Master Degree / Yüksek Lisans Tezleri

Permanent URI for this collectionhttps://hdl.handle.net/11147/3008

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Publisher: search.filters.publisher.Izmir Institute of TechnologySubject: search.filters.subject.Fiber optics

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Now showing 1 - 3 of 3
  • Master Thesis
    Development of Simulation Tool for Fbg-Based Phase-Otdr Vibration Sensors
    (Izmir Institute of Technology, 2021) Yüksel Aldoğan, Kıvılcım; Yüksel Aldoğan, Kıvılcım; 03.05. Department of Electrical and Electronics Engineering; 03. Faculty of Engineering; 01. Izmir Institute of Technology
    Over the last twenty years, phase-sensitive optical time-domain reflectometer based distributed vibration sensors have attracted increasing attention as a research topic and commercial product. Due to their capability to detect external perturbations along fiber in real-time, they have found applications in several sectors such as oil\&gas pipeline monitoring, border security, transportation, and so on. Recently, the implementation of fiber Bragg gratings into these systems has become popular in order to improve sensing performance and several system configurations adopting a wide range of signal processing techniques have been presented. In this thesis, a simulation tool for distributed vibration sensing systems based on FBG-based phase-sensitive optical time-domain reflectometry is developed. In this context, firstly, the operation principle of optical time-domain reflectometry and system parameters affecting the performance are investigated. Secondly, fiber Bragg grating structures and their implementation into phase-sensitive optical time-domain reflectometer based vibration sensors are studied. By utilizing the one-dimensional impulse response model, fiber Bragg grating implementation-related problems, spectral shadowing crosstalk and multiple reflection crosstalk, and different sensor configurations are analyzed. Based on this model, the numerical simulation tool is developed by constructing fiber, fiber Bragg grating, and probe pulse structures. The operation of the developed simulation tool is verified by comparing obtained results with theory. Then, the capabilities of the developed tool are demonstrated by conducting several simulations with conventional Phase-OTDR and FBG-based Phase-OTDR configurations.
  • Master Thesis
    Optical Characterization of Nanoscale Dielectric Films on Curved Surfaces Using Near Field Diffraction Method
    (Izmir Institute of Technology, 2019) Ataç, Enes; Dinleyici, Mehmet Salih; Dinleyici, Mehmet Salih; 03.05. Department of Electrical and Electronics Engineering; 03. Faculty of Engineering; 01. Izmir Institute of Technology
    Demand on the high-quality optical thin films has increased because of the importance in the optical sensor technologies. The thicknesses of such films are usually shorter than the wavelength of visible light. Therefore, the optical characterization of these films is not a routine procedure especially on curved surfaces such as optical fiber. Besides, the methods in the literature and commercially available systems are either expensive, destructive or non-real time. In this thesis, it is aimed to propose a simple, inexpensive and non-destructive optical characterization method of nano-scale dielectric films on curved surfaces. The methodology of that approach can be described as the near field wavefront tracing diffraction by using structured light. In this way, it has been shown that sub-wavelength film thicknesses can be estimated. The proposed diffraction method is organized in four main stages. These are the coating of optical fibers, generation of structured light, determination of wave propagation via the near field Huygens-Fresnel wave-front tracing and sensing and processing of signal from the sensor array. Layer by layer assembly technique is used in coating process to keep under control the thickness of transparent film. Selection of various source types is about to changing of point spread function of applied field and observe the effects on intensity pattern. Using near field diffraction technique, sub-wavelength thickness of thin films can be predicted by taking the higher order components of diffraction pattern by recording at very close proximity to object. In this way, determination of thickness beyond the diffraction limits can be realized. Furthermore, the resolution of sensor array in sensing part is important since pixel size of the sensor array determines your detection limits to catch all variations on diffraction pattern. The whole process has a mathematical model with numerical analysis methods. This dissertation is about the proposing a mathematical estimation model for the optical properties of nano-scale dielectric films coated on curved surfaces. The experimental results show that near field Huygens-Fresnel wave-front tracing method by using structured light is a powerful technique.
  • Master Thesis
    Analysis and Modelling of a Novel Approach for the Interrogation Unit of Fiber Bragg Grating Sensors Using Optical Frequency Domain Reflectometry Techniques
    (Izmir Institute of Technology, 2014) Pala, Deniz; Yüksel Aldoğan, Kıvılcım; Yüksel Aldoğan, Kıvılcım; 03.05. Department of Electrical and Electronics Engineering; 03. Faculty of Engineering; 01. Izmir Institute of Technology
    The main purpose of this thesis is to demonstrate the feasibility of using polarization properties of FBGs interrogated by OFDR for quasi-distributed sensing applications. A fiber Bragg grating (FBG) is a constant and periodic refractive index value modulation within the core along an optical fiber. This modification is generally obtained by exposing the fiber core of a photosensitive optical fiber to an intense ultraviolet (UV) interference pattern. At the fabrication process of Bragg gratings, only one side of the fiber expose to UV light. As a result, refractive index change is not constant at the cross section of fiber. This non-uniformity on the refractive index gives rise to photo-induced birefringence which combines with the birefringence resulting from the slightly elliptical shape of the optical fiber and creates a global birefringence value. In the presence of the birefringence, the reflection (transmission) spectrum of Bragg grating is degenerated into two reflection (transmission) spectra corresponding to a pair of orthogonal polarization modes (x and y modes). The ratio between maximum and minimum optical transmitted power of these modes are defined as Polarization Dependent Loss (PDL). We analyzed the reflection spectrum, transmission spectrum and the PDL of the cascaded FBGs interrogated by an OFDR by the way of simulations. Based on the simulation results, we demonstrated the feasibility of a novel FBG interrogation method which can be implemented in quasi-distributed strain sensors embedded into composite materials.