18-06-2010, 11:22 PM
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Introduction to underwater optical communication
¢ 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
¢ Under water, where radio waves do not propagate, acoustic methods will continue to be used.
Free Space Optics Concepts
Free-space optics (FSO) is a line-of-sight (LOS) page link that utilizes the use of lasers or light emitting diodes, LEDs, to make optical connections that can send/receive data information, voice, and video through free space.
Appling FSO Concepts to Seawater
The transmitter and receiver for an underwater page link can be very similar to a FSO page link in air, the major difference being the wavelength of operation.
ocean water has widely varying optical properties depending on location, time of day, organic and inorganic content, as well as temporal variations such as turbulence.
Water Types
Attenuation Underwater
Attenuation underwater is the loss of beam intensity due to intrinsic absorption by water, dissolved impurities, organic matter and scattering from the water, and impurities including organic and inorganic particulate.
Each water type also contains different levels of biomass known as:
Phytoplankton-unicellular plants with light absorbing chlorophylls,
Gelbstoffe -dissolved organic compounds know as yellow substance, Other optical effects of the biomass include:
Fluorescence -re-emission of light at a lower frequency by absorber illuminated with optical energy,
Bioluminescence- emission of light by marine organisms.
Absorption by Pure Seawater
Phytoplankton “ Chlorophyll-a
Scattering by Pure Seawater
Because of the inherent scattering of molecules, and of the salts dissolved in the water, a certain amount of initial scattering has to be taken into account before calculating the total scattering coefficient.
This type of scattering is Raleigh scattering, with strong wavelength dependence seen with the increased scattering at shorter wavelengths.
Scattering by Suspended Particulate
Scattering in the ocean is due to both inorganic and organic particles floating within the water column. Where in the open ocean scattering comes mainly from organic particles (phytoplankton, etc.),
Scattering from the particulates depends on the degree of external reflection and diffraction by their geometric form, and the internal refraction and reflection from the index of refraction from the particulates
Link Budget
Judging the relative merits of optical communication systems can be difficult due to the wide variety of different methods that can be used to communicate.
This FOM has been used to discuss the evolution of communications systems, especially the fiber optic communication systems.
Power Budget
Assuming a specific wavelength of operation, and a goal of a specific Bit Error Rate(BER) to construct the power budget for a simple point to point link, one should consider:
The transmitter output power,
Coupling efficiency of the source and receiver to the fiber,
Rise time Budget
The purpose of a rise-time budget is to ensure that the complete system is able to operate at the intended bit-rate
The rise-time characteristics of the transmitter and receiver are usually known. The allocated rise time will depend on the format used by the system, i.e.Return to Zero (RZ) or NonReturn to Zero (NRZ). With NRZ format able to accommodate twice the Bit-rate as the RZ format.
RF Link Budget
Geometric Effects in Link Budget
Antenna and aperture theories are also typically used for RF communication systems that are static in configuration.
At high frequencies, where line of sight dominates the situation, it is similar to that of the laser beam while at lower frequencies multi-path effects dominate.
Acoustical Link Budget
Geometric Effects in Link Budget
Similarly in acoustic communications, the deep water environment will be different than the shallow water environment, purely from the geometric effects of multiple reflections from the bottom and surface.
For acoustic single transducers the emitter can be considered omni directional, although there shadowing effects that attenuate the signal
Amount of Bandwidth available in an acoustical page link with respect to page link distance
High Powered LEDS
Recently, high powered LEDs have been commercialized for applications requiring high efficiency including automobile head lights, brake lighting, and indoor lamp lighting.
As a possible future application the possibility of using high powered LEDs for underwater communications were explored where the desired wavelengths of operation are between 450nm and 550nm
For optical communication purposes the following aspects should be considered:
Optical wavelength
Optical output power
Reliability
Beam parameters.
LumiLEDs are presently the industry leader in high powered LEDs. They have focused on
efficiently extracting light from the LEDs and packaging the LEDs so that heat can
efficiently be removed.
FM Optical Wireless System
The next experiment demonstrated the use of LEDs in a simple communication page link using human voice.
FM modulation was chosen over AM modulation since it was viewed as being more resistant to fading and variations in the signal amplitude.
Receiver
Applications for underwater optical communication systems include:
Diver-to-diver communication links
Diver-to-submarine links
Submarine-to-UAV links
Submarine-to-submarine links
UAV-to-UAV links
Submarine-to-satellite links
The model describing the inherent optical properties of ocean water, included:
Absorption by Pure Seawater
Absorption by Chlorophyll
Absorption by Color Dissolved Organic Matter
Scattering by Pure Seawater
Scattering by Small & Large Particle
Beam Spread through a horizontal water column
