Fibre optic hydrophone array - evaluation of techniqe and possibilities

A field experiment with fibre optic hydrophones was conducted at Oxdjupet Sept. 9-11 2002. The purpose was to collect experimental data suitable for evaluation of the fibre-optic hydrophone performance characteristics, especially at low frequencies. The question is, does a signature measured with a fibre-optic hydrophone have the potential to give better classification compared to a signature measured with a conventional mine sensor? In the report, we compare data from the fibre optic hydrophones measured at different ship passages with corresponding data from a dynamic pressure sensor and a reference hydrophone. Gained experiences from the use of fibre optic hydrophones in a full-scale experiment are summarized in the report. The high intensity caused by the passage of large ships gave unexpected disturbances in the signal. Basically, the fibre optic hydrophones were over-driven meaning that the used demodulating algorithm ceased to work correctly. Another disturbance, a low frequent oscillation around 2 Hz, observed in the fibre optic hydrophone recordings was not at all visible in the reference recordings. A possible explanation of these oscillations can be related to defects in the embedment of the hydrophone elements. The disturbances in the fibre optic signals have to a great extent complicated the analysis and by that the comparison with the reference sensors. Based upon these results we have not found any advantages of fibre-optic hydrophones for the use in mine sensor applications. However, fibre-optic techniques could make it possible to construct new types of hydrophones such as large, are covering sensors.