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.
