Resonantly Induced Refractive Index Changes in Yb-doped Fibers: the Origin, Properties and Application for All-Fiber Coherent Beam Combining

Abstract

Frontiers in Guided Wave Optics and Optoelectronics 210 enable those who are not expert in the field to appreciate the subject of the research.We give a clear physical insight into the electronic and thermal RIC mechanisms, and present important estimations helping the reader to clearly understand the scientific context, motivation and aims of this work.The main body of the work is centered around two experimental studies (Fotiadi et al., 2008c; Fotiadi et al., 2009c), reproduced in parts 3 and 4. Part 3 is dedicated to the fundamental properties of the Yb-doped fibers sensitive to the diode pumping at 980 nm.In this part we show our original RIC experiments performed with different samples of aluminum-silicate fibers and report on the dynamical characteristics of the RIC effect measured in the spectral range 1460 -1620 nm.Detailed analysis of these experimental data has brought us to the conclusion that the process is predetermining by the electronic RIC mechanism that can be quantitatively described by a two-level population inversion model with the polarizability difference (PD) of the Yb-ion in the excited and ground states as the only material model parameter determined from the experimental data.The absolute PD value measured in the experiment is found to be independent on the testing wavelength and the fiber specification parameters, i.e. the cross-section profile, the size, the length and the Yb-ion concentration.Our results ensure a predominant contribution to the RIC effect from far-resonance UV electronic transitions rather than near-resonant IR transitions.Part 4 of the Chapter focuses on potential applications of the RIC effect in advanced fiber laser systems implemented through our original concept of all-fiber coherent beam combining.The concept employs Yb-doped fibers as optically controllable phase shifters to be conjugated with Er-doped fiber amplifiers operating at ~1.55 µm.The coherent combining of 500-mW Er-doped amplifiers successfully demonstrated in our experiment confirms the validity of the concept.The performance characteristics of the multichannel fiber laser system associated with the proposed power scaling method are also under discussion. RICs in Yb-doped optical fibers: the origins and fundamentalsIn this part we summarize fundamentals of the electronic and thermal mechanisms of the refractive index changes (RIC) in single-mode ytterbium-doped optical fibers induced by optical pulses at 980 nm and we report the general equations describing RIC evolution.In particular, the electronic RIC dynamics is shown to follow the dynamics of the population of the excited/unexcited ion states with a factor proportional to their polarizability difference (PD).The thermal contribution to the RICs induced in standard Yb-doped fibers is shown to be negligible under condition of all-fiber low-power experiments discussed in the Chapter. Electronic and thermal RIC mechanisms caused by nonlinear pump absorptionYtterbium-doped silicate glass fibers are widely used for high-power, high-beam-quality laser sources due to their impressing gain efficiency, single transverse mode quality, relaxed thermal management problems, and overall robustness to environmental disturbances.Resulting from the confined mode structure and typically long propagation distances, nonlinear phenomena such as stimulated Brillouin scattering, stimulated Raman scattering, optical Kerr effect, inter-mode four-wave mixing, soliton and supercontinuum generation are observed in fiber lasers at relatively low power due to the high field intensity in the fiber www.intechopen.comResonantly Induced Refractive Index Changes in