Solid-liquid compound hydroacoustic lens of low aberration

A underwater acoustic imaging system capable of working in extremely turbid conditions and producing high resolution dimensioned images has been developed. It is suitable for inspecting pipelines and offshore structures. A large development program has been carried out in Australia as a collaborative effort between a number of organisations to overcome the traditional problems with underwater imaging. The system uses megahertz frequency sound, sparse array technology, one-bit digitisation and chirp illumination. Acoustic three dimensional images with range and orthogonal dimensions have been obtained from a prototype underwater acoustic imaging system and examples are presented. Resolutions of millimeters have been obtained. This high resolution, large image (103x103x103 voxels) system is a new-to-the-world capability. This paper is a step in diffusing the new technology so that underwater acoustic imaging may become an innovation in offshore inspection and measurement. Introduction A new technology has been demonstrated for generating three dimensional near video quality images using underwater acoustics. There are two technologies for producing focused acoustic images, lenses and beam forming from twodimensional arrays. The lens has been demonstrated for two dimensional images by Belcher (1998) using a onedimensional array in the focal plane. The lens system places an array of acoustic sensors at the focal plane of the system and avoids the processing load of beam forming. Erikson (1997) discusses a more capable two-dimensional focal plane array. Beam formed images rely on the ability of the signals from an appropriately spaced array of acoustic sensors to be delayed in time and added to make the reflected pressure signals in phase for a particular voxel. For a large imaging volume this can become a very large processing load. The Underwater Acoustic Imaging technology is founded on five elements. Sparse arrays, one-bit digitisation, chirp ensonification, economical acoustic tiles and correlation flattening. While all of these elements have been used in some context before, it took the implementation of them all in a single system to produce a potential innovation. The sonar equation sets limits to performance that are determined by the source level and the system noise. While the source level is felt to be limited by the cavitation pressure, even before that magnitude the acoustic signal is nonlinear. Non linear acoustic signals distort during propagation and make replicate correction more difficult. System noise limits the range at which an object of given target strength can be seen above this noise. Arrays do not focus exactly on a single voxel but allow acoustic energy from adjacent voxels to leak into the picture. Replicate correlation of the received signals with the chirp ensonification, which provides focusing in the range direction, does the same. The clutter from elements of the target limits the size of objects that can be viewed clearly by restricting the contrast. Technology For an active acoustic imaging system such as the present, the acoustic pressure on the face of the receive array depends on the source level, the range of the target, its target strength and the absorption and scattering of water.

Acoustic vision systems are gaining prominence due to ocean surveillance for internal security along coastal waters, in particular defence and engineering. Image generation is an effective method for underwater target exploration. Underwater acoustic imaging systems can identify objects according to the characteristic information obtained from the acoustic reflections from the object. Acoustic lens is an attractive device for underwater imaging sonar because of its imaging capability without complex beamforming. This paper discusses the design and development of spherical and cylindrical acoustic lenses with plano-concave and double concave shapes and also development of imaging transducer array operating at 500 kHz. Acoustic lenses in combination with transducer arrays were evaluated underwater for acoustic imaging applications.

Underwater acoustic imaging systems are useful in providing underwater observation and inspection capabilities whenever optical systems cannot. Underwater acoustic imaging is a part of the larger field of acoustic imaging, and as such, has its own peculiarities and attributes as compared with the rest of the acoustic imaging field. In this report, the general characteristics of underwater acoustic imaging are presented. Three basic techniques of acoustic imaging that are appropriate for underwater imaging are discussed (focused, beamformed, and holographic) and examples of each of them are described. Then the limitations and promise for the future in underwater acoustic imaging are explored.

Wavefront distortion can affect the performance of underwater acoustic communication (a-comms) and imaging systems. A measure of wavefront distortion, the background structure functions (BSFs) are functions of sound speed profiles (SSPs) that are functions of depth. Long term, BSFs can use established commercial software to reduce wavefront distortion and forecast phase statistics for coherent systems. Historically, a suite of path-integral, ray tracing, and modal methods replaced the initial methods for statistical acoustics developed in the latter half of the last century. Underwater (UW) acoustics methods provide the amalgamation of deterministic and statistical expressions to help estimate Doppler and delay for UW acoustics by integrating the classical structure functions from the source to the receiver. In the interim years, non-acoustic communications industries used the older methods with algorithms and hardware that alleviated some of the old shortcomings in order to forecast communication performance. For acoustics, the use of databases of BSFs and SSPs recently helped simulate proof-of-concept forecasting of a-comms operating envelopes. These purely statistical BSFs have less than a quarter of the degrees of freedom (DOFs) in classical UW structure functions. Symmetries also drastically reduce the required background DOFs. For decades industry used these purely statistical structure functions to estimate wavefront wander, lensing (ducting), and fade. Commercial methods to estimate comms fade assume the medium has BSFs that are sufficiently symmetric in order to simplify the calculations. While these metrics can require re-calculation at different stations along the propagation path, extra phase screens are compatible with the evolution of computer technology during the past half century. Conditions of acoustical symmetry and invariance allow simplification of the acoustical calculations in most of the deep-water ocean and in littorals under iso-speed conditions. Results from shallow-sea measurements show that background BSFs in the littorals validate the simplified calculations for sufficiently symmetric sound speed fields. However, the complex calculations that arise from removing the simplifications are tractable with recent algorithms. Adaptive wavefront methods can use BSFs with arrays to guide the wavefront through less lossy paths. Nevertheless, even the value of the ability to forecast phase and throughput statistics in locations where measurements or estimates of sound speed fields exist is of adequate value to provide this foundation for the future use of these purely statistical methods.

Region-specialized underwater image restoration in inhomogeneous optical environments

To address the inability to quantitatively analyze the performance of the direct sequence spread spectrum (DSSS) communication system in the presence of short-period pseudo-noise (PN) sequences, this study derives the interference components resulting from correlation processing of single-frequency interference. Given the short period of the spread spectrum sequence used in underwater acoustic systems, a theoretical expression is proposed for analyzing the anti-single-frequency interference performance of DSSS communication systems in underwater acoustics. This expression combines the interference power output by the system to enable quantitative analysis. Simulations are conducted to validate the accuracy of the theoretical expression. The results reveal that the interference sequence generated by correlation processing assumes either a direct current (DC) or sinusoidal distribution. The bit error rate (BER) of the system is influenced by signal-to-interference ratio (SIR), interference frequency offset, phase deviation, and the structure of the spreading code itself. The BER exhibits regular variation with changes in the interference phase. In all working conditions where the interference frequency offset is an integer multiple of the bit bandwidth, single-frequency interference with a frequency offset equivalent to the bit bandwidth has the greatest impact on the system. The concise theoretical expression presented in this study has significant practical value as it allows for quick prediction of variation in the anti-single-frequency interference performance of underwater acoustic DSSS communication systems using short-period PN sequences.

Range-gated laser imaging technology as a way of underwater detection featuring long detecting distance has a very wide range of applications in underwater exploration, reconnaissance, rescue and other fields. Digital image enhancement algorithm has been proved to have a significant effect to further improve the image quality of underwater range-gated laser imaging system. Based on lab's previous research on underwater range-gated laser imaging technology, we realize the local non-linear enhancement algorithm on FPGA hardware platform, which obtained real-time underwater laser range-gated imaging enhancement. Experiments were carried out in 18m long pipes full of water to compare and analyze several enhancement algorithms together. The results showed that the local non-linear enhancement algorithm in the paper significantly improved image quality compared to the conventional methods.

This work studies the steps to optimize the sensor placement for underwater acoustic positioning system when the ranges between the investigated points and sensors are measured only. It is assumed that the measurements of the ranges are corrupted with white Gaussian noise for the mathematical tractability. Firstly, the optimal geometric sensor formation that will yield the best positioning accuracy in 3D space is achieved by minimizing the values of geometric position dilution of precision(GDOP). Secondly, more computed positioning accuracy parameters, geometric positioning accuracy(GPA), horizontal positioning accuracy(HPA) and vertical positioning accuracy(VPA) are utilized to assess the optimal geometric sensor formation. Thirdly, determine whether increase the number of sensors into the underwater acoustic positioning system according to the compare between the computed positioning accuracy and the positioning accuracy that users require. Finally, simulative experiments are designed to show the steps to optimize the sensor placement for underwater acoustic positioning system.

Pre-set secret keys are generally used to encipher and decipher the message in communication systems, including underwater acoustic (UWA) systems. Communication is not secure if the keys are compromised, so we need to generate an adaptive shared secret key. The secret keys are generated adaptively using correlated channel estimation, which reckons on the randomness in the environment and does not require the secrecy of data transferred during the process. This concept is extended in UWA channels with orthogonal frequency-division multiplexing (OFDM) systems and implements a protocol to generate secret keys adaptively based on the channel frequency response (CFR). A secret key generated from the CFR estimation manages the channel coding system, namely a turbo-code. This scheme allows to jointly performing an error control and adaptive coding in only one step. This scheme allows an adaptive security coding without change of the hardware structure of the transceiver devices. In our analysis, BER is used as a measure of the performance of channel estimation and mutual information is used as a measure of secretkey performance.

Underwater image restoration and enhancement using a Hybrid algorithm to evaluate the undersea activities like underwater vehicles to carry optical imaging systems for recording. The captured images and videos frequently suffered from two displeasing problems: 1. Color distortion; 2. Poor visibility. Those factors are the most notorious threats in underwater imaging systems because the light is exponentially attenuated while penetrating through water and the strength of attenuation is color dependent. Under these inferences, an effective single underwater image restoration, and enhancement framework-based Sea-thru algorithm has been proposed for image restoration, depth estimation, and transmission compensation to enhance the image. To address the consequences of scattering and absorption, a new restoration algorithm outperformed by the state-of-the-art method both qualitatively and quantitatively. A wide variety of underwater images with various scenarios were exploited to assess the restoration performance of the proposed algorithm. The proposed underwater image restoration technique is a promising result for undersea activities that required high-quality images. Sea-thru method estimates backscatter using the dark pixels and machine learning algorithms, to create exciting opportunities for future underwater image exploration and conservation

Range-gated technology has been a hot research field in recent years due to its high effective back scattering eliminating. As a result, it can enhance the contrast between a target and its background and extent the working distance of the imaging system. The underwater imaging system is required to have the ability to image in low light level conditions, as well as the ability to eliminate the back scattering effect, which means that the receiver has to be high-speed external trigger function, high resolution, high sensitivity, low noise, higher gain dynamic range. When it comes to an intensifier, the noise characteristics directly restrict the observation effect and range of the imaging system. The background noise may decrease the image contrast and sharpness, even covering the signal making it impossible to recognize the target. So it is quite important to investigate the noise characteristics of intensifiers. SNR is an important parameter reflecting the noise features of a system. Through the use of underwater laser range-gated imaging prediction model, and according to the linear SNR system theory, the gated imaging noise performance of the present market adopted super second generation and generation Ⅲ intensifiers were theoretically analyzed. Based on the active laser underwater range-gated imaging model, the effect to the system by gated intensifiers and the relationship between the system SNR and MTF were studied. Through theoretical and simulation analysis to the image intensifier background noise and SNR, the different influence on system SNR by super second generation and generation Ⅲ ICCD was obtained. Range-gated system SNR formula was put forward, and compared the different effect influence on the system by using two kind of ICCDs was compared. According to the matlab simulation, a detailed analysis was carried out theoretically. All the work in this paper lays a theoretical foundation to further eliminating back scattering effect, improving image SNR, designing and manufacturing higher performance underwater range-gated imaging systems.

Event Abstract Back to Event Properties of Silicone fluids before and after purification Can B. Hu1* and Mike Lowery1* 1 Abbott Medical Optics, United States Introduction: Silicone is a widely used biomaterial[1]. For ophthalmologic applications such as intraocular lenses (IOLs), many properties such as amount of leachables, clarity, refractive index (RI) and viscosity are all important factors for the IOL design. Freshly synthesized high RI silicone fluid usually needs to go through the filtration[2] process to improve the clarity; however, filtration alone can’t remove the low molecular weight species which may leach out after long term implantation. Purification of silicone fluid would be important to remove the undesired low molecular weight species. In this study, properties of silicone fluid before and after purification by solvent washing process are discussed. Material and Methods: High molecular weight silicone with refractive index of 1.47±0.01 was synthesized by anionic ring-opening polymerization[3]-[5]. After filtration through 0.2 µm PTFE filters, silicone fluid was purified by solvent washing process to remove low molecular weight species. Washing process was repeated multiple times (3X and 5X) to check the effect of purification on properties of refractive index (RI), Abbe number, UV-Vis percent transmission (%T), haze level and viscosity. Results: Table 1 shows RI and Abbe number before and after multiple purification process. Although RI of purified silicone fluid was slightly dropped (0.0015 and 0.0019 respectively with 3 and 5 times purification when compared to the filtered control sample only, nevertheless, the difference is significant with p value <0.05 for both purification processes. Abbe number increased slightly after purification. The haze levels before and after purification was summarized in Table 2. It was found that haze level was increased from 2.6 to 3.9 and 5.2 respectively after 3 times and 5 times purification. Although the haze difference of silicone fluid after purification was statistically significant, however, the difference was difficult to tell by naked eyes. It was found that haze level increased with number of purification process. Table 1 and 2. RI, Abbe number and Haze level of silicone fluid before and after purification. An overlap UV/Vis scan of silicone fluid before and after multiple purification process was shown in Figure 1. As in Figure 1, percent transmission (%T) of UV/Vis of purified silicone fluids were also slightly lower than that of non-purified control sample. This observation is consistent with the results of haze study and confirmed our previous study[2]. Figure 1. UV/Vis scans of silicone before and after purification Results of viscosity measurement of silicone fluid before and after purification were summarized in Table 3. It was found that viscosity increased significantly after purification which indicated that low molecular weight species were removed effectively. Table 3. Viscosity of silicone fluid before and after filtration Conclusions: In this study, we found that purification is a very effective method to remove the low molecular weight species of silicone fluid as indicated by the significantly increased the viscosity of silicone fluid. However, other properties such as refractive index, abbe number, haze level and percent transmission of silicone are also impacted by the purification process as well and worth studying further. Thanks approval from Abbott Medical Optics to presented this work SC2015CT0038

Underwater acoustic communications are required for controlling and networking a variety of underwater or sea surface objects. Due to the unique characteristics of the underwater acoustic channel, e.g., complicated multipath propagation, selective frequency fading, extreme Doppler effect and heavy ambient noise, FH-MFSK techniques play a vital role in robust underwater acoustic communication, especially in shallow water. However, based on detecting the frequency of the receiving signals, the demodulation scheme for FH-MFSK may be sensitive to the frequency shifting caused by underwater Doppler shifts. This paper focuses on how to design the frequency demodulation scheme and to improve the performance of underwater FH-MFSK acoustic communication system. By using an original Double-Line Interpolation algorithm with Hanning window, we propose a demodulation scheme to enhance the tolerance of the Doppler shift and improve the detection rate. Simulations and demodulation performance were tested. Experimental results show a promising future for FH-MFSK demodulation in underwater acoustic communication system.

In recent years, underwater acoustic (UWA) communications have received much attention as their applications have begun to shift from military toward commercial. Digital communications through UWA channels differ substantially from those in other media, such as radio channels, due to severe signal degradations caused by multipath propagation and high temporal and spatial variability of the channel conditions. The design of underwater acoustic communication systems has until recently relied on the use of noncoherent modulation techniques. However, to achieve high data rates on the severely band-limited UWA channels, bandwidth-efficient modulation techniques must be considered, together with array processing for exploitation of spatial multipath diversity. The new generation of underwater communication systems, employing phase-coherent modulation techniques, has a potential of achieving at least an order of magnitude increase in data throughput. The emerging communication scenario in which the modern underwater acoustic systems mill operate is that of an underwater network consisting of stationary and mobile nodes. Current research focuses on the development of efficient signal processing algorithms, multiuser communications in the presence of interference, and design of efficient modulation and coding schemes. This paper presents a review of recent results and research problems in high-speed underwater acoustic communications, focusing on the bandwidth-efficient phase-coherent methods. Experimental results are included to illustrate the state-of-the-art coherent detection of digital signals transmitted at 30 and 40 kb/s through a rapidly varying one-mile shallow water channel.

In order to improve underwater range-gated imagery quality, image restoration and super-resolution reconstruction techniques based on maximum a posteriori framework are combined with the point spread function (PSF) of the system. A model based on beam propagation considering time-dependent backscatter is established including the extreme attenuation properties of water and the diffraction limit of sensor for obtaining the PSF. The proposed method is applied for an underwater range-gated imaging system. Performance of the approach is compared with the traditional restoration and reconstruction algorithms. Objective image quality metrics are used to quantify the effectiveness of the restoration and reconstruction. Experimental results showed that the proposed method can effectively improve the resolution and quality of underwater range-gated imaging.