Modeling the photodegradation of large mode area Yb-doped fiber power amplifiers Pierre Laperle, Louis Desbiens, Karine Le Foulgoc, Mathieu Drolet, Pascal Deladurantaye, Antoine Proulx, and Yves Taillon INO, 2740 Einstein Street, Quebec City, Quebec, Canada, G1P 4S4 ABSTRACT Photodarkening is presently a major concern for the long term reliability and efficiency of high power Yb-doped fiber lasers and amplifiers. This phenomenon has been associated with the formation of color centers in the fiber core of single-clad and large mode area Yb-doped fibers. However, its origin is still not well understood and to date no comprehensive model that could predict the lifetime of Yb-doped fiber-based devices has been put forward. A semi- empirical approach seems at the moment the best way to gain a better understanding of the growth behavior of photo- induced losses in Yb-doped fibers in the presence of both photodarkening and photobleaching processes. A rate equation describing the activation and deactivation of color centers involving stretched exponential functions has been developed. For this approach to be effective and reliable, a minimum of parameters is used, four to describe photodarkening and three for photobleaching. A large mode area Yb-doped fiber fabricated at INO using the MCVD process has been characterized. By properly choosing the initial pumping conditions, each parameter of the stretched exponential functions has been measured separately from the others. The model has then been used to simulate the power decay from a 1 kW, 10 ns-pulse, 100 kHz Yd-doped LMA fiber power amplifier. We show that the photodarkening behavior predicted by the model is in good agreement with the experimental results over more than 6000 hours. Such a model is general in its application but the stretched exponential parameters are unique to the type of fiber tested. The model will be a useful characterization tool for developing photodarkening-resistant fibers and for evaluating the lifetime of Yb- doped fiber-based devices affected by photodegradation. Keywords: Fiber amplifier, large mode area, ytterbium, triple-clad, photodarkening, photobleaching, modeling, fiber characterization 1. INTRODUCTION Photodarkening in Yb-doped fibers has received a lot of attention in recent years due to its detrimental effect on the performance of fiber lasers and amplifiers. Several techniques for characterizing LMA and single-clad fibers have been developed [1-3] to get a better understanding of the underlying mechanism of photodarkening. The following key facts have been reported in the literature thus far. Yb 3+ -doped silica fibers with high content of Al and/or P and phosphate fibers are more resistant to photodarkening [4-9]. Photodarkening depends on Yb population inversion and is spatially distributed along the length and cross section of the fiber [4,10-13]. The excess loss spans the UV to near-infrared spectrum and has a finite maximum value [14-16]. UV light and heat restore photodarkened fibers to their pristine conditions [15,17-19]. And finally, O 2 or H 2 loading reduces photodarkening [20,21]. Attempts have also been made to explain the origin of photodarkening by invoking oxygen deficiency centers (ODC) or charge-transfer-states (CTS) and the presence of Yb 2+ [7,22-26]. The debate is still on as whether ODC or CTS is the main mechanism responsible for photodarkening [27,28]. On the theoretical side, a model has been proposed linking excess loss to non-binding oxygen [29] but bi-exponential functions [30] or stretched exponential functions [16,25] are most often used to fit experimental data. However, these approaches are not suited for long-term prediction of fiber lasers and amplifiers output power degradation and fiber lifetime evaluation. We developed a semi-empirical model of photodegradation and a characterization method for Yb-doped LMA and single-clad fibers in terms of photodarkening and photobleaching processes with rate coefficients dependent on Yb population inversion and signal intensity. The model is not restricted to Yb but is applicable to any of the rare earth ions used in fiber lasers and amplifiers. A rate equation and seven parameters determined separately from experimental data Fiber Lasers VI: Technology, Systems, and Applications, edited by Denis V. Gapontsev, Dahv A. Kliner, Jay W. Dawson, Kanishka Tankala, Proc. of SPIE Vol. 7195, 71952C © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.808189 Proc. of SPIE Vol. 7195 71952C-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/18/2013 Terms of Use: http://spiedl.org/terms