Silica Soot Research Articles

YbPO4 crystals in as-drawn silica-based optical fibers

MCVD germanosilica glass embedded with YbPO4 crystals were for the first time drawn into optical fibers. Solution doping was used to embed the crystals in the silica soot prior to the collapsing step. We demonstrate, using scanning/transmission electron microscopes and confocal Raman microscope, that YbPO4 crystals survive not only the MCVD process but also the drawing process despite the high temperature involved (up to 2100 °C) during fabrication processes. The fiber contains 100 nm-crystals with the same composition and structure as the as-prepared crystals. During the drawing process, these crystals tend to have a preferential orientation of their c-axis along the drawing direction. These results open a new route to fabricate glass-based composite fibers containing crystalline particles without additional post heat-treatment.

Thermal stability and photoluminescence properties of RE-doped (RE = Ho, Er, Tm) alumina nanoparticles in bulk and fiber-optic silica glass

We present the thermal stability and the photoluminescence properties of RE-doped (RE = Ho, Er, Tm) alumina nanoparticles in the phase system Al 2 O 3 -SiO 2 with respect to the chemical composition and the thermal processing conditions applied in the fiber-optic technology. The alumina and silica soot reacted together to form mullite when the Al 2 O 3 concentration was higher than 5 mol. %. We have demonstrated that the solubility limits of RE ions in the mullite nanocrystals are strongly limited. The RE ions preferentially occupy highly disordered positions on the nanoparticle surface or in the amorphous Al 3+ -enriched shell around the nanoparticles, exhibiting maximal lifetime of approx. 1.2 ms, 10.0 ms and 0.6 ms in the Ho-, Er- and Tm-doped samples. Rapid cooling of the samples with stoichiometric composition 3Al 2 O 3 ·2SiO 2 managed to prepare highly defective mullite nanocrystals with embedded RE ions, exhibiting promising photoluminescence lifetimes of 5.6 ms and 2.4 ms in the case of Ho 3+ and Tm 3+ ions, respectively. In optical fibers with 5 mol. % Al 2 O 3 , the formation of amorphous Al 3+ -enriched nanoparticles was observed and the photoluminescence lifetime was in a good agreement with corresponding bulk samples. Exploitation of the RE-doped stoichiometric mullite in the fiber-optic technology may be a perspective way to improve the photoluminescence efficiency of active optical fibers for high-power applications. • Alumina nanoparticles and silica soot form mullite. • The solubility of Ho 3+ , Er 3+ and Tm 3+ ions in mullite is strongly limited. • Ho 3+ , Er 3+ and Tm 3+ ions embedded in mullite exhibit lifetime of 5.6, 6.2 and 2.4 ms, resp. • Ho 3+ , Er 3+ and Tm 3+ ions in Al-enriched amorphous matrix have lifetime of 1.3, 10.0 and 0.6 ms, resp.

Microwave consolidation of UV photosensitive doped silica for integrated photonics

Rapid thermal consolidation of doped silica soot in a silicon carbide lined kiln subjected to microwave radiation is reported. The silica soot is fabricated through flame hydrolysis deposition and peak temperatures within the kiln reach >1350 °C after 18 minutes of irradiation. The glass underwent Newtonian cooling with an e−1 of approximately 5 minutes. Optical characteristics of the planar glass layer are compared against traditional furnace consolidation approaches. Single-mode waveguides and Bragg gratings were direct UV written into the consolidated doped silica, and a propagation loss of 0.34 ± 0.15 dB cm−1 was measured.

Carbon‐doped fused silica glass

AbstractCarbon‐doped fused silica glass is fabricated by consolidating zinc stearate‐doped porous silica soot blank. The carbon doping concentration is tunable and a maximum of ~0.3 wt% carbon can be incorporated into densified silica. Carbon dopants assemble into nanorod structures which are uniformly distributed inside glass. Transmission electron microscopy electron energy loss spectroscopy and Raman studies identify that carbon dopants are a mixture of nanosized silicon carbide and multiple layer graphene, which produce strong and flat optical absorption in a wide wavelength range of 400‐2500 nm. The electrical resistivity of silica glass is reduced by about 15 orders of magnitude with 0.28 wt% carbon doping.

Influence of process parameters on the incorporation of phosphorus into silica soot material during MCVD process

The incorporation of phosphorus into silica soot material strongly changes during the multistep preparation process of the MCVD technology in combination with solution doping for Al and rare earths. We report on the influence of various process parameters on the phosphorus concentration, the bond types of phosphorus atoms and the relative density of the soot material. By optimization of the process the phosphorus concentration of the presintered soot could be increased by around 10% in comparison to the conventional treatment. The understanding of the interdependencies allows an improvement of the preparation process of phosphorus co-doped RE doped silica laser fibers with MCVD technology.

The Dependence of N2 Carrier Gas Flow Rate on Deposition Rate and Density Changes of Porous Silica Preform Synthesized by Eco‐Friendly Octamethylcyclotetrasiloxane

A porous silica preform for the fabrication of optical fibers is generally synthesized by flame hydrolysis deposition (FHD). To synthesize the porous silica preform, SiCl4 containing chlorine (Cl) is normally used. However, the Cl generated through the FHD process is highly reactive and generates toxic substances, which seriously affect the environment. In this study, the porous silica preform with a radius of more than 250 mm is synthesized using vaporized octamethylcyclotetrasiloxane (OMCTS), which does not contain the Cl. In addition, the effect of the N2 carrier gas flow rate on the deposition rate of the silica soot and the density of the porous silica preform are investigated. As the carrier gas flow rate increases, the deposition rate decreases and the density of the porous silica preform tends to increase. This result may be due to the difference in the substrate and flame temperature gradients depending on the carrier gas flow rate. The characteristics of the porous silica preform are investigated at the top, middle, and bottom positions. Regardless of the position, the porous silica preform is composed of spherical silica particles with the size of 100–200 nm.

Advances in the monitoring of the SiO2 evaporation loss in transparent YAG ceramics by LIBS

Abstract The effect of silica on the densification of transparent YAG ceramics has long been investigated. The present paper makes a step forward on the quantification of the Si amount in YAG. The study shows how to measure with a high level of accuracy the silicon concentration in YAG in different steps of the fabrication process. The Laser-Induced Breakdown Spectroscopy (LIBS) has been used to quantify the amount of silicon in a set of samples with 0–0.275 wt% silica. Nanometric silica soot or its precursor, TEOS, has been used as silica source. The analysis shows that more than 80 wt % of SiO2 evaporates during reactive-sintering under vacuum, most probably because SiO gas forms during sintering under low oxygen partial pressure. In-depth concentration maps were obtained by laser drilling with LIBS pulses. The results show the possibility to modify the amount of residual Si in YAG ceramics by adjusting the sintering and annealing process.

Inside view

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Consolidation of flame hydrolysis deposited silica with a 9.3 µm wavelength CO 2 laser

Consolidation of flame hydrolysis deposition silica soot with a 9.3 µm CO2 laser has been demonstrated. A range of laser parameters were investigated and the surface roughness of the resulting silica layers were characterised with a stylus profiler and white light interferometer. The surface roughness parameters were Ra = 68.9 nm, Rq = 83.8 nm perpendicular to the trajectory of the translated laser beam for a speed of 300 mm s−1 and an average laser power of 42.5 W, and Ra = 29.5 nm, Rq = 36.18 nm along the trajectory of the translated laser beam for a speed of 250 mm s−1 and an average laser power of 35.8 W.

Control of optical properties of silica glass synthesized by VAD method for photonic components

High-purity silica glass (SiO 2) synthesized by vapor-phase methods has been extensively used as optical material for photolithography due to its high transmittance in the UV range. Furthermore, low birefringence is an essential property in order to attain distortion-free images of fine IC patterns. This research reports the development of low birefringence silica glass for photonic components that excludes the annealing requirement by controlling the processing parameters of the VAD (Vapor-phase Axial Deposition) method. Nanoparticles size radial homogeneity of silica soot boules was characterized by scanning electron microscopy (SEM). It was established a relationship with the birefringence of consolidated boules characterized through polarization spectrophotometry. As the most interesting results, low birefringence silica glass (⩽ 2 nm/cm) was synthesized with reduced fabrication time and cost besides simple processing stages by controlling the silica boule bottom shape during deposition processing in correlation with the consolidation time.

Effect of using aqueous/alcohol solution during solution doping on the physical and chemical characteristics of pre-sintered silica soot and the resultant native glass species concentration

This paper presents the effect of solution doping on pre-sintered silica soot prepared by flame hydrolysis deposition. Additionally, the effects of using either an aqueous or alcohol solution on the soot layer, and the resultant concentration of native glass species (pre- and post-consolidation) are also discussed. Soot consisting native glass species such as SiO 2, GeO 2, P 2O 5, and B 2O 3 was pre-sintered with temperatures ranging from 550 °C to 950 °C before undergoing solution doping. Cross-sectional thickness of the soot layer following solution doping was measured via optical microscopy and subsequently compared with those prior to solution doping. Significant variations of GeO 2 and P 2O 5 concentration in the soot layer after solution doping were detected via energy dispersive X-ray analysis. It was found that apart from the pre-sintered soot layer integrity during solution doping, the type of solution used plays an essential role in ensuring retention of native glass species both prior to, and following consolidation. Measurements using inductively coupled plasma atomic emission spectroscopy confirmed that significant amounts of GeO 2 and P 2O 5 were lost during the immersion in solution stage of the process.

Effect of the Nanostructure Control on the Novel Optical Properties of Silica Photonic Glass Synthesized by VAD Method

This research reports the development of high homogeneity silica glass for photonic components synthesized by the VAD (Vapor-phase Axial Deposition) method. Structural radial homogeneity of silica soot were characterized by scanning electron microscopy (SEM) and small angle X-ray scattering (SAXS). It was established a relationship with the refractive index homogeneity, n, and the birefringence of consolidated boules characterized through interferometry, optical spectrometry, and polarization spectrophotometry. By controlling the silica soot nanostructure during the deposition stage, a material with high radial homogeneity of refractive index of 1 ppm and birefringence of 2 nm/cm can be synthesized.

Germania nanocrystals in Modified Chemical Vapour Deposition

The Modified Chemical Vapour Deposition process for fabrication of silica-based optical fibres is used to generate nanoscale soot particles which are deposited on a substrate by thermophoresis and then sintered and collapsed into a preform from which optical fibre can be drawn. To increase the refractive index of the fibre core silica and germania soots are often generated together and co-deposited. The silica soot is invariably amorphous in character but for certain configurations of the preform lathe it has been found that germania soot particles may be crystalline. A systematic study has identified parameters controlling germania crystallinity in soot nanoparticles and transition from crystalline germania nanoparticles to glassy germania has been observed. The implications of possible retention of germania nanocrystals in preform and fibre are discussed with regard to fibre quality and optical performance.

Multiple solution-doping in optical fibre fabrication II – Rare-earth and aluminium co-doping

This work is part of a study of solution-doping in Modified Chemical Vapour Deposition, as used for fabrication of specialty silica optical fibres. It has been shown in an earlier paper that multiple soakings of a porous silica soot layer with intermediate heat-treatment stages between soakings can be used to introduce high levels of aluminium into an optical fibre core. The present paper demonstrates that the same technique can be applied to doping a fibre core with the rare-earth erbium and to co-doping aluminium and erbium simultaneously. Dopant levels have been tracked through the various stages, from soaked soot layer to the sintered glass layer to the collapsed preform. Experimental data are presented for both erbium soaking and co-doping with aluminium and erbium. The role of aluminium in preventing rare-earth segregation is confirmed.

Multiple solution-doping in optical fibre fabrication I – Aluminium doping

This work is part of a study of solution-doping in Modified Chemical Vapour Deposition, as used in fabrication of specialty silica optical fibres. The present paper concentrates on aluminium solution-doping and the effects of introducing additional heat-treatment stages into the process. The behaviours of the aluminium salt when heat-treated independently and after deposition in the silica soot have been studied by X-ray diffractometry, thermo-gravimetric analysis with mass spectrometry, electron microscopy and microanalysis. It has been shown that a procedure involving multiple cycles of solution-soaking and heat-treatment of the soot layer increases the level of aluminium doping obtainable. Efficiency of aluminium incorporation has been measured in the soaked soot, the sintered glass layer and in the final collapsed preform. An interpretive model of the multiple cycle soak/heat-treatment process is suggested.

Spectroscopic analysis of silica soot deposited by flame hydrolysis deposition

Flame hydrolysis deposition is widely used to fabricate SiO 2/Si passive optical waveguide devices for optical communication network. In flame hydrolysis deposition, silica soot doped with various elements such as B, P, and Ge is formed in the form of thin film layer and the composition of this layer is not predictable due to complex process variables. In this research, it was intended to clarify the quantitative relationship between simple process parameters and resulting chemical composition. In FHD process, compositional analysis of soot by FTIR and ICP-AES under the control of the amount of dopant is carried out to obtain the compositional information, which will affect the optical and thermo-mechanical properties of films. By measuring absorbance spectra with FTIR, the variation of B–O, Si–O, OH (H 2O) composition in FHD soot was investigated and could be quantified in B–O absorbance by ICP-AES analysis.

Nanoscale characterization of silica soots and aluminium solution doping in optical fibre fabrication

Visual and microanalytical studies have been made of the nanostructures and mechanisms involved in aluminium solution doping of silica soot generated by Modified Chemical Vapour Deposition used in the fabrication of silica optical fibres. Three aspects were studied – the nature of the soot particles, the structures in thermophoretically deposited soot layers and the way aluminium bonds to the surface of the soot particles. Soot particles were shown to be non-crystalline using both X-ray diffraction and electron diffraction. Particle size distributions, particle morphologies and deposited soot layer structures were characterised by electron microscopy and microprobe analysis. Atomic distributions of aluminium on surfaces of soot particles soaked in aluminium chloride solution were studied by high resolution transmission electron microscopy. The surface of solution doped particles was shown to have high aluminium content and nanoscale crystalline regions were identified amongst the mainly amorphous material. Soot layer structures and porosity were examined by electron and optical microscopy and distribution of the aluminium impregnation was mapped across the thickness of the deposited soot layer. Heterogeneous porosity distributions were observed and the effect of different types of pores, holes and cavities in the silica soot layer are discussed with regard to efficiency of deposition and solution doping.

A structural investigation of a synthesized precursor for optical fiber applications; the heterobimetallic ErNb2(OPri)13

A structural investigation of a synthesized precursor in a silica glass matrix is performed. Silica soot samples are doped with the heterobimetallic precursor ErNb2(OPri)13 by using a conventional solution doping technique and heat treatments to different temperatures. The precursor has also been introduced into a silica fiber preform by using the modified chemical vapor deposition technique. Analyses are made by using ultraviolet–visible–near infrared absorption spectroscopy, scanning electron microscopy, energy dispersive spectroscopy and powder X-ray diffraction. It is concluded that an immiscible system of ErNbO4 crystallites and Nb2O5 is formed in the silica soot samples at high temperatures. Colloidal particles of ErNbO4 are also formed in the silica glass fiber preform showing interesting features.

Pore Structure Simulation of Gels with a Binary Monomer Size Distribution

The pore size distribution in silica gels can be tailored by the addition of silica soot particles during the gel formation. We introduce a numerical model in order to simulate the structure of this “composite gel”. The algorithm is based on Diffusion-Limited Cluster-Cluster Aggregation model with an initial binary distribution of monomer sizes. The textural properties of the simulated gels are calculated using a simple triangulation method. Nitrogen adsorption-desorption experiments show that with the powder addition the mean pore size is shifted towards larger pore size and the specific surface area decreases. Numerical results of the mean pore size, specific surface area, and particles are in good agreement with experimental data. Because of these textural properties this new type of gels and aerogels has larger permeability and interesting properties as host matrix. The composite gels and the numerical model could also be helpful to simulate the natural allophanic gel found in volcanic soils.

Large Scale Porous Structure in Gels and Relationship with Their Plastic Behavior

Silica gels (classical aerogels and composite aerogels) have been prepared by classical gelation and addition of silica soot in the gelifying solution before gelation. Due to the aggregation mechanisms, these structures are characterized by a fractal organization. The fractal network previously described in the literature (1-100 nm) which results from the aggregation mechanism of the organosiloxane is affected by the addition of the silica soots. Ultra Small Angle X-ray Scattering (USAXS) experiments (done at ESRF) shows that besides the fractal network built by the organosiloxane, the silica soots are forming another porous structure at a higher scale. The mechanical properties seem to be dependent on this large pore structure. Under isostatic pressure, aerogels display an irreversible shrinkage caused by plastic deformation. As a consequence of this plastic shrinkage it is possible to densify, by the pore collapse tending towards the silica glass. The densification mechanism is different from the one obtained by a sintering at high temperature. The pore collapse mechanism is favored by the large pores structure of the composite aerogels, in contrast to viscous sintering.