DESIGN AND ANALYSIS OF PHOTONIC CRYSTAL FIBER FOR SENSING APPLICATIONS

Abstract

Over the past few years, photonic crystal fibers (PCFs) have emerged as a promising class of optical fibers, offering immense potential across a wide range of applications, particularly in telecommunications and sensing technologies. Initially, research on PCFs was mainly focused on enhancing key optical characteristics such as dispersion, nonlinearity, confinement losses, and birefringence. However, with ongoing advancements in fiber fabrication techniques, PCF-based sensors have gained increasing attention, especially in the domain of sensing. A notable development in this field is the rapid progress of surface plasmon resonance (SPR)-based sensing technologies. Traditional prism-based SPR sensors, known for their label-free detection and real-time monitoring capabilities, are becoming less favorable due to limitations such as low sensitivity, bulky configurations, and high manufacturing costs. In response, PCF-based sensors have been introduced as a more efficient alternative, offering compact design and improved performance while eliminating the need for extensive experimental setups. This thesis explores the design and analysis of PCF-based sensors tailored for both physical and biological sensing applications. By carefully optimizing the structural parameters of the PCF-based refractive index (RI) sensor, it is possible to achieve high sensitivity, a broad sensing range, and simplified fabrication processes. This thesis begins by providing a concise overview of the development of fiber-optic sensors. Optical fibers serve as effective sensing elements by continuously monitoring variations in the surrounding analyte. While traditional optical fibers are applicable for SPR sensing, their structural and optical limitations hinder further advancement in this area. To overcome these challenges, PCFs have been introduced an innovative class of fibers that integrate the benefits of both optical fibers and photonic crystals. PCFs exhibit unique features that surpass the capabilities of conventional fibers. Among their many applications, PCF-based RI sensors have shown remarkable adaptability in diverse sensing domains, including physical, biological, and chemical detection. The following chapter presents a comprehensive literature review on PCF-based RI sensors for physical and biomedical applications. The review begins with an overview of the fundamental concepts, including mode coupling theory, birefringence, and wavelength sensitivity. It then provides a concise explanation of the suitability of PCFs for RI-based sensing applications, followed by an in-depth discussion on PCF-based SPR sensors. Key areas covered in the discussion include temperature monitoring, analysis of blood components, malaria detection, and sensing of various fluid analytes. The next chapter examines a circular-shaped hollow-core PCF filled with ethanol. The study focuses on analyzing key optical properties such as dispersion, effective mode area, confinement loss, and nonlinear coefficient across a wavelength range of 800 nm to 1600 nm. The primary objective is to attain a near-zero dispersion wavelength (ZDW) using the finite element method (FEM). By varying the filling configuration, air in the entire ring, ethanol in the central ring, and ethanol in the entire ring, ZDW values of approximately 880 nm, 1220 nm, and 1250 nm are achieved, ix respectively. This type of PCF holds significant potential for applications in sensing, nonlinear optics, laser systems, and telecommunications. The following chapter presents a detailed study of a twin-core photonic crystal fiber (TC-PCF) structure designed for temperature and chemical sensing applications. The proposed design features two solid cores separated by a vertically aligned elliptical air hole, which allows for independent light propagation in each core and results in high birefringence. The sensing mechanism is based on mode coupling between the two cores, which significantly enhances sensitivity. The performance of the TC-PCF sensor has been evaluated through simulations using the FEM. The findings demonstrate the sensor's high sensitivity and its suitability for both temperature and chemical detection. Numerical simulations reveal that the 3 cm long TC-PCF sensor has been optimized to exhibit a high temperature sensitivity of approximately 21.5 pm/°C across a broad temperature range of 0 to 1200 °C. Additionally, for chemical sensing, a sensor with a fiber length of just 0.03 cm achieves a maximum sensitivity of 6667 nm/RIU. This chapter covers the theoretical background, structural design, and performance analysis, emphasizing the real-world applicability of TC-PCF-based sensors. Next chapter focuses on the design and investigation of advanced biosensing methods utilizing PCFs for the early identification of a range of diseases, such as multiple forms of cancer, essential blood constituents, and malaria. The research employs FEM simulations conducted in the terahertz (THz) frequency range to analyze mode coupling behaviour in TC-PCF structures, aiming to achieve superior sensitivity and accuracy in biomedical sensing applications. By thoroughly examining key design parameters of PCF, the proposed sensors demonstrate exceptional sensitivity and performance in identifying diseases, thereby supporting progress in biomedical diagnostics and enhancing healthcare technologies. The following chapter introduces a SPR-based PCF sensor specifically designed for the detection of diabetes. Gold is employed as the plasmonic material and is incorporated in a layered structure to enhance the sensor’s overall performance. The design is analyzed using FEM simulations to assess its capability in identifying diabetic conditions. In this sensor configuration, two concentric layers of air holes are organized in a hexagonal lattice, and a thin layer of gold is coated onto the fiber to enable the excitation of the SPR effect. This effect arises when the surface plasmon polariton (SPP) mode interacts with the core-guided mode under phase-matching conditions. Diabetes-related samples, each characterized by a distinct RI, are introduced into the fiber structure. Variations in RI between normal and diabetes-affected samples cause a measurable shift in the SPR resonance wavelength during confinement loss analysis. The sensor achieves a sensitivity of 2400 nm/RIU, as determined by tracking changes in the loss spectrum. With its simple sensing approach, the proposed SPR-PCF sensor offers a practical and cost-efficient solution for diabetes monitoring.