Novel photonic systems and devices exploiting the Raman effect in optical fiber

Abstract

The Raman effect is an important nonlinear effect with applications mainly in the fields of spectroscopy and fiber optics. Since the end of the twentieth century and thanks to the output power improvement and price reduction of pump lasers, its application to optical amplification in optical fiber communications has become widespread. In this thesis, two particular types of Raman amplifiers are studied: ultralong Raman fiber lasers (URFLs) and Raman polarizers. The advantages of distributed amplification based on URFLs have been demonstrated over the last decade in several optical communications systems, in which the efficient distribution of the gain over long distances offered by these amplifiers allows for a nearly optimal balance between noise and nonlinear effects, which leads to improved performance. Nevertheless, one of the main sources of errors in this type of amplifiers is the RIN transfer between the pumps and the signal. The first chapter of results in this thesis is committed to study this impairment, focusing on the numerical analysis of a specific case of URFLs, the random distributed feedback fiber lasers (RDFLs) in which the feedback is distributedly provided by Rayleigh scattering, instead of relying on a classical cavity delimited by reflectors. A second batch of results explores three applications of URFL amplification (to distributed sensing based on Brillouin optical time-domain reflectometry, to gyroscopic measurements using Sagnac interferometers and to long haul, high-speed, coherent communications) from a theoretical and, in the case of long-haul communications, experimental perspective. In all cases we demonstrate that the use of URFLbased amplification can lead to performance improvements. Raman polarizers are an special kind of Raman amplifiers which not only amplify but also produce a polarized output thanks to the polarization dependence of Raman gain. In this thesis, the first complete and general analysis of the evolution of the state of polarization of a signal in Raman amplification system, in the presence of other nonlinear effects, is presented. As a result a system of differential equations which can be solved numerically in order to describe the main characteristics of Raman polarizers is presented. This system is valid for all cases of interest and requires less computational time than previous approaches. An analytical approximation applicable to most situations of interest is also presented. Finally, a method for the suppression of RIN in Raman polarizers based upon the fast scrambling of the input signals is presented.