A 1 Mbps Underwater Communication System Using a 405 nm Laser Diode and Photomultiplier Tube

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.