Alan Laux

Characterization of the Beam-Spread Function for Underwater Wireless Optical Communications Links

Optical links are currently being considered for high-bandwidth underwater communications at short ranges (<100 m). To predict the performance of these links, a firm understanding of how the inherent optical properties of water affect the encoded optical signal is needed. Of particular interest is the impact of scattering due to particulates. Typically, the link loss is computed using the beam attenuation coefficient, which describes the attenuation of nonscattered light due to absorption and scattering. This approach is insufficient, as it neglects the contribution of scattered light to the total received signal. Given the dynamic nature of underwater platforms, as well as the dynamic nature of the environment itself, knowledge of the angular dependence of forward-scattered light is imperative for determining pointing and tracking requirements as well as overall signal to noise. In this work, the theory necessary to describe spatial spreading of an optical beam in the presence of scattering agents underwater is reviewed. This theory is then applied to a performance prediction model that is validated via laboratory experiments. Finally, the model is used to study the impact of spatial spreading on an underwater optical link.

Amplitude-modulated laser imager

Laser systems have been developed to image underwater objects. However, the performance of these systems can be severely degraded in turbid water. We have developed a technique using modulated light to improve underwater detection and imaging. A program, Modulated Vision System (MVS), which is based on a new theoretical approach, has been developed to predict modulated laser imaging performance. Experiments have been conducted in a controlled laboratory environment to test the accuracy of the theory as a function of system and environmental parameters. Results show a strong correlation between experiment and theory and indicate that the MVS program can be used to predict future system performance.

Propagation of modulated light in water: implications for imaging and communications systems

Until recently, little has been done to study the effect of higher modulation frequencies (>100 MHz) or short (<2 ns) pulse durations on forward-scattered light in ocean water. This forward-scattered light limits image resolution and may ultimately limit the bandwidth of a point-to-point optical communications link. The purpose of this work is to study the propagation of modulated light fields at frequencies up to 1 GHz. Results from laboratory tank experiments and their impact on future underwater optical imaging and communications systems are discussed.

Effects of Multiple Scattering on the Implementation of an Underwater Wireless Optical Communications Link

Recent interest in ocean exploration has brought about a desire for developing wireless communication techniques in this challenging environment. Due to its high attenuation in water, a radio frequency (RF) carrier is not the optimum choice. Acoustic techniques have made tremendous progress in establishing wireless underwater links, but they are ultimately limited in bandwidth. A third option is optical radiation, which is discussed in this paper. One drawback of underwater wireless optical communications is that the transmission of the optical carrier is highly dependent on water type. This study examines some of the challenges in implementing an optical link in turbid water environments and attempts to answer how water clarity affects the overall link

Phase Coherent Digital Communications for Wireless Optical Links in Turbid Underwater Environments

Previous studies by the authors have included a theoretical and experimental investigation of the spatial distribution of an optical signal used for communications in underwater scattering environments. Presented here is an experimental study of how scattering affects the temporally encoded information bearing component of the optical signal. Short range underwater optical links employing BPSK, QPSK, 8- PSK, 16-QAM, and 32-QAM modulation are implemented in a laboratory setting, yielding data rates up to 5 Mb/s. The effect of link quality is examined versus water turbidity.

Spatial and temporal dispersion in high bandwidth underwater laser communication links

A resurgence is occurring in the area of underwater laser communications. While acoustic systems are currently the more mature technology, they are ultimately band-limited to sub-MHz type data rates due to the frequency dependent absorption of acoustic energies in water. Advances in fiber optic and free space links have shown promise for optical links to provide data rates in excess of a gigabit per second. It is not surprising then, that laser links are being considered for Naval applications involving high bandwidth communications undersea. A major challenge in implementing optical links underwater arises from the spatial dispersion of photons due to scattering. Spatial spreading of the optical beam reduces the photon density at the receiver position. As such, optical links are only expected to be of greatest utility in links <100 m. Nonetheless, it appears that end users may accept limited link range in exchange for the gain in information bandwidth that optical links may provide. Additionally, researchers continue to study how spatial spreading affects the time encoded portion of the transmitted optical signal. Temporal dispersion arising from multiple scattering events may result in inter-symbol interference (ISI), further limiting link range and/or capacity. Researchers at NAVAIR in Patuxent River MD are currently investigating both the spatial and temporal effects of scattering on a laser link in turbid underwater environments. These links utilize an intensity modulated beam to implement coherent digital modulation schemes such as PSK and QAM. Through both modeling and experiment, the underwater channel is characterized both spatially and temporally. Results are providing insight to system requirements of link range, pointing accuracy, photo-receiver requirements, modulation frequency, and optimal modulation format.

Demodulation techniques for the amplitude modulated laser imager

A new technique has been found that uses in-phase and quadrature phase (I/Q) demodulation to optimize the images produced with an amplitude-modulated laser imaging system. An I/Q demodulator was used to collect the I/Q components of the received modulation envelope. It was discovered that by adjusting the local oscillator phase and the modulation frequency, the backscatter and target signals can be analyzed separately via the I/Q components. This new approach enhances image contrast beyond what was achieved with a previous design that processed only the composite magnitude information.

Modulated laser line scanner for enhanced underwater imaging

Current Laser Line Scanner (LLS) sensor performance is limited in turbid water and in bright solar background conditions. In turbid water, backscattered and small angle forward scattered light reaching the receiver decreases underwater target contrast and resolution. Scattered solar energy reaching the detector also decreases detection sensitivity by increasing receiver noise. Thus, a technique which rejects unwanted, scattered light while retaining image-bearing photons is needed to improve underwater object detection and identification. The approach which we are investigating is the application of radar modulation and detection techniques to the LLS. This configuration will enable us to use optical modulation to discriminate against scattered light. A nonscanning mock-up of an existing LLS, the Electro-Optic Identification sensor, has been developed with off-the-shelf components. An electro-optic modulator will be added to this system to create a modulated LLS prototype. Laboratory tank experiments will be conducted to evaluate the performance of the modulated LLS as a function of water clarity and solar background levels. The new system will be compared to its unmodulated counterpart in terms of target contrast.

Optical modulation techniques for underwater detection, ranging and imaging

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.

Hybrid technique for enhanced optical ranging in turbid water environments

Abstract. A hybrid approach is described that enhances the performance of an underwater optical ranging system. This approach uses high-frequency modulation and a spatial delay line filter to suppress unwanted backscatter. A dual frequency approach is also implemented to reduce the effects of forward scatter and remove the ambiguity associated with using the phase of the single, high-frequency modulation envelope to measure range. Controlled laboratory experiments were conducted to evaluate the effectiveness of the hybrid technique to reject multiple scattered light and improve range precision. The experimental results were compared with data generated from a theoretical model developed to predict the performance of the technique as a function of system and environmental variables. Model and experimental results are shown that reveal the ability of the approach to provide accurate ranging to an underwater object in a variety of water environments. Model predictions also indicate that advancements in transmitter and receiver technology will extend the range and improve the accuracy of the technique beyond what has been achieved thus far.