This paper will demonstrate an 18 aperture Digital Adaptive Optics system imaging through laboratory-controlled turbulence. Comparisons to a standard imaging system, with data that was captured simultaneously under the exact same conditions, will be shown. System performance relative to increasing turbulence strength will be quantified using QR codes as an imaging metric.
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.
NIWC Pacific will present a novel, cost-effective method for in situ measurement and characterization of atmospheric turbulence, as quantified by the atmospheric seeing parameter, r0. The technique will leverage spatially encoded QR codes that are imaged using normal imaging optics. The presentation will cover the theory of the technique along with simulation and experimental results compared to a commercial turbulence measurement system.
Turbulence estimation theory is presented and demonstrated by imaging a series of spatially encoded quick response (QR) codes in ambient radiation through atmospheric scintillation. This remote sensing concept was verified though preliminary feasibility experiments and detailed MATLAB simulations using QR codes displayed on a low-power digital e-ink screen. Of note, knowledge of propagation range and QR code dimensions are not required ahead of time, as each code contains information detailing its block size and overall physical size, enabling automated calculations of spatial resolution and target range. Estimation algorithms leverage the extracted resolution and range information to determine path-integrated optical turbulence, as quantified by the Fried parameter, r0. The estimation criterion is obtained by cycling a series of QR code sizes on an e-ink screen and determining the transition point at which the QR code can no longer be read, resulting in a system capable of automatically estimating path-integrated optical turbulence.
This paper will propose a new method of measuring path integrated turbulence using Quick Response (QR) codes. The QR turbulence estimation theory will be presented, and results using a normal camera and the Digital Adaptive Optics system under development at the Naval Information Warfare Center Pacific will be explored.
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.
Digital adaptive optics created using homodyne encoding can mitigate atmospheric turbulence in passive imaging systems. This work demonstrates a self-referencing homodyne interferometry technique that combines the passive imaging utility of multi-frame algorithmic procedures with the single-frame correction capability of the Shack-Hartmann adaptive optics technique. As an expansion of recent progress on three sub-aperture assemblies, this work showcases the latest results from the NIWC Pacific team to include coherently reconstructing 18 sub-apertures.
Digital adaptive optics (DAO) created using homodyne encoding can mitigate atmospheric turbulence in passive imaging systems. This work demonstrates a self-referencing homodyne interferometry technique that combines the passive imaging utility of multiframe algorithmic procedures with the single-frame correction capability of the Shack–Hartmann adaptive optics technique. This paper presents image reconstruction improvements through the addition of (1) phase diversity modulation techniques within the interferometry reconstruction algorithm and (2) temporal image processing techniques applied after the interferometry reconstruction algorithm. By imaging quick response codes through a turbulent air chamber in the laboratory, it was possible to quantify the machine-readable performance gain provided by DAO when compared with a standard imaging camera. Results from this research verify that DAO from homodyne encoding provides turbulence mitigation for single frames of data, paving the way for environmentally robust, high-speed, self-contained imaging systems.
Optical link prediction capabilities are important for planning and operating a free-space, optical communication link outdoors. The impact of the atmospheric channel on the optical link is dependent on factors like geographic location and local atmospheric conditions. Currently, link predication and analysis tools, like the Navy Atmospheric Vertical Surface Layer Model (NAVSLAM) and the Path Characterization System (PCS), are used to predict the atmospheric characteristics and received power. Additionally, the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) is a model that has recently been integrated into PCS and needs to have its prediction of refractive index structure parameter (Cn2) validated from outdoor range data. The bit error rate is a measure of error probability and when combined with baud rate, can be used to predicted the data transmission rate. In this experiment, the characterization of a one-way laser communication link was conducted outdoors on a 1 km range in a littoral environment. During communication, the bit error rate and received power were measured on the receiver side of the link. Additionally, the local atmospheric conditions were measured and utilized by link analysis tools, including the NAVSLaM, COAMPS, and PCS. These tools have been used to quantify the turbulence and path losses and evaluated against range measurements. The bit error rate and received power were also computed and compared with the measurements on the range to evaluate link predictive capabilities.
This publication will demonstrate recent advances of a self-referencing homodyne interferometry technique for mitigating atmospheric turbulence. The results will be quantified by using QR codes to document the machine-readable performance gain by using Digital Adaptive Optics when compared to a traditional imaging camera.
This publication will demonstrate recent advances of a self-referencing homodyne interferometry technique for mitigating atmospheric turbulence. The results will be quantified by using QR codes to document the machine-readable performance gain by using Digital Adaptive Optics when compared to a traditional imaging camera.
The paper will present an overview of atmospheric turbulence, turbulence limited imaging, and a Digital Adaptive Optics system that mitigates atmospheric turbulence in passive imaging systems.
This effort designed and tested new algorithms and deployable scintillometer hardware for ocean optical turbulence characterization. Novel features include a hand-deployable design, a non-laser optical source, a rapidly adjustable propagation length, and a collocated multi-instrument environmental sensor package. Undersea testing was contingent on several accomplishments, including developing robust algorithms and data logging methods, integrating compact optics and electronics, and engineering handheld-sized pressure vessels suitable for field experimentation. The test assembly was deployed in 428-m Pacific Ocean water from a small boat. Direct measurements revealed the ocean’s refractive-index structure parameter (Cn2 from 1.9×10−11 m−2/3 to 2.3×10−10 m−2/3) and the inner scale of optical turbulence (l0 from 0.5 mm to 1.5 mm). Onboard temperature, depth, beam attenuation, and backscattering sensors corroborated key regions of interest, namely the thermocline. By integrating turbulence, temperature, depth, attenuation, and backscattering measurements within a single hand-portable assembly, we increased our understanding of ocean optical dynamics while demonstrating the practicality of a low size, weight, and power scintillometer.
The inherent and apparent optical properties (IOPs and AOPs) of seawater limit the performance of free-space optical (FSO), underwater wireless optical communication (UWOC), and imaging systems. Absorption, scattering, and downwelling irradiance are three such properties that influence system performance and often evolve independently. In situ measurements of multiple IOPs and AOPs would provide environmental sensing for fielded optical systems, but such comprehensive measurements are typically expensive or impractical. This effort analyzed existing oceanographic data sets to uncover wavelength-dependent correlations between IOPs, AOPs, test depths, and ocean depths. We then employed machine learning (ML) methods to predict the optical properties of diffuse attenuation (Kd) and backscatter (bb) using beam attenuation (c) and compared these results to ground-truth values. Predicted values of Kd and bb were well matched to their ground-truth data. Furthermore, we demonstrate ML-based Jerlov optical water type classification using beam attenuation as the optical data input. With our methods validated, we collected new optical data sets and processed them using our ML models. Results are promising and indicate future in situ classification and prediction capability. To highlight one practical application, we present a preliminary ML-enabled performance estimator for a generic FSO or UWOC system.
In this work, we investigate the guided mode properties of planar anisotropic aluminum-doped zinc oxide waveguides (air/metamaterial/silicon oxide) at the epsilon-near-zero spectral point. Our calculations predict two fundamentally different propagation regimes for the lowest order guided TM and TE modes over a broad spectral range (400-2000 nm). Our study shows that excitation of the TM guided mode is possible for wavelengths higher than epsilon-near-zero spectral point. However the propagation distance for such a TM mode will strongly depend on the spectral position of the mode’s eigenfrequency. E.g., propagation distance is maximized for eigenfrequencies near epsilon-near-zero point and reduced by an order of magnitude as the eigenfrequency is tuned by ~50 nm.
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