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This study develops an intermodal fiber interferometer (IFI) system based on spectral interrogation and Fourier analysis for high-precision measurement of external impacts. Addressing limitations of conventional fiber sensors in dynamic range, demodulation speed, and environmental robustness, we establish a linear mapping between mode group phase differences and external perturbations through theoretical modeling and experimental validation. Theoretically, a mathematical model reveals the linear response mechanism of Fourier phase spectra to displacement. Experimentally, a 40-meter graded-index MMF integrated with a piezoelectric ceramic modulator achieves linear measurement over 0.1–200 μm dynamic range using high-speed wavelength-swept laser scanning and real-time FFT processing. Results demonstrate that Fourier phase extraction effectively overcomes nonlinearity and fading in traditional interferometric signals, offering a robust solution for structural health monitoring in aerospace and industrial applications.

• ABSTRACT
• CONTENTS
• INTRODUCTION
• CHAPTER 1. Literature Review
  • 1.1. Technical Classification and Limitations of Fiber Optic Displacement / Deformation Sensors
    • 1.1.1. Intensity-Modulated Sensors
    • 1.1.2. Fiber Bragg Grating (FBG) Sensors
    • 1.1.3. Interferometric Fiber Optic Sensors
    • 1.1.4. Intermodal fiber interferometer Sensors
  • 1.2. Technical Challenges and Future Directions
  • 1.3. Research Motivation
  • 1.4. Research Objectives
• CHAPTER 2. Theoretical Modeling of Intermodal Fiber Interferometers
  • 2.1. Fundamental Principles of IFI
    • 2.1.1 Interferometer Structure and Light Wave Propagation Characteristics
    • 2.1.2. Sensing Mechanism for External Perturbations
    • 2.1.3. Technical Challenges and Solutions
  • 2.2. Theoretical Framework of Fourier Analysis Method
    • 2.2.1. Physical Essence of Fourier Transform
    • 2.2.2. Derivation of Linear Phase-Deformation Relationship
    • 2.2.3. Complete Measurement Theory and Procedure
  • 2.3. Numerical Simulation and Parameter Optimization
    • 2.3.1. Foundation of Simulation Model Construction
    • 2.3.2. Simulation Validation of Mode Group Characteristics: Influence of Mode Group Count 𝑀
    • 2.3.3. Simulation Analysis of Parameter Influence Mechanisms
    • 2.3.4. Linear Phase-Deformation Response
• CHAPTER 3. Experimental System Design and Validation
  • 3.1. Experimental Platform Construction
    • 3.1.1. Hardware System Configuration
    • 3.1.2. Software Architecture and Data Flow
  • 3.2. Experimental Results and Analysis
    • 3.2.1. Fundamental Performance Validation
    • 3.2.2. Linear Response Between Phase Difference and Displacement
    • 3.3.3. Analysis of mode pair response characteristics
• CONCLUSION
• REFERENCES
• APPENDICES