Fiber-Optic Bottom-Mounted Array

C.K. Kirkendall and G.A. Cranch
Optical Sciences Division

The Fiber-Optic Bottom-Mounted Array Program: The Fiber-Optic Bottom-Mounted Array (FOBMA) program has advanced the underlying technology for large-area, electrically passive, lightweight, seabed fiber-optic hydrophone arrays. We have developed a high dynamic range digital demodulator system and have incorporated three new optical technologies into the existing fiber-optic sensing technology base. These are: dense wavelength division multiplexing (DWDM), remote optical amplification, and high-performance fiber laser sources. The use of DWDM allows large numbers of hydrophones to be multiplexed onto a single optical fiber, while the use of remote amplification increases the passive array span by several tens of kilometers. Our prototype system, comprising 96 hydrophones interrogated through a 40-km fiber link, has been successfully tested in a sea trial off the coast of Nova Scotia, and demonstrated excellent acoustic performance and target tracking capability. This program is a collaboration between the Naval Research Laboratory and the Defence Science and Technology Laboratory (DSTL) sponsored by the U.S. Navy International Program Office and the U.K. Ministry of Defence.

Project Summary: Fiber-optic hydrophone arrays have been under development at NRL since the late 1970s. This technology has reached a level of maturity such that prototype systems comprising around 100 sensors have been successfully demonstrated in realistic sea trials.¹ The continued interest of the U.S. Navy in developing remote, electrically passive underwater acoustic surveillance arrays for littoral water applications has further driven the development of these systems. Figure 1 shows the deployment configuration of our system. Systems of this type can be deployed in either a vertical or horizontal configuration; our system is designed for horizontal deployment (i.e., a seabed array). The array is linked to the optoelectronic interrogation unit located either on a surface ship or at a shore station.

The underlying sensing mechanism uses fiber-optic interferometry to measure the acoustically induced strain in a fiber-wrapped mandrel hydrophone. Multiple sensors are interrogated sequentially using time division multiplexing (TDM), resulting in up to 64 sensors multiplexed per wavelength. Wavelength division multiplexing can then be added to increase the number of TDM signals carried by a single fiber. One aim of this project was to incorporate three new optical technologies into the existing fiber-optic sensor TDM technology base. These are DWDM, remotely pumped erbium-doped fiber amplifier (RPEDFA), and high-performance erbium fiber laser sources. Incorporating DWDM technology increases the number of sensors that can be interrogated over a single fiber by a factor equal to the number of wavelengths used. RPEDFA technology is incorporated to increase the distance between the optoelectronic interrogation system and the array. Finally, fiber laser technology demonstrates a potentially low-cost, high-performance laser source, compatible with DWDM technology.

Figure 2 shows the system architecture. The launch optics comprises six erbium-doped distributed feedback fiber lasers multiplexed onto a single fiber in the multiplexer (MUX). The output of the lasers is modulated using acousto-optic modulators in a compensating interferometer and amplified with a power EDFA. Attenuators (ATTEN) are included to allow pre-emphasis of the signal powers and control the launch power. The receive opto-electronics contains the wavelength demultiplexers, optical preamplifier, RPEDFA pump diode, and the digital demodulators. The digital demodulator incorporates a bi-cell polarization diversity receiver (PDR) to alleviate polarization-induced signal fading. While the demodulator supports > 60 kHz of acoustic bandwidth, the majority of the recorded acoustic data was band-limited to ~700 Hz to conserve storage space. A personal computer stores the acoustic data from the array. The RPEDFA pump power is delivered through a separate fiber. Thus, three fibers are required to interrogate the array.

FIGURE 2
FOBMA system architecture.

The FOBMA system is designed to be directly scalable to a full-size system and comprises two nodes of 48 hydrophones (total 96) separated by a 3-km fiber-optic cable. Each wavelength interrogates 16 TDM sensors (expandable to 64) arranged in the architecture described in Ref. 2. The RPEDFA is in-corporated into the first node and provides ~20 dB of small-signal gain per wavelength. A 5-km fiber-optic cable connects the first node to the opto-electronics at a shore station. A 40-km link was simulated by adding bare fiber coils located with the optoelectronics.

Results and Conclusion: The array was deployed for 9 days at a depth of ~50 m off the coast of Halifax, Nova Scotia. Around 40 Gbytes of data were recorded over this period from various targets, both real and simulated. Figure 3 is an example of the target tracking capability of one of the arrays in this system. The target was a 450 Hz tone generated by a towed acoustic source. The image shows true bearing relative to the center of a 48-hydrophone array vs time. Color indicates sound pressure level. Curvature in the array, induced during the deployment process, allows the left-right ambiguity in the response of the line array to be resolved (the sensor locations were determined using a chirp-based element location measurement conducted shortly after deployment). The actual track of the towed source can be identified to be along the narrower track following a bearing of ~150°. The tone is visible to a range of ~9 km. Figure 3 was generated using a shape-corrected conventional beamformer. These data were recorded with a total standoff distance of 40 km. However, standoff distances in excess of 50 km are possible, depending on the array size and required acoustic performance.³ This technology enhances the U.S. Navy's capabilities in littoral water antisubmarine warfare and large-area surveillance.

Acknowledgments: NRL was responsible for developing the optoelectronics system and data recorder, and a number of people made significant contributions to this program. These are G. Cogdell and A. Dandridge of NRL and A. Bautista, K. Daley, S. Motley, and J. Salzano of SFA, MD. QinetiQ, UK, was responsible for developing the RPEDFA and underwater array portion and carried out the target tracking processing. Cogent Defense Systems, UK, designed and constructed the underwater array. The sea trial was part of the international Rapidly Deployable Sensors 4 trial.

[Sponsored by ONR and Navy IPO]

References

¹ C.K. Kirkendall, A.R. Davis, A. Dandridge, and A.D. Kersey, "64-channel All-optical Deployable Acoustic Array," NRL Review, 1997, 63-65.
² G.A. Cranch and P.J. Nash, "Large-scale Multiplexing of Interferometric Fiber-optic Sensors using TDM and DWDM," J. Light. Tech. 19(5), 687-699 (2001).
³ G.A. Cranch, P.J. Nash, and C.K. Kirkendall, "Large-scale Remotely Interrogated Arrays of Fiber-optic Interferometric Sensors for Underwater Acoustic Applications," to be published in IEEE Sensors Journal special edition on Fiber Optic Sensors, February 2003.