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Recently, significant power losses in ytterbium-doped fibers have been interpreted as resulting from the formation of ytterbium ion pairs [A. V. Kir’yanov et al., Opt. Express 14, 3981 (2006)]. However, there appears to be strong evidence against this model.
©2006 Optical Society of America
Significantly non-ideal performance of ytterbium-doped fibers in terms of the power conversion efficiency of ytterbium-doped lasers and amplifiers has been observed by a number of researchers. Reference [1] recently also reported on such phenomena, in particular a strongly reduced nonlinear transmission, and attributed the observations to the formation of ytterbium ion pairs. Even though the presented numerical model appears to be in reasonable quantitative agreement with experimental nonlinear transmission data, there is strong evidence against the given interpretation.
First, the green cooperative emission as typically observed from such fibers is orders of magnitude too weak to explain strong power losses. On the other hand, there seems to be no compelling reason to postulate a multiphonon process which can strongly quench ion pairs but not single ions. In principle, such a mechanism could exist if ytterbium ion pairs would have intermediate electronic levels, but these should also lead to significant absorption at longer wavelengths, which has to our knowledge not been observed in such fibers.
Second, the observed concentration dependence of the additional decay process is not compatible with the expectation that the density of ytterbium ion pairs should grow at least in proportion to the square of the ytterbium concentration. The quantity 1/τp, indicating the rate of decay per excited ion, should thus rise at least in proportion to the ytterbium concentration, i.e., the parameter β should either stay constant or decrease with increasing concentration. (Note that a smaller value of β indicates a larger decay rate.) The values of 1/τp fitted to the experimental data for different fibers, however, appear to be nearly independent of the doping concentration, apart from the first data point; β thus increases with increasing N 0. This observation is obviously not compatible with the assumption that ion pairs are responsible for the decay process.
We also note that earlier work already presented significant evidence for other decay mechanisms. In particular, there appear to exist color centers to which ytterbium ions can transfer their excitation energy [2]. There is also a gradual photodarkening as reported in Ref. [3]. Further work is required to definitely clarify the origin of these technologically important phenomena. So far it seems that there is little evidence for a particular role of ytterbium ion pairs.
References
1. A. V. Kir’yanov, Y. O. Barmenkov, I. L. Martinez, A. S. Kurkov, and E. M. Dianov, “Cooperative luminescence and absorption in Ytterbium-doped silica fiber and the fiber nonlinear transmission coefficient at λ=980 nm with a regard to the Ytterbium ion-pairs’ effect,” Opt. Express 14, 3981–3992 (2006). [CrossRef] [PubMed]
2. R. Paschotta, J. Nilsson, P. R. Barber, J. E. Caplen, A. C. Tropper, and D. C. Hanna, “Lifetime quenching in Yb-doped fibres,” Opt. Commun. 136, 375–378 (1997). [CrossRef]
3. J. J. Koponen, M. J. Söderlund, S. K. T. Tammela, and H. Po, “Photodarkening in ytterbium-doped silica fibers,” in Optically Based Materials and Optically Based Biological and Chemical Sensing for Defence II; J. C. Carrano, A. Zukauskas, A. W. Vere, J. G. Grote, and F. Kajzar; Eds., Proc. SPIE 5990, 72–81 (2005).