| Underwater acoustic sensing for mapping energy resources below the ocean floor involves measuring weak reflections of acoustic pulses from the rock layers. The acoustic pulses are generated by a ship-mounted air gun. The reflected acoustic signals are detected by thousands of acoustic hydrophones distributed along miles of bulky electrical wires and towed behind a large ship.; For the last two decades, fiber optics has held the promise of providing longer arrays, more sensors, and higher sensitivity, on a 'pencil-thin' fiber optic cable. Fiber sensors with high sensitivity and wide dynamic range have been available for years, but multiplexing many sensors on a single pair of fibers has been difficult. Until recently, only passive multiplexing of fiber sensors was available, with its inherent limit of approximately 10 sensors per pair of fibers, due to passive splitting and dissipative losses. A separate fiber pair for each group of 10 sensors leads to an unmanageable fiber count for a practical passive array with thousands of sensors.; The general objective of our work is to design and test new fiber array topologies that maximize the number of sensors per fiber pair while maintaining high signal-to-noise ratio and meeting all operational requirements. We discovered that multiplexing hundreds of sensors per fiber is possible by adding low-gain fiber amplifiers to compensate for the losses. The amplifiers are distributed throughout an array and pumped by a remote source. This application uses erbium-doped fiber amplifiers, borrowed from the communication industry. We demonstrated a sensor array, incorporating 20 amplifiers, that exhibits excellent signal-to-noise ratio, confirming our theory. We predict that this new design can be scaled to support 300 sensors per fiber pair, and achieve a great reduction in cost, complexity, and weight.; We also investigated a second array topology using fiber optic switches to route the signals dynamically through the array. Both a theoretical model and an experimental prototype were developed. This work shows that when practical fiber switches become available, this topology will be effective in multiplexing hundreds of sensors, and will compete directly with other telemetries. |
