Joan J. Montiel i Ponsoda

Ytterbium-doped fibers fabricated with atomic layer deposition method

We report on a new fabrication method of producing ytterbium doped fibers by atomic layer deposition (ALD) in combination with the conventional modified chemical vapor deposition (MCVD) technique. An MCVD soot-preform with a porous layer of SiO(2) is coated with layers of Yb(2)O(3) and Al(2)O(3) prior to sintering, using the gas-phase ALD method. An SEM/EDS material analysis study shows that the dopants successfully penetrate the full thickness of 320 µm of the soot layer. An Yb-doped fiber fabricated by this technique shows a background attenuation of 20 dB/km, a uniform longitudinal Yb-doping profile, and good laser characteristics with a slope efficiency of 80%. Furthermore, we present a comparison in terms of photodarkening between the MCVD-ALD fiber and a solution doped fiber, fabricated with the same MCVD recipe. The new MCVD-ALD fiber appears to be more photodarkening resistant.

Heat-induced darkening and spectral broadening in photodarkened ytterbium-doped fiber under thermal cycling

We study thermal bleaching of photodarkening-induced loss in a 20-microm core diameter, large-mode-area ytterbium-doped silica fiber. Pristine and photodarkened samples are subjected to thermal cycling pulses. Recovery of the photodarkened fiber absorption coefficient initiates at approximately 350 degrees C and complete recovery is reached at approximately 625 degrees C. However, prior to recovery, the photodarkened fiber exhibits further heat-induced increase of absorption loss. This increase of loss is attributed to both a permanent increase of loss-inducing color centers and a temperature-dependent broadening of the absorption spectrum. Post-irradiation heat-induced formation of color centers suggests the presence of an intermediate energy state in the near-infrared photochemical mechanism for photodarkening.

Thermal bleaching of photodarkening-induced loss in ytterbium-doped fibers.

We study the thermal bleaching process of photodarkening-induced loss in ytterbium-doped fibers. The observed power-law time dependence of the normalized absorption coefficient is well represented by the thermal decay model of Erdogan et al. [J. Appl. Phys. 76, 73 (1994)]. Using this model, we derive the activation energy associated with thermal recovery of the induced loss to prephotodarkened state. For the studied commercial 20 microm core diameter large-mode-area fiber, the energy distribution consists of a single peak, located at 1.32 eV with a FWHM of approximately 0.31 eV. The results presented are of particular importance to understanding photodarkening and bleaching processes in high-power fiber lasers.

Refractive index change in ytterbium-doped fibers induced by photodarkening and thermal bleaching

High-accuracy measurements and analysis of refractive index change induced by photodarkening and thermal bleaching in ytterbium-doped fibers are presented, based on a modal interference method. Photodarkening-induced refractive index change is positive at the ytterbium lasing wavelengths near 1080 nm, and it approaches a saturated level, which is in the order of 10(-6)∼10(-5) for the tested fiber samples. It is found that the value of this refractive index change is linearly proportional to the photodarkening-induced excess loss at an arbitrary probe wavelength in the visible band. Thermal bleaching can only partially erase the photodarkening-induced refractive index change, leaving a residual index change of (0.2∼0.3)×10(-5). The influence of the photodarkening-induced refractive index change on fiber lasers is discussed.

Mode-induced transverse photodarkening loss variations in large-mode-area ytterbium doped silica fibers

Transverse photodarkening loss variations in an LMA Yb-DCF are experimentally studied. Photodarkening rate depends on inversion, which is affected by the pump induced increase and signal induced depletion of the inversion. In double-clad fibers, intensity distributions of the pump and signal modes and their overlap with the core are significantly different, leading to transverse differences in inversion within the core. Moreover, practical fiber laser configurations aim at generating and preserving only the fundamental transverse-mode thus creating a high contrast in inversion within the core. Therefore, dramatically different rates of photodarkening across the core of the active fiber can be expected. We demonstrate the existence of transverse mode-induced photodarkening loss variations in an LMA Yb-DCF laser and discuss its implications. Composition-related transverse photodarkening loss variations are measured to be negligible in the studied Yb-DCF.

Combined photodarkening and thermal bleaching measurement of an ytterbium-doped fiber

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.

Analysis of temperature dependence of photodarkening in ytterbium-doped fibers

We examine the temperature dependence of photodarkening in ytterbium-doped silica fibers. A sequence of consecutive photodarkening experiments are performed over the same fiber sample, which shows good repeatability with no apparent changes in the glass structure. We find that during infrared irradiation, the level of saturation of the losses can be determined by the fiber core temperature, independent of the previous state of photodarkening losses and fiber temperature, and also at low temperatures where the thermal bleaching is not activated. We observe that variations in the fiber core temperature, induced by pump absorption due to photodarkening, affect the inversion level and photodarkening processes. These effects in turn cause a discrepancy in determining the ion dependence. We highlight the importance of performing the experiments under isothermal conditions and we propose a new approach to control the fiber temperature at room temperature and at elevated temperatures. The approach is based on an isothermal Galinstan bath. The appropriateness of this method is shown by comparing it to different cooling methods, and the results are supported by simulations.

Photodarkening-induced increase of fiber temperature

We examine the effect of photodarkening-induced pump light absorption on Yb-doped fiber thermal loading. In these experiments, the fiber is cladding pumped at 915nm, air cooled by natural convection, and monitored with a midinfrared thermal camera. The fiber temperature is found to correlate with progressive photodarkening. The maximum observed fiber temperature was 120 degrees C, recorded at a pump power of 10.5W. The observed increase in fiber temperature can be explained by a model that takes into account the combined effects of the Yb-silica quantum defect, the temperature dependence of the Yb-absorption cross section, and photodarkening-induced loss at 915nm. We hypothesize that the latter effect results in the progressive activation of temperature-dependent photochemical processes (e.g., bleaching) and is an important consideration with regard to the accurate modeling of Yb-doped fiber photodarkening kinetics.

Measurement of thermal binding energy of photodarkening-induced color centers in ytterbium-doped silica fibers

We report on thermal bleaching measurements of photodarkening-induced color centers in ytterbium-doped silica fibers. We make use of the similarity between UV and photodarkening-induced defects and apply a model, developed to describe and predict the performance of thermal decay of UV-induced fiber Bragg gratings [1], to study the thermal binding energy associated with photodarkening-induced color centers. This model assumes a broad distribution of trap sites, with the rate of thermal depopulation being an activated function of the trap depth [1]. Fraction of depopulated traps is uniquely defined by the demarcation energy E<inf>d</inf>, with traps having energy greater than E<inf>d</inf> remaining unchanged. Time t and temperature T are related through E<inf>d</inf> = k<inf>B</inf>T·ln(v<inf>0</inf>t), where k<inf>B</inf>=8.62·10⁻⁵ eV/K is Boltzmanns constant and v<inf>0</inf> is a frequency term (attempt frequency), obtained by acquiring datasets at different temperatures and fitting them together.

Confined-doped ytterbium fibers for beam quality improvement: fabrication and performance

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 (M2~1.3) is experimentally demonstrated.