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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Collective effects in composites: an enhancement of index of refraction and distortion of dispersive spectra

Yan Xue

Published May 2, 1997• 1 citation
Publisher

Abstract

This paper investigates the resonantly enhanced nonlinearity in active-medium densely doped composite materials the interparticle collective effects due to the high concentration of dopants significantly enhances the nonlinear index of refraction and results in a density- dependent distortion of the dispersive spectra. Two types of active composite materials, organically modified silicate (ORMOSIL) incorporated with perylimide dye molecules and silica incorporated with erbium ions, which represent, respectively, molecular and atomic nonlinear system, are examined. Perylimide dye doped ORMOSILs exhibit more obvious collective effects at a relative lower doping level than erbium ions in silica. The enhancement of nonlinear index of refraction in active-medium densely doped composites provides a new possibility for the low-intensity operation of switching devices and self-guiding nonlinear devices. The gain properties are also studied.

DOI
10.1117/12.271166

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This paper investigates the resonantly enhanced nonlinearity in active-medium densely doped composite materials the interparticle collective effects due to the high concentration of dopants significantly enhances the nonlinear index of refraction and results in a density- dependent distortion of the dispersive spectra. Two types of active composite materials, organically modified silicate (ORMOSIL) incorporated with perylimide dye molecules and silica incorporated with erbium ions, which represent, respectively, molecular and atomic nonlinear system, are examined. Perylimide dye doped ORMOSILs exhibit more obvious collective effects at a relative lower doping level than erbium ions in silica. The enhancement of nonlinear index of refraction in active-medium densely doped composites provides a new possibility for the low-intensity operation of switching devices and self-guiding nonlinear devices. The gain properties are also studied.
We have performed a study of the population dynamics in a composite, in which the active medium is densely doped, by considering the interparticle interaction between the nearest neighboring active particles. A model is presented to describe the nonlinear modulation induced by interparticle collective effects and upconversion in the resonantly enhanced nonlinearity and its response to an input optical signal in such a densely doped composite. The results show that the nonlinear refractive index can be greatly increased with unchanged or slightly reduced transient response time. This is expected to have potential in lowering the critical switching intensity of bistable switching devices without sacrificing switching speed. We examined two typical composites: rare-earth densely doped silica and perylimide-dye densely doped polymer/glass composites. It is found that perylimide-dye densely doped polymer/glass composite is more suitable for the switching application due to its large nonlinear refractive index and fast response time at relatively low optical intensity.
We perform a study of extremely slow pulse propagation speed in densely-doped erbium fibers by using a detuned signal. A quantum coherence effect, coherent population oscillations, produces a very narrow spectral "hole" in the homogeneous broadened absorption profile of the erbium-doped fiber. The resulting rapid spectral variation of the refractive index leads to a large value of the group index. The interparticle interaction between the nearest neighboring active particles due to the high density of erbium ions is considered. The result shows that the nonlinear refractive index can be greatly increased which leads to the smaller group velocity and the larger time delay.
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
Relationships are obtained for the nonlinear resonant polarization of a multilevel system in the field of a monochromatic light wave under noncoherent interaction conditions allowing for anharmonicity of the electron oscillator and the dynamic Stark shift. Estimates are made of the resonant nonlinear refractive index of glasses and crystals activated with Cr 3+ and Nd 3+ ions and it is found that this may be of the order of (or greater than) the same index of the insulating base of the active medium.
A theory for the refractive-index enhancement due to quantum coherence and interference has been developed in dispersive polaritonic and photonic band-gap materials doped with an ensemble of noninteracting three-level atoms. Quantum coherence is introduced by driving the atoms with a coherent monochromatic laser field. The real and the imaginary parts of the susceptibility have been calculated by using the master equation and the Laplace transform methods. It is found that the energy gap in these materials plays an important role in the refractive-index enhancement. Numerical simulations for the real and the imaginary parts of susceptibility are performed for SiC as a function of the probe laser frequency. It is found that there is a giant refractive-index enhancement with vanishing absorption when the resonance frequencies lie near the lower band edge.