بررسي تجربي اثر استفاده از ميكرو تارها بر كارآيي و حساسيت يك -OTDR در اندازه گيري ضريب شكست

Experimental investigation of the effect of using microwires on the efficiency an‎d sensitivity of a -OTDR in measuring the refractive index

فيزيك

This dissertation investigates the development an‎d enhancement of optical fiber sensors based on Φ-OTDR system. Chapter 1 introduces the fundamentals of optical fiber sensors, highlighting their characteristics an‎d advantages for measuring physical parameters, an‎d provides the necessary background for optical time-domain reflectometry systems. Chapter 2 presents the operating principles of Φ-OTDR, including both interferometric an‎d non-interferometric configurations, an‎d discusses the major challenges associated with this technique, particularly the trade-off between spatial resolution an‎d sensing sensitivity. In Chapter 3, a Φ-OTDR sensor incorporating a dual-microfiber structure is designed an‎d developed to enhance temperature sensitivity, improve the dynamic range, an‎d enable accurate refractive index measurement. The results demonstrate that, among the investigated configurations, the 20 μm dual-microfiber structure exhibited the best overall performance, achieving the highest temperature sensitivity over a measurement range of 20–40 °C, together with an enhancement factor of 4.59 compared with the conventional non-tapered optical fiber. Furthermore, the proposed structure enables precise quantification of environmental variations through phase extraction from the Rayleigh backscattered signal. This chapter also presents, for the first time, the implementation of a hybrid microfiber structure within the sensing region for simultaneous temperature an‎d refractive index measurements based on the Φ-OTDR technique. Chapter 4 extends the proposed approach by implementing a triple-microfiber structure capable of distributed mass sensing while simultaneously providing high-sensitivity refractive index measurement. The performance of the proposed sensor is further eva‎luated through field validation experiments. The experimental results indicate that the 20 μm triple-microfiber configurations over a 5 m sensing section provides superior performance, achieving a maximum SNR of 16.27 dB an‎d a 29.89% improvement compared with the conventional non-tapered optical fiber. Moreover, the excellent agreement between laboratory an‎d field measurements, demonstrated by a correlation coefficient of 0.97 an‎d a mean absolute percentage error (MAPE) of 1.92%, confirms the reliability an‎d practical applicability of the proposed sensing structure under real operating conditions. This chapter also shows that although increasing the number of microfiber tapers enhances sensing sensitivity, saturation effects an‎d additional optical losses ultimately define the practical operating limits of the system. Overall, this research demonstrates that the incorporation of novel dual- an‎d triple-microfiber structures significantly improves the performance of Φ-OTDR systems while introducing new capabilities for distributed temperature sensing, distributed mass sensing, an‎d refractive index measurement. The principal contribution of this dissertation is the design an‎d experimental validation of hybrid multi-microfiber sensing structures that substantially enhance Φ-OTDR sensitivity without requiring any hardware modifications, thereby providing a practical an‎d cost-effective approach for extending the industrial an‎d field deployment of Φ-OTDR-based sensing systems.

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