U.S. Naval Academy, was used to study heat driven convective turbulence in a systemic manner. A Gaussian laser beam was characterized though synchronized phase and intensity measurements obtained by a Shack-Hartmann wavefront sensor and high-speed camera, respectively. The beam’s instantaneous intensity and phase measurements were analyzed in space and time, and the synchronicity between the wavefront sensor and camera allows for the temporal statistics to be directly compared. Phase time series were analyzed to obtain an ensemble averaged power spectrum that was fit to a bounded Kolmogorov model. Wavelet analysis was leveraged to process the turbulence frequency rates at weak and moderate turbulence levels. Estimates for the turbulence turnover rates were obtained from the temporal statistics. Upon applying the same methods to the intensity time series, the statistics appeared subtly different compared to the phase statistics. It was shown within the wavefront frequency statistics that features changed on the time scale of seconds. However, intensity features changed on timescales of seconds to a tenth of a second.

We consider the difficult problem of ground-based propagation of 90 femtosecond laser pulses through the turbulent atmosphere in the range of 1 mile, where the atmospheric properties change significantly according to weather conditions. Our experimental data indicates that use of femtosecond laser pulses for optical communications at up to 1.35 Gb/sec is significantly advantageous compared to CW lasers, offering much reduced error rates and higher consistency in transmission in diverse atmospheric conditions. We develop a new model using Gaussian beamlets to simulate the effects of propagation in different refractive index conditions within the beam. The results are quite consistent with the experimental observations indicating significantly less scintillation and small- scale fluctuations in received data when fs laser pulses at 1540 nm are used. In the new model, we simulate the transmitter as a coherent sum of Gaussian beamlets (5-500) and propagate them with slightly different index of refraction, corresponding to the normal variations in average index of refraction due to changing atmospheric conditions. For instance, when transmitting over one mile of air, a temperature difference within the beam of only 0.02 °C causes the Gaussian beamlets to arrive at the receiver with time delays differing by about 200 fs, exceeding the coherence time of the 90 fs laser pulses, thereby causing incoherent summation at the detector.

Spatial diversity is a technique widely used in wireless communications to enhance the signal quality at the receiver. We propose a multiple-input single-output system that utilizes this technique to enhance a free-space optics quantum communication link by reducing the amount of photon losses caused by atmospheric turbulence, thus increasing the capacity of the quantum channel. The system consists of two transmitters with uncorrelated optical paths, and a single receiver. A 515-nm quantum signal is transmitted through the transmission path with the highest gain, dynamically chosen by comparing the signal distortions of a 660-nm classical signal. Preliminary experiments with a single transmitter have been conducted in a laboratory environment with atmospheric turbulence generated via heat guns. We observed that the single-photon channel is highly correlated with the fluctuations of the 660-nm classical signal, so that an improvement in the former is expected when selecting the path with highest gain. The number of photon counts received was compared with the turbulence-free scenario, revealing that the mean number of counts decreased, and its standard deviation increased when turbulence is present, as expected.

Free space optics propagation through the atmosphere experiences wavefront phase deformation, beam distortion, and aberrations due to the refraction index variations fluctuation along the optical path . Many studies have been done to analyse the atmospheric layers and understand their effects on beam quality. Different applications need to tackle the atmospheric effect like satellite to ground optical communication, astronomy, beam sensing, and power beaming where the atmospheric effect leads to beam wandering that result on beam mis-pointing and power loss at the receiver/target. This paper covers the design of an atmospheric turbulence generator and its characterization and capabilities to create different atmospheric turbulent strength levels. Temperature variation, wind speed/direction, and humidity effects are considered and monitored to emulate different turbulences regimes (corresponding to different spatial coherence levels) that will impact a Gaussian wavefront beam with different turbulence strengths. The turbulence emulator has various apertures for the incoming beam, outcoming beam, ambient air flow, and tuneable temperature air flow. The mixture of these two airflows results in high-speed refractive index fluctuations. A 1064 nm collimated optical beam is used to characterize the turbulence generator and illustrate the impact of the environmental condition on the outcoming optical beam.

In free-space optical (FSO) communication, photodetectors are used for data reception and position sensing for pointing, acquisition, and tracking. These two functions are typically performed by separate photodetectors in a split-path optical receiver architecture. The U.S. Naval Research Laboratory has developed advanced heterostucture, large area avalanche photodetector (APD) arrays that combine both position sensing and data detector functions in a single device and can perform these functions simultaneously. Here, we describe the development of single period, low dark current APD arrays that are scaled to larger active areas than previously demonstrated. In addition to laboratory test-bed development, initial sensitivity performance as a function of bandwidth is presented. The results are then compared to previous generation APD array performance.

Free-space optical communication (FSOC) holds unmatched potential for high bandwidth and secure communications while minimizing size, weight, and power (SWAP). However, the effects of atmospheric scintillations on high bandwidth signals limits data link performance by degrading OSNR (Optical signal-to-noise ratio) and Q-factor. A critical component due to which a communication signal quality deteriorates is timing jitter. Jitter may be due to timing of the data signal or it may be due to the amplitude variations in the data bit stream as it propagates through free-space. As the data bandwidth increases, these effects become more significant. A small-time deviation in a lower data rate signal which would be tolerable or be above a receiver sensitivity, turns into an intolerable signal at higher data rates as jitter increases. The total jitter (TJ) can be further broken down to deterministic jitter (DJ) and random jitter (RJ). These may help understand signal behavior and the root cause of degradation in a FSOC or any data communication link. Thus, for a system to achieve desired BER (bit-error-rate and bit-error-ratio), an in-depth analysis of jitter by investigating each of the subclass of both timing jitters, DJ and RJ, would be extremely helpful and enhance the robustness of the link. In this paper, we report in-depth jitter analysis from a FSOC data link at 10 Gbps propagating at 1550 nm.

The integration of Free Space Optics (FSO) inter–satellite links within low Earth orbit (LEO) satellite constellations will bring a revolutionary change to communication systems in the near future. These links offer a multitude of advantages over conventional methods, including smaller antenna sizes, reduced weight and volume, compact form factor, cost–effective for satellite launching and deployment resources, lower power consumption, and higher data rates. Furthermore, FSO links ensure enhanced security by advantage of their high directivity and narrow beam width, effectively eliminating interference. However, there is still requiring for improvement in FSO's channel capacity when compared to radio–frequency links (RF–Links). The purpose of this article is to present an optical design for a dual–channel free space optical communication system, which has been specifically optimized to achieve a compact form factor suitable for CubeSat inter–satellite links.

This paper discusses a testbed that implements a beam acquisition and tracking system essential for inter-satellite optical communication. It summarizes the results of optical and control system design. Recent research on free-space optical communication has been thriving due to the increasing demand for data transmission in clustered operations of small satellites. However, since the distances between satellites are often hundreds of kilometers, a beam acquisition and tracking system is necessary to establish communication by accurately detecting and tracking the position of the target satellite. The optical system of the testbed, developed by Telepix, shows the attenuated output and flatness of wavefront propagated over long distances. The control system applies an adaptive control method to manage disturbances, resulting in the successful reduction of beam pointing errors to the desired level, as demonstrated by simulation results. In the future, this technology holds potential for various applications, including clustered operations of small satellites using free-space optical communication, as well as communication between ground stations and deep-space optical communication.

The refractive index structure parameter (C_n²) is of interest because it characterizes turbulence, which affects optical propagation through the atmosphere, including free space optical communications, laser propagation, and imaging. This work seeks to develop a geography-agnostic model that can predict C_n² and received signal strength index (RSSI), with as few input parameters as possible. This work trains models including the Gaussian process regression, neural network, and bagged decision tree types, and use r-squared and root-mean squared error to compare model performance. Most of the data used to train and test the algorithms is collected in San Diego, a Csa-type climate (hot-summer Mediterranean climate) according to Köppen climate classification. We then demonstrate application of the trained models to a different site with similar climate, using available common input parameters, and quantitatively assess each model's respective efficacy.

The optical in-phase and quadrature (IQ) modulator is a crucial component in coherent optical communication transmitter. An automatic bias control technology of optical IQ modulator for optical quadrature phase-shift keying (QPSK) transmitter based on envelope detection is proposed in this paper. The implementation basis of this method is provided and the feasibility of this method has been demonstrated through experiments. This method applies dither signals to the three direct current (DC) arms of optical IQ modulator in time sharing, and provides envelope detection to the feedback optical-to-electrical signal, and controls the real-time DC bias of the three arms according to the spectrum of the signal that completes envelope detection calculated by Fast Fourier transform (FFT). The described dither signals are all sine single frequency signals and the specific basis refers to the amplitude value of the point corresponding to the frequency of the dither signal on the spectrum. This method has simple judgment basis, and is easy to implement in practice, and does not have high requirements for the feedback photodetectors. The local optimal algorithm can serve this method well. By using this ABC method based on envelope detection, QPSK modulation was completed with an error vector amplitude (EVM) of 8.89%.

In the phase control of multiple beams, when multiple phase shifters reach the boundary at the same time, the traditional method is to reset multiple phase shifters at the same time. However, this approach leads to a significant drop in the combined beam optical power. To address this issue, we propose a new reset method. This method resets the phase shifters that reach the boundary at the same time in sequence. It helps to avoid the problems that cause large phase fluctuations and dramatic decreases in optical power. This paper presents the principle of coherent multibeam beam combining and compares the power fluctuations resulting from two different resetting methods. Theoretical derivations and numerical simulations are employed to analyze the effects. The results demonstrate that the sequential reset method yields smaller power jitter compared to the traditional reset method. Furthermore, the suppression of power jitter becomes more pronounced as the number of beams reaching the boundary simultaneously increases. To validate the feasibility of the sequential reset method, an experimental system is constructed. The experimental results reveal a remarkable 36.78% improvement in power stability when compared to the traditional reset method.