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

Resonantly enhanced nonlinearaties in rare-earth-doped fibers and waveguides

John W. Arkwright, Graham R. Atkins, John Canning, P. L. Chu, Mark Janos, Mark G. Sceats, Bin Wu

Published Nov 20, 1996• 4 citations
Publisher

Abstract

The use of rare-earth dopants for all-optical switching is of interest because of the very low pump powers required to achieve full switching. In addition, the ease with which rare-earth ions can be incorporated into silica based fibers and waveguides makes them ideally suited for the fabrication of fiber compatible components. The principal disadvantages of this type of nonlinearity are that the relaxation times are characteristically slow, and relatively long interaction lengths are required to achieve phase changes of the order of (pi) . In this presentation, an overview of the work being carried out in Australia will be given, concentrating on the techniques developed to minimize the relaxation times and power length products of these nonlinearities.

DOI
10.1117/12.258969

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The use of rare-earth dopants for all-optical switching is of interest because of the very low pump powers required to achieve full switching. In addition, the ease with which rare-earth ions can be incorporated into silica based fibers and waveguides makes them ideally suited for the fabrication of fiber compatible components. The principal disadvantages of this type of nonlinearity are that the relaxation times are characteristically slow, and relatively long interaction lengths are required to achieve phase changes of the order of (pi) . In this presentation, an overview of the work being carried out in Australia will be given, concentrating on the techniques developed to minimize the relaxation times and power length products of these nonlinearities.
We report low loss, low pump power, optical-optical switches in rare-earth-doped fibers based on the third-order optical nonlinearity resonantly enhanced by the dopant. In a 0.95-m Er- doped two-mode fiber switch pumped with a 1.48-micrometers diode laser, the absorbed pump power required for switching a 906-nm signal was 8 mW, for a signal loss of only 0.25 dB. This is an enhancement by a factor of 6200 in power-length product over undoped silica. The phase shift was found to be due in part to a non-resonant contribution, thought to arise from a strong UV-VUV transition, and in part to a resonant term from the 980-nm transition. In a 0.98-m Nd-doped, elliptical-core, two-mode fiber switch, switching of a 632.8-nm signal was achieved with only 6.6 mW of absorbed power at 900 nm. The dynamic response of the switch was found to have two components, a slow component equal to the metastable level lifetime (approximately equals 380 microsecond(s) ) and a fast component (approximately equals 2 microsecond(s) ). The latter is believed to arise from rapid cross-relaxation between paired ions, a mechanism which shows promises for low- power, microsecond switching in fibers.
A theoretical model of switches utilizing the resonantly enhanced nonlinearity present in doped fibers is presented. It establishes the dependence of the switching pump power, fiber length, residual resonant signal loss, and response time of the switch on the dopant spectroscopic parameters. Simple factors of merits are derived for the power and the length requirements and are evaluated for selected rare-earth transitions. The most promising candidates require only a fraction of a mW and a few cm of fiber for full switching. Similar power characteristics, but with considerably shorter response times (ns) and lengths (sub-mm), are predicted for dopants with a high oscillator strength transition, or a power-length product ten orders of magnitude smaller than for switches based on the Kerr effect in undoped silica fibers.>
To explain the high third-order nonlinearity observed in rare earth doped fibers, we examine the role of the various transitions of the dopants. We present experimental evidence that in fibers doped with Er3+, Nd3+, or Yb3+, most of the nonlinearity is caused by the dopant's UV transitions, i.e., it is non-resonant in nature. The magnitude of the nonlinearity is shown to be in excellent quantitatively agreement with a model. Fiber modulators and switches based on this widespread mechanisms are predicted to operate over a large range of signal wavelengths and with negligible signal loss.
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.