Experimental Study: Underwater Propagation of Super-Gaussian and Multi-Gaussian Schell-model Partially Coherent Beams with Varying Degrees of Spatial Coherence

We report on experiments where super-Gaussian and flat-top, multi-Gaussian Schell-model spatially partially coherent beams, with varying degrees of spatial coherence, were propagated underwater. Two scenarios were explored—calm and mechanically agitated water. The main objective of our study was the experimental comparison of the scintillation statistics. For a similar degree of coherence widths, the results show a potentially improved performance of scintillation index for the multi-Gaussian Schell-model beams as compared to the super-Gaussian beams. It should be noted that the presented results pertain only to the given experimental scenarios and further investigation is necessary to determine the scope of the findings

Bidirectional Underwater Visible Light Communication

In this paper, a novel bidirectional underwater visible light communication (BiUVLC) is proposed. The VLC transmitter transmits an information signal using the one of RGB LED through the water tank that represents an underwater environment and then is received by VLC receiver via a color filter. The color LEDs and color filters are utilized in bidirectional systems. The single link is created by a LED on the transmitter and the color filter on the receiver with the same color which represents a single wavelength. The performance of the proposed BiUVLC system was evaluated via implementations. The experimental result shows that the transmitted signal undergoes attenuation over the underwater optical channel and the pair of the blue wavelength in link 1 and the green wavelength in link 2 have the best performance than the other wavelength pair. In the crosstalk measurement, the red wavelength color is the worst in the underwater environment

Visual summaries for low-bandwidth semantic mapping with autonomous underwater vehicles

A fundamental problem in autonomous underwater robotics is the high latency between the capture of image data and the time at which operators are able to gain a visual understanding of the survey environment. Typical missions can generate imagery at rates orders of magnitude greater than highly compressed images can be transmitted acoustically, delaying that understanding until after the robot has been recovered and the data analyzed. We present modifications to state-of-the-art online visual summary techniques that enable an autonomous robot to select representative images to be compressed and transmitted acoustically to the surface ship. These transmitted images then serve as the basis for a semantic map which, combined with scalar navigation data and classification masks, can provide an operator with a visual understanding of the survey environment while a mission is still underway

Light emitting diodes and lasers for high-speed underwater optical communications

During the last decade, a lot of research has been carried-out around Underwater Wireless Optical Communications (UWOC) as they are considered as a promising technology for high data rate transmission in underwater environments

Remote Control Submarine Robot

Tato bakalářská práce se zabývá návrhem dálkově řízeného podvodního robota použitelného k inspekční a servisní činnosti. V úvodu práce je nastíněn historický vývoj podvodních servisních robotů a jejich rozdělení. Dále se práce zabývá jednotlivými prvky robotu, srovnáním jejich konstrukčních řešení a výběrem vhodné varianty řešení.This bachelor thesis deals with the design of a remotely controlled underwater robot that can be used for inspection and service operations. The first part of the thesis outlines the historical development of underwater service robots and their classification. Furthemore, the thesis deals with the individual parts of the robot; compares their design solutions; and selects suitable ones.

Study on Underwater Optical Wireless Communication

In underwater environment, a sonar is mostly used as a communication tool. However, its speed is not fast enough to transfer large data such as image. To speed up the data transfer speed, the LED light is used as a communication tool in this paper. The overall structure of the system is introduced including the control system of the LED lamp, light transmission and receiver system. Finally, experiments were performed on the developed underwater communication system in sea water and fresh water and results are described.1. 서 론 1.1 연구배경 1 1.2 수중 LED 통신 시스템의 국내외 연구동향 1 1.3 연구 목적 4 2. 수중광학 이론 2.1 전자기 파장에 따른 광 감쇠율 5 2.2 해수의 광학적 특성 7 2.3 수중 LED 통신 성능 이론 9 3. LED 송신기 설계 및 제작 3.1 수중통신용 LED 선정 14 3.2 LED 방열 16 3.3 콜리메이트 렌즈 설계 18 3.4 액추에이터를 이용한 LED 광 확산 제어기 설계 22 3.5 LED 송신기 방수 하우징 외압 해석 29 3.6 LED 송신기 제작 34 4. 광 수신기 설계 및 제작 4.1 광 검출센서의 선정 36 4.2 집광렌즈 설계 44 4.3 트랜스임피던스 증폭기 설계 44 4.4 신호처리기 설계 46 5. 수중 LED통신 성능시험 및 결과 5.1 LED 송신기의 광 확산 제어기 성능실험 48 5.2 수중통신 성능평가 프로그램 제작 51 5.3 PIN 광수신기의 담수 수중통신 성능실험 52 5.4 PIN 광수신기의 해수 수중통신 성능실험 53 5.5 PIN광수신기, APD광수신기의 탁도에 따른 수중통신 성능실험 57 5.6 수중 LED통신을 이용한 수중동영상 통신 실험 60 6. 결론 6.1 결론 66 참고문헌 6

Estudo de viabilidade de utilização de soluções óticas de baixo custo

A forte atenuação dos sinais eletromagnéticos na água é a principal razão pela qual a exploração subaquática, a vigilância marítima e outras aplicações, atualmente dependem de cabos submarinos para enviar ou receber dados de um terminal de comunicação. Embora a comunicação acústica se tenha tornado o método de comunicação sem fios padrão para as aplicações subaquáticas devido ao seu longo alcance, elas incorrem em alto custo e têm baixas taxas de dados. A necessidade por comunicações de alto débito binário e largura de banda estimulou a exploração de métodos não acústicos que antes eram negligenciados devido às limitações de distância. Contrariamente aos sistemas acústicos e de radiofrequência, que apresentam limitações como custo, velocidade de propagação das ondas e largura de banda disponível, o que se traduz na taxa de transmissão de dados, a comunicação ótica sem fios é apresentada como solução alternativa para ultrapassar as limitações mencionadas. À vista disso, a solução proposta nesta dissertação baseia-se num sistema de comunicação ótica de baixo custo, viável para as comunicações sem fio subaquática à curta distância entre veículos não tripulados. Assim sendo, um sistema transmissor baseado em LED de alto brilho de 3 com uma faixa de comprimento de onda entre 460 – 470 e um sistema recetor baseado em fotodíodo com uma sensibilidade espectral entre 400 – 1100 , foram desenvolvidos e testados em diferentes canais de comunicação, ao ar livre, num tanque de água doce e por fim na água do mar. O resultado das experiências com o protótipo mostrou com sucesso que, na presença da luz ambiente, a distância de transmissão - receção dos dados chega a 4,5 e, na ausência da luz ambiente 5,50 ambos com uma taxa de transmissão de 300 . Por fim, o trabalho apresenta sugestões para trabalhos futuros de modo a melhorar o desempenho, a eficiência e a robustez do sistema.The strong attenuation of electromagnetic signals in water is the main reason why underwater exploration, maritime surveillance and other applications currently rely on underwater cables to send or receive data from a communication terminal. Although acoustic communications have become the standard wireless communication method for underwater applications due to their long range, they incur high cost and have low data rates. The need for high data rate and bandwidth communications has stimulated the exploration of non-acoustic methods that were previously neglected due to distance limitations. Contrary to acoustic and radio-frequency systems, which present limitations such as cost, wave propagation speed and available bandwidth, which translates into data transmission rate, wireless optical communication is presented as an alternative solution to overcome the mentioned limitations. In view of this, the solution proposed in this dissertation is based on a low-cost optical communication system, feasible for underwater wireless communications at short distance between unmanned vehicles. Therefore, a transmitter system based on 3W high-brightness LED with a wavelength range between 460 - 470 nm and a receiver system based on photodiode with a spectral sensitivity between 400 - 1100 nm, were developed and tested in different communication channels, outdoors, in a freshwater tank and finally in seawater. The result of the experiments with the prototype successfully showed that, in the presence of ambient light, the data transmission-reception distance reaches 4,5 m and, in the absence of ambient light 5,50 m both with a transmission rate of 300 bps. Finally, the work presents suggestions for future work in order to improve the performance, efficiency and robustness of the system

Ultrasonic wireless broadband communication system for underwater applications

Tese de doutoramento do Programa Doutoral em Engenharia Eletrónica e de ComputadoresUnderwater wireless communication systems are becoming a priority in terms of research and technological development due to the increasing demand for exploring the oceans’ potential in areas such as pharmaceutical, oil, minerals, environmental and biodiversity. This demand is increasing exponentially with the need for high data rate and near-real-time communications between submerged mobile and static agents. The existing wireless communication technologies using electromagnetic waves or lasers are not very efficient due to the large attenuation in aquatic environment. Ultrasound reveals a lower attenuation, and thus has been used in underwater long-distance communications. But the underwater acoustic medium is one of the less reliable communication channels which represent major challenges for communications. With relatively slow sound speed propagation (~1500 m/s) the delay may represent a problem for communications with real-time applications. A theoretical model of an underwater communication system was also developed. The model allows to emulate the emitter, the hydrophone and the underwater acoustic channel, which includes attenuation, environmental noise, Doppler Effect, multipath and propagation delay. This model supported the study of wireless communications by emulating the transmission of acoustic signals using different types of digital modulations. The acoustic signal attenuation, multipath, ambient noise in several environments theoretical results were compared to those obtained experimentally. Allowing to conclude that the model represents a suitable approximation to the real subaquatic communication channel for the evaluation of digital acoustic communications. An optimization study of ultrasound transducers for underwater communications was addressed, focusing on a piston type emitter operating in the thickness mode (d33). It was discussed how the acoustic impedance, thickness, resonance frequency and structure affect the transducer performance. This work allowed a better understanding of the emitter transducer characteristics allowing reaching the optimum point of operation for specific applications. Focusing on underwater communication, the transducer was optimized by finite element computer simulations. The results were compared with experimental tests and show that four-layer structures increase up to 16 dB in performance when compared to single-layer transducer disks. For high data-rates and real-time applications it was necessary to develop ultrasound transducers able to work at high frequencies and wideband, with suitable responses to digital modulations. It was thus also included a comparison study that shows how the acoustic impedance influences the performance of an ultrasonic emitter when using different digital modulations and operating at frequencies between 100 kHz and 1 MHz and some tens of meters of distance. It is presented a Finite Element Method (FEM) and a MATLAB/Simulink simulation with an experimental validation to evaluate two types of piezoelectric materials: one based in ceramics (high acoustic impedance) with a resonance design and a polymer based (low acoustic impedance) system, designed to optimize the performance when using digital modulations. The transducers performance for Binary Amplitude Shift Keying (BASK), On-Off Keying (OOK), Binary Phase Shift Keying (BPSK) and Binary Frequency Shift Keying (BFSK) modulations with a 1 MHz carrier at 125 kbps baud rate were compared. The transducers materials used were the ceramics PZT-5H and the polymer PVDF. The results show that PVDF transducer has a better performance to digital modulations than PZT-5H transducer, providing the signal full demodulation for all digital modulations tested. On the other hand, the PZT-5H transducer showed a higher output, but fails to perform accurate modulated signals. Finally, the system was validated by the implementation of a full duplex point-to-point communication at 1 Mbps using OOK modulation with a 1 MHz single carrier. The system was successfully tested in a swimming pool at a distance of 6 meters with a 1 Mbps rate, achieving a 3x10-3 Bit Error Rate (BER) using just 1.4 W of power consumption. These results represent an advance in underwater acoustic communications, being the first practical system to achieve data rates up to 1 Mbps.O desenvolvimento de sistemas de comunicação subaquáticos sem fios está a tornar-se uma prioridade na comunidade científica no sentido de aumentar o desenvolvimento tecnológico. Este facto deve-se à crescente necessidade de exploração do potencial dos oceanos em áreas científicas diversas como farmacêutica, petrolífera, mineral, ambiental e até do próprio estudo da biodiversidade. Essa necessidade aumenta exponencialmente com a necessidade de comunicações de alto débito e em tempo real entre agentes submersos móveis e estáticos. As tecnologias de comunicações sem fios existentes, nomeadamente as que utilizam ondas eletromagnéticas ou lasers não são muito eficientes, devido, em grande parte, à atenuação no ambiente subaquático. Os ultrassons revelam uma menor atenuação tendo sido, por isso, utilizados em comunicações subaquáticas em longas distâncias. Contudo o canal acústico subaquático definisse como um dos mais difíceis, devido em parte as suas características únicas, o que apresenta ser um enorme desafio. Como a velocidade de propagação do som é relativamente lenta (~1500 m/s), o atraso pode representar um problema para as aplicações em tempo real. Foi desenvolvido um modelo teórico do sistema de comunicações subaquáticos que permite emular o emissor, o hidrofone e o canal acústico subaquático. No canal acústico subaquático foi simulado o efeito da atenuação, ruído ambiente, efeito de Doppler, multipath e atraso de propagação. Este modelo é indicado para o estudo das comunicações subaquáticas, emulando a transmissão de sinais acústicos utilizando diferentes tipos de modulações digitais. Neste estudo foram testados, a atenuação do sinal acústico, multipath, ruído em diversos ambientes e os resultados teóricos foram comparados com os obtidos experimentalmente. Permitindo concluir que o modelo representa uma aproximação adequada do canal de comunicação, permitindo a avaliação das comunicações digitais acústicas. Inclui ainda um estudo de otimização de transdutores de ultrassons para comunicações subaquáticos, tendo como base o emissor do tipo pistão, operando ao longo da espessura (d33). Foi analisada ainda a forma como a impedância, espessura, frequência de ressonância acústica e estrutura afetam o desempenho do transdutor. Este trabalho permitiu uma melhor compreensão das características do transdutor emissor que permitem atingir o ponto ótimo de operação para aplicações específicas. Tendo como base a comunicação subaquática, o transdutor foi otimizado usando os resultados de simulações pelo Método dos Elementos Finitos. Os resultados foram comparados com os testes experimentais, onde se mostra que as estruturas de quatro camadas podem aumentar até 16dB no desempenho quando comparados com discos de transdutor de única camada. Para aplicações em tempo real e de elevado debito, foi necessário desenvolver transdutores de ultrassons capazes de operar em banda larga a altas frequências, com resposta adequada às modulações digitais. Foi, portanto, incluído também um estudo comparativo que mostra como a impedância acústica influencia o desempenho do emissor de ultrassons quando se utilizam modulações digitais a operar com frequências entre 100 kHz e 1 MHz abrangendo distâncias de algumas dezenas de metros. São apresentadas simulações por Método de Elementos Finitos (MEF) e MATLAB/Simulink com validação experimental de modo a avaliar dois tipos de materiais piezoelétricos: um com base cerâmica PZT-5H (alta impedância acústica) com um design de ressonância e outro de base de polimérica PVDF (baixa impedância acústica), otimizado para modulações digitais. O desempenho dos transdutores foi comparado para as modulações: Binary Amplitude Shift Keying (BASK), On-Off Keying (OOK), Binary Phase Shift Keying (BPSK) e Binary Frequency Shift Keying (BFSK) com uma portadora de 1 MHz a 125 kbps. Os resultados mostram que o transdutor de PVDF tem um melhor desempenho do que transdutor PZT-5H, proporcionando a desmodulação completa do sinal para todas as modulações digitais testadas. Por outro lado, o transdutor de PZT-5H mostrou uma potência acústica mais elevada, embora não consiga produzir sinais modulados precisos. Finalmente, o sistema foi validado através da implementação de uma comunicação ponto-aponto bidirecional de 1 Mbps utilizando uma modulação OOK com uma portadora de 1 MHz. O sistema foi testado com sucesso numa piscina a uma distância de 6 metros com uma taxa de 1 Mbps, com um BER (Bit Error Rate) de 3x10-3, utilizando apenas 1,4 W de consumo de potência. Estes resultados representam um avanço nas comunicações acústicas subaquáticas, sendo o primeiro sistema prático de atingir velocidades até 1 Mbps

III-nitride LEDs and lasers for underwater wireless optical communications

This thesis delves into the deployment of III-nitride micro-LED and laser-based devices for Underwater Wireless Optical Communications (UWOC), an emerging field with considerable potential for underwater data transfer. UWOC offers a compelling solution owing to the vast bandwidth available in the visible spectrum (hundreds of THz), the substantial modulation bandwidth of micro-GaN-based devices (hundreds of MHz), and the underwater “window” characterised by low light attenuation at visible wavelengths. Throughout this thesis, extensive experiments were conducted within a 1.5 m water tank, employing various transmitters (Tx’s) and receivers (Rx’s). The results showcase error-free data transmission rates in the gigabits per second (Gb/s) and megabits per second (Mb/s) range, even in the presence of varying water turbidity levels and attenuation lengths (ALs). Notably, data rates of 4.92 Gb/s and 1.32 Gb/s were achieved using a series-connected micro-LED array of 6 pixels, spanning distances of 1.5 m and 4 m, respectively, and achieved 15 Mb/s over 5.33 ALs. Different underwater conditions, influenced by water turbidity, lead to distinct light attenuation minima underwater. Consequently, I highlight the applicability of wavelength division multiplexing (WDM) in real UWOC scenarios. Using transfer-printed (TP) dual-colour micro-LED arrays, data rates of 200 Mb/s were demonstrated over 8.5 ALs. In most UWOC scenarios, establishing a reliable point-to-point underwater link is challenging due to the unpredictable subsea environment and terrestrial weather conditions. To mitigate this, I introduced the deployment of a diffusing fibre coupled with a laser diode, functioning as a Tx for omnidirectional data transmission (underwater “beacon”). This approach circumvented the alignment restrictions between the Tx and Rx, and data at 5 Mb/s were successfully transmitted over 11.81 ALs, showcasing the robustness of this solution.This thesis delves into the deployment of III-nitride micro-LED and laser-based devices for Underwater Wireless Optical Communications (UWOC), an emerging field with considerable potential for underwater data transfer. UWOC offers a compelling solution owing to the vast bandwidth available in the visible spectrum (hundreds of THz), the substantial modulation bandwidth of micro-GaN-based devices (hundreds of MHz), and the underwater “window” characterised by low light attenuation at visible wavelengths. Throughout this thesis, extensive experiments were conducted within a 1.5 m water tank, employing various transmitters (Tx’s) and receivers (Rx’s). The results showcase error-free data transmission rates in the gigabits per second (Gb/s) and megabits per second (Mb/s) range, even in the presence of varying water turbidity levels and attenuation lengths (ALs). Notably, data rates of 4.92 Gb/s and 1.32 Gb/s were achieved using a series-connected micro-LED array of 6 pixels, spanning distances of 1.5 m and 4 m, respectively, and achieved 15 Mb/s over 5.33 ALs. Different underwater conditions, influenced by water turbidity, lead to distinct light attenuation minima underwater. Consequently, I highlight the applicability of wavelength division multiplexing (WDM) in real UWOC scenarios. Using transfer-printed (TP) dual-colour micro-LED arrays, data rates of 200 Mb/s were demonstrated over 8.5 ALs. In most UWOC scenarios, establishing a reliable point-to-point underwater link is challenging due to the unpredictable subsea environment and terrestrial weather conditions. To mitigate this, I introduced the deployment of a diffusing fibre coupled with a laser diode, functioning as a Tx for omnidirectional data transmission (underwater “beacon”). This approach circumvented the alignment restrictions between the Tx and Rx, and data at 5 Mb/s were successfully transmitted over 11.81 ALs, showcasing the robustness of this solution