An investigation of Q-switched induced quenching of the resonant nonlinearity in neodymium doped fibers

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 μs to ~1 μs and a phase modulation of π-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