Spatial Coherence, Multimode Dynamics, and Nonlinear Self-Organization in Multimode Optical Fibers

Abstract

Multimode optical fibers (MMFs) are increasingly attractive for high-power delivery, high-capacity transmission, and advanced imaging. Yet complex modal interactions often yield speckled, low-coherence output fields. Recent observations of beam self-cleaning and multimode supercontinuum generation show that nonlinear effects can spontaneously generate structured light, motivating full-field studies of multimode propagation.

This thesis develops and applies advanced diagnostics to examine MMF dynamics in linear and nonlinear regimes. First, spatial-coherence evolution is characterized in step-index and graded-index (GRIN) fibers. Experiments show strong coherence degradation in linear propagation, while controlled excitation, spectral filtering, and nonlinearities enable partial recovery. A numerical model supports interpretation of intermodal coupling.

Second, spatio–spectral dynamics are resolved using single-shot bandwidth-tunable digital holography. Varying the reference bandwidth reveals direct coupling between spectral and spatial intensities. Experiments and simulations show how nonlinear interactions re-shape beams and redistribute spectral energy.

Third, the thesis demonstrates spontaneous formation of orbital-angular-momentum (OAM) beams in a standard step-index MMF without wavefront shaping. Intermodal coupling and cascaded Raman gain generate high-purity OAM states at new wavelengths, with power-dependent redistribution of OAM orders. Modal decomposition identifies the evolving modal content.

Overall, the thesis links multimode propagation, coherence control, nonlinear beam re-shaping, and the emergence of structured light. The results establish MMFs as viable platforms for self-organized photonics with applications in high-brightness fiber lasers, structured-light sources, adaptive delivery, and multimode or neuromorphic photonic processing.