PhD defence of Robi Kormokar – Soliton self-frequency shift in nonlinear optical fibers

Abstract

Since its discovery, soliton self-frequency shift (SSFS) has been subject of many applications including widely wavelength tunable femtosecond laser sources, broadband supercontinuum sources, optical buffering, photonic analog-to-digital conversion, and multiphoton imaging. Several factors influence SSFS in an optical fiber, and their collective impact is still unclear. This thesis presents an analytical description of the dynamics of SSFS in nonlinear optical fibers, accounting for loss, gain, and high-order linear and nonlinear effects. After an introduction (chapter 1) and the presentation of basic physical concepts (chapter 2), chapter 3 presents a high-order analytical expression of SSFS experienced by a fundamental soliton. This expression is a high-order extension of Gordon’s well-known formula, but also including propagation losses, third-order dispersion, and self-steepening. It also provides relative magnitudes of third-order dispersion and self-steepening responsible for the nonlinear evolution of SSFS. Chapter 4 presents an analytical expression for the energy conversion efficiency (ECE) of Raman solitons resulting from high-order soliton fission. The theoretical framework accounts for interpulse Raman scattering, enhancing the accuracy of predicting ECE of Raman solitons compared to the inverse scattering method. An experiment is also developed to validate the enhancement of ECE of Raman solitons in the fission process. Chapter 5 presents moment equations that are developed to quantify SSFS in amplifying fibers. These equations yield the SSFS in amplifying fibers without extensive computational resources and unveil the optimal initial pulse chirp needed to achieve maximum SSFS and ECE. Chapter 6 presents the experimental demonstration of the effect of pre-chirping on the SSFS and ECE in an amplifying fiber. An initially chirped pulse at a wavelength of 1880 nm undergoes SSFS within a thulium-doped fiber amplifier, generating tunable solitons in the 2000 nm spectral region. The experimental results show that SSFS and ECE are optimized when the initial pulse chirp aligns with the theoretical prediction in Chapter 3. In conclusion, this thesis provides fundamental analytical tools that predict SSFS, as well as they serve to optimize wavelength conversion and supercontinuum generation systems.