Silica Glass Preforms Research Articles

Heat treatment and fiber drawing effect on the luminescence properties of RE-doped optical fibers (RE = Yb, Tm, Ho).

We investigate the influence of various optical fiber fabrication processes on the fluorescence decay of RE ions commonly used in fiber lasers and amplifiers, i.e. Yb3+, Tm3+ and Ho3+. Optical fiber preforms were prepared using the MCVD method combined with Al2O3 nanoparticle doping and subjected to subsequent heat treatment processes such as preform elongation and fiber drawing. The fluorescence decay of RE ions was measured in multiple stages of optical fiber preparation: in an original preform, in an elongated preform (cane), in a standard fiber, and in an overcladded fiber. It was found that heat treatment processing of the preforms generally leads to a faster fluorescence decay, which can be explained by the diffusion of dopants and clustering of RE ions. The fiber drawing exhibited a greater effect compared to preform elongation, which was ascribed to a faster cooling rate of the process. In general, the heat treatment of RE-doped silica glass preforms leads to the decline of fluorescence decay.

FTIR multispectral imaging of P/Al-doped silica glass preforms prepared by MCVD in combination with the high temperature evaporation of AlCl3

For high power fiber lasers, codoping with Al, P or both is necessary to prevent rare earth (RE) clustering in the silica network of the laser active core material. Here, we present a complementary infrared (IR) based multispectral method combined with elemental analysis data on core/cladding to describe the structure of the doped core material as fabricated by chemical vapor deposition with gas phase doping of Al and P. The resulting 2D image and its corresponding 3D visualization of the data enable an alternative and convenient way to characterize the main species of the dopants aside from NMR measurements.

Investigation of the Incorporation of Cerium Ions in MCVD-Silica Glass Preforms for Remote Optical Fiber Radiation Dosimetry.

The incorporation of Ce3+ ions in silicate glasses is a crucial issue for luminescence-based sensing applications. In this article, we report on silica glass preforms doped with cerium ions fabricated by modified chemical vapor deposition (MCVD) under different atmospheres in order to favor the Ce3+ oxidation state. Structural analysis and photophysical investigations are performed on the obtained glass rods. The preform fabricated under reducing atmosphere presents the highest photoluminescence (PL) quantum yield (QY). This preform drawn into a 125 µm-optical fiber, with a Ce-doped core diameter of about 40 µm, is characterized to confirm the presence of Ce3+ ions inside this optical fiber core. The fiber is then tested in an all-fibered X-ray dosimeter configuration. We demonstrate that this fiber allows the remote monitoring of the X-ray dose rate (flux) through a radioluminescence (RL) signal generated around 460 nm. The response dependence of RL versus dose rate exhibits a linear behavior over five decades, at least from 330 µGy(SiO2)/s up to 22.6 Gy(SiO2)/s. These results attest the potentialities of the MCVD-made Ce-doped material, obtained under reducing atmosphere, for real-time remote ionizing radiation dosimetry.

Tm3+-doped silica-glass fiber for ∼2-μm fiber laser.

Tm3+-doped silica-glass preforms were fabricated by the modified chemical vapor deposition process combined with the solution doping technique. Its spectroscopic properties were evaluated according to the detected absorption spectrum and the fluorescence spectrum. The calculated maximum emission cross section of Tm3+ ions in this core glass is 5.66×10-21 cm2, and the tested fluorescence lifetime is 544μs at 1816nm. According to the theory of Dexter and Forster, the cross-relaxation rate was calculated by the integral overlap of the absorption and emission cross sections. A double-cladding fiber with a core NA of 0.14 was prepared by high temperature drawing. A maximum 14.5-W fiber laser output centered at 2001nm with a slope efficiency of 50.6% was obtained from a 4.5-m-length double-cladding fiber.

Evidence of AlOHC responsible for the radiation-induced darkening in Yb doped fiber

Using a combination of experimental techniques such as optical absorption, Raman scattering, continuous wave and pulse Electron Spin Resonance (ESR), we characterize a set of γ-irradiated Yb(3+) doped silica glass preforms with different contents of phosphorous and aluminum. We demonstrate that when P is introduced in excess compared to Al, nearly no radiodarkening is induced by γ-rays. On the other hand, when Al>P, a large absorption band is induced by radiation. Thermal annealing experiments reveal the correlation between the decrease of the optical absorption band and the decrease of the Al-Oxygen Hole Center (AlOHC) ESR signal, demonstrating the main role of AlOHC defects in the fiber darkening. HYSCORE (HYperfine Sublevel CORElation) pulse-ESR experiments show a high Al-P nuclear spin coupling when P>Al and no coupling when Al>P. This result suggests that both AlOHC and POHC creation is inhibited by Al-O-P linkages. Confronting our data with previous works, we show that the well-known photodarkening process, meaning losses induced by the IR pump, can also be explained in this framework.

Experimental evidence for the formation of divalent ytterbium in the photodarkening process of Yb-doped fiber lasers

In this work we present experimental evidence that the valence instability of the ytterbium ion play a key role for the observed photodarkening mechanism in Yb-doped fiber lasers. Luminescence and excitation spectroscopy performed on UV irradiated Yb/Al doped silica glass preforms and near-infrared diode pumped photodarkened fibers show a concentration increase of Yb(2+) ions. A concentration decrease in Yb(3+) could also be observed for the UV irradiated preform. The findings contribute to an increased understanding of the kinetic processes related to photodarkening in Yb-doped high power fiber lasers.

Yb/P/Al共掺双包层光纤的制备与激光性能

Yb/P/Al codoped silica glass preforms are fabricated by MCVD process combined with the solution doping technique.Highly and homogeneously doping of Yb3+ is realized by optimization of the core composition and fabrication process.The Yb2O3 concentration in preform core is up to ~4 wt.% and Yb3+ fluorescence lifetime at 1080nm reaches 1 780 μs.Double-clad fibers with octagonal inner cladding are successfully drawn from the preform,the fiber core diameter is 7.5 μm and the clad absorption coefficicent is up to ~5 dB/m@976 nm.The operation of the cladding pumped all-fiber laser is demonstrated using Yb-doped double-clad fibers.Fiber lasers with output power of 5.15 W and slope efficiency of 76% are obtained.

Yb2O3-doped YAG nano-crystallites in silica-based core glass matrix of optical fiber preform

Abstract Yb2O3-doped yttrium aluminium garnet (YAG) nano-crystals within the silicate glass-based optical fiber preforms were obtained through the conventional-modified chemical vapour deposition (MCVD) and solution doping technique. Nano-crystals were developed with soaking of the porous phospho-silica or pure silica core layer in a solution containing the ytterbium, yttrium and other co-dopants, including fluorine using 10–15% fluorosilicic acid, and through post-annealing of the preform at a temperature of around 1450 °C. The size, shape and nature of Yb2O3-doped phase-separated nano-crystallites were evaluated from HRTEM images along with the electron diffraction pattern based on the doping levels of phosphorous and fluorine. The size of nano-crystallites was maintained within 6–10 nm when doped with 0.25 mole% of fluorine. X-ray analyses EDX data reveals that the nano-particles are rich in Yb:YAG, and uniformly dispersed into the amorphous silica glass matrix. The novelty of this technique involves the direct synthesis of rare-earth doped phase-separated nano-crystallites within the core region of silica glass preforms. This class of fibers containing the nano-particles with or without Yb:YAG crystalline nature will keep the advantage of the mechanical properties as well as good lasing properties under high power application compared to the Yb:YAG ceramic laser.

掺镱石英光纤预制棒的制备与表征

掺镱石英光纤预制棒的制备与表征

Nanoindentation study on germania-doped silica glass preforms: evidence for the compaction-densification model of photosensitivity

Nanoindentation technique was employed to measure the changes in mechanical properties of a glass preform subjected to different levels of UV exposure. The results reveal that short-term exposure leads to an appreciable increase in the Young's modulus (E), suggesting the densification of the glass, confirming the compaction-densification model. However, on prolonged exposure, E decreases, which provides what we believe to be the first direct evidence of dilation in the glass leading into the Type IIA regime. The present results rule out the hypothesis that continued exposure leads to an irreversible compaction and prove that index modulation regimes are intrinsic to the glass matrix.

Preparation and properties of NiAl(Si)/Al 2O 3 co-continuous composites obtained by reactive metal penetration

Reactive metal penetration was used to prepare intermetallic–ceramic composites with co-continuous structure, starting from silica glass preforms. Two subsequent metal penetrations were performed: first, the silica was immersed in a liquid Al bath, obtaining an Al(Si)/Al 2O 3 composite, then Ni was put in contact with the composite at high temperature, bringing to the substitution of Al with a Ni–Al intermetallic. The obtained composites present both phases continuous, and the whole process is a near net-shape one. The intermetallic phase is based on the Ni–Al system, with small Si content (lower than 2%) and its composition ranges from Al 3Ni 2 to a mixture of NiAl and Ni 3Al depending on the Ni content during the second penetration step. The composites present high hardness and melting point, low thermal expansion coefficient and good mechanical properties.

Influences of Yb 3+ ion concentration on the spectroscopic properties of silica glass

We investigated optical spectroscopic properties of silica glass doped with ytterbium (Yb 3+) ions to assess their lasing performance in diode-laser pumped systems. The Yb-doped silica glass preforms were fabricated by solution doping technique using the MCVD process. The stimulated emission cross-section and laser performance parameters were determined from the measured absorption spectra using the method of reciprocity. Fluorescence decay characteristics were observed to be deviated from exponential behavior as the Yb 3+ ions doping level increases and there exists a lifetime quenching behavior related to the cluster effect of Yb 3+ ions into silica glass. Nevertheless, lasing parameters indicated that clustering of Yb 3+ ions does not significantly affect the spectral properties relevant to the predicted lasing performance when concentration is low, but becomes predominant at higher concentration.

Fabrication of Yb-doped silica glass through the modification of MCVD process

A method to deliver Yb and Al compounds in vapor phase to the reaction/deposition zone has been devised for modified chemical vapor deposition (MCVD) process. By using this MCVD setup, we succeeded to prepare silica glass preforms presenting uniform dopants concentrations in the radial and longitudinal directions with good reproducibility. Preforms with a core diameter larger than 5 mm were easily prepared by depositing around 40 layers. By changing some parameters in the deposition step, such as carrier gases flow and temperatures of Yb(DPM) 3 and AlCl 3 furnaces, different concentrations of Yb and Al were incorporated into the core region of the silica glass preforms. By adjusting the Yb and Al concentrations, we succeeded to prepare preforms for large mode area (LMA) fiber with a change less than 10% of nominal refractive index.

Characterization of Yb 3+ clusters in silica glass preforms

The aim of this study is the optimisation of Yb doped silica preforms which are used for the realization of high power laser fibres. A spectroscopic study has thus been carried on using a stable setup conducing to quantify in the same time the Yb 3+ emission in the visible and in the infrared spectral ranges. In this way, the impact of the Al ions on the silica solubility as well as the low Yb 3+ doping levels reachable when the silica preforms are simply doped with Yb ions, have been clearly put in evidence.

Intrinsic loss of optical fibers

Rayleigh scattering is a dominant factor in terms of optical fiber loss. We investigated the dopant concentration and fictive temperature dependence of Rayleigh scattering in silica-based optical fibers and their glass preforms fabricated by the vapor-phase axial deposition (VAD) technique. The lowest Rayleigh scattering coefficient, which was obtained for a P 2O 5-doped silica preform, was about 80% of the pure silica value. In contrast, the Rayleigh scattering coefficients of F-doped and GeO 2-doped and pure silica glass preforms increased by 5–10% when they were heated to 1800 °C because the density fluctuation is proportional to their fictive temperature. We also obtained very low Rayleigh scattering coefficients in fabricated fibers by controlling the preform composition or the fiber drawing conditions. We used these results to reevaluate the intrinsic losses of P 2O 5-doped, pure, and GeO 2-doped silica core fibers and found them to be 0.095–0.130 dB/km at 1.55 μm by using the Rayleigh scattering coefficients of their preforms.

High-rate IR laser evaporation of silica glass

IR (10.6 μm) laser irradiation of silica glass preforms ensures rapid evaporation of the surface layer. The resulting deposit consists of amorphous SiO2 with a particle size of ∼ 70 nm.

Effect of H 2/O 2 ratio on the GeO 2 concentration profile in SiO 2:GeO 2 glass preforms prepared by vapor-phase axial deposition

GeO 2 radial concentration in silica glass preforms, prepared by vapor-phase axial deposition (VAD) method, was analyzed by X-ray fluorescence (XRF) measurements. The results were used to evaluate the effect of the H 2/O 2 ratio used during the deposition process in the formation of the GeO 2 concentration profile. Considering four different H 2/O 2 ratios, GeO 2 distribution was observed to decrease monotonically with the increasing radius for H 2/O 2⩽1.5, and the acute shape around the core center of the GeO 2 concentration profile, decreased with increasing H 2/O 2 ratio. When the ratio H 2/O 2=2.5, the central doping of GeO 2 was minimal, and a constant distribution was obtained along glass preform radii. The results seem to indicate that the control of spatial distribution in the concentration of GeO 2 deposited is favorable for the ratio H 2/O 2⩽1.5.

Scattering property of fluorine-doped silica glasses

The Rayleigh scattering coefficients of fluorine (F) doped silica glasses are invertigated experimentally and theoretically. They are measured for F-doped silica glass preforms fabricated by the VAD method. It is clarified that the Rayleigh scattering coefficients of F-doped silica glasses increase with increasing F concentration because the concentration fluctuation increases.

Consolidation-atmosphere influence on drawing-induced defects in pure silica optical fibers

Drawing-induced paramagnetic defects (E'centers and oxygen-associated hole centers (OHCs)) and optical absorption bands at 630 nm and in the UV region are investigated for optical fibers. These fibers are drawn from pure silica glass preforms consolidated in atmospheres containing O/sub 2/ and Cl/sub 2/. UV absorption bands at 245 nm and 275 nm are clearly observed through deconvolution of the absorption spectra of optical fibers. The defects and absorption bands are influenced by the atmosphere in the consolidation process: (1) the OHCs and the absorption bands at 630 nm and 275 nm increase the intensity with increasing O/sub 2/ content and with decreasing Cl/sub 2/ content; (2) the E' centers decrease under the same conditions; and (3) the absorption band at 245 nm is independent of the atmosphere. >