KEYWORDS: LIDAR, Temperature metrology, Signal detection, Turbulence, Electromagnetism, Synthetic aperture radar, Signal attenuation, Particles, Numerical simulations, Medium wave
Submersible vehicles, such as Uncrewed Underwater Vehicles (UUVs), can be lost underwater and it is desirable to locate them before they go missing or collide with other vehicles. These objects generate inherent signatures when they move in the ocean. With a better understanding of these signatures, more information regarding the motion of underwater vehicles can be inferred using LiDAR technology. In this paper, the authors review existing literature on various non-acoustic signatures of a submerged body, namely, the electromagnetic, biological and thermal signatures, turbulent wake patterns, internal waves and vortex structures. The authors discuss how these signatures evolve both spatially and temporally. Furthermore, the review investigates how environmental and operational parameters such as the stratification of the medium, vehicle speed, shape and depth affect the non-acoustic signatures. Underwater vehicles operating at low speeds and large depths have bioluminescent, Kelvin wake and Bernoulli hump signatures that are difficult to detect on the surface. Thermal signatures, vortex wakes and far wakes are only likely to be detected at the vehicle’s depth. This is primarily due to the ocean stratification which suppresses the vertical motion of underwater turbulent signatures. Thermal signatures appear to be most likely to be detected. The study concludes that relying on a single signature to detect submersibles is not advisable and future methods for underwater vehicle detection should use multiple sensors to detect complementary signatures.
Traditional acoustic and magnetic techniques for detecting underwater vehicles are becoming less reliable due to advances in underwater technology such as composite materials, miniaturized electronics, and more space efficient battery technologies. Optical remote sensing technologies, such as Light Detection and Ranging (LiDAR) systems, are promising alternatives due to their high measurement accuracy, independence of operating environments, and simple integration onto airborne platforms. However, the penetration depth of direct detection methods is limited by the strong attenuation of light by the water column. Detection techniques that rely on monitoring changes to the inherent optical properties (IOPs) and other remotely sensed properties of water are thus being considered. Effects of underwater vehicles on ocean properties such as salinity and temperature have been well studied, but a stronger understanding of their effect on IOPs and optical constituents of the ocean is required for these new detection techniques. In this paper, the authors develop a system to measure the effect of underwater turbulence equivalent to an inspection-class Remotely Operated Vehicle (ROV) on the IOPs and other physical properties of the water column. Measurements are taken in an indoor water tank, freshwater reservoir, coastal waters and oceanic waters. Four vertical thrusters are used as a turbulence generator. Four commercially available sensors monitor the changes in IOPs and optical constituents of water above the turbulence generator. Preliminary results are presented on the effect of underwater turbulence on the optical and ocean properties measured. We conclude that the turbulence was able to be detected via changes in the IOPs at a distance of 10m under most conditions, with caveats and qualifiers discussed.
The first higher power 3.5 μm pumping technique. Initial rapid power scaling has stagnated with no higher average power results in recent years. Power levels in excess of 10 W are necessary to unlock industrial and defense applications. Utilizing our rate equation based numerical model we investigate the expected performance of 3.5 μm fiber lasers configurations operating on the 10 W-20 W range. Fiber geometries and pump configurations are investigated. We also compare resonator with the simulated performance of master-oscillator power amplifiers configuration as a potential alternative for power scaling.
The next-generation gravitational wave detectors aim to enhance our understanding of extreme phenomena in the Universe. The high-frequency sensitivity of these detectors will be maximized by injecting squeezed vacuum states into the detector. However, the performance advantages offered by squeezed state injection can be easily degraded by losses in the system. A significant source of loss is the mode mismatch between optical cavities within the interferometer. To overcome this issue, new actuators are required that can produce a highly spherical wavefront change, with minimal higher order aberrations, whist adding low phase noise to the incident beam.
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