A 1 Mbps Underwater Communication System Using a 405 nm Laser Diode and Photomultiplier Tube
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Date
2008-12-07
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Abstract
Radio frequency communications in seawater are impractical due to high conductivity of
seawater limiting the propagation of electromagnetic waves. Current methods, such as
acoustic communication, are limited in bandwidth, and the use of cables, such as fiber optic,
are expensive and not practical for autonomous vehicles. Underwater tethered
communication systems are also very costly to repair if damaged. Optical wireless
communications that exploit the blue/green transparency window of seawater potentially
offer high bandwidth, although short range, communications.
The goal of this Masters thesis was to build sufficient infrastructure to experimentally
validate the performance of underwater optical communication systems under laboratory, but
hopefully realistic, water conditions.
An optical transmitter based on a 405nm blue laser diode was constructed. The
transmitter is capable of sourcing 200mA of current to a blue laser diode at speeds of up to
200MHz. The receiver was based on a photomultiplier tube. The high gain and blue/green
sensitivity of a photomultiplier tube make it ideal for underwater optical communications.
Finally, a 1,200 gallon water tank was constructed that allows the water conditions to be
appropriately controlled to simulate an ocean environment
Experiments were conducted to validate the design and construction of the receiver,
transmitter and water tank. An underwater optical data link was demonstrated that was
capable of transmitting data at 500kpbs in return-to-zero format, or 1Mpbs in non-return-tozero
format. The transmitted signal could then be optically detected, digitized and stored on a
PC for later signal processing.
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Keywords
communications, optical, underwater, laser diode, laser, free space optical, PMT, photomultiplier tube
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Degree
MS
Discipline
Electrical Engineering
