Study and Control of Nonlinearity in Large-Mode-Area Fibers.

Abstract

Practical advantages and high power of fiber lasers make them important in many scientific and industrial applications. However, relatively small mode-area and long propagation-length in an optical fiber also enhances the nonlinear interactions, posing certain limits on achievable average and peak powers in fiber lasers. In this dissertation, we explore such nonlinear effects and their control in CCC fibers, a practically important type of large-core effectively-single-mode fibers. Many applications require short wavelengths. We study use of four-wave-mixing (FWM) for wavelength conversion in CCC fibers. Our theoretical analysis shows that under proper conditions CCC fibers can be used for efficient and high-power wavelength conversion from ~1µm to yellow-red visible wavelengths. We study use of spectral filtering properties of CCC fibers for suppressing stimulated Raman scattering (SRS). SRS suppression has been experimentally achieved in two types of spectrally-tailored CCC fibers, demonstrating an additional degree of design freedom, combining core-size scalability and SRS suppression. Average powers in large-core amplifying fibers are limited by the thermally induced transverse mode instability (TMI). We show that TMI is essentially a two-beam coupling process, causing stimulated scattering from the fundamental to higher-order modes. We show that increasing higher-order mode suppression in CCC fibers increases TMI threshold power. CCC fibers are low-birefringence fibers, in which fiber coiling and twisting produces externally induced linear and circular birefringence. Presence of the later complicates nonlinear polarization evolution (NPE) at high peak powers, which can degrade polarization preservation at the amplifier or laser output. Our experimental and theoretical analysis shows that with proper signal excitation and fiber packaging conditions linear output polarization can be maintained under a wide range of output peak powers. Additionally, this dissertation also includes a study of some design aspects of large-core polygonal-CCC fibers, directly related to fiber modal properties used in controlling nonlinear interactions. Results of this work are important for using CCC, as well as other types of flexible (i.e. non-rod type) effectively-single-mode fibers, in high power and energy fiber lasers.