Philip John Nash

Large-scale remotely interrogated arrays of fiber-optic interferometric sensors for underwater acoustic applications

The fiber-optic interferometric acoustic sensor array has established itself as a potential alternative to the conventional sonar array based on electroceramic transducers. In this paper, we discuss all the aspects of a large-scale fiber-optic interferometric sensor array. We review the basic operating principles of the fiber-optic interferometric sensor, signal processing, and multiplexing techniques, we present results from a noise model for a full size system, and we determine the benefit of incorporating a remotely-pumped optical amplifier in the array. As a practical example we describe the design and construction of a prototype array with 96 hydrophones incorporating a remotely pumped erbium-doped fiber amplifier, called the fiber-optic bottom mounted array, which is based on a dense wavelength division and time division multiplexed architecture. These arrays have applications in military sonar and seismic surveying.

Large-scale multiplexing of interferometric fiber-optic sensors using TDM and DWDM

This paper describes multiplexing schemes for interferometric fiber sensors based on time-division multiplexed and dense wavelength-division multiplexing using optical add/drop multiplexers. The results of an experimental arrangement, which is based on one of the architectures, is also presented. Topics include a discussion of the noise sources in the system, dynamic range, and a characterization of the distributed feedback fiber laser source noise. We show the crosstalk levels in the experimental arrangement to be between -47 and -76 dB depending on the mechanism involved. The multiplexing schemes demonstrate the potential to address at least 192 interferometric sensors through two fibers based on a system with six wavelengths with a phase resolution less than 20 /spl mu/rad//spl radic/Hz. For application to sonar arrays, our analysis has shown that hydrophones multiplexed in this type of architecture would achieve ambient acoustic noise-limited pressure resolution with an in-water dynamic range up to 135 dB at frequencies up to 10 kHz. In general, these architectures would find application in systems requiring very large numbers of sensors with a minimum of telemetry cabling required.

High-responsivity fiber-optic flexural disk accelerometers

This paper presents performance measurements of fiber-optic flexural disk accelerometers. The flexural disk acts as a mass-spring element to which the fiber is bonded, such that an acceleration causes a strain to be imposed on the fiber which is measured interferometrically. Simple analytical models have been written to calculate the responsivity and resonant frequency of disks under various boundary conditions and the results of the models have been shown to be in good agreement with the measured responsivity for the case of moderately thick disks. Six optical fiber accelerometers based on flexural disks of different thickness and supports have been demonstrated to exhibit a responsivity in the range from 28 to 39 dB re 1 rad/g with a resonant frequency between 2.4 kHz to greater than 5 kHz, respectively. Of the designs considered, the centrally supported disk is shown to give the highest combination of responsivity and bandwidth. A centrally supported disk has been demonstrated to exhibit a flat response up to 2 kHz and a responsivity of 37 dB re 1 rad/g which when combined with an interferometric phase resolution of 6 /spl mu/rad//spl radic/Hz, would give a minimum detectable acceleration of 84 ng//spl radic/Hz. We have attempted to cover all aspects of the sensor design including responsivity, bandwidth, cross-responsivity, phase response and size and find that a complicated compromise between all of these design parameters is required to achieve the optimum performance.

Acoustic performance of a large-aperture, seabed, fiber-optic hydrophone array

A large-aperture, seabed mounted, fiber-optic hydrophone array has been constructed and characterized. The system is designed for use as a large area surveillance array for deployment in shallow water regions. The underwater portion comprises two arrays of 48 hydrophones separated by a 3 km fiber-optic link, which are connected to a shore station by 40 km of single-mode optical fiber. The hydrophone is based on a fiber-optic Michelson interferometer and the acoustic transduction mechanism is a fiber-wrapped mandrel design. No electrical power is required in the underwater portion. The performance of the system is described, characterized during laboratory measurements and during a recent sea trial. Specifically, measurements of the acoustic resolution, array shape, beam patterns, array gain, and target tracking capability of this array. The system demonstrates self-noise levels up to 20 dB (typically 10 dB) lower than the ambient acoustic noise experienced in the sea trial and array gains close to the the...

Measurement of sensor axis misalignment in fibre-optic accelerometers

A method is described for the measurement of sensor axis misalignment relative to its mounting can for a fibre-optic accelerometer. The accelerometers investigated were based on the common cylindrical compliant mandrel design and mounted accelerometers showed typical angular misalignments of 2°. The influence of the misalignment on cross-axis sensitivity is also described for accelerometers orthogonally mounted in a three-component package.

Highly Scalable Amplified Hybrid TDM/DWDM Array Architecture for Interferometric Fiber-Optic Sensor Systems

We present a distributed amplified hybrid dense wavelength division multiplexing (DWDM) and time division multiplexing (TDM) array architecture for large scale interferometric fiber-optic sensor array systems. This architecture employs a distributed Erbium doped fiber amplifier (EDFA) scheme to decrease the distribution loss among multiplexed wavelengths, and employs TDM at each wavelength to increase the total number of sensors that can be supported. The first experimental demonstration of this system is reported including results which show the potential for multiplexing and interrogating up to 4096 sensors using a single telemetry fiber pair with good system performance. The number of interrogation sensors could be further increased by increasing the number of wavelength channels. These architectures would be of great importance in the application of systems requiring very large number of sensors with limited telemetry cabling.

Design, development and construction of fibre-optic bottom mounted array

We report the development of a novel seabed hydrophone array, the Fibre Optic Bottom Mounted Array (FOBMA) which uses a combination of time and dense wavelength division multiplexing (TDM/DWDM) to achieve 96 hydrophone channels, in an architecture which can readily be expanded to much higher channel counts. The array also makes use of remotely pumped optical amplification techniques to increase the standoff between the array and interrogation system. The FOBMA system has been developed as a collaboration between the Naval Research Laboratory and QinetiQ (formally DERA), and has tackled all aspects of the system development, from system architecture studies through electronic system design, sensor design, array mechanical design, array construction and array acoustic testing.

High performance architecture design for large scale fibre-optic sensor arrays using distributed EDFAs and hybrid TDM/DWDM

A distributed amplified dense wavelength division multiplexing (DWDM) array architecture is presented for interferometric fibre-optic sensor array systems. This architecture employs a distributed erbium-doped fibre amplifier (EDFA) scheme to decrease the array insertion loss, and employs time division multiplexing (TDM) at each wavelength to increase the number of sensors that can be supported. The first experimental demonstration of this system is reported including results which show the potential for multiplexing and interrogating up to 4096 sensors using a single telemetry fibre pair with good system performance. The number can be increased to 8192 by using dual pump sources.

High performance fibre-optic acoustic sensor array using a distributed EDFA and hybrid TDM/DWDM, scalable to 4096 sensors

An amplified 16 channel dense wavelength division multiplexing (DWDM) array architecture is presented for interferometric fibre optic sensor array systems. This architecture employs a distributed Erbium doped fibre amplifier (EDFA) scheme to decrease the array insertion loss, and employs time division multiplexing (TDM) at each wavelength to increase the number of sensors that can be supported. The first experimental demonstration of this system is reported including results which show the potential for multiplexing and interrogating up to 4096 sensors using a single telemetry fibre pair with good system performance.