A distributed feedback (DFB) fiber laser strain sensor was implemented to measure acoustic emission induced by the hydraulic fracturing process. A study of practical sensor mounting configurations and their characteristics was carried out to find a practical solution. Combining the suitable mounting configuration and ultrahigh strain sensitivity of the DFB fiber laser, the evolution of the hydraulic fracturing process was well monitored. This study shows that fiber lasers can be useful alternatives to piezoelectric sensors in the field of hydraulic fracturing for gas and oil extraction.
Shock wave transmission and propagation in solid media is studied using fiber optic pressure and velocity probes. Shock waves are generated in two experiments using a high power laser facility as well as conventional explosives. Shock wave properties including peak overpressure, mass velocity, shock duration, impulse, arrival time and shock velocity are characterized using fiber tip interferometric displacement sensors and Fabry-Perot pressure sensors. Measurements are conducted in polymethyl methacrylate and limestone. The probes recorded shock pressures up to 0.1 GPa (1 kbar). Measurements from the fiber optic sensors are shown to be in close agreement with measurements from an electrical sensor based on a Dremin loop.
The direct interaction of an acoustic field with a planar fiber laser cantilever in a fluid is investigated.
It is shown that a sensor based on this transduction mechanism responds to acoustic particle
velocity with a responsivity around 5×104 Hz/Pa yielding a resolution of 1.6 mPa/Hz1/2 at 100Hz, which is
adequate for many underwater acoustic applications. Issues related to developing a practical, miniaturized
sensor are discussed.
Fiber laser strain sensors achieve fundamentally limited strain resolution, resulting in their ability to resolve
axial fiber displacements at the sub-femtometer level. This ultra-high resolution enables the development of
miniaturized sensors capable of achieving the performance necessary for high resolution marine sensing. The
reduction in size also facilitates an increased operating bandwidth for broadband acoustic transducers.
The last decade has seen considerable development of this technology, moving it from a laboratory curiosity to
deployable demonstrations. Significant advances have been made in understanding the fundamental properties
of erbium fiber laser strain sensors as well as laser multiplexing, signal demodulation and sensor design.
This talk will describe the basic properties of fiber laser strain sensors and show how the technical challenges
involved in developing deployable, multiplexed arrays of miniature transducers have been overcome. In particular
the development of miniature wideband hydrophones, low power DC magnetometers with sub-nT resolution and
miniature acoustic vector sensors will be described. Finally, a view towards potential future applications of this
technology will be given.
The fiber laser strain sensor promises a dramatic performance benefit impacting many high
performance sensor applications. This would lead to smaller, lower cost sensors operating over a broader
frequency range. This paper explains why.
This manuscript presents a fully engineered fiber-optic magnetic field sensor based on the
Lorentz force. The sensor comprises a gold coated optical fiber that is soldered to a current carrying aluminum
ribbon. The ribbon vibrates in the presence of a magnetic field which is detected with fiber optic
interferometry. The mechanical-Q of the mechanism is measured to be 200±10% which is greater than
previous prototype designs by a factor between 2.7 and 11.3. This yields a higher resonant responsivity
and lower thermo-mechanical noise.
A two-axis inclinometer is created by affixing a glass capillary tube to the end of a multicore fiber, forming the proof
mass of a fiber cantilever. As the sensor tilts, fiber Bragg gratings inscribed in individual cores of the multicore fiber
experience strain and are interrogated interferometrically. A phase generated carrier demodulation technique is then
used to measure the differential phases between cores. Within a measurement cycle, the instrument reports a maximum
phase excursion of ±0.55 mrad, corresponding to a curvature uncertainty of ±3.7×10-4 m-1. The experimental calibration
and performance of the sensor are presented.
It is known that the strain resolution of fibre laser sensors is determined by frequency noise of the laser sensor element.
We compare the frequency noise of a number of distributed feedback fibre laser sensors and find that all exhibit similar
dominant 1/f noise below 10kHz. Examination of current theories of thermodynamic noise and 1/f noise in fibre lasers
suggests that these lasers may be close to the theoretical limits of strain sensitivity.
An optically powered DFB fiber laser based magnetometer free from hysteresis, incorporating drift compensation is
demonstrated. Optical power is delivered through a dedicated fiber to provide the required dither current yielding an
electrically passive sensor.
The DFB fiber laser strain sensor is shown to provide an improvement in the minimum detectable length
change by a factor of 275 at 2 kHz for an applied strain when compared with an interferometric sensor interrogated by an
equivalent DFB laser. This corresponds to a strain resolution enhancement by a factor of 18, if the strain is applied over a
10cm length of fiber. The ability of the fiber laser sensor to operate in its thermodynamic noise limit is shown to be the
primary reason for this enhanced sensitivity. Thermodynamically limited noise performance is demonstrated with three
interrogation methods.
We report interferometric interrogation of fiber Bragg gratings in separate cores of a multicore fiber for high resolution quasi-static and dynamic bend measurements. Two axis curvature measurements are made by measuring the differential strain between three FBG sensors formed in a singlemode four-core fiber using a common interrogating interferometer.
Therefore a measurement of the differential phase from each FBG yields the differential strain and compensates for the common-mode random drift of the interrogating interferometer. A DC curvature accuracy of 3.4×10-3m-1, and an AC
curvature resolution of 1.2×10-4m-1 / Hz1/2 are reported.
Fiber Bragg gratings have been demonstrated as a versatile sensor for structural health monitoring. We present an
efficient and cost effective multiplexing method for fiber Bragg grating and fiber Fabry-Perot sensors based on a
broadband mode-locked fiber laser source and interferometric interrogation. The broadband, pulsed laser source permits
time and wavelength division multiplexing to be employed to achieve very high sensor counts. Interferometric
interrogation also permits high strain resolutions over large frequency ranges to be achieved. The proposed system has
the capability to interrogate several hundred fiber Bragg gratings or fiber Fabry-Perot sensors on a single fiber, whilst
achieving sub-microstrain resolution over bandwidths greater than 100 kHz. Strain resolutions of 30nε /Hz1/2 and 2
nε/Hz1/2 are demonstrated with the fiber Bragg grating and fiber Fabry-Perot sensor respectively. The fiber Fabry-Perot
sensor provides an increase in the strain resolution over the fiber Bragg grating sensor of greater than a factor of 10. The
fiber Bragg gratings are low reflectivity and could be fabricated during the fiber draw process providing a cost effective
method for array fabrication. This system would find applications in several health monitoring applications where large
sensor counts are necessary, in particular acoustic emission.
An efficient multiplexing method for fiber Bragg grating sensors based on a broadband mode-locked laser source and interferometric interrogation is described. The system has the capability to interrogate several hundred fiber Bragg grating sensors on a single fiber, whilst achieving sub-microstrain resolution over bandwidths greater than 100 kHz. A demonstration system with 8 sensors is presented, which achieves strain resolutions less than 213 nε/Hz1/2 up to 100 Hz.
The emission characteristics of a passively mode-locked erbium fiber laser for use as a source for interrogating multiplexed fiber Bragg grating sensors are reported. Specifically investigated are the temporal and spectral characteristics, optical extinction ratio, and relative intensity noise. This source is demonstrated to achieve the performance required for highresolution interferometric interrogation of time division multiplexed fiber Bragg grating sensors.
We demonstrate interrogation of a single, high performance Mach-Zehnder interferometric sensor over a total of 166 km of single-mode optical fiber, using a remotely pumped EDFA and Raman amplification. A phase resolution less than 5 μrad/Hz1/2 at 1 kHz is achieved.
Phase-shifted DFB fiber lasers (DFB-FL) can have two orthogonally polarized laser modes with a frequency offset proportional to the fiber birefringence. In order to achieve true single frequency operation one polarization mode must be extinguished. On the other hand, the existence of a polarization beat frequency (PBF) between the two states of polarization in a dual-mode DFB-FL can be utilized in a frequency-based fiber sensor. The underlying mechanisms leading to the quenching or preservation of the PBF must be controlled in either case. Twisting the DFB-FL to induce circular birefringence has been used to achieve single polarization-mode operation, though an explanation has been lacking. We present results of the reflection and transmission spectra, and the PBF, as a function of twist angle for two phase-shifted fiber gratings, and compare the results with recent theoretical calculations. We have also measured the PBF as a function of twist angle for the active case of a phase-shifted DFB-FL. Our results show that the PBF decreases monotonically as a function of twist angle for angles less than 700 degrees (over a 20 cm length), where the magnitude of the PBF signal can be suppressed by 20 dB or more. For larger twist angles, the magnitude of the PBF signal was not in general extinguished, in contrast to previously reported work. A possible explanation of such behavior will be presented in addition to a discussion of the applications of DFB-FLs as sensors.
Fiber optic sensors are becoming a well-established technology for a range of geophysical applications, and static pressure and temperature sensors in particular are now comparatively well developed. However, rather less attention has been paid to systems for measuring dynamic quantities such as acoustic and seismic signals. Furthermore, the very large multiplexing potential of fiber optic sensing systems has yet to be fully explored for geophysical applications. However, development of fiber optic sonar systems for military applications has proven the viability of large multiplexed arrays, and demonstrated advantages which include electrically passive arrays, long term reliability and the potential for operation in very deep ($GTR3000m) water. This paper describes the applications for large scale fiber optic sensing arrays in geophysical metrology. The main applications considered here are ocean bottom cables and streamers for marine seismic, and downwell seismic systems. Systems can require up to several thousand channels and the use of multi- component sensors, which include 3-axis geophones and hydrophones. The paper discusses the specific requirements for each application, and shows how these requirements can be met using a system approach based on time and wavelength multiplexing of interferometric sensors. Experimental and theoretical studies at DERA into the performance of highly multiplexed systems are also described, together with initial development work on fiber optic hydrophones and geophones.
We investigate the performance of a large-scale array of interferometric sensors using time and dense wavelength division multiplexing. By consideration of the noise sources in the system and the bandwidths available for the signals to occupy we have calculated the signal resolution and dynamic range expected from a single sensor. A discussion on the potential applications for these arrays is also given.
Optical hydrophone arrays have reached a sufficiently advanced stage of development that prototype systems have been successfully deployed at sea, however, one of the remaining areas of development required for a full scale system is multiplexing gain. Many different multiplexing schemes have been proposed based on techniques such as time, code, frequency, coherence and wavelength division multiplexing which typically limit the number of sensors addressable to around 60. Described here is a demonstration interferometric hydrophone array using time and dense wavelength division multiplexing. The system uses a 3 wavelength fiber laser source, with a wavelength spacing of 1.6 nm, in a configuration where two of the wavelengths each interrogate a pulsed reflectometric array module with up to thirty-two time division multiplexed hydrophones. The fiber architecture uses optical add/drop multiplexers (OADM) to drop wavelengths from a telemetry fiber,and launch them into an array module and the return signals are re-combined with the telemetry fiber. The received optical signals are separated, detected and processed separately. The experiment demonstrates the potential to address in excess of 96 sensors through a single fiber pair with 32 hydrophones per wavelength. The measured level of array-sensor crosstalk is shown to be less than 72 dB.
This paper describes the design and development of an optical hydrophone array for a seabed application. Optical hydrophones have been under development in a number of countries [1] as alternatives to piezoelectric based sensors, and the technology has now reached the stage where arrays with significant numbers of sensors can be constructed and deployed in realistic (and frequently hostile) environments.
The use of Bragg gratings for measurement of pressure, strain and temperature has been the subject of considerable research over the last decade. Techniques for temperature/strain discrimination are summarised in Reference 1 and often involve either a specialised interrogation system or a dual grating approach
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