12-01-2011, 04:44 PM
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DR.AIT,Bangalore
INTRODUCTION
The future tactical ocean environment will be increasingly complicated. In addition to traditional communication links there will be a proliferation of unmanned vehicles in space, in the air, on the surface, and underwater. Above the air/water interface wireless radio frequency communications will continue to provide the majority of communication channels.Underwater, where radio waves do not propagate, acoustic methods will continue to be used.However, while there have been substantial advances in acoustic underwater communications, acoustics will be hard pressed to provide sufficient bandwidth to multiple platforms at the same time. Acoustic methods will also continue to have difficulty penetrating the water/air interface. This suggests that high bandwidth, short range underwater optical communications have high potential to augment acoustic communication methods.
The variations in the optical properties of ocean water lead to interesting problems when considering the feasibility and reliability of underwater optical links. Radio waves do not propagate underwater, however with the proliferation of unmanned autonomous vehicles the need to communicate large amounts of data is quickly increasing. Making physical connections underwater to transfer data is often impractical operationally or technically hard to do. Traditionally most underwater communication systems have been acoustic and relatively low bandwidth. However, the development of high brightness blue/green LED sources, and laser diodes suggest that high speed optical links can be viable for short range applications. Underwater systems also have severe power, and size constraints compared to land or air based systems. Underwater vehicles also encounter a wide range of optical environments. In shallow water the effects of absorption by organic matter and scattering by inorganic particulates can be severe compared to deep ocean water. Where the system operates in the water column can also have strong influence. Near the sea floor, ocean currents and silt can play a factor, while in the middle of the water column the medium may be considered more homogeneous, but with its optical properties varying as a function of depth. Near the surface, sunlight can provide a strong background signal that needs to be filtered, and the amount of wave action can have significant effects.In this thesis the use of free space optical links will be investigated for underwater applications. With the use of MathCAD, optical link budgets for three different scenarios are considered:
• A blue/green LED based, bottom moored buoy system operating in relatively shallow water.
• A blue/green laser based system operating in deep clear ocean water with unlimited power and size constraints.
• A power and size constrained, diode laser system suitable for small unmanned underwater vehicle operation.
Inputs into the link budget include: light source type, wavelength, optical power, beam divergence, ocean water optical parameters based on depth, geographic location and time of day, and photodetector type. As a point of comparison, the relative merits of these systems are compared to a conventional acoustic communications links.
A secondary focus of the thesis was to construct light emitting diode based links. The choice of using LEDs instead of Lasers was largely economic, however in the underwater environment can be very challenging optically and many of the advantages that lasers have in terms of beam quality can be rapidly degraded by scattering and turbulence.
Free Space Optics Concepts
Free-space optics (FSO) is a line-of-sight (LOS) link that utilizes the use of lasers or light emitting diodes, LEDs, to make optical connections that can send/receive data nformation, voice, and video through free space. FSO also has attractive characteristics of dense spatial reuse, low power usage per transmitted bit, and relatively high bandwidth. FSO is license-free and offers easy to deploy, fast, high bandwidth connections. Moreover, the optical spectrum is not regulated by the FCC allowing the use of large amounts of unlicensed bandwidth. Due to the large investment in traditional fiber based optical communications networks, LED’s, lasers, photodetectors are available today cheaply and in large volumes. A free space link requires a light source, modulation/demodulation device, and transmitting and receiving telescopes. For moving targets, the transmitter and receiver are placed on gimbal system with feedback controls1 . Instead of propagating through silica glass, as with optical fiber, the light travels through free space.
The main disadvantage of FSO networks is that the transmission medium is uncontrolled. The effects of atmospheric distortions, scintillation, weather and attenuation can only be minimized or compensated by the transmitter/receiver hardware. Free-space optics above and below water have similar issues that need to be accommodated when building a system.