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IEEE Journal of Quantum Electronics

Large refractive index and interparticle-interaction-induced nonlinear modulation in densely doped composite materials

Yan Xue, Geoffrey B. Smith, Anthony Thomas Baker

Published Jan 1, 1998• 6 citations
Publisher

Abstract

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.

DOI
10.1109/3.704325

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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.
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 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.
A method of reducing the decay time of the population-inversion-dependent refractive index change in rare-earth doped fibers is investigated, and its suitability for use in all-optical switching is considered. Normally, the temporal response of this type of nonlinearity is dominated by the slow fluorescent decay from the metastable level present in the doped fiber. However, in this instance, the nonlinearity is rapidly quenched by the internally generated lasing signal produced by Q-switching a lasing cavity containing the doped fiber. Using this technique the observed decay time was reduced from 370 /spl mu/s to /spl sim/1 /spl mu/s and a phase modulation of /spl pi/-radians was observed, demonstrating that the effect could be used to achieve complete optical switching in an interferometric device. A model of the temporal behavior of this effect is developed using a rate equation analysis. The induced index change is modeled by summing the effects of individual transitions in the region of the signal wavelength, and by including the effects of assumed transitions in the VUV. An extension to this method is discussed in which the depopulating signal is generated using gain-switched techniques, obviating the need for intracavity modulation, thus making the depopulation mechanism more suitable for practical purposes.
We present a theory that enables us to account for the pump-induced change of refractive index in a rare-earth-doped fiber. Experimental results carried out on an erbium-doped fiber with cw pump and cw signal operation confirm the accuracy of this theory at the steady state. Both theoretical and experimental results show that significant index changes can be obtained for the order of 1–2 mW of cw optical pump or signal power. In addition we studied the transient dynamics of the resonance-enhanced refractive index in a wide range of pulse widths from millisecond to tens of femtoseconds. The result shows that a fast response of refractive-index changes is achievable since the effects of spontaneous emission and nonradiative decay are negligible in the short-pulse case.