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.
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 (Cn2), 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.
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.
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.
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 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.
This effort develops and tests algorithms and a user-portable optical system designed to autonomously optimize the laser communication wavelength in open and coastal oceans. In situ optical meteorology and oceanography (METOC) data gathered and analyzed as part of the auto-selection process can be stored and forwarded. The system performs closedloop optimization of three visible-band lasers within one minute by probing the water column via passive retroreflector and polarization optics, selecting the ideal wavelength, and enabling high-speed communication. Backscattered and stray light is selectively blocked by employing polarizers and wave plates, thus increasing the signal-to-noise ratio. As an advancement in instrumentation, we present autonomy software and portable hardware, and demonstrate this new system in two environments: ocean bay seawater and outdoor test pool freshwater. The next generation design is also presented. Once fully miniaturized, the optical payload and software will be ready for deployment on manned and unmanned platforms such as buoys and vehicles. Gathering timely and accurate ocean sensing data in situ will dramatically increase the knowledge base and capabilities for environmental sensing, defense, and industrial applications. Furthermore, communicating on the optimal channel increases transfer rates, propagation range, and mission length, all while reducing power consumption in undersea platforms.
The wireless, high-data-rate transmission of information is becoming increasingly important for undersea applications that include defense, environmental monitoring, and petroleum engineering. Free-space optical (FSO) communication addresses this need by providing an undersea high-data-rate link over moderate distances (up to 100s of meters). Light transmission through seawater is maximal in the blue-green part of the optical spectrum (475 nm–575 nm), but turbidity conditions, which are dynamic, strongly influence the actual maximum. We describe the development of a laser-wavelength auto-selection algorithm and system for optimized underwater FSO communications. The use of a passive corner cube retroreflector allows all transmitter and receiver electronics to be collocated, which will be beneficial for any fielded system. First, we describe the laser test bed and retroreflector system. Next, we describe the development of the algorithm and hardware. We then describe the creation of various water types (from clear to turbid) in the laboratory using particle suspensions and dyes, which will enable wavelength-dependent transmission tests. Finally, we show experimental results from water tube tests, demonstrating wavelength auto-selection within one minute.
Wirelessly transmitting large volumes of information at high data rates underwater is becoming increasingly important for such applications as environmental monitoring and petroleum exploration and maintenance. Underwater free-space optical (FSO) communication addresses the aforementioned need by providing wireless high-data-rate links. Visible light transmission through seawater typically peaks in the blue-green spectrum (475 nm–575 nm), but local clarity conditions, which are dynamic, strongly influence the actual maximum. We describe the development of a new laser-wavelength auto-selection algorithm and system for optimized underwater FSO communication. This system has the potential to improve underwater optical link reliability for high-data-rate communications. First, we describe the laser system and water tube setup for performing optical experiments. Next, we present research on recreating various seawater types (from clear to turbid) in the laboratory using particle suspensions and dye, which will enable wavelength-dependent transmission tests. Finally, we show experimental results from optical water tube tests, and describe the development of the autoselection algorithm.
We observe critical coupling to surface phonon-polaritons in silicon carbide by attenuated total reflection of
mid-infrared radiation. Reflectance measurements demonstrate critical coupling by a double-scan of wavelength
and incidence angle. Critical coupling occurs when prism coupling loss is equal to losses in silicon carbide and
the substrate, resulting in maximal electric field enhancement.
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