Fiber Optic Array Scans the Ocean for Submarines and Oil Wells

Breck Hitz

A group of researchers from the US Naval Research Laboratory in Washington and from QinetiQ in Dorchester, UK, has designed and demonstrated an array of fiber optic sensors that can detect weak underwater acoustic signals across a spectrum of frequencies. These signals could be generated by submarines or other vessels, or they could be the return signals of an active sonar system.

Sonar systems often are employed in the oil and gas industry to survey the geophysical properties of the ocean floor in search of undiscovered petroleum deposits. The new fiber optic design has several advantages over conventional piezo-ceramic sensors, including smaller size, better reliability and imperviousness to electromagnetic interference.

The array, which is 40 km from the interrogating optics package, comprises six subarrays, each operating at a separate wavelength (Figure 1). In the interrogating optics package, six fiber lasers generate signal pulses at approximately 150 kHz to interrogate each of the subarrays.

Figure 1. Six wavelengths are generated in the interrogating optics package and multiplexed onto a single fiber. In the sensor array, each wavelength is demultiplexed and sent to a separate subarray.

The six wavelengths are multiplexed onto a 40-km-long fiber and sent to the array, where drop multiplexers sequentially pick off each wavelength and send it to the subarrays. On the other side of the subarrays, an add multiplexer combines the signal from each subarray onto a common fiber.

It's frequently desirable to detect acoustic signals at different locations. The distance to a source, for example, can be calculated by observing the time the same signal is detected at two locations. To allow for two different listening "nodes," the scientists separated three subarrays from the others with 3 km of fiber.

To boost the weak signal from the sensor array to the strength it needs for the 40-km trip back to the interrogating optics package, an erbium-doped fiber amplifier (EDFA) is included in the array package. The EDFA is pumped through a third fiber linking the array and the interrogating optics package.

Within each of the subarrays are 16 in-line fiber optic Michelson interferometers, each of which comprises a coupler at the left side of the figure (analogous to the beamsplitter in a free-space Michelson) and two reflective directional couplers (analogous to the two mirrors in a free-space Michelson) (Figure 2). The small coils in the figure represent 80 m of fiber, so the signals from each of the 16 interferometers are temporally multiplexed, separated from each other by the time it takes the signal to travel 160 m. Each coil is wrapped around an air-backed mandrel hydrophone that has a flat frequency response over the acoustic band of interest. Altogether, the array consists of 96 (6 X 16) independent acoustic channels.

Figure 2. The signals from 16 fiber optic Michelson interferometers in each subarray are temporally multiplexed and returned to the interrogating optics package.

To test the array, the researchers substituted piezo-ceramic cylinders for the hydrophones in each Michelson interferometer so that they could simulate the acoustic signals received. They found that the system's inherent noise was 10 dB less than the expected ocean background noise and that crosstalk between the sensors was from –39 to –66 dB, depending on the relative location of the sensors.