Distributed Fiber Ultrasonic Sensor and Pattern Recognition Analytics

Ultrasound interrogation and structural health monitoring technologies have found a wide array of applications in the health care, aerospace, automobile, and energy sectors. To achieve high spatial resolution, large array electrical transducers have been used in these applications to harness sufficient data for both monitoring and diagnoses. Electronic-based sensors have been the standard technology for ultrasonic detection, which are often expensive and cumbersome for use in large scale deployments. Fiber optical sensors have advantageous characteristics of smaller cross-sectional area, humidity-resistance, immunity to electromagnetic interference, as well as compatibility with telemetry and telecommunications applications, which make them attractive alternatives for use as ultrasonic sensors. A unique trait of fiber sensors is its ability to perform distributed acoustic measurements to achieve high spatial resolution detection using a single fiber. Using ultrafast laser direct-writing techniques, nano-reflectors can be induced inside fiber cores to drastically improve the signal-to-noise ratio of distributed fiber sensors. This dissertation explores the applications of laser-fabricated nano-reflectors in optical fiber cores for both multi-point intrinsic Fabry–Perot (FP) interferometer sensors and a distributed phase-sensitive optical time-domain reflectometry (φ-OTDR) to be used in ultrasound detection. Multi-point intrinsic FP interferometer was based on swept-frequency interferometry with optoelectronic phase-locked loop that interrogated cascaded FP cavities to obtain ultrasound patterns. The ultrasound was demodulated through reassigned short time Fourier transform incorporating with maximum-energy ridges tracking. With tens of centimeters cavity length, this approach achieved 20kHz ultrasound detection that was finesse-insensitive, noise-free, high-sensitivity and multiplex-scalability. The use of φ-OTDR with enhanced Rayleigh backscattering compensated the deficiencies of low inherent signal-to-noise ratio (SNR). The dynamic strain between two adjacent nano-reflectors was extracted by using 3×3 coupler demodulation within Michelson interferometer. With an improvement of over 35 dB SNR, this was adequate for the recognition of the subtle differences in signals, such as footstep of human locomotion and abnormal acoustic echoes from pipeline corrosion. With the help of artificial intelligence in pattern recognition, high accuracy of events’ identification can be achieved in perimeter security and structural health monitoring, with further potential that can be harnessed using unsurprised learning

Development of a multi-point temperature fiber sensor based on a serial array of optical fiber interferometers

M.Ing. (Electrical and Electronic Engineering)An experimental study of a multi-point optic fibre sensor for monitoring temperature changes is presented. The multi-point optic fibre sensor is made of a serial array of weak-reflectivity identical gratings. The weak-reflectivity identical gratings form the interferometric cavities UV printed on the single mode fibre. The ability to measure temperatures changes at different cavities along the serial array is particularly interesting for the monitoring of power transformers, high temperature furnaces and jet engines. Changes in temperature in each respective cavity is measured based on the spectral shift in the phase of the light from each respective cavity. The performance of the multi-point fibre sensor system is evaluated. Further, a theoretical and experimental investigation of a serial array composed of two cavities of different lengths is conducted. This investigation is aimed at measuring the impact of the overlap of the two distinct cavities in their respective frequency domain and determining the accuracy of the measurement. The result found shows that the sensor phase response is no more linear to temperature changes. It is also found that the nonlinear response of the sensor to temperature changes increases with the magnitude of the overlap

Advanced Interrogation of Fiber-Optic Bragg Grating and Fabry-Perot Sensors with KLT Analysis

The Karhunen-Loeve Transform (KLT) is applied to accurate detection of optical fiber sensors in the spectral domain. By processing an optical spectrum, although coarsely sampled, through the KLT, and subsequently processing the obtained eigenvalues, it is possible to decode a plurality of optical sensor results. The KLT returns higher accuracy than other demodulation techniques, despite coarse sampling, and exhibits higher resilience to noise. Three case studies of KLT-based processing are presented, representing most of the current challenges in optical fiber sensing: (1) demodulation of individual sensors, such as Fiber Bragg Gratings (FBGs) and Fabry-Perot Interferometers (FPIs); (2) demodulation of dual (FBG/FPI) sensors; (3) application of reverse KLT to isolate different sensors operating on the same spectrum. A simulative outline is provided to demonstrate the KLT operation and estimate performance; a brief experimental section is also provided to validate accurate FBG and FPI decoding

Detection of Magnetic Fields Using Fibre Optic Interferometric Sensors.

The principle aim of the work described in this thesis is to determine a suitable optical detection system for d.c. and low frequency magnetic fields which are likely to be encountered in practical magnetometer applications. To construct a sensitive magneotmeter one arm of an optical fibre Mach-Zehnder interferometer has been magnetically sensitised using a magnetostrictive material. Since the signal frequency range of interest was in the region of 0.01 to 10Hz, clearly the signal was in the same frequency band as the environmental noise associated with ambient temperature and pressure variations. Initially, a technique was developed to measure the magnetic field from the shift of the total fringe pattern generated by a modified Mach-Zehnder interferometer and a minimum detectable magnetic field of 10e-7 tesla.m. was obtained. This minimum detectable magnetic field has been improved by a number of modifications. A technique has been developed which utilises an a.c. bias field to put the magnetic signal on a carrier so that it can be measured at a frequency where the amplitude of the interferometer 1/f noise is much reduced. To maintain maximum interferometric sensitivity to this signal active homodyne demodulation techniques have been developed to maintain the interferometer at quadrature by compensating for the environmental noise. A minimum detectable magnetic field of 5x10e-10 tesla.m. has been achieved with this system. As an alternative to the Mach-Zehnder interferometer a Fabry-Perot interferometer, which utilises multiple-beam interference, has been considered. This type of interferometer consists of a single fibre with high reflectivity coatings on its ends. Such an interferometer has been used as a sensor and as an external cavity in laser frequency stabilisation scheme

Solid-state lasers for coherent communication and remote sensing

Semiconductor-diode laser-pumped solid-state lasers have properties that are superior to other lasers for the applications of coherent communication and remote sensing. These properties include efficiency, reliability, stability, and capability to be scaled to higher powers. We have demonstrated that an optical phase-locked loop can be used to lock the frequency of two diode-pumped 1.06 micron Nd:YAG lasers to levels required for coherent communication. Monolithic nonplanar ring oscillators constructed from solid pieces of the laser material provide better than 10 kHz frequency stability over 0.1 sec intervals. We have used active feedback stabilization of the cavity length of these lasers to demonstrate 0.3 Hz frequency stabilization relative to a reference cavity. We have performed experiments and analysis to show that optical parametric oscillators (OPO's) reproduce the frequency stability of the pump laser in outputs that can be tuned to arbitrary wavelengths. Another measurement performed in this program has demonstrated the sub-shot-noise character of correlations of the fluctuations in the twin output of OPO's. Measurements of nonlinear optical coefficients by phase-matched second harmonic generation are helping to resolve inconsistency in these important parameters

Techniques in Laser Interferometry for the Detection of Gravitational Radiation

General relativity leads to the expectation of travelling gravitational waves which manifest themselves as strains in space. Massive relativistic systems are the only sources of gravitational radiation which are expected to be detectable on earth. A range of such sources is reviewed in the first chapter in order to estimate the likely gravitational wave amplitudes at the earth. It is seen that in the frequency range accessible to ground-based detectors (tens of hertz to several kilohertz) the expected strain amplitudes are at most ~ 10 -21. At present the most promising technique for the detection of gravitational radiation is laser interferometry between nearly-free test masses. The signal and noise properties of the simplest configuration of this system are considered to show that the performance can approach that required to detect the predicted sources of gravitational radiation. In order to reach this level of performance more advanced optical techniques are needed and these are discussed in Chapter 2. The lasers used to illuminate the interferometers have excess intensity noise at low frequencies and this forces all measurements to be made using high frequency (at least several MHz) modulation techniques. Two modulation and signal recovery schemes for laser interferometers (internal and external modulation) are compared. It is seen that external modulation should provide the same signal as internal modulation but without the need to add lossy modulation components to the arms of the interferometer. The effect of the modulation technique on the signal to noise ratio of the interferometer is evaluated and it is found that both the technique used and the particular modulation waveform can alter the potential signal to noise ratio slightly. There is an optimum length of the arms of an interferometer designed to detect gravitational waves at a given frequency. The time the light spends in each of the arms should be half of the period of the gravitational wave (i. e. several milliseconds). This can be achieved by multiple reflection of the light along arms (perhaps several km in length) using optical delay-lines or Fabry-Perot cavities. Some aspects of both of these schemes are considered in this thesis. The application of internal modulation to an interferometer with cavities in its arms is considered. The signals which can be expected from this system are investigated to reveal if there should be any problems with the control of such an interferometer. It is found that there should be no unexpected control problems. The very important techniques of power and dual recycling should both allow the performance of a basic interferometer to be enhanced considerably. Both of these are discussed in Chapter 2 in order to enable an experiment to be done to test the optical properties of these techniques and to test suitable control systems. Power recycling is concerned with the maximisation of the light amplitude in the interferometer for a given laser power by making the laser light resonant in the system. Dual recycling provides a method of enhancing the frequency response of the interferometer, especially at low frequencies or in a narrow range of frequencies, by resonating the signal sidebands. In this respect it can replace the need for cavities in the arms of the interferometer and also provides a method of optimising the detector response if the frequency of an expected signal is known. The techniques presented in Chapter 2 were tested as described in Chapter 3. The results suggested that the behaviour of the optical systems were understood and that control systems could be made to operate these relatively complicated arrangements. The effects of internal modulation on measurement noise were also confirmed. The above systems require extremely low loss mirrors. A method based on the measurement of the storage time of a Fabry-Perot cavity was adapted to allow the losses of such mirrors to be evaluated. This has revealed that sufficiently low loss mirrors are now becoming available in the large sizes required for gravitational wave detectors. (Abstract shortened by ProQuest.)

Opto-mechanical noise cancellation

The experiments presented in this thesis investigate the interaction between radiation and an optical cavity, in which one mirror of the cavity is mounted on a flexure which could be moved by radiation pressure. The cavity was shown to exhibit non-linear behaviour with high input power. The radiation pressure force was shown to change the mechanical resonance frequency of the moveable mirror. Motion was induced through amplitude modulation of a high power input beam and the extent of this motion measured using the cavity control loop. To demonstrate the way quantum correlations could be used to beat the SQL, the laser light incident on the cavity was prepared, using classical modulation techniques, with classical correlations between the quadratures that cause shot noise and radiation pressure noise. A level of modulation much higher than the quantum level was used to make the cancellation effects more visible. The effect of radiation pressure induced motion was cancelled by the addition of correlated frequency modulation. The input amplitude was then modulated by a white noise source. The resulting noise was partially cancelled when the same white noise source was used to drive the frequency modulator with a dierent phase. This cancellation demonstrably improved the signal to noise ratio of a signal injected into the system

Fourierdomänen modengekoppelte Laser: Aufklärung der Funktionsweise und Erschließung neuer Anwendungsbereiche

Mit der Fourierdomänen Modenkopplung (FDML) wurde vor kurzem ein neuer Operationsmodus für sehr schnell in der Wellenlänge abstimmbare Laser entdeckt, bei dem ein schmalbandiger Spektralfilter resonant zur Lichtumlaufzeit im Resonator abgestimmt wird. Diese FDML-Laser, deren genaue Funktionsweise noch unverstanden ist, gehören zu den schnellsten weit abstimmbaren Lichtquellen und eignen sich besonders für die optische Kohärenztomografie (OCT), die ein junges dreidimensionales Bildgebungsverfahren darstellt. In dieser Arbeit wurden zwei Ziele parallel verfolgt. Zum einen sollten durch Optimierungen und Erweiterungen des Lasers neue Anwendungsmöglichkeiten insbesondere in der OCT ermöglicht, gleichzeitig aber auch neue Erkenntnisse über die genaue Funktionsweise der Fourierdomänen Modenkopplung auf physikalischer Ebene gewonnen werden. In dem eher anwendungsorientierten Teil dieser Arbeit wurde zunächst eine neue Methode entwickelt, mit der es in der OCT mit schnell abstimmbaren Lasern möglich ist, zweidimensionale Schnitte bei einer bestimmten Tiefe, sogenannte en face Schnitte, ohne aufwändige Computerberechnung zu erhalten. Während diese Schnitte bisher nur sehr rechenintensiv durch Nachbearbeitung und Extraktion aus einem vollständig aufgenommenen dreidimensionalen Datensatz gewonnen werden konnten, lassen diese sich nun um ein Vielfaches schneller aufnehmen und darstellen. Weiterhin wurde durch den Eigenbau eines auf Abstimmgeschwindigkeit optimierten Spektralfilters die Aufnahmegeschwindigkeit eines mit einem FDML-Laser betriebenen OCT-Systems um über eine Größenordnung erhöht, so dass dieses nun mit Abstand zu den schnellsten Systemen gehört. Obwohl bei diesen hohen Geschwindigkeiten das Signal-Rausch-Verhältnis durch Schrotrauschen bereits limitiert wird, konnten Aufnahmen sehr hoher Qualität erzeugt werden, was a priori nicht selbstverständlich war. Im Grundlagenteil dieser Arbeit wurde das Verständnis der Operationsweise der Fourierdomänen Modenkopplung erweitert. Da FDML-Laser vollständig aus Glasfaserkomponenten aufgebaut sind und Längen von mehreren Kilometern aufweisen, wurde der Einfluss der Faserdispersion auf sowohl Linienbreite und Rauschverhalten untersucht. Mit einer speziellen dispersionskompensierten Resonatorgeometrie konnte dabei ein einfaches Modell des Einflusses der Dispersion auf die Kohärenzlänge validiert und eine deutliche Erhöhung dieser erreicht werden. Ein umfassenderes Modell der Operationsweise von FDML-Lasern ist wünschenswert, um experimentell schwer zugängliche Fragestellungen beantworten zu können. Auf dem Weg dahin müssen zunächst alle physikalischen Effekte im Resonator, welche zur Lasertätigkeit beitragen, aufgeklärt werden. Hierzu wurde die zeitabhängige Leistung eines FDML-Lasers durch verschiedene Terme in der nichtlinearen Schrödingergleichung modelliert, numerisch ausgewertet und mit experimentellen Daten verglichen. Dadurch konnten wichtige an der Laseroperation beteiligte Prozesse aufgeklärt und eine Basis für weitergehende Simulationen geschaffen werden