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By confining the Yb doping within a smaller radius in the center of the core of the few-mode large-mode-area fiber, the fundamental mode, which overlaps better with the Yb ions, sees higher gain than the higher order modes, and dominates the output. Hence, improved beam quality can be achieved. A confined-doped fiber with 41/395μm core/cladding diameters is fabricated by Direct Nanoparticle Deposition (DND) process. The fiber is characterized in an amplified spontaneous emission (ASE) source setup. Near-diffraction-limited beam quality (M 2 ~1.3) is experimentally demonstrated.

Atomic layer deposition (ALD) was used to fabricate an ytterbium (Yb)-doped silica fiber in combination with the conventional modified chemical vapor deposition (MCVD) method. An MCVD soot-preform with a porous layer of SiO 2 doped with GeO 2 was coated with layers of Yb 2 O 3 and Al 2 O 3 prior to sintering, using the ALD method. ALD is a surface controlled CVD-type process enabling thin film deposition over large substrates with good thickness control, uniformity and high conformality. A materials analysis study showed that the dopants successfully penetrated the full thickness of 320 μm of the soot layer. Preliminary preform and fiber experiments on refractive index profiles, background losses, lifetime and the characteristic gain-loss curve were performed demonstrating the potential of this method for fabricating Yb-doped fibers with high concentration of dopants.

By using a modal interference method, photodarkening-induced refractive index change in ytterbium doped fibers is measured with high accuracy. This refractive index change is positive at the ytterbium lasing wavelengths near 1080nm, and it gradually approaches a saturated level over time. It is found that the refractive index change is linearly proportional to the photodarkening-induced excess loss at an arbitrary probe wavelength in the visible band. The maximum refractive index change in our experiment is close to 1×10 -5 . The origin and influence of the photodarkening-induced refractive index change on fiber lasers is discussed.

We study the temperature of an Yb-doped large-mode-area (LMA) fiber during an accelerated photodarkening experiment. In these measurements, photodarkening is optically induced by IR irradiation (i.e. 915 nm) while the fiber temperature is measured by a thermal camera. Fiber temperature is observed to exceed 120 °C under conditions of 10.5 W of pump power and unforced air cooling. We show evidences that this temperature increase is caused by the lost pump power due to photodarkening. A thermal model is used to explain the fiber temperature in terms of pump power absorbed by photodarkening-induced defects. Furthermore, the effect of temperature on the rate of photodarkening and saturation of the losses is studied. Both the photodarkening saturation level and the photodarkening rate are observed to show significant temperature dependence that result on a variation of the photodarkening rate ion dependency. The use of an air cooling system and low inversion measurements is shown to reduce the ion dependency from 7 to 4.5.

We study thermal bleaching of photodarkening-induced loss in ytterbium-doped fibers. Post-irradiation heating of a photodarkened fiber is shown to result in further increase the loss which is attributed to both a permanent increase of loss-inducing color centers and a temperature-dependent broadening of the absorption spectrum. The permanent heat-induced increase of loss is believed to indicate presence of an intermediate energy state in the NIRpho tochemical mechanism for photodarkening. Further, we apply the demarcation energy curve approach to derive the thermal activation energy of the induced defects. For the studied commercial 20-μm-core-diameter LMA fiber, the energy distribution consists of a single peak, located at 1.3 eV with a FWHM of 0.31 eV.

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.

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.

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.