Vapor-phase Axial Deposition Research Articles

Spectroscopic and lasing characteristics of an Yb-doped silica rod fabricated by the vapor-phase axial deposition technology

Spectroscopic and lasing characteristics of the ytterbium (Yb)-doped silica-based rod fabricated by the vapor-phase axial deposition technology is reported. Using a high-quality Yb-doped silica (Yb:silica) rod with a diameter of 1.5 mm, we calculated the absorption and emission cross sections based on measurement of the absorption spectrum, resulting in a maximum absorption cross section of 3.84 × 10−19 cm2 at 978 nm and a maximum emission cross section of 4.23 × 10−19 cm2 at 980 nm. In addition, the fluorescent lifetime was measured to be 974 μs. We applied the Yb:silica rod to a laser amplifier, yielding 72 W of amplified continuous-wave output at 1030 nm with a slope efficiency of 28%. Prospects of power scaling along with achievement of good beam quality are discussed.

Vapor-Phase Axial Deposition Synthesis of SiO<sub>2</sub> and SiO<sub>2</sub>-TiO<sub>2</sub> Sponge-Shaped Nanostructures

Amorphous silica can be synthesized with controlled dimensions and morphologies by the Vapor-phase Axial Deposition (VAD) method, so we performed an exploratory study for verifying the possibility of using this technique for the obtention of a sponge-shaped ramified nanostructure with high superficial area. We were successful in synthesizing SiO2 and SiO2-TiO2 with this desired morphology and characterized the materials by Scanning Electron Microscopy and X-Ray Diffraction. The obtained ceramics present pores of adequate dimensions for the use as microfiltration devices and proved to be capable of adsorbing important commercial dyes. These materials show physical characteristics that make them promising for applications on liquid and gas separations, and as very selective photocatalysts for chemical reactions and for waste and water treatment.

A method to synthesize SiO 2–TiO 2 glasses based on the synergy between VAD and ALD techniques: study of TiO 2doping profile along radial direction

This study aimed the synthesis of titania–doped silica glasses with controlled TiO 2 concentrations and distribution profiles through the synergy between Vapor-Phase Axial Deposition (VAD) and Atomic Layer Deposition (ALD) techniques. In a first stage, porous boules of pure silica were synthesized by the VAD technique. Afterwards, these boules were submitted to a cyclic doping process with TiCl 4 vapor and H 2O vapor using the principles of ALD technique. Silica–titania on glassy state was obtained after the consolidation of the porous boule at 1400 °C. The effects of VAD and ALD parameters were studied on the radial doping profile homogeneity and glass structure in the consolidated material. Concerning to the radial profile, a more homogeneous glass was obtained for porous boules doped at 90 °C and with higher average density. This temperature was enough to suppress the effect of TiCl 4 condensation on the porous boule surface even for a long TiCl 4 flow time, providing a relatively high doping homogeneity for one and four ALD cycles. The opacity in some consolidated samples was attributed to a segregated phase, which could be completely eliminated by a second heat treatment in flame.

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 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.

Development of a cascaded controller for temperature and core growth rate in vapour-phase axial deposition

A cascaded feedback control strategy for an industrial vapour-phase axial deposition (VAD) process is investigated in this paper. VAD is a widely used process in the creation of high-purity glass for optical fibre. In previous work a soot tip surface temperature controller was developed for the VAD process to reduce the effects of core soot temperature variation on deposition geometry, leading to a more stable process. However, it is desired to regulate both the core soot and clad soot such that they deposit at the same axial rate to provide a more uniform product. This paper presents the development of a cascaded controller strategy and process model to couple and regulate the surface temperature and deposition rates of core and clad soot. Simulation studies demonstrate a potential improvement in the uniformity of the core and clad soot geometry over the soot product length.

SiO2–GeO2 Soot Preform as a Core for Eu2O3 Nanocoating: Synthesis and Photophysical Study

Nowadays solid state chemists have the possibility of work with low temperature strategies to obtain solid state materials with appropriate physical and chemical properties for useful technological applications. Photonic core shell materials having a core and shell domains composed by a variety of compounds have been synthesized by different methods. In this work we used silica-germania soot prepared by vapor-phase axial deposition as a core where a nanoshell of Eu(2)O(3) was deposited. A new sol-gel like method was used to obtain the Eu(2)O(3) nanoshell coating the SiO(2)-GeO(2) particles, which was prepared by the polymeric precursor method. The photophysical properties of Eu(3+) were used to obtain information about the rare earth surrounding in the SiO(2)-GeO(2)@Eu(2)O(3) material during the sintering process. The sintering process was followed by the luminescence spectra of Eu(3+) and all the samples present the characteristic emission related to the (5)D(0)-->(7)F( J ) (J = 0, 1, 2, 3 and 4). The ratios of the (5)D(0)-->(7)F(2)/(5)D(0)-->(7)F(1) emission intensity for the SiO(2)-GeO(2)@Eu(2)O(3) systems were calculated and it was observed an increase in its values, indicating a low symmetry around the Eu(3+) as the temperature increases.

Relationship between refractive index and density of synthetic silica glasses

The effect of impurity and density on refractive index was studied for silica glasses synthesized with direct, sol-gel, and vapor phase axial deposition (VAD) processes. Linear relationships between the refractive index and density n=Cρ+D were found in a wide wavelength region from ultraviolet to infrared. The correlation coefficient C decreased from 0.081±0.007cm3∕g at 1.707μm to 0.061±0.008cm3∕g at 0.238μm. The relationship was different from those of the VAD or direct glasses formed by high-pressure densification, that is, the coefficient increased from 0.200±0.011cm3∕g at 1.707μm to 0.229±0.012cm3∕g at 0.238μm for VAD glass. Chlorine and hydroxyl impurities affected the refractive index; however, these impurities did not cause the linear relationship. After adjusting the index according to the chlorine related effect and the effect of the sample density, the difference between the sample’s refractive indices Δ(n2−1) showed a similar dispersion relation irrespective of the synthetic process. An analysis using a two-term harmonic oscillator model suggested that the origin of the refractive index dispersion is the oxygen 2p-silicon 3s interband transition, as is the dispersion relation of silica glass. The difference in the refractive indices in these synthetic silica glasses is deduced to be due to a stoichiometric excess or deficiency of oxygen atoms of forms ≡Si–OHHO–Si≡, ≡Si–Si≡, ≡Si––SI, etc., and the density arising from these forms in these synthetic processes.

Effect of processing parameters on control of defect centers associated with second-order harmonic generation and photosensitivity in SiO2:GeO2 glass preforms

In this research, the effect of the H2/O2 ratio and the processing temperature parameters on the inducing and enhancement of the defect centers associated to the second-order optical non-linearity in SiO2:GeO2 glass preforms, prepared by vapor-phase axial deposition method, have been investigated. The formation of germanium oxygen deficient centers and the development of paramagnetic structures induced in the glass preforms after X-ray irradiation were investigated using UV–Vis absorption spectroscopy and electronic spin resonance. The results indicate that the concentration of germanium oxygen deficient centers increases exponentially when the H2/O2 ratio decreases, while the processing temperature increases, simultaneously. The electronic spin resonance spectra profiles, shows that defects of the electron trapped centers type [Ge(1), Ge(2)] are induced by the effect of X-ray irradiation. An efficient generation of defect centers associated to the second-order optical non-linearity in SiO2:GeO2 glass preforms, occurring in samples prepared with low H2/O2 ratios and high processing temperatures, have been observed.

Automation for monitoring of the refractive index profile of vapor-phase-deposited soot preforms for optical fiber

The vapor-phase axial deposition process is currently one of the most advantageous methods to produce preforms for optical fibers, due to its high efficiency and reduced production cost. However, this method has great difficulty in determining the refractive index profile, since it is influenced by too many process parameters. In this work, an automation system to determine the refractive index profile by monitoring the preform deposition surface profile during the soot preform deposition stage is presented. Based on a previous study that showed a strong correlation between these two profiles, an automation system was developed in LABVIEW to monitor the deposition surface profile during the preform deposition stage in order to estimate the preform germanium doping profile and refractive index profile, as well as a theoretical study to develop this system in order to minimize the performance impairment. As a result, not only preforms with a predetermined index profile were produced but also a reduction in production cost was obtained by decreasing the number of preform rejects.

Correlation study between VAD preform deposition surface and Germanium doping profiles

The refractive index profile of germanium doped preforms for optical fibers is determined by the radial distribution of germanium concentration. Knowing that there is a correlation between the germanium doping profile and the deposition surface profile of vapor-phase axial deposition (VAD) preforms, the study of this correlation has been carried out in order to estimate, indirectly, the refractive index profile of VAD preforms for optical fibers during the deposition stage. This correlation was studied through the parameterization of the preform deposition surface using two parameters: the power law index profile that best fits the preform bottom profile (/spl alpha/) and the axial distance from the bottom tip to a reference height (h). A range of values of these parameters to produce VAD preforms with standard and special doping profiles has been presented. Preforms with triangular index profile can be fabricated with /spl alpha/ and h values of about 2.0 and 5.0 mm, respectively, and preforms with parabolic index profiles can be produced with /spl alpha/ and h values of about 2.0 and 4.0 mm, respectively.

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.

Dependence of H 2/O 2 ratio and GeO 2 content on the enhancement of second-order non-linearity related defects in Ge-doped optical fiber preforms

In this research, we studied the influence of the process conditions through the H 2/O 2 ratio and the GeO 2 content, on the formation of germanium oxygen deficient centers and paramagnetic defects center induced by X-ray irradiation in SiO 2:GeO 2 glass preform prepared by the vapor-phase axial deposition method. The absorption band at around 5.1 eV, characteristic of germanium oxygen defect center was observed in all glass preform sample absorption spectra. The absorption coefficient increased when the GeO 2 concentration increased, however, the absorption was higher in samples prepared with high H 2/O 2 ratio. The electronic spin resonance spectrum profiles suggest that paramagnetic structures, such as, electron trapped centers and positively charged germanium oxygen deficient centers, assigned as Ge(1) and Ge(2), respectively, are induced in all samples after being X-ray irradiated for 1 h. It was found that the concentration of induced paramagnetic centers is linearly proportional to germanium oxygen defect center concentration and that the efficiency in the enhancement of paramagnetic defect centers associated with second-order optical non-linearity increased as H 2/O 2 ratio used in the fabrication process was decreasing. It was concluded that SiO 2:GeO 2 glass preform with low H 2/O 2 ratio and higher GeO 2 content must be prepared for the enhancement of the second-order non-linearity.

Dependence of H2/O2 ratio and GeO2 content on the enhancement of second-order non-linearity related defects in Ge-doped optical fiber preforms

In this research, we studied the influence of the process conditions through the H2/O2 ratio and the GeO2 content, on the formation of germanium oxygen deficient centers and paramagnetic defects center induced by X-ray irradiation in SiO2:GeO2 glass preform prepared by the vapor-phase axial deposition method. The absorption band at around 5.1 eV, characteristic of germanium oxygen defect center was observed in all glass preform sample absorption spectra. The absorption coefficient increased when the GeO2 concentration increased, however, the absorption was higher in samples prepared with high H2/O2 ratio. The electronic spin resonance spectrum profiles suggest that paramagnetic structures, such as, electron trapped centers and positively charged germanium oxygen deficient centers, assigned as Ge(1) and Ge(2), respectively, are induced in all samples after being X-ray irradiated for 1 h. It was found that the concentration of induced paramagnetic centers is linearly proportional to germanium oxygen defect center concentration and that the efficiency in the enhancement of paramagnetic defect centers associated with second-order optical non-linearity increased as H2/O2 ratio used in the fabrication process was decreasing. It was concluded that SiO2:GeO2 glass preform with low H2/O2 ratio and higher GeO2 content must be prepared for the enhancement of the second-order non-linearity.

Radiation-hard KS-4V glass and optical fiber, manufactured on its basis, for plasma diagnostics in ITER

Transmission spectra and optical losses before and after γ-irradiation (60Co) at a dose up to 11 MGy in 1 cm thick glass plates and in “all-silica” type optical fibers were examined. KS-4V, KU-1, KUVI, Suprasil F-300 glasses and fluorine-doped glass (vapor-phase axial deposition (VAD) technology, Japan) were used as a core material in the fibers. Comparison tests of four types of high purity fused silica (HPFS) showed that the highest ORR is characteristic of KS-4V (the lowest chlorine content). Optical Radiation Resistance (ORR) testing of “all-silica” type fibers (KS-4V glass, Japan VAD glass, Suprasil F-300 and KU-1 glasses as a core material) showed that the highest ORR is characteristic of KS-4V glass, which contains minimum of impurities. The level of optical losses in the visible band of the KS-4V spectrum is twice lower than the level of fluorinated glass (VAD), and 4–5 times lower as the level of losses of Suprasil F-300 and KU-1 respectively.

Crystallization Behaviors with Pre-Treatments of Thermal Annealing and Ultraviolet Laser Irradiation in Ge-Doped SiO2 Glass Fiber Preforms.

Crystallization behaviors in Ge-doped SiO 2 fiber preforms fabricated by vapor-phase axial deposition (VAD) method have been investigated. Since X-ray diffraction (XRD) peaks at around 20=22° in the crystallized glasses are reported to be strongly related with the induced second- and third-order optical nonlinear effects in poled glasses, enhancement of such crystalline phases is quite important and useful for fabrication of nonlinear functional glasses. It has been found that crystallization behaviors are drastically affected by thermal annealing and ultraviolet (UV) laser irradiation performed prior to heat-treatments for crystallization. In particular, it has been clarified that a crystalline phase, which is known to be highly effective for increase of induced optical nonlinearity, can be increased by pre-treatments of UV-laser irradiation at λ = 193 nm wavelength. These results suggest that modification of initial defect states in Ge-doped SiO 2 fiber preforms could lead to fabricate crystallized glasses with optical nonlinearity induced by poling and/or crystallization.

Early days of VAD process

Describes early days of a preform fabrication process for optical fibers named vapor-phase axial deposition (VAD) method. This includes not only technical aspects but also the processes of the invention and research environment. It was about 10 years from 1970 to 1980 just before the optical fiber communication systems were introduced into a practical application.

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.

Crystalline and amorphous phases of GeO 2 in VAD silica–germania soot preform

In germanium doped optical fiber, produced by vapor-phase axial deposition (VAD) technique, the refractive index profile is controlled by the radial distribution profile of germanium. At the porous soot state, GeO 2 may be deposited as amorphous and/or crystalline phases in a SiO 2–GeO 2 system. To correlate the quantitative deposition between amorphous and crystalline phases of GeO 2, several silica–germania soot preforms were prepared under different deposition conditions and their X-ray diffraction (XRD) patterns and X-ray fluorescence spectra were analyzed. The presence of GeO 2 crystalline (hexagonal) phase was detected in all soot samples, and its concentration was affected by deposition conditions, mainly the soot surface temperature and H 2/O 2 ratio in the flame. The concentration of the crystalline phase is minimum at the center of the silica–germania soot preform and increases along the radial direction. The greater concentrations at the outer diameter of the silica–germania soot preform are due to the lower deposition temperature (<700°C) in comparison to the one at the center (>850°C). The GeO 2 is eliminated at the center of preform in reductive atmosphere (H 2/O 2>2) and higher temperature ( T ∼ 1000°C), independently of SiCl 4/GeCl 4 ratio. These results are important to understand the main parameters of the doping process, to produce high quality materials with a given germanium concentration profile.

Local structure and persistent spectral hole burning of Sm 2+ in silica-based fibers

Silica-based glasses doped with Sm 2+ have been prepared by using vapor-phase axial deposition method under a reducing atmosphere. The fluorescence spectra reveal that the reduction of Sm 3+ to Sm 2+ ions is accomplished in the silica glass codoped with Al 2O 3, while the Sm 2+ ions are not incorporated into the silica glass without Al 2O 3. The effects of Al 2O 3 codopant on the coordination change around divalent rare-earth ions are discussed according to our previous studies of 151Eu Mössbauer effects. It is assumed that the divalent rare-earth ions are preferentially coordinated by Al polyhedra, resulting in the stability of the lower valence state of the rare-earth ions. Persistent spectral hole burning is also demonstrated at room temperature for the 7F 0– 5D 0 transition of Sm 2+ in the aluminosilicate glass fiber. A hole width of 6 cm −1 is much narrower than the inhomogeneous width of 100 cm −1 , leading to the ratio of inhomogeneous-to-homogeneous width of ∼40.