The underwater world represents 70% of our planet. As the number of human underwater activities grows, with the advent of underwater sensor networks, remotely operated vehicles and autonomous underwater vehicles, the need for an efficient and flexible means of communicating through seawater has become of utmost importance. Common terrestrial technologies, such as fibre-optic communication, are difficult to employ underwater due to the constraint of a physical link, which reduces the maximum operational depth and limits the manoeuvrability . Underwater acoustic-wave-based communication, widely used in the past, are limited due to their low-speed and low energy efficiency . However, the recent development of high bandwidth GaN-based sources has further renewed interest in the development of underwater optical communication systems (UOCS), particularly within the low loss transmission window of water in the blue-green region .
Modelling the channel is a fundamental step in an UOCS due to the different medium compared to other free-space optical communications. The ocean water has widely varying optical properties. The optimal wavelength is highly dependent on water type and the two main processes affecting light propagation, absorption and scattering, are both wavelength-dependent and influenced by the rich chemical and biological environment. Hence, the underwater channel design is based on an understanding of the optical properties of ocean water. Moreover, sunlight has a maximum intensity in the blue-green region. The solar background can be reduced by using a transmission wavelength that matches one of the Fraunhofer lines, which are narrow intervals of relative intensity minima that act as natural filters [4,5].
An important step in the design of an UOCS is the trade-off analysis, taking into account the channel model and setting the system parameters, such as: power efficiency, bandwidth efficiency, transmission reliability and cost. At least three different system configurations are possible , depending on the transmitter source (LED or LD). LEDs have a medium modulation speed, a wide field of view and an incoherent output light, making them more suitable for low cost, moderate data rates and short range links such as small platforms. In contrast LDs, thanks to their coherent, highly directional and well-collimated beam profile as well as high modulation speed, allow longer transmission links, provide an active pointing mechanism to persistently maintain optical alignment toward the receiving station with an acceptable accuracy. Indeed, the pointing-and-tracking between the transmitter and the photodetector is crucial to maximise the amount of optical power density received. A highly sensitive array of photodetectors is required, in particular for non-stationary vehicles in clear seawater, where the intensity decreases rapidly with lateral distance from the beam. The results of numerical simulations indicate that links under 15 m are achievable with all configurations . So, the choice of the source, as well as of the link geometry, is based on the water scattering conditions and the system performance requirements.
The total attenuation in clear ocean water is limited by the absorption whereas in harbour water the scattering from large particles is the main limiting factor. Thus, it is more challenging to design a UOCS near the shore than open ocean. The development of an UOCS will have a great impact in the defence and civil sectors with commercial products and solutions that take advantage of the unexplored opportunities offered by oceanic optics.
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