Auswahl der wissenschaftlichen Literatur zum Thema „Photodarkening (PD)“

In this paper, an improved mathematical model is proposed by taking the factors of high-energy photons and temperature into consideration, which is verified and explained by the experimental data in our experiments and other papers. By fitting and analyzing the experimental data, we can quantitatively determine the relationship between the pump power Pλ and the photon frequency ν in the fiber core, the core area A and the temperature T of the fiber core and PD loss, and explain the mechanism of the PD phenomenon to a certain extent. We believe that the excitation of color centers by high-energy photons is the main reason for photodarkening. Furthermore, there is a positive correlation between the power of high-energy photons and the photodarkening rate, and the temperature is positively correlated with the saturated photodarkening absorption.

We investigated photo-induced effects in Ge-Ga-Se films. It was found that, thermal annealing causes the increase of the bandgap in the as-prepared films, and all three annealed films show photodarkening (PD) behavior with light illumination but the change of optical bandgap is minimum in Ge25 film. Such PD behavior can be recovered by reannealing of the film. The kinetic process of Tf/Ti exhibits part recovering of PD during the cycle of laser on and off. Moreover, no obvious structural change can be observed in the Raman spectra of the films at different states, especially in that of Ge25 film.

Abstract We present the first demonstration of visible laser oscillation in the Dy3+-doped silica fiber pumped by a 451 nm InGaN laser diode. It was found that Ge-co-doping plays the following important roles of laser oscillation: (1) to reduce the Rayleigh scattering loss, (2) to suppress the X-ray-induced and pump-induced photodarkening (PD), and (3) to increase lasing slope efficiency. In a fiber with 0.46 wt% Dy, 1.8 wt% Ge, and 0.54 wt% Al, the slope efficiency is 22.0% at 582.5 nm, and the maximum output power is 18.4 mW.

Amorphous chalcogenide glasses are intrinsically metastable, highly photosensitive, and therefore exhibit numerous light-induced effects upon bandgap and sub-bandgap illumination. Depending on the pulse duration of the excitation laser, ChGs exhibit a series of light-induced effects spanning over femtosecond to seconds time domain. For continuous wave (CW) illumination, the effects are dominantly metastable in terms of photodarkening (PD) and photobleaching (PB) that take place via homopolar to heteropolar bond conversion. On the other hand, under nanosecond and ultrafast pulsed illumination, ChGs exhibit transient absorption (TA) that is instigated from the transient bonding rearrangements through self-trapped exciton recombination. In the first part of the review, we pay special attention to continuous wave light-induced PD and PB, while in the second part we will focus on the TA and controlling such effects via internal and external parameters, e.g., chemical composition, temperature, sample history, etc.

Abstract Effects of P codoping on X-ray/450 nm-LD induced photodarkening in Dy3+ doped silica glass and fiber were investigated to develop a visible fiber laser. P and Dy concentrations in Dy-P-doped silica glass were optimized to reduce Rayleigh scattering. It was found in optimized glass that X-ray induced photodarkening can be well suppressed by P co-doping. 450-nm LD pumped laser oscillation was firstly observed in the fiber using optimized glass. Slope efficiency is 4.2% which is lower than those in Dy-Al-and Dy-Al-Ge-doped fibers previously reported. Contrary to the results of X-ray induced PD, 450 nm-LD induced photodarkening is easily occurred.

Abstract Difference between X-ray-induced photodarkening (X-PD) and 451 nm-LD induced PD (LD-PD) was investigated in DyAlGe-doped silica glasses. The same defect species (Al-OHC, Dy2+, GEC, and GLPC+) are generated in X-PD and LD-PD, although the LD-PD defect population is 10-3 of the X-PD defect population. There is a big difference in the defect population ratio of GEC and Al-OHC and its Ge concentration dependence. The ratio in LD-PD is higher than that in X-PD. GEC poputarion of X-PD almost saturates when Ge > 2 wt%. On the other hand, GEC of LD-PD continues to increase even if Ge exceeds 2 wt%. Based on these facts, possible mechanisms of LD-PD and the role of Ge codoping were proposed.

ABSTRACTIn the chalcogenide glasses the primary defects are thought to involve under- and over-coordinated chalcogen atoms which are diamagnetic under quasiequilibrium conditions because of the presence of a strong electron-lattice interaction. These defects can be optically, or in some cases thermally, excited into metastable paramagnetic configurations which are well described as chalcogen “dangling bonds.” Alloying the chalcogenide elements with group V or group IV atoms produces additional diamagnetic defects which in their excited paramagnetic configurations also appear to be well described as dangling bonds. Some of these defects may play a role in the photodarkening (PD) process which is a metastable shift of the absorption edge to lower energies after irradiation with light of energy greater than or equal to that of the band gap. The PD process requires the presence of a non-bonding plike valence band. The microscopic description of the PD process may involve the creation of defects or the tunneling of chalcogen atoms.

We presented an experimental comparison of the core-composition difference on the suppression of the photodarkening and transverse mode instability effects. Two core-composition fibers, entailing Yb/Al/Ce and Yb/Al/P co-doped fibers, were fabricated by MCVD process combined with solution doping technique. The parameters of two fibers were almost the same. The PD-induced loss at equilibrium was 3.94 dB/m at 702 nm in Yb/Al/Ce fiber, while it was 0.99 dB/m in Yb/Al/P fiber. To obtain a deeper understanding of the impact of PD on laser performance, a bidirectional pumping fiber amplifier was constructed. Compared with Yb/Al/Ce co-doped fiber, the TMI thresholds of Yb/Al/P co-doped fiber were enhanced in co-pumped and counter-pumped schemes. Meanwhile, the slope efficiency in bidirectional scheme was promoted by 4%. Moreover, the transmittance at 638 nm confirmed the superior PD resistance of Yb/Al/P co-doped fiber. These experimental results pave the way for the further development of high-power fiber lasers.

Les travaux présentés dans ce manuscrit s’inscrivent dans le cadre d’une thèse CIFRE en colla-boration entre le laboratoire Xlim et le centre technologique optique & lasers « ALPhANOV ». L’objectif principal de cette thèse consiste à développer des préformes dopées à l’ytterbium, conçues pour résister au phénomène de photo-noircissement (PN), en vue de leur utilisation dans des applications laser à haute puissance. Ce phénomène dégrade les performances des lasers et entraîne une réduction de leur puissance de sortie. Les travaux de cette thèse sont articulés autour de deux principaux axes. Le premier vise à définir la composition du cœur ainsi que la méthode de synthèse appropriée pour fabriquer des cœurs de préformes résistant face au photo-noircissement (PN). Le deuxième axe repose sur l’élaboration, la caractérisation et l’évaluation des performances optiques de la fibre obtenue. La matrice choisie pour être étudiée est la matrice alumino-phosphosilicate. Elle a été obtenue par une méthode de synthèse appelée « poudre en suspension ». Les différentes étapes de ce procédé de fabrication ont été validées depuis la synthèse de la préforme jusqu’à l’étirage de la fibre optique. Les tests optiques effectués sur la fibre optique ont révélé d’excellentes performances, avec une faible atténuation de 0.1 dB/m et une efficacité de conversion optique- optique très prometteuse de 76 %. L’évaluation de cette fibre face au phénomène de photo- noircissement a révélé une résistance exceptionnelle. Aucune décroissance en puissance n’a pu être constatée au cours des expériences effectuées sur plus de 100 h
The work presented in this manuscript is part of a PhD thesis carried out by CIFRE in collabora-tion between the laboratory of Xlim and the technological center of optics and lasers "ALPhA-NOV". The main objective of this thesis is to design ytterbium doped preforms that are resistant to the photodarkening (PD) phenomenon for use in high power laser applications.This phenomenon degrades laser performance and leads to reduced output. Two main areas are focused in this thesis. The first is the definition of the core composition and the appropriate synthesis method for the production of preform cores which are resistant to photodarkening (PD). The second is to develop, characterize and evaluate the optical performance of the resulting fiber. The matrix that has been chosen for the study is the alumino-phosphosilicate matrix. To obtain this matrix, a synthesis method called "powder in suspension" was used.From the synthesis of the preform to the drawing of the optical fiber, the different stages of the process have been validated. Excellent performance, with low attenuation of 0.1 dB/m and a very promising optical-to-optical conversion efficiency of 76%, was obtained from the optical tests performed on the fiber. The evaluation of the fiber against photodarkening showed an exceptional resistance. No performance degradation was observed over 100 hours