The single hydrophone localization problem is considered. Single hydrophone localization is a special case of matched field localization where measurements from only one hydrophone are available. The time series of the pressure at the hydrophone is compared with predicted times series calculated using an ocean acoustic propagation model for many different source locations. The source location that gives the best match between the predicted time series and the measurement is assumed to be the correct source location. Single hydrophone localization algorithms from the literature are reviewed and a new algorithm is introduced. The new algorithm does not require knowledge of the source signal and does not assume the use of a particular ocean acoustic model, unlike some algorithms in the literature. Source location estimates calculated from the new algorithm are compared with ground truth using simulated ocean acoustic measurements and experimental measurements. Source location estimates calculated using other algorithms from the literature are shown for comparison. The simulated measurements use three source signals with bandwidths of 10 Hz, 100 Hz, and 200 Hz and the ocean is modeled as a Pekeris waveguide. The new algorithm estimates the source location accurately for all three source signals when several of the localization algorithms from the literature give inaccurate estimates. Gaussian white noise signals are added to the measured signals to test the impact of signal-to-noise ratio (SNR) on the algorithm. Four signal-to-noise ratios of 60 dB, 40 dB, 20 dB, and 0 dB are used. The new algorithm gives accurate source location estimates down to an SNR of 20 dB for two of the source signal bandwidths. Source location estimates using other algorithms from the literature break down at either 20 dB or 0 dB. Source location estimates are calculated using two hydrophone measurements taken at different depths in an experiment conducted near the Bahamas. The new algorithm accurately estimates the source location in both cases. In one case, only two other localization algorithms from the literature locate the source accurately. In the other case, only one other localization algorithm succeeds.

In this thesis, my main focus was to establish a novel composite-cavity fibre laser (CCFL) and to apply it in sensing, particularly in the hydrophone application. The CCFL that I have proposed is formed by writing three wavelength matched fibre Bragg gratings directly into a continuous length of doped fibre. I have also examined the relative advantages and disadvantages of interferometric and intensity-based hydrophone systems, and have established a hydrophone system that can be switched between the two modes of operation, by making use of digital signal processing. I have established a theoretical model to study the lasing and spectral characteristics of the CCFL. My analysis showed that whilst the CCFL have significantly different phase and threshold conditions from the common semiconductor diode lasers with external cavity, the CCFL also have mode-limiting properties that are often sought after. Through simulations, I was able to identify that a non-uniform straining scheme, that is, when one of the sub-cavities of the CCFL is restrained from strain, can improve the sensitivity with respect to existing single cavity fibre lasers, in both the frequency and intensity domains. My simulations also showed that the sensitivity of such a straining scheme can be optimised by tuning the reflectivity of the gratings, sub-cavity lengths, doping concentration and pump power. I have fabricated multiple CCFLs using the in-house grating writing facilities, and have experimentally assessed their power and spectral related lasing characteristics. Whilst having a significantly longer total cavity length compared to typical fibre lasers, the CCFLs demonstrated stable single longitudinal mode operation and narrow linewidth in the order for a few tens of kHz. Asymmetric output power and frequency as a result of unequal sub-cavity lengths were also examined. Finally, I conducted sensing experiments by applying the CCFLs in strain monitoring and intensity-based hydrophone. My results showed that the non-uniform straining scheme had significantly improved the intensity response of the CCFL, and that the acoustic pressure and frequency can be determined by directly sampling and applying Fourier transform to the output intensity of the fibre laser.

Hydroacoustic tertiary (T-) waves are seismically generated acoustic waves that propagate over great distances in the ocean sound channel with little loss in signal strength. Hydrophone recorded T-waves can provide a lower earthquake detection threshold and an improved epicenter location accuracy for oceanic earthquakes than land-based seismic networks. Thus detection and location of NE Pacific ocean earthquakes along the Blanco Transform Fault (BTFZ) and Gorda plate using the U.S. Navy's SOSUS (SOund SUrveillance System) hydrophone arrays afford greater insight into the current state of stress and crustal deformation mechanics than previously available. Acoustic earthquake information combined with bathymetry, submersible observations, earthquake source-parameter estimates, petrologic samples, and water-column chemistry renders a new tectonic view of the southern Juan de Fuca plate boundaries. Chapter 2 discusses development of seismo-acoustic analysis techniques using the well-documented April 1992 Cape Mendocino earthquake sequence. Findings include a hydrophone detection threshold estimate (M~2.4), and T-wave propagation path modeling to approximate earthquake acoustic source energy. Empirical analyses indicate that acoustic energy provides a reasonable magnitude and seismic moment estimate of oceanic earthquakes not detected by seismic networks. Chapters 3 documents a probable volcanogenic T-wave event swarm along a pull-apart basin within the western BTFZ during January 1994. Response efforts yielded evidence of anomalous water-column ��He concentrations, pillow-lava volcanism, and the first discovery of active hydrothermal vents along an oceanic fracture zone. Chapter 4 discusses the detection of a NE-SW trending microearthquake band along the mid-Gorda plate which was active from initiation of SOSUS recording in August 1991 through July 1992, then abruptly ceased. It is proposed that eventual termination of the Gorda plate seismicity band is due to strain reduction associated with the Cape Mendocino earthquake sequence. Chapter 5 combines bathymetric, hydro-acoustic, seismic, submersible, and gravity data to investigate the active tectonics of the transform parallel Blanco Ridge (BR), along the eastern BTFZ. The BR formation mechanism preferred here is uplift through strike-slip motion (with a normal component) followed by formation and intrusion of mantle-derived serpentinized-peridotite into the shallow ocean crust. The conclusion considers a potential link between the deformation patterns observed along the BTFZ and Gorda plate regions. / Graduation date: 1998

The main purpose of the optical fiber sensing technology is to detect perturbation of physical fields. By means of some demodulating scheme, we can extract the real signal from those light beams which modified by physical fields. In the thesis, we proposed a configuration of modified Sagnac Interferometer as a sensing system. The optical sensing and demodulation system are exploited separately. Next, we study the advantages and disadvantages of the configuration. Besides, we are also measured the sensitivity and dynamic range. The sensing system used a low coherence light source to reduce cost. This system also improves the shortage of a Sagnac Interferometer which has a blind point in the middle position. In addition, the structure is easily implemented and can detect weak signal in a high noisy water environment. For matching the main structure, we make many kinds of sensing heads for detecting signals under water. We also use the mathematical model as the base of the theory. The dynamic range is 40 dB and the sensitivity is -231.47 dB re V/uPa.

The interferometeric optical fiber sensor has high sensitivity for sound signal. This characteristic is used to design hydrophones. The sound pressure causes the optical fiber to change its shape. So as to induce phase difference between sensing and reference arms. Using the demodulation system, we can get the signal we want. In this thesis, we plan to analyze three different kinds of optic configurations, such as Michelson, compensating Mach-Zehnder, hybrid configuration of Mach-Zehnder and Sagnac interferometers. The mathematical methods are used to compare their characters. We also use software to simulate the relation among sensitivity, delay fiber and frequency character of the Sagnac interferometer. In our experiment, we use PGC modulation technology and compare the results with a standard hydrophone B&K 8103 for calibration. We also measure the dynamic range of proposed three interferometers. The measurement result of this paper is as following: Michelson and compensating type Mach-Zehnder interferometer dynamic range were about 24.90 dB and 13.98 dB, the acoustic signal sensitivity was -201.67 dB re V/1uPa and -205.97 dB re V/Pa, respectively. The dynamic range of the hybrid of Mach-Zehnder and Sagnac type interferometer was 33.67 dB and acoustic signal sensitivity was -212.47 dB re V/1uPa.

In this paper, the fesibility of using a Fiber Bragg Grating (FBG) as a sensing scheme to detect the underwater acoustic signals is analyzed. When a FBG is disturbed by an underwater sound, the wavelength of the FBG is changed. Therefore, the central spectrum of the reflected light is shifted according to the wavelength change of the FBG. This spectrum can be detected by an imbalanced two-arm interferometer. Its transfer function will be studied. Also, the polarization induced signal fading of those two-arm interferometers will be studied.

No description available.

In this thesis, my main focus was to establish a novel composite-cavity fibre laser (CCFL) and to apply it in sensing, particularly in the hydrophone application. The CCFL that I have proposed is formed by writing three wavelength matched fibre Bragg gratings directly into a continuous length of doped fibre. I have also examined the relative advantages and disadvantages of interferometric and intensity-based hydrophone systems, and have established a hydrophone system that can be switched between the two modes of operation, by making use of digital signal processing. I have established a theoretical model to study the lasing and spectral characteristics of the CCFL. My analysis showed that whilst the CCFL have significantly different phase and threshold conditions from the common semiconductor diode lasers with external cavity, the CCFL also have mode-limiting properties that are often sought after. Through simulations, I was able to identify that a non-uniform straining scheme, that is, when one of the sub-cavities of the CCFL is restrained from strain, can improve the sensitivity with respect to existing single cavity fibre lasers, in both the frequency and intensity domains. My simulations also showed that the sensitivity of such a straining scheme can be optimised by tuning the reflectivity of the gratings, sub-cavity lengths, doping concentration and pump power. I have fabricated multiple CCFLs using the in-house grating writing facilities, and have experimentally assessed their power and spectral related lasing characteristics. Whilst having a significantly longer total cavity length compared to typical fibre lasers, the CCFLs demonstrated stable single longitudinal mode operation and narrow linewidth in the order for a few tens of kHz. Asymmetric output power and frequency as a result of unequal sub-cavity lengths were also examined. Finally, I conducted sensing experiments by applying the CCFLs in strain monitoring and intensity-based hydrophone. My results showed that the non-uniform straining scheme had significantly improved the intensity response of the CCFL, and that the acoustic pressure and frequency can be determined by directly sampling and applying Fourier transform to the output intensity of the fibre laser.

Passive acoustic systems composed of hydrophone array have been shown useful for underwater acoustic source detection and tracking. The work presented here demonstrates use of a passive acoustic system for an Autonomous Underwater Vehicle (AUV) composed of a 2D hydrophone array along with a post processing algorithm for real time detection and tracking of surface vessels. Important design decisions for development of the hydrophone array are taken based on different factors such as the frequency range of broadband surface vessel noise, review of literature, financial as well as structural constraints of the AUV. The post-processing algorithm, developed using a phased array principle called acoustic beamforming, outputs real-time heading angles of the target surface vessels. Initial measurements conducted at Claytor Lake with the developed passive acoustic system to locate a white noise acoustic source showed better performance with functional beamforming technique among others. Various hydrophone array configurations are tested during these measurements to determine the optimal hydrophone placement. Furthermore, field tests are conducted at Norfolk Bay area to assess the performance of the developed system to real time detect and track surface vessels of different sizes in mission relevant environment. Cross-spectral matrix subtraction approach to subtract AUV's self noise is investigated to improve signal range and thus the detection range of these different surface vessels. This approach showed improvement in detection range of up to 350%. Another set of measurements again at Claytor Lake demonstrates real time detection and tracking of a small boat using an AUV integrated with the developed passive acoustic system operating at different propeller conditions. Results showed that low signal to noise ratio at higher AUV propeller rpm makes the detection and tracking difficult limiting the operating AUV propeller rpm up to 1500. This work also explores custom build hydrophones based on piezoelectric material of different shapes and sized to replace the expensive industry purchased hydrophones to lower the cost of developed system. / Master of Science / In field of underwater acoustic, hydrophone arrays have gained popularity for the detection and tracking of sound sources by just listening to them. This study presents design, development and testing of such hydrophone array attached to an AUV for real time detection and tracking of surface vessels. Multiple hydrophones in an array collect the underwater noise radiated by the target surface vessel which are essentially the unsteady pressure fluctuations. The phase difference between signals acquired by different hydrophones is then used to predict the direction of arrival of a sound wave from the target ship. Such a phased array principle called acoustic beamforming is used to develop a post processing algorithm which takes hydrophone array signals as input and outputs the heading angle of the target ship. This work first demonstrates capability of the developed hydrophone array and the algorithm to detect a white noise acoustic source (speaker) placed inside water at Claytor Lake. These measurements investigated performance of different acoustic beamforming techniques as well as different hydrophone array configurations. Furthermore, measurements conducted with actual surface vessel at Norfolk Bay area proved capability of the developed hydrophone array and the algorithm to detect and track ships in real time. The performance of the hydrophone array is characterized in terms of detection range and was observed to improve by 350% when the AUV's self noise is removed from the acquired hydrophone signals. Combined single unit of AUV and developed hydrophone array system also demonstrated real time detection and tracking of a small boat at Claytor Lake for different AUV operating conditions. Moreover, custom build hydrophones manufactured using piezoelectric material are found to be a feasible replacement for the expensive industry purchased hydrophones in order to reduce cost of the array.

No description available.