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Electronics Letters

Resonantly enhanced nonlinearity using stimulateddown-pumping inneodymium doped twin-core fibre

John W. Arkwright, Bin Wu, Iain Skinner, P. L. Chu

Published Feb 3, 1994• 10 citations
Publisher

Abstract

Preliminary observations and measurements of increased relaxation speeds in resonantly enhanced nonlinear systems are demonstrated. The effect is observed using a neodymium-doped twin-core fibe interferometer, and uses stimulated down-pumping of the 4F3/2 metastable state in neodymium. The relaxation time has been reduced from ~390 µs to less than 120 µs, and it is shown that the reduction is dependent on the power of the down-pumping source.

DOI
10.1049/el:19940128

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•May 11, 1993•2 citations
Preliminary observations and measurements of increased relaxation speeds in resonantly enhanced nonlinear systems are demonstrated. The effect is observed using a neodymium-doped twin-core fibe interferometer, and uses stimulated down-pumping of the 4F3/2 metastable state in neodymium. The relaxation time has been reduced from ~390 µs to less than 120 µs, and it is shown that the reduction is dependent on the power of the down-pumping source.
In this presentation, preliminary observations and measurements of increased switching speeds in resonantly enhanced nonlinear systems are shown. The method uses stimulated down- pumping of the 4F3/2 metastable state in neodymium, but is applicable to any four- level system, or pseudo-four-level system such as erbium. Since the stimulated down-pumping is only initiated at the end of each switching period, the lifetime of the metastable state is not affected. Thus high up-pumping efficiencies are still possible. This allows both low power operation and fast relaxation to be achieved in the same switching system. The relaxation time has been reduced from approximately 390 microsecond(s) to less than 120 microsecond(s) , and it is shown that the reduction is dependent on the power of the down-pumping source.
We report on optically induced optical switching of a 632.8-nm signal in a Nd3+-doped, two-mode fiber based on a nonlinearity resonantly enhanced by the dopant. A fast, 0.38-μs component was observed in the switched signal, which we believe is due to cross relaxation between clustered ions. We demonstrated a 600-kHz switch in a 1.14-m elliptical-core fiber with 0.31 μJ of launched 807-nm pump energy. Information about cluster size and speed of the cross-relaxation process is inferred from the switch dynamics.
The resonant nonlinearity observed in rare-earth doped fiber is of interest for all-optical switching due to the very low pump powers needed to achieve complete switching. Large nonlinear effects have been observed previously in both erbium and neodymium doped fibers, however, these large effects cannot be explained by taking into account the change in absorption of local transitions alone. Two mechanisms have been suggested to account for these anomalously large index changes, namely thermally induced effects and far from resonance population dependent effects. In this work, an ytterbium doped twin-core fiber is used to investigate the tow possible mechanisms, and by allowing the doped fiber to lase and observing the clamping of the induced phase change, it is demonstrated that the effect is population dependent in nature. The observed wavelength response of the effect shows an increase in induced phase towards shorter wavelengths, which suggests that the effect is due principally to changes in strong absorptions in the UV. This far-from-resonance effect has been used to demonstrate large phase changes at the preferred telecommunications windows of 1300nm and 1550nm.
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 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.