MEASURING PHOTODARKENING FROM Yb DOPED FIBERS

OF DOCTORAL DISSERTATION HELSINKI UNIVERSITY OF TECHNOLOGY P.O. BOX 1000, FI-02015 TKK http://www.tkk.fi Author Joona Koponen Name of the dissertation Measuring photodarkening from Yb doped fibers Manuscript submitted 12.9.2008 Manuscript revised 7.11.2008 Date of the defence 12.12.2008 Monograph Article dissertation (summary + original articles) Faculty Faculty of Electronics, Communications and Automation Department Department of Micro and Nanosciences Field of research Photonics Opponent(s) Dr. Lars Norin Supervisor Prof. Seppo Honkanen Instructor Abstract 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 pumpor 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 7 power dependency to inversion; and more generally, to have a 7 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).