High multiplexing gain using TDM and WDM in interferometric sensor arrays

Optical hydrophone arrays have reached a sufficiently advanced stage of development that prototype systems have been successfully deployed at sea, however, one of the remaining areas of development required for a full scale system is multiplexing gain. Many different multiplexing schemes have been proposed based on techniques such as time, code, frequency, coherence and wavelength division multiplexing which typically limit the number of sensors addressable to around 60. Described here is a demonstration interferometric hydrophone array using time and dense wavelength division multiplexing. The system uses a 3 wavelength fiber laser source, with a wavelength spacing of 1.6 nm, in a configuration where two of the wavelengths each interrogate a pulsed reflectometric array module with up to thirty-two time division multiplexed hydrophones. The fiber architecture uses optical add/drop multiplexers (OADM) to drop wavelengths from a telemetry fiber,and launch them into an array module and the return signals are re-combined with the telemetry fiber. The received optical signals are separated, detected and processed separately. The experiment demonstrates the potential to address in excess of 96 sensors through a single fiber pair with 32 hydrophones per wavelength. The measured level of array-sensor crosstalk is shown to be less than 72 dB.