Measuring photodarkening from Yb doped fibers

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TKK dissertations, 147
Yb doped fibers are used widely in applications requiring high beam quality and efficiency. A detrimental phenomenon, photodarkening, is known to reduce the efficiency of fiber devices under some operating conditions, and in extreme cases to prevent lasing altogether. Photodarkening manifests as an incrementally increasing spectrally broad transmission loss in the Yb doped core of a fiber, and it is caused by the exposure to pump- or signal photons. This work focused on development of methods to benchmark fibers for photodarkening, and to develop methods to study the temporal and spatial characteristics of photodarkening. A measurement method for conducting benchmarking and spectral studies on single-mode fibers was developed, and a similar alternative method was developed for large-mode-area fibers. Both approaches were studied experimentally and by simulations. Additionally, an experimental setup was built to study the spatial photodarkening differences in large-mode-area fibers. Observations agreed with the simulated results. Photodarkening rate was found to have a repeatable spectral response with fibers of similar but varying compositions, and the photodarkening induced temporal transmission loss was found to follow a stretched-exponential decay rate, and a bi-exponential decay rate. Photodarkening was found to be proportional to the inversion of a fiber sample; and also to have a 7th power dependency to inversion; and more generally, to have a 7th power dependency to the excited state Yb ion density. The methodology obtained through this work enable benchmarking of Yb doped single-mode and large-mode-area fibers. The observed excited state ion density dependency to the photodarkening rate has strong implications to Yb fiber devices, as a given fiber may photodarken with a high rate in one application (having a higher inversion), and non-measurably in another application (having a lower inversion).
ytterbium, photodarkening, optical fiber, metrology
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  • [Publication 1]: J. J. Koponen, M. J. Söderlund, H. J. Hoffman, and S. K. T. Tammela. 2006. Measuring photodarkening from single-mode ytterbium doped silica fibers. Optics Express, volume 14, number 24, pages 11539-11544. © 2006 Optical Society of America (OSA). By permission.
  • [Publication 2]: Joona Koponen, Mikko Söderlund, Hanna J. Hoffman, Dahv Kliner, and Jeffrey Koplow. 2007. Photodarkening measurements in large-mode-area fibers. In: Donald J. Harter, Andreas Tünnermann, Jes Broeng, and Clifford Headley III (editors). Fiber Lasers IV: Technology, Systems, and Applications. Proceedings of SPIE, volume 6453, 64531E.
  • [Publication 3]: Joona Koponen, Mikko Söderlund, Hanna J. Hoffman, Dahv A. V. Kliner, Jeffrey P. Koplow, and Mircea Hotoleanu. 2008. Photodarkening rate in Yb-doped silica fibers. Applied Optics, volume 47, number 9, pages 1247-1256. © 2008 Optical Society of America (OSA). By permission.
  • [Publication 4]: Joona Koponen, Marko Laurila, and Mircea Hotoleanu. 2008. Inversion behavior in core- and cladding-pumped Yb-doped fiber photodarkening measurements. Applied Optics, volume 47, number 25, pages 4522-4528. © 2008 Optical Society of America (OSA). By permission.
  • [Publication 5]: J. Koponen, M. Laurila, and M. Hotoleanu. 2008. Demonstration of spatial distribution of photodarkening in Yb-doped large-mode-area fibre laser. Electronics Letters, volume 44, number 16, pages 960-961.