Optical methods to communicate or sense in the ocean environment can be effected inhomogeneities in the index of refraction called optical turbulence. Beam wander introduced by optical turbulence is of particular interest for optical means relying on the propagation of a well-defined laser beam such as free space communication and laser line scan. Here we present a comprehensive study of beam propagation simulations, lab experiments, and field measurements of laser beams propagating through varying degrees of optical turbulence. For the computational part of the investigation a true end to end simulation was performed. Starting with a CFD simulation of Rayleigh–Bénard convection the temperature fields where converted to index of refraction phase screens which then where used to simulate the propagation of a focused Gaussian laser beam via the split-step Fourier method. Lab experiments where conducted using the same parameters as in the simulation using a good quality TEM00 beam and a CCD camera to record data. For the field experiments a Telescoping Ridged Underwater Sensor Structure (TRUSS) was equipped with a transmitter and a receiver capable of analyzing a multitude of laser beams simultaneously. The TRUSS was deployed in the Bahamas to record beam wander under weak optical turbulence conditions above and stronger optical turbulence conditions inside the thermocline. The data from the experimental and lab experiments are compared and the strength of the optical turbulence in terms of the structure parameter Cn2 are extracted. We also extract Cn2 from the TRUSS experiments and in doing so provide, for the first time, a quantitative estimate for the strength of optical turbulence in the ocean.
Particle image velocimetry (PIV) is a well-established tool to collect high-resolution velocity and turbulence data in the laboratory. PIV measurements are based on using a laser sheet to illuminate a flow seeded with small particles and taking quick successive images or image pairs of the illuminated particle field with a CCD or CMOS camera. The movement of the particles between images can be used to infer flow field velocities over an image area. During experiments at the Simulated Turbulence and Turbidity Environment (SiTTE) laboratory tank, we observed a marked influence of optical turbulence, i.e. strong temperature gradients leading to changes in the index of refraction, on particle imaging in PIV. The particles look blurred and have a “shooting star” appearance. PIV is routinely used in flows with very high temperature gradients, such as nuclear reactor cooling rods, but the optical path length is typically very short (on the order of cm), and no such effect is generally considered for measurements in liquids. We investigated the effect of optical turbulence on PIV imaging for various optical path lengths (0.5m to 2m) and turbulence strengths. Velocities from the PIV measurements were calculated using the algorithms provided within Dantec’s Dynamic Studio and compared to velocities from concurrent velocity point measurements with a Laser Doppler Velocimetry system. The results indicate that optical turbulence can affect PIV measurements in liquids, and that depending on the strength of the optical turbulence and path length, care needs to be taken to mediate this effect using appropriate post-processing techniques when inferring velocities from PIV data.
Laser beam propagation underwater is becoming an important research topic because of high demand for its potential applications. Namely, ability to image underwater at long distances is highly desired for scientific and military purposes, including submarine awareness, diver visibility, and mine detection. Optical communication in the ocean can provide covert data transmission with much higher rates than that available with acoustic techniques, and it is now desired for certain military and scientific applications that involve sending large quantities of data. Unfortunately underwater environment presents serious challenges for propagation of laser beams. Even in clean ocean water, the extinction due to absorption and scattering theoretically limit the useful range to few attenuation lengths. However, extending the laser light propagation range to the theoretical limit leads to significant beam distortions due to optical underwater turbulence. Experiments show that the magnitude of the distortions that are caused by water temperature and salinity fluctuations can significantly exceed the magnitude of the beam distortions due to atmospheric turbulence even for relatively short propagation distances. We are presenting direct measurements of optical underwater turbulence in controlled conditions of laboratory water tank using two separate techniques involving wavefront sensor and LED array. These independent approaches will enable development of underwater turbulence power spectrum model based directly on the spatial domain measurements and will lead to accurate predictions of underwater beam propagation.
Originally proposed in SPIE DSS’13, the compressive line sensing (CLS) imaging system adopts the paradigm of
independently sensing each line and jointly reconstructing a group of lines. Such system achieves “resource
compression” and is still compatible with the conventional push-broom operation mode. This paper attempts to extend
the CLS concept, originally developed to effectively acquire scene intensity images in a scattering medium, to 3D scene
reconstruction through the adoption of a temporal-spatial measurement matrix. The sensing model is discussed.
Simulation results are presented as part of this work.
The compressive line sensing (CLS) imaging system adopts the paradigm of independently sensing each line and jointly reconstructing a group of lines. This system achieves “resource compression” and is compatible with the conventional push-broom line-by-line sensing mode. This paper discusses the development of a prototype system to enable the experimental study the CLS imaging system. The results from an initial turbidity cycle experiments are presented.
Compressive sensing (CS) theory has drawn great interest and led to new imaging techniques in many different fields. Over the last few years, the authors have conducted extensive research on CS-based active electro-optical imaging in a scattering medium, such as the underwater environment. This paper proposes a compressive line sensing underwater imaging system that is more compatible with conventional underwater survey operations. This new imaging system builds on our frame-based CS underwater laser imager concept, which is more advantageous for hover capable platforms. We contrast features of CS underwater imaging with those of traditional underwater electro-optical imaging and highlight some advantages of the CS approach. Simulation and initial underwater validation test results are also presented.
This paper will discuss and compare some recent oceanic test results from the Bahamas Optical Turbulence Exercise (BOTEX) cruise, where vertical profiling was conducted with both time-resolved laser backscatter measurements being acquired via a subsurface light detection and ranging (lidar) profiling instrument, and laser beam forward deflection measurements were acquired from a matrix of continuous wave (cw) laser beams (i.e. structured lighting) being imaged in the forward direction with a high speed camera over a one-way path, with both transmitter and camera firmly fixed on a rigid frame. From the latter, it was observed that when within a natural turbulent layer, the laser beams were being deflected from their still water location at the image plane, which was 8.8 meters distance from the laser dot matrix transmitter. As well as suggesting that the turbulent structures being encountered were predominately larger than the beam diameter, the magnitude of the deflection has been confirmed to correlate with the temperature dissipation rate. The profiling lidar measurements which were conducted in similar conditions, also used a narrow collimated laser beam in order to resolve small-scale spatial structure, but with the added attribute that sub-nanosecond short pulse temporal profile could potentially resolve small-scale vertical structure. In the clear waters of the Tongue of the Ocean in the Bahamas, it was hypothesized that the backscatter anomalies due to the effect of refractive index discontinuities (i.e. mixed layer turbulence) would be observable. The processed lidar data presented herein indicates that higher backscatter levels were observed in the regions of the water column which corresponded to higher turbulent mixing which occurs at the first and second themoclines. At the same test stations that the laser beam matrix and lidar measurements were conducted, turbulence measurements were made with two non-optical instruments, the Vertical Microstructure Profiler (VMP) and a 3D acoustical Doppler velocimeter with fast conductivity and temperature probes. The turbulence kinetic energy dissipation rate and the temperature dissipation rates were calculated from both these setups in order to characterize the physical environments and corroborate with the laser measurements. To further investigate the utility of elastic lidar in detecting small-scale turbulent structures, controlled laboratory experiments were also conducted, with the objective of concurrently acquiring both the laser beam spatial characteristics in the forward direction and the laser backscatter temporal profile from each transmitted sub-nanosecond pulse. An artificial refractive index discontinuity was generated in clear test tank conditions by placing a clean ice-filled carboy above the laser beam propagation path. The results from both field and laboratory experiments confirm our hypothesis that turbulent layers are detectable by lidar sensors, and motivates that more research and lidar instrumentation development is needed to better quantify turbulence, especially for mitigating associated performance degrading effects for the U.S. Navy’s next generation electro-optic (EO) systems, including active laser imaging and laser communications.
Compressive sensing (CS) theory has drawn great interest in recent years and has led to new image-acquisition techniques in many different fields. This research investigates a CS-based active underwater laser serial imaging system, which employs a spatial light modulator (SLM) at the source. A multiscale polarity-flipping measurement matrix and a model-assisted image reconstruction concept are proposed to address limitations imposed by a scattering medium. These concepts are also applicable to CS-based imaging in atmospheric environments characterized by fog, rain, or clouds. Simulation results comparing the performance of the proposed technique with that of traditional laser line scan (LLS) sensors and other structured illumination-based imager are analyzed. Experimental results from over-the-air and underwater tests are also presented. The potential for extending the proposed frame-based imaging technique to the traditional line-by-line scanning mode is discussed.
The Bahamas Optical Turbulence Exercise (BOTEX) was conducted in the coastal waters of Florida and the Bahamas
from June 30 to July 12 2011, onboard the R/V FG Walton Smith. The primary objective of the BOTEX was to obtain
field measurements of optical turbulence structures, in order to investigate the impacts of the naturally occurring
turbulence on underwater imaging and optical beam propagation. In order to successfully image through optical
turbulence structures in the water and examine their impacts on optical transmission, a high speed camera and targets
(both active and passive) were mounted on a rigid frame to form the Image Measurement Assembly for Subsurface
Turbulence (IMAST). To investigate the impacts on active imaging systems such as the laser line scan (LLS), the
Telescoping Rigid Underwater Sensor Structure (TRUSS) was designed and implemented by Harbor Branch
Oceanographic Institute. The experiments were designed to determine the resolution limits of LLS systems as a function
of turbulence induced beam wander at the target. The impact of natural turbulence structures on lidar backscatter
waveforms was also examined, by means of a telescopic receiver and a short pulse transmitter, co-located, on a vertical
profiling frame. To include a wide range of water types in terms of optical and physical conditions, data was collected
from four different locations. . Impacts from optical turbulence were observed under both strong and weak physical
structures. Turbulence measurements were made by two instruments, the Vertical Microstructure Profiler (VMP) and a
3D acoustical Doppler velocimeter with fast conductivity and temperature probes, in close proximity in the field.
Subsequently these were mounted on the IMAST during moored deployments. The turbulence kinetic energy dissipation
rate and the temperature dissipation rates were calculated from both setups in order to characterize the physical
environments and their impacts. Beam deflection by multiple point patterns are examined, using high speed camera
recordings (300 to 1200 fps), in association with measured turbulence structures. Initial results confirmed our hypothesis
that turbulence impacted optical transmissions. They also showed that more research will be needed to better quantify
and mitigate such effects, especially for the U.S. Navy's next generation EO systems, including active imaging, lidar and
We describe a method that we believe will for the first time allow diffraction-limited imaging through ground-level
turbulence with large apertures and at large distances (e.g., 1mm resolution at 1km and at a wavelength of 1μm). The
key lies in collecting image data in the spatial frequency domain via the method of Fourier telescopy and in taking
suitable time averages of the Fourier telescopy signal magnitude and phase. The method requires active illumination of
the target with laser light, and the time averages required will likely be of many seconds duration, if not minutes. The
scheme will thus not be suitable for time-varying scenes.
The compressive sensing (CS) theory has drawn great interest in signal processing community in recent years and led
to new image acquisition techniques in many different fields. This research attempts to develop a CS based underwater
laser serial imaging system. A Digital Mirror Device (DMD) based system configuration is proposed. The constraints
due to scattering medium are studied. A multi-scale measurement matrix design, the "model-assisted" image
reconstruction concept and a volume backscattering reduction technique are proposed to mitigate such constraints. These
concepts are also applicable to CS based imager in other scattering environment such as fog, rain or clouds. Simulation
results using a modified imaging model developed by HBOI and Metron and experimental results using a simple optical
bench setup are presented. Finally the proposed technique is compared with traditional laser line scan (LLS) design and
other structured illumination based imager.
This paper examines imaging performance bounds for undersea electro-optic identification (EOID) sensors that use
pulsed laser line scanners to form serial images, typically utilizing one laser pulse for each formed image element. The
experimental results presented include the use of two distinct imaging geometries; firstly where the laser source and
single element optical detector are nearly co-aligned (near monostatic) and secondly where the laser source is deployed
on a separate platform positioned closer to the target (bistatic) to minimize source-to-target beam spread, volumetric
scatter and attenuation, with the detector being positioned much further from the target. The former system uses
synchronous scanning in order to significantly limit the required instantaneous angular acceptance function of the
detector and has the desired intention of acquiring only ballistic photons that have directly interacted with the target
element and the undesirable property of acquiring snake photon contributions that indirectly arrive into the detector
aperture via multiple forward scattering over the two-way propagation path. The latter system utilizes a staring detector
with a much wider angular acceptance function, the objective being to deliver maximum photon density to each target
element and acquire diffuse, snake and ballistic photon contributions in order to maximize the signal.
The objective of this work was to experimentally investigate pulse-to-pulse detection statistics for both imaging
geometries in carefully controlled particle suspensions, with and without artificially generated random uncharacterized
scattering inhomogeneities to assess potential image performance in realistic conditions where large biological and
mineral particles, aggregates, thin biological scattering layers and turbulence will exist. More specifically, the study
investigates received pulse energy variance in clear filtered water, as well as various well-characterized particle
suspensions with and without an artificial thin random scattering layer. Efforts were made to keep device noise constant
in order to assess the impact of the environment on extrapolated image quality.
Understanding the nonlinear optical processes in semiconductor nanostructures leads to possible applications in areas
including laser amplifiers, optical switches, and solar cells. Here we present a study of the frequency degenerate two-photon
absorption (2PA) spectrum of a series of PbS and PbSe quantum dots (QDs). The influence of the quantum
confinement is analyzed using a four-band model which considers the mixing of valence and conduction bands. In
contrast to our observations of CdSe QDs, the present results point to an increase of the 2PA cross-section (normalized
by the QD volume) as the quantum dot size is made smaller. This is explained by the symmetry between the valence and
conduction bands which allows the density of states to remain high even for small QDs. A study of the ultrafast carrier
dynamics of the PbS quantum dots is also presented. Through nondegenerate femtosecond pump-probe experiments we
show evidence of multi-exciton generation with quantum yield (number of excitons generated per absorbed photon) up
to 170% for excitation with hω> 3 Eg (where Eg is the bandgap energy).
We present degenerate and nondegenerate two-photon absorption spectra in a series of CdSe and CdTe quantum dots. The measurements show that the two-photon absorption (2PA) spectrum is strongly dependent on the quantum dot size and that the 2PA coefficient decreases as the quantum dot size decreases, and it is larger for the frequency nondegenerate process. Previously we had shown a theoretical analysis of these results based on a simple model using the effective mass approximation. Although this model works well for larger quantum dots, it fails for the smaller ones. Here we use the more (formula available in manuscript) model for the band structure and consider the hole band mixing in quantum dots to describe our data. This theory better describes the spectral structures for smaller quantum dots and also predicts the decrease of the 2PA coefficient with the decrease of quantum dot size. This is due to the reduction of the number of possible transitions and the blue shift of the optical bandgap from quantum confinement. This theory predicts the reduction of the 2PA coefficient with size, although our experimental results show an even stronger reduction.
In an effort to develop an improved medium for optical communication, chalcogenide glasses are being investigated for waveguide and integrated optical components. These glasses are attractive for integrated optics applications due to their good infrared transmission and high nonlinear Kerr effects. The fact that these glasses can be fabricated in thin films and optical fiber forms constitute a major advantage for future high-speed optical devices applications. However, to advance these novel characteristics, it is crucial to identify the structure/property relationship in the glass, in both bulk and film materials. Rutherford Backscattering Spectroscopy (RBS) is an analytical tool that gives very useful information regarding compositional and structural analysis of the films, as well as a precise measurement of the film's layer thickness. Results obtained showed no apparent variation in composition and small (less than 10%) density variation in single layer As2S3 films. Multilayer films, which thickness were measured using SEM images, displayed compositional and density modifications associated with the annealing process. The same calculations were conducted after almost a year from the previous measurements to study changes induced due to film aging. Stoichiometric and thickness modifications, caused by aging, were observed in unannealed structures. No apparent changes were detected in annealed films. Waveguide Raman Spectroscopy was used as a complementary tool to identify the molecular features responsible for the changes.