A new measurement technique for characterizing the magnitude of power coupling from the fundamental mode to higher
order core- and cladding-bounded modes occurring in a fiber Bragg grating (FBG) inscribed in a large-mode-area (LMA)
fiber is demonstrated and studied. The method is based on inducing mode selective fiber bending losses on the modes
propagating in the core and monitoring the power guided by the cladding of the LMA fiber. Besides transmitted, also
reflected distributions of modes can be resolved in terms of the relative powers carried by them and thus the fraction of
higher order modes (HOMs) can be quantified. Additionally, the method can distinguish the mode content spectrally
with high resolution. Sample FBGs having a chirped index profile are characterized using the method. It is shown that
the method can yield information that is useful for better design and optimization of fiber optic devices utilizing FBGs in
LMA fibers, such as fiber lasers.
Photodarkening is a detrimental phenomenon known to affect ytterbium doped fibers. Methods to study the spectral and
temporal properties of the photodarkening induced loss were developed. The spectral shape of the photodarkening loss
measured from multiple aluminosilicate samples indicate that visible wavelength(s) could be used in benchmarking
fibers for their PD induced loss. Two principal methods, core and cladding pumping, were introduced to induce a known
and repeatable inversion to fiber samples. The photodarkening rate could be parameterized using a single variable,
inversion. More generally, the photodarkening rate was found to follow a simple power law and to be proportional to
[Yb]7±1 (the excited state Yb ion density). Two methods, stretched exponential and bi-exponential, were used to fit the
rate measurements. Both fitting methods were found suitable, with the bi-exponential method having more potential in
increasing the understanding of the mechanism(s) behind photodarkening. Coiling induced spatial changes in the
inversion and subsequent photodarkening performance were demonstrated for a large-mode-area fiber laser.
A combined photodarkening and thermal bleaching measurement of a large-mode-area (LMA) ytterbium-doped fiber
(YDF) is presented. Photodarkened YDF sample is recovered to pre-photodarkened state by thermal annealing. As a
result, this approach enables repeated measurements with the same sample and therefore eliminates uncertainties related
to changing of the sample (such as sample length and splice losses). Additionally, our approach potentially improves the
accuracy and repeatability of the photodarkening rate measurement, and also allows automation of the measurement
procedure.
Accuracy at which population inversion in an ytterbium-doped fiber can be determined by modeling is studied. Here inversion refers to the percentage of ions excited to a higher energy level by the various optical fields. Knowledge of rare-earth ion inversion is crucial for determining the photodarkening behavior of a fiber, but can also be used to study the gain and noise properties of amplifiers. Sample fibers are first characterized for their optical and mechanical properties (e.g. absorption and fluorescence spectrum, excited state lifetime, rare-earth concentration and geometrical dimensions). Fiber specific absorption and emission cross-sections are then derived from the measured fiber parameters. Two methods (i.e. McCumber theory and Fuchtbauer-Ladenburg relation) are used to determine the detailed shape and the relative level of absorption to emission at different wavelengths. A full numerical model is used to simulate both core and cladding pumped YDFs. In order to validate the inversion results produced by the simulator a comparison between the measured and simulated ASE spectra is made. Sensitivity of the simulated ASE spectrum on the different parameters is investigated. Uncertainty analysis is made to show the contributions of various measured parameters on the uncertainty of the inversion. The principal contributor of uncertainty on the inversion was found to be the cross-section values.
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