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

Large off-resonance nonlinear index changes at 1310 nm and 1545 nm observed in ytterbium-doped fiber

John W. Arkwright, P. Elango, Trevor Whitbread, Graham R. Atkins

Published Nov 20, 1996• 3 citations
Publisher

Abstract

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.

DOI
10.1117/12.258972

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Contribution of UV transitions to the strong third-order nonlinearity of doped fibers

Michel J. F. Digonnet, Robert W. Sadowski, H. J. Shaw +1 more

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.

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Nov 20, 1996•1 citation
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.
Resonance properties of the all-fiber ring cavity filled with nonlinear material - saturable rare-earth-doped fiber are analyzed and experimentally investigated. Unlike the earlier investigated erbium-doped fiber at 1550nm where the optical absorption photo-induced change (saturation) is observed only, the ytterbium-doped fiber at 1064nm demonstrates the saturation of the refractive index mainly. For this configuration we report experimental observation of the optical bistability and hysteresis in the transmitted output light at the 10mW-scale incident light power. The experimental results are in qualitative agreement with the theoretical analysis that takes into account the saturation of both parameters: the optical absorption and the refractive index of the doped fiber. The reported results seem promising for applications in high-sensitivity interferometric configurations at 1064nm operation wavelength.
A resonance-enhanced nonlinearity is used to demonstrate a digital switching response in a two-mode fiber interferometer constructed from ytterbium(III)-doped optical fiber. The fiber used was single-mode at the pump wavelength (980 nm) and two-moded at the signal wavelength (514 nm). Interferometer-based optical switches are generally sensitive to fluctuations in pump power, because of the sinusoidal dependence of the output state on pump power. However by allowing the doped fiber to lase, thus clamping the phase shift, this sensitivity can be eliminated to yield a bi-stable, digital switching response. In our interferometer configuration, a laser cavity was defined in the ytterbium-doped fiber by a matched pair of Bragg gratings, and the magnitude of the clamped phase shift was controlled by stretching one of the gratings to adjust the laser threshold. Digital switching of a 514 nm signal with 11 dB isolation and a power-length product of 1.3 mW.m was demonstrated. The relaxation time of the switch was determined by the ytterbium(III) excited state lifetime, while the rise time could be reduced by increasing the pump power without compromising the isolation. The clamping technique demonstrated in this paper should be applicable to any optical switch incorporating a gain medium.
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.