Research on Spatiotemporal Light Propagation in Multimode Fibers and its Control for Fiber Laser Applications
- 주제(키워드) Multimode fiber , Ultra-fast fiber laser , Adaptive optics
- 주제(DDC) 621.042
- 발행기관 아주대학교 일반대학원
- 지도교수 염동일
- 발행년도 2024
- 학위수여년월 2024. 2
- 학위명 박사
- 학과 및 전공 일반대학원 에너지시스템학과
- 실제URI http://www.dcollection.net/handler/ajou/000000033769
- 본문언어 영어
- 저작권 아주대학교 논문은 저작권에 의해 보호받습니다.
초록/요약
Multimode optical fibers, characterized by a large core diameter and high numerical aperture, support multiple spatial modes beyond the fundamental spatial mode of optical fibers. The larger core diameter with a high numerical aperture allows for higher power transmission and higher spatial resolution compared to single-mode fibers, making multimode fibers suitable for not only various high- power laser applications but also high-resolution imaging systems. However, the ease and unstable energy exchange among spatial modes in multimode fibers often lead to irregular beam distributions that are challenging to control in response to environmental effects. This dissertation aims to understand and control various linear and nonlinear phenomena that occur when light propagates through multimode optical fibers, pursuing the development of ultrashort pulse multimode fiber lasers. The research first explores methods to control the beam distribution at the output end of multimode fibers by manipulating the spatial phase of the incident light. A transmission matrix method is employed to control the intensity distribution of the beam at the fiber output by adjusting the spatial phase distribution of the input beam using a spatial light modulator. This research then studies the Kerr beam self-cleaning phenomenon, observing how the laser pulse with high peak power undergoes spatial mode cleaning by nonlinear effects in multimode-graded index (GRIN) fibers. Furthermore, by polishing the side of a multimode GRIN fiber and applying an appropriate over- cladding, a device was developed that selectively experiences loss only in higher- order spatial modes. It is expected that an all-optical fiber saturable absorber operating in a multimode optical fiber can be realized by combining the developed side-polished optical fiber device with the Kerr beam self-cleaning effect. In addition, the studies extend to the development of mode-locked lasers using multimode GRIN fibers. By utilizing wavelength and spatial filters, spatial modes of multimode fibers are coupled to the cavity longitudinal modes of the laser resonator, resulting in the realization of spatiotemporal mode-locked (STML) lasers. Stable single soliton and bound soliton pulse lasers are developed by modulating the cavity conditions, and their characteristics are investigated. In addition, analyzing spectral characteristics according to location at the output end of the bound soliton STML laser confirmed that each spatial mode has different spectral output characteristics. This research will contribute to developing new mode-locked multimode fiber lasers and related devices that overcome the limitations of single-mode fiber lasers.
more목차
Chapter 1. Introduction 1
1.1. Thesis outline 4
Chapter 2. Beam Propagation in Optical Fibers 6
2.1. Maxwell's equations 7
2.2. Fiber Modes 12
2.2.1. Fiber Modes in Step-index Fiber 12
2.2.2. Fiber Modes in Graded-index Multimode Fibers 16
2.3. Nonlinear Propagation in Optical Fibers 19
2.3.1. Nonlinear Schrödinger Equation in Single Mode Fibers 21
2.3.2. Generalized Multimode Nonlinear Schrödinger Equations 23
Chapter 3. Active Output Beam Distribution Control in Multimode Fibers 26
3.1. Wavefront Shaping in Scattering Medium 28
3.2. Speckle Effects in Multimode Fibers 30
3.3. Transmission-matrix with Hadamard Matrix 31
3.4. Spatial Phase Conjugation Method by Transmission-matrix of Multimode Fibers 35
Chapter 4. Kerr Beam Self-cleaning effects in Graded-index Multimode Fibers 43
4.1. Modal Instability in Graded-index Multimode Fiber 44
4.2. Kerr Beam Self-cleaning Effect through ns Pulse Propagation 47
4.3. Nonlinear transmission by Kerr beam self-cleaning effect 52
Chapter 5. Spatiotemporal Mode-locking in Multimode Fiber Lasers 58
5.1. Mode-locked Fiber Lasers 59
5.2. Saturable Absorbers 60
5.3. All-fiber Spatiotemporal Mode-locked Laser 61
Chapter 6. Conclusion 69
Reference 72
