Laser beams carrying orbital angular momentum (OAM) in underwater environments have been a topic of research for underwater communications and remote sensing applications. When a laser beam propagates through turbid water, the dominant form of attenuation and spatial dispersion is scattering due to small particles. The goal of this experimental study is to measure the transmitted OAM mode and its intermodal crosstalk via measurements of the OAM spectrum after propagation through turbid water. An initial beam is encoded with a single OAM state using a spiral phase hologram displayed on a high-resolution spatial light modulator. The optical receiver performs a phase cancellation measurement to decode the OAM on the incident beam. After recording images of the phase canceled beam, the OAM spectrum is found in post-processing. Three methods of post-processing are presented and compared to account for beam wander and an astigmatism in the experiment. After determining which method of post-processing gives the most accurate results, our results are compared to those in the literature. Our results show that an OAM beam maintains mode purity up to an optical depth (OD) of 12, whereas previous literature saw a loss of mode purity at an OD of 6. This is attributed to differences in receiver field of view, scattering volume, scattering length, and beam size.
In recent years the study of the orbital angular momentum (OAM) of light has gained traction for applications of remote sensing in underwater environments. When a laser beam propagates through turbid water, the dominant form of attenuation is scattering by large particles relative to visible wavelengths. The volume scattering function (VSF) describes the intensity distribution of light versus angle from an infinitesimal volume of scatterers. Recent computational studies have suggested that the distribution of scattered light due to a single scattering particle differs depending on whether the light is encoded with OAM or not. Other computational studies suggest that these differences are minimized when a volume of particles is illuminated. However, none of these computational projects provide experimental evidence to validate their predictions. This paper sets out to determine the experimental behavior of the VSF in the single scattering regime with and without OAM encoding on the transmitted beam. The experimental results are directly compared to Mie theory and a mixed numerical and analytical method.
We demonstrate a novel approach for transmissometry that uses an optical vortex for e↵ective discrimination between scattered and ballistic light. Current commercial transmissometers reject unwanted scattered light using a narrow field-of-view (FOV), but this technique fails in multiple scatter environments, or in Mie scattering regimes where large particles create a high probability of scatter near the beam axis. In the optical vortex approach to transmissometry, received light passes through a di↵ractive spiral phase plate. Coherent nonscattered light that passes through the phase plate will create an intensity vortex, while incoherent scattered light will not. The resulting spatial dependence can be exploited to discriminate between the scattered and ballistic light. We present experimental results demonstrating the e↵ectiveness of this approach.
We present an investigation of the optical property of orbital angular momentum (OAM) for use in the detection of objects obscured by a turbid underwater channel. In our experiment, a target is illuminated by a Gaussian beam. An optical vortex is formed by passing the object-reflected and backscattered light through a diffractive spiral phase plate at the receiver, which allows for the spatial separation of coherent and non-coherent light. This provides a method for discriminating target from environment. Initial laboratory results show that the ballistic target return can be detected 2-3 orders of magnitude below the backscatter clutter level. Furthermore, the detection of this coherent component is accomplished with the use of a complicated optical heterodyning scheme. The results suggest new optical sensing techniques for underwater imaging or LIDAR.
We present a novel chaotic lidar system designed for underwater impulse response measurements. The system uses two recently introduced, low-cost, commercially available 462 nm multimode InGaN laser diodes, which are synchronized by a bi-directional optical link. This synchronization results in a noise-like chaotic intensity modulation with over 1 GHz bandwidth and strong modulation depth. An advantage of this approach is its simple transmitter architecture, which uses no electrical signal generator, electro-optic modulator, or optical frequency doubler.
Remotely operated vehicles (ROVs) typically use traditional optical imaging systems, such as cameras, for high resolution imaging. Cameras are effective in clear water, but have extremely poor performance in degraded visual environments (DVEs) such as turbid coastal waters and harbors. This is due to the multiple scattering of the light from the particulates and organic matter in the water. Laser-based sensors have been developed to enhance optical imaging in DVEs1,3,4,5,6. However, since conventional approaches require that the illuminator and receiver be located on the same platform, the size, weight, and power (SWaP) requirements are incompatible with small ROVs. Researchers at NAVAIR have developed a low cost optical imager utilizing a bistatic geometry where the illuminator and receiver are mounted on separate, smaller platforms. The illuminator steers a modulated laser beam with a microelectromechanical system (MEMS) scanner to sequentially illuminate an underwater object. A distant receiver collects the object reflected laser light and reconstructs the imagery. Communications information, including a synchronization sequence, is encoded onto the modulation which is used by the receiver to build the image. The SWaP of the illuminator’s components have been optimized and integrated into a modified version of the OpenROV, a miniature, commercial off-the-shelf ROV. This paper reports on the efforts to reduce the SWaP of the modulated illuminator and the results of testing this system in a laboratory water tank environment.
This work examines the propagation properties of two superimposed coherent orbital angular momentum (OAM) modes for use in underwater systems as an alternative to amplitude modulation. An OAM mode of l=+2 is interfered with OAM mode l=-1 from a λ = 540 nm laser source. These OAM modes are superimposed using a Mach-Zehnder (MZ) interferometer combined with diffractive optical elements. By manipulating the optical path length of one of the MZ legs, the interference of these beams can be temporally controlled. The spatial profile is maintained in a turbid environment up through 4.9 attenuation lengths for both cases.
Underwater optical communication has recently become the topic of much investigation as the demands for underwater data transmission have rapidly grown in recent years. The need for reliable, high-speed, secure underwater communication has turned increasingly to blue-light optical solutions. The blue-green visible wavelength window provides an attractive solution to the problem of underwater data transmission thanks to its low attenuation, where traditional RF solutions used in free-space communications collapse. Beginning with GaN laser diodes as the optical source, this work explores the encoding and transmission of digital data across underwater environments of varying turbidities. Given the challenges present in an underwater environment, such as the mechanical and optical turbulences that make proper alignment difficult to maintain, it is desirable to achieve extremely high data rates in order to allow the time window of alignment between the transmitter and receiver to be as small as possible. In this paper, work is done to increase underwater data rates through the use of orbital angular momentum. Results are shown for a range of data rates across a variety of channel types ranging in turbidity from that of a clear ocean to a dirty harbor.
Recent interest in high speed laser communications underwater has restimulated theoretical studies in laser propagation in turbid media. In particular, the characterization of temporal dispersion is of paramount importance in order to predict the bandwidth and capacity of underwater optical channels. While the temporal aspects of underwater laser propagation have received attention from the modeling community in the past, few if any of these models have been validated with experimental data. However recent advances in hardware technology now enable experimental characterization at high speeds (~GHz). Such measurements have been made by the authors.1 In this work, we develop a Monte Carlo model, and present initial results validated against the aforementioned experimental data.
A modulated pulse laser imaging system has been developed which utilizes coded/chirped RF modulation to mitigate the adverse effects of optical scattering in degraded visual underwater environments. Current laser imaging techniques employ either short pulses or single frequency modulated pulses to obtain both intensity and range images. Systems using short pulses have high range resolution but are susceptible to scattering due to the wide bandwidth nature of the pulse. Range gating can be used to limit the effects of backscatter, but this can lead to blind spots in the range image. Modulated pulse systems can help suppress the contribution from scattered light in generated imagery without gating the receiver. However, the use of narrowband, single tone modulation results in limited range resolution where small targets are camouflaged within the background. This drives the need for systems which have high range resolution while still suppressing the effects of scattering caused by the environment. Coded/chirped modulated pulses enable the use of radar pulse compression techniques to substantially increase range resolution while also providing a way to discriminate the object of interest from the light scattered from the environment. Linearly frequency chirped waveforms and phase shift keyed barker codes were experimentally investigated to determine the effects that pulse compression would have on intensity/range data. The effect of modulation frequency on the data produced with both wideband and narrowband modulation was also investigated. The results from laboratory experiments will be presented and compared to model predictions.
Techniques have been developed to mitigate many of the issues associated with underwater imaging in turbid environments. However, as targets get smaller and better camouflaged, new techniques are needed to enhance system sensitivity. Researchers at NAVAIR have been developing several techniques that use RF modulation to suppress background clutter and enhance target detection. One approach in particular uses modulation to encode a pulse in a synchronous line scan configuration. Previous results have shown this technique to be effective at both forward and backscatter suppression. Nearly a perfect analog to modulated pulse radar, this technique can leverage additional signal processing and pulse encoding schemes to further suppress background clutter, pull signals out of noise, and improve image resolution. Additionally, using a software controlled transmitter, we can exploit this flexibility without the need to change out expensive hardware. Various types of encoding schemes were tested and compared. We report on their comparative effectiveness relative to a more conventional non-coded pulse scheme to suppress background clutter and improved target detection.
Conventional underwater laser imaging systems are configured such that the laser illuminator and the optical receiver co-exist on the same platform. The main challenge for optical imaging in turbid water is the collection of light that is scattered multiple times on its path to and from the scene of interest. Sophisticated techniques using pulsed or modulated lasers and gated, high speed photo receivers have been developed to reject scattered light and enhance image quality. Although advancements in laser and receiver hardware have made it possible to reduce the size, weight and power of these laser imaging systems, it is still a significant challenge to make these systems compatible with small, autonomous platforms that are desirable for undersea surveillance. Researchers at NAVAIR have developed a multistatic laser imaging approach where the transmitter(s) and receiver(s) are located on separate, smaller platforms. Each laser illuminator is temporally encoded with information that is used by the receiver(s) to construct an image. Recently, this multistatic configuration was modified so that both 2D (amplitude) and 3D (amplitude and range) underwater imagery was collected. This new system design enables the system to identify low contrast objects that contain distinct range relief features. Results from recent laboratory water tank experiments will be presented to evaluate the performance of this new 3D, multistatic laser imaging system in different water environments.
The detection and identification of underwater threats in coastal areas are of interest to the Navy. When identifying a potential target, both two-dimensional (amplitude versus position) and three-dimensional (amplitude and range versus position) information are important. Laser imaging in turbid coastal waters makes this task challenging due to absorption and scattering in both the forward and backward directions. Conventional imaging approaches to suppress scatter rely on a pulsed laser and a range-gated receiver or an intensity-modulated continuous wave laser and a coherent RF receiver. The modulated pulsed laser imaging system is a hybrid of these two approaches and uses RF intensity modulation on a short optical pulse. The result is an imaging system capable of simultaneously acquiring high-contrast images along with high-precision unambiguous ranges. A working modulated pulsed laser line scanner was constructed and tested with a custom-built transmitter, a large-bandwidth optical receiver, and a high-speed digitizing oscilloscope. The effectiveness of the modulation to suppress both backscatter and forward scatter, as applied to both magnitude and range images, is discussed.
Laser imaging through a turbid medium is complicated by scattering. Backscattered photons reduce image contrast as weak target returns compete against a large background of backscattered light. Forward scattering broadens the interrogating laser beam, thereby reducing the spatial resolution of the target. Prior research has shown that intensity modulation (<100 MHz ) can be used to “wash-out” the backscatter, resulting in better discrimination of the target and higher contrast. We show that the higher modulation frequencies (>100 MHz ) can be also used to suppress forward scattered light, thereby increasing spatial resolution.
The detection and identification of underwater threats in coastal areas is of interest to the Navy. Conventional optical imaging systems are limited to scenarios where the number of attenuation lengths between the system and the object are less than 4. With a desire to operate at extended ranges and threats becoming smaller and better camouflaged, new approaches are needed. In response to these challenges, new transmitters and receivers are being developed to support the next-generation of underwater optical imaging systems. One of these systems is based on the modulated pulse concept where a pulsed laser source is encoded with a radar signal, and a range-gated, high-speed optical receiver recovers the radar modulation envelope. Subsequent processing of the radar signal provides a way to discriminate against multiply scattered light and to enhance image contrast and resolution. The challenge is developing transmitter and receiver hardware that meets the requirements of the modulated pulse technique. We report recent progress that has been made in developing modulated pulse transmitter and receiver hardware. A working prototype was demonstrated and tested in a controlled laboratory environment. The results of these initial experiments are presented.
A new, modulated-pulse, technique is currently being investigated for underwater laser detection, ranging, imag-
ing, and communications. This technique represents a unique marriage of pulsed and intensity modulated sources.
For detection, ranging, and imaging, the source can be congured to transmit a variety of intensity modulated
waveforms, from single-tone to pseudorandom code. The utility of such waveforms in turbid underwater envi-
ronments in the presence of backscatter is investigated in this work.
The modulated pulse laser may also nd utility in underwater laser communication links. In addition to
exibility in modulation format additional variable parameters, such as macro-pulse width and macro-pulse
repetition rate, provide a link designer with additional methods of optimizing links based on the bandwidth,
power, range, etc. needed for the application. Initial laboratory experiments in simulated ocean waters are
The focus of this paper is to describe research being conducted at NAVAIR in Patuxent River, MD to improve optical
detection, ranging and imaging in the underwater environment through the use of optical modulation techniques. The
modulation provides a way to discriminate against unwanted scattered light that would otherwise reduce detection
sensitivity. Another benefit of modulating the transmitted light is that coherent detection of the modulation envelope
results in the ability to accurately measure the range to the underwater object. Ways to use the hardware and methods
developed for the detection, ranging, and imaging scenario to satisfy other mission requirements are also being
investigated. The requirements for the modulation scheme, modulation frequency, and laser characteristics (pulsed,
continuous, optical power level) depend on the targeted application. The implementation of this optical modulation
technique in a variety of underwater sensors has become possible due to recent advances in laser and receiver
technology. A review of the work being done in this area of research will be presented, and results from laboratory
experiments will be discussed.
Optical communication links using retro-reflectors underwater are investigated. In the retro-reflector geometry,
backscattered light from turbid waters can interfere with the retro-reflected information signal. Presented here are
polarization techniques to reduce the contribution of backscatter, as well as an evaluation on the impact of
communication link range and reliability.
Optical imaging and optical communications systems are limited in the underwater environment due to optical
scattering. For an imaging system, light that scatters back to the receiver before reaching an underwater object degrades
image contrast. Light that scatters multiple times on its way to and from the object of interest tends to blur the image and
further reduce its contrast. This forward-scattered light may also ultimately limit the bandwidth of a point-to-point
optical communications link. While continuous wave sources with low frequency modulation (<100MHz) and pulsed
sources with several nanosecond pulse durations have been used to mitigate the backscatter problem, little has been done
to study the effect of higher modulation frequencies (>100MHz) or short (<2nsec) pulse durations on forward-scattered
light. The challenge has been the lack of hardware and theoretical models required to examine events at these short time
scales. Fortunately, short pulse and modulated pulse sources have now been developed, along with optical receivers with
sufficient bandwidth and sensitivity to measure the response of the water to these sources. The purpose of this work is to
use these tools to study the propagation of modulated light fields at frequencies up to 1GHz. Results from laboratory
tank experiments and their impact on future underwater optical imaging and communications systems will be discussed.
Optical imaging in turbid ocean water is a challenge due to the high probability that light will scatter multiple times as it propagates to and from the object of interest. Techniques have been developed to suppress the contribution from scattered light and increase the image contrast, such as those using a pulsed source with a gated receiver or a modulated source with a coherent RF receiver. While improving the amplitude contrast of underwater images, these two approaches also have the capability of providing target range information. The effectiveness of each approach for both 2D and 3D imagery depends highly on the turbidity of the intervening water medium. This paper describes a system based on the optical modulation approach, the Frequency Agile Modulated Imaging System (FAMIS), and the techniques that have been developed to improve both amplitude and range imaging in turbid water.