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

Theoretical and experimental characterization of thermal effects in doped fibers

Monica K. Davis, Michel J. F. Digonnet

Published Dec 21, 1998• 1 citation
Publisher

Abstract

Nonradiative decay mechanisms in doped fibers introduce heat into the fiber core and often decrease the upper state lifetime of a dopant. This can be detrimental to doped-fiber lasers, amplifiers, and in some cases all-optical switches. In this paper we report theoretical and experimental studies of thermal effects in doped fibers, with particular emphasis on their impact on all-optical nonlinear switches using resonant nonlinearities. We observe significant thermal effects in transition metal-doped silica fibers. We determine that sub-microsecond(s) switching using a resonant nonlinear effect in a conventional switch architecture requires dopant oscillator strengths of 3.6 10-3 or greater when nonradiative processes are the predominant decay mechanism.

DOI
10.1117/12.333760

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Nonradiative decay mechanisms in doped fibers introduce heat into the fiber core and often decrease the upper state lifetime of a dopant. This can be detrimental to doped-fiber lasers, amplifiers, and in some cases all-optical switches. In this paper we report theoretical and experimental studies of thermal effects in doped fibers, with particular emphasis on their impact on all-optical nonlinear switches using resonant nonlinearities. We observe significant thermal effects in transition metal-doped silica fibers. We determine that sub-microsecond(s) switching using a resonant nonlinear effect in a conventional switch architecture requires dopant oscillator strengths of 3.6 10-3 or greater when nonradiative processes are the predominant decay mechanism.
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.>
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.
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.
We report a new theoretical interpretation of the strong resonantly-enhanced third-order nonlinearity observed in fibers doped with Nd/sup 3+/, Er/sup 3+/, or Yb/sup 3+/. For each fiber, the absolute contributions of the near resonance (IR-visible) and far from resonance (UV) transitions to the nonlinearity are calculated and compared to experimental values. We show that in all cases, a large fraction of the measured nonlinearity is due to strong UV transitions. The consequences of this observation on the bandwidth, switching power and response time of switches based on this effect are discussed.
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.