Emerging underwater optical imaging and sensing applications rely on phase-sensitive detection to provide added functionality and improved sensitivity. However, underwater turbulence introduces spatio-temporal variations in the refractive index of water which can degrade the performance of these systems. Although the influence of turbulence on traditional, non-interferometric imaging has been investigated, its influence on the optical phase remains poorly understood. Nonetheless, a thorough understanding of the spatio-temporal dynamics of the optical phase of light passing through underwater turbulence are crucial to the design of phase-sensitive imaging and sensing systems. To address this concern, we combined underwater imaging with high speed holography to provide a calibrated characterization of the effects of turbulence on the optical phase. By measuring the modulation transfer function of an underwater imaging system, we were able to calibrate varying levels of optical turbulence intensity using the Simple Underwater Imaging Model (SUIM). We then used high speed holography to measure the temporal dynamics of the optical phase of light passing through varying levels of turbulence. Using this method, we measured the variance in the amplitude and phase of the beam, the temporal correlation of the optical phase, and recorded the turbulence induced phase noise as a function of frequency. By bench marking the effects of varying levels of turbulence on the optical phase, this work provides a basis to evaluate the real-world potential of emerging underwater interferometric sensing modalities.
Fiber-optic interferometric acoustic sensors were first proposed for US Navy applications 36 years ago. This paper will review the origin, development and deployment of these sensors. Future applications will also be discussed.
Under the Composite Hull Embedded Sensor System (CHESS) program, which is a co-operative program run jointly by FFI and NRL, a drop test with a composite sandwich plate has been performed [1]. The plate was designed to have similar mechanical properties to a bottom panel in the Norwegian Navy’s new fast patrol boat (MTB).
Alan Kersey, Michael Davis, Timothy Berkoff, Anthony Dandridge, R. Jones, Tsung-Ein Tsai, Gary Cogdell, Gunnar Wang, G. Havsgaard, Karianne Pran, Sverre Knudsen
We describe strain data recorded using fiber optic Bragg grating sensors mounted on the hull of a GRP composite ship. Twelve gratings were attached to the structure, in three arrays of four elements. The electro-optic system used was able to monitor a single set of four elements at a time. The preliminary results indicate the usefulness of distributed fiber Bragg grating sensor systems for monitoring transient loading events on such structures.
We briefly review the theory of fundamental thermal induced phase fluctuations in the fiber- loop of a Sagnac interferometer and compare it to novel experimental data. Loop lengths of 1.1 and 2.2 km are considered, showing phase noise levels between approximately 0.2 - 1.4 (mu) rad rms/(root)Hz in the frequency range 1 - 100 kHz. Excellent agreement between theory and experiment is demonstrated, showing that this noise source can have considerable impact on the threshold detection of a Sagnac interferometer based sensor for both detection of reciprocal and non-reciprocal measurands, using practical system parameters. We discuss the impact of this noise source when the Sagnac interferometer is configured for rotation sensing, and show the dependence of the random walk coefficient when the bias frequency and system parameters are varied.
An array of eight fiber optic vector magnetometers was designed and constructed for use in undersea applications such as harbors, ports, and waterways. We describe the design and performance characteristics of the array including the single-mode fiber optic system and magnetostrictive transducers. Data is presented from both laboratory measurements and from actual undersea operation.
We describe the design and performance of a novel planar ultrasonic fiber sensor for
detection of air acoustic signals. The sensor is formed from a potted single layer of spirally wound
optical fiber, and configured such that the static backing pressure can be varied. Experimentally we
demonstrate normalized sensitivities (?Ø /Ø?P) of-295 to .265dB re. 1 µPa1.
The design and performance of single element fiber optic hydrophones for shallow water applications is described. Hydrophone responsivities up to -113.8 dB re rad/?Pa were measured while maintaining a usable depth capability. A system with this design of hydrophone is being used to monitor the deterioration of water carrying siphons in Arizona.
Laboratory and field tests have proven that magnetostrictive fiber-optic
interferometric sensors can detect small low-frequency disturbances of the earth magnetic
field [1]. With proper sensor design, the thermal noise limitation [2] does not rise above the
interferometer baseband noise of around 0.3 µrad?Hz.
We demonstrate the feasibility of optically demodulating and processing the output produced by an
interferometric fiber sensor. A technique for providing the time filtering of an optical signal in the
optical domain based on the use of a doped fiber section is reported.
Optical processing is a field which offers enormous potential for rapid, parallel processing of
information, and is an area which will continue to experience significant growth over the coming
years. Optical sensors have a natural inherent compatibility with this form of processing, which
could be utilized to great advantage for processesing the data from large arrays of sensors. Many
forms of fiber optic sensor, however, produce an optical output which does not vary in a uniform
fashion with the measurand field, and consequently require a linearization circuit. or 'demodulator'
to provide a linear transfer function before a signal could be converted back to the optical domain
and operated on by an optical processor. This is particularly true with interferometric fiber sensors
which exhibit a cosine squared transfer function. To demodulate this form of output, a range of
demodulation schemes have been developed, such as active phase tracking homodyne [1], phase
generated carrier (PGC) passive homodyne [2], and synthetic heterodyne [3J. These techniques
allow the detection of measurand-induced phase shifts down to the irad/'JHz level over a wide range
of frequencies with high linearity and wide dynamic range. Certain forms of interferometric sensors
designed for low frequency measurands, such as magnetometers and electhc field sensors, require
the additional demodulation of a 'dither' induced phase carrier signal which is modulated by the DC
and low frequency measurand information. In such systems, the overall demodulation process
requires several electronic processing blocks after photodetection of the interferometer signal. In
applications where arrays of such sensors are of interest, a significant amount of electronics may be
required to fully demodulate the sensor outputs. We have examined the practicality of using optical
techniques to perform much of the demodulation process, and in this paper demonstrate the
feasibility of this concept.
The performance of fiber optic interferometers constructed with 3x3 couplers is investigated.
Deviations from ideal behavior are noted at low fringe visibilities. The use of Faraday
mirrors in a 3x3 Michelson configuration is shown to eliminate these effects.
Two fiber optic pressure gradient hydrophones have been designed, constructed
and tested. These devices both exhibit good sensitivity and appropriate directionality.
Their responsivities are linear in the 100 Hz to 1000 Hz frequency band.
Frank Bucholtz, Carl Villarruel, Clay Kirkendall, Dominique Dagenais, J. McVicker, A. Davis, S. Patrick, Kee Koo, Anthony Dandridge, Gunnar Wang, Terje Lund, H. Valo
The 1/r3 dependence of the magnetic field with distance r makes the vector magnetometer an ideal sensor in undersea systems for the localization and tracking of vessels. Since the undersea system typically spans several hundred meters to several kilometers, use of fiber optics for both telemetry and sensing offers inherent advantages of low cable weight, immunity to electromagnetic interference on the signal transmission paths, and high channel capability.
A number of the recent developments in the field of fiber optic interferometric sensing will be discussed. The major areas in which these developments occurred will be briefly reviewed.
We demonstrate measurements of fundamental thermal induced phase
fluctuations in the fiber of a Sagnac interferometer. Loop lengths of 1.1 and 2.2 km are
considered, showing phase noise levels between ~0.2-1.4/?rad rms/?HZ in the 1-100 kHz frequency range.
Real world operational environments place stringent requirements upon the
fidelity of the demodulator output to the acoustic signal which is present. This paper
discusses the optimization of a demodulator circuit for hydrophones in time-varying
hydrostatic pressure environments as well as in environments which contain large
amplitude acoustic signals.
Passive birefringence compensation using Faraday rotator and mirror combinations in Michelson interferometer systems has been shown to produce an interferometric output signal which is free of the effects of polarization induced fading. This technique promises to have a strong impact on the development of practical interferometric sensor systems, and is particularly important in the area of multiplexed sensors where the problem of polarization fading in conventional arrays is a serious technical issue. In this paper we describe various topologies for implementing passive birefringence compensation to achieve polarization independent operation in multiplexed sensor arrays. Results obtained on a time-division multiplexed system with 4 sensor elements are presented.
The advantages of networking fiber sensors using optical telemetry techniques to form all-optical arrays of sensor-elements are well recognized. Interest in this general area began in the early 1980s, and over the past six years at NRL we have attempted to experimentally and theoretically characterize various different array configurations for interferometric sensor arrays based on frequency-division, time-division and coherence multiplexing schemes. Recently a number of tests by the U.S. Navy have successfully demonstrated the operation of multiplexed sensors at sea. Work has also been conducted on other novel multiplexing schemes for non-linear phase transduction based sensors and intensity-modulation based schemes. This paper will review the progress made in this area in recent years, and discuss some aspects of current research interest.
The results are presented of the first comprehensive experimental investigation of the effects of the state of polarization (SOP) in the common input of the multielement array of sensors on the interferometric signal visibilities of the individual sensors in the array. In one case of a 10-sensor array with randomly set initial visibilities, an empirical determination of the optimum input SOPs is shown to maintain the visibilities of all sensors larger than 0.55, or signal fade less than 6 dB. A theoretical model is developed which predicts the minimum visibility obtained from controlling the input SOP in N-element arrays. The results agree very well with experimental results obtained from a 10-sensor array, including the worst case situation in which all sensors are initially set to the complete polarization-fade condition.
Two displacement sensing techniques based on determination of the phase shift induced in an unbalanced interferometer due to laser frequency modulation are described. Experimental results demonstrating the operation of the schemes are presented; optical displacements over a 50 mm range have been recorded with a resolution of 25 microns. Both approaches are suitable for passive remote sensing applications. The authors also discuss the use of this type of sensing approach for the measurement of 'D.C.' parameters, such as quasi-static pressure and structural strain.
The issue of excess14 noise is of interest with respect to the
broadband optical sources commonly used in interferometric fiber optic
gyroscopes because it can limit the ultimate sensitivity of the device.
Excess noise has been studied in superluminescent diodes (SLD's) by Yurek,
et .al. 1 and in an Erbium doped fiber by Morkel, et.al.2 . The latter group
also presented a simple model for the excess noise and demonstrated a
quantitative fit with the linewidth of their source.
In this letter we report measurements of excess noise in three
potential fiber gyroscope sources, SLD's at 0.83 pm and 1.3 .tm and a
superfluorescent Nd doped fiber at 1 .06 pm. These noise measure-ments
are shown to be in good agreement with the model of Ref. 2. The model
for excess noise is used to calculate the random walk coefficient due to
shot and excess noise in a interferometric fiber gyro to demonstrate the
impact excess noise in these sources will have on such a gyroscope. We
show that the gyros utilizing SLD sources are just barely impacted by
excess noise due to their limited output power (1-3 mW in a single mode
fiber). The fiber source at 1 .06 tim, with its higher potential output power,
will likely be limited by excess noise.
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