Vapor Axial Deposition Research Articles

Ultrafast Laser Writing in Different Types of Silica Glass

AbstractIt is demonstrated that ultrafast laser writing in silica glass depends on the grade of silica glass associated with the method of its manufacture. Moreover, laser‐written modifications, in particular birefringent modifications, reveal a dependence on the geometry of writing, that is, the modification strength of voxels is smaller than that of single line structures and multi‐line scanned areas, which can be explained by free carrier diffusion and reduced electric field in scanning writing. The retardance of scanned birefringent region produced in the regime of anisotropic nanopores formation in silica glass manufactured by vapor axial deposition (VAD) is about five times higher than that in an electrically fused sample at the same laser writing parameters, while the difference in retardance of a nanograting based modification in synthetic and fused silica is only about 10%. The phenomenon is interpreted in terms of the higher concentration of oxygen deficient centers in the electrically fused silica glass, which can confine self‐trapped holes and prevent the nanopores formation. Improvement of high transmission optical elements is demonstrated in the VAD sample, and low cost multiplexed optical data storage with higher capacity and readout accuracy is realized in the electrically fused silica glass.

Plasma‐based VAD process for multiply doped glass powders and high‐performance fiber preforms with outstanding homogeneity

AbstractAn innovative approach using the vapor axial deposition (VAD), for the preparation of silica‐based high‐power fiber laser preforms, is described in this study. The VAD uses a plasma deposition system operating at atmospheric pressure, fed by a single, chemically adapted solution containing precursors of laser‐active dopants (e.g., Yb2O3), glass‐modifier species (e.g., Al2O3), and the silica matrix. The approach enables simultaneous doping with multiple optically active species and overcomes some of the current technological limitations encountered with well‐established fiber preform technologies in terms of dopant distribution, doping levels, and achievable active core diameter. The deposition of co‐doped silica with outstanding homogeneity is proven by Raman spectroscopy and electron probe microanalysis. Yb2O3 concentrations are realized up to 0.3 mol% in SiO2, with simultaneous doping of 3 mol% of Al2O3.

Dispersion-optimized optical fiber for high-speed long-haul dense wavelength division multiplexing transmission

Four non-zero-dispersion-shifted fibers with almost the same large effective area (A(eff)) and optimized dispersion properties are realized by novel index profile designing and modified vapor axial deposition and modified chemical vapor deposition processes. An A(eff) of greater than 71 μm(2) is obtained for the designed fibers. Three of the developed fibers with positive dispersion are improved by reducing the 1550 nm dispersion slope from 0.072 ps/nm(2)/km to 0.063 ps/nm(2)/km or 0.05 ps/nm(2)/km, increasing the 1550 nm dispersion from 4.972 ps/nm/km to 5.679 ps/nm/km or 7.776 ps/nm/km, and shifting the zero-dispersion wavelength from 1500 nm to 1450 nm. One of these fibers is in good agreement with G655D and G.656 fibers simultaneously, and another one with G655E and G.656 fibers; both fibers are beneficial to high-bit long-haul dense wavelength division multiplexing systems over S-, C-, and L-bands. The fourth developed fiber with negative dispersion is also improved by reducing the 1550 nm dispersion slope from 0.12 ps/nm(2)/km to 0.085 ps/nm(2)/km, increasing the 1550 nm dispersion from -4 ps/nm/km to -6.016 ps/nm/km, providing facilities for a submarine transmission system. Experimental measurements indicate that the developed fibers all have excellent optical transmission and good macrobending and splice performances.

Thermal equilibrium of Ge-related defects in a GeO2-SiO2 glass

In a 15.7 GeO2⋅84.3 SiO2 glass prepared by a vapor axial deposition method, optical absorptions at 4.92 and 5.08 eV were observed from 300 to 600 K. The results were then compared with electron spin resonance (ESR) measurement using the same samples. The absorption at 4.92 eV was found to increase with increasing temperature, while the optical absorption at 5.08 eV decreased with increasing temperature. These changes of optical absorption indicate that the concentration of neutral oxygen monovacancy is reduced at higher temperatures. The changes were quite reversible with the temperature. Concentration of Ge E′ center, estimated from ESR, increased with increasing temperature, which is in contrast with the decrease of neutral oxygen monovacancy. The present result strongly suggests the thermal equilibrium reaction between neutral oxygen monovacancy and Ge E′ center.

Particle deposition on a rotating disk in application to vapor axial deposition (VAD) process

Vapor axial deposition (VAD), one of the optical waveguide preform fabrication processes, is performed by deposition of silica particles that are synthesized by combustion of raw chemical materials. In this study, flow field in a VAD process is assumed to be a forced uniform flow perpendicularly impinging on a rotating disk. A set of exact solutions for velocity and temperature distributions are developed by employing a similarity variable obtained from force balance on a control volume near the disk. The solutions depend on the rotating velocity of the disk and the forced flow velocity toward the disk. The solutions are then incorporated into the particle dynamics/transport equation. The particles are approximated to be in a lognormal size distribution. A moment model is used in order to predict distributions of particle number density and size simultaneously. Deposition of the particles on the disk is examined, considering convection, diffusion, thermophoresis, and coagulation with variations of the forced flow velocity and the disk rotating velocity. The deposition rate and the deposition efficiency directly increase as the flow velocity increases, resulting from that the increase of the forced flow velocity causes thinner thermal and diffusion boundary layer thicknesses and, thus, causes the increase of thermophoretic drift and Brownian diffusion of the particles toward the disk. However, the increase of the disk rotating velocity does not result in the direct increase of the deposition rate and the deposition efficiency. Slower flow velocity causes extension of the time scale for coagulation and thus, yields larger mean particle size and its geometric standard deviation at the deposition surface. In the case of coagulation starting farther from the deposition surface, coagulation effects increase, resulting in the increase of the particle size and the decrease of the deposition rate at the surface.

Particle deposition on a rotating disk in application to vapor deposition process (VAD)

Vapor Axial Deposition (VAD), one of optical fiber preform fabrication processes, is performed by deposition of submicron-size silica particles that are synthesized by combustion of raw chemical materials. In this study, flow field is assumed to be a forced uniform flow perpendicularly impinging on a rotating disk. Similarity solutions obtained in our previous study are utilized to solve the particle transport equation. The particles are approximated to be in a polydisperse state that satisfies a lognormal size distribution. A moment model is used in order to predict distributions of particle number density and size simultaneously. Deposition of the particles on the disk is examined considering convection, Brownian diffusion, thermophoresis, and coagulation with variations of the forced flow velocity and the disk rotating velocity. The deposition rate and the efficiency directly increase as the flow velocity increases, resulting from that the increase of the forced flow velocity causes thinner thermal and diffusion boundary layer thicknesses and thus causes the increase of thermophoretic drift and Brownian diffusion of the particles toward the disk. However, the increase of the disk rotating speed does not result in the direct increase of the deposition rate and the deposition efficiency. Slower flow velocity causes extension of the time scale for coagulation and thus yields larger mean particle size and its geometric standard deviation at the deposition surface. In the case of coagulation starting farther from the deposition surface, coagulation effects increases, resulting in the increase of the particle size and the decrease of the deposition rate at the surface.

Optical absorption and luminescence of germanium oxygen-deficient centers in densified germanosilicate glass

Spectroscopic properties of germanium oxygen-deficient centers were studied in vapor axial deposition and modified chemical-vapor deposition germanosilicate glass after hydrostatic densification by as much as 19%. Gaussian decomposition showed a broadening and a significant shift of the initial absorption bands at 5.11 and 5.42 eV. A threefold decrease of the 3.15-eV luminescence band was revealed, whereas the 4.25-eV luminescence band was only slightly sensitive to densification. This shows that these two bands belong to different luminescence centers. A drastic growth of a new metastable absorption band near 7.3 eV was observed. The pressure-induced changes disappeared as a result of thermal annealing. These changes are discussed with respect to in-fiber grating phenomena.

Characteristics of 5-eV absorption band in sputter deposited GeO2-SiO2 thin glass films

Thin films (4.6 μm thick) of 5 GeO2-95 SiO2 and 55 GeO2-45 SiO2 (mol %) glasses were prepared by rf sputtering method in an Ar-O2 atmosphere. An intense absorption band at around 5 eV was distinctly observed in both films after the as-deposited films were annealed at 350 °C for 30 min in a vacuum. A part of this 5-eV band was gradually decreased by UV irradiation. Saturated absorptivity changes (−Δα∞) of the UV bleached component, which is considered to be the origin of Hill gratings [K. O. Hill, Y. Fujii, and B. S. Kawasaki, Appl. Phys. Lett. 32, 647 (1978)] and second-harmonic generation in SiO2 glass fibers doped with GeO2, after a prolonged irradiation were 50 cm−1 for 5 GeO2-95 SiO2 films and 400 cm−1 for 55 GeO2-45 SiO2 films. These values are greater by one or two orders of magnitude than those (∼2 cm−1) of bulk germanosilicate glasses prepared by the vapor axial deposition method.

Characterization of a vapor axial deposition (VAD) flame impinging on a disk for a study o felectric field enhanced deposition

Methane-oxygen-nitrogen flames produced in a vapor axial deposition (VAD) burner and impinging on a disk were characterized with application to the problem of increasing particle deposition (transport) rate by an electric field applied between the disk (powdered) and an inserted ring (grounded). Two-dimensional temperature and electric potential distributions in flames were measured using a B-type thermocouple and an electrostatic probe, respectively. The temperature measurements demonstrate that the concentric tube burner design is effective because it allows thermophoretic focusing of the particle distribution within the flame, thus leading to higher deposition rates. The potential measurements show that that the electrical boundary layer (Debye sheath) appears adjacent to the powered electrode and grows in thickness as the strength of the applied potential increases. For the experimental conditions studied, the probe data show that the sheath extends outside of the thermal boundary layer at a large applied potential; this is expected to increase the effective capture area for particle deposition, depending on the particle-charging mechanisms associated with the polarity of the applied potential. The probe data imply that even for a sheath of thickness smaller than the themal boundary layer thickness the additional electrophoretic effect superimposed on the thermophoresis effect also increases the particle capture area for deposition. Sample calculations of particle drift velocities were carried out based on the probe results. For an electric field of 1 kV/mm the electrophoretic drift velocity becomes higher than the thermophoretic drift velocity when the number of charge carriers attached to a particle exceeds 4.

Fabrication and characterization of erbium‐doped silica single‐mode fibers

AbstractThis paper discusses erbium‐doped (Er‐doped) optical‐mode silica fibers for which vigorous research efforts have been carried out in recent years. The fabrication techniques and the amplification characteristics with semiconductor lasers used as the excitation source are investigated. As the fabrication of Er‐doped optical fibers, the vapor axial deposition (VAD) method, which is one of the fiber manufacturing techniques, is used. The rare earth doping into the porous material (in vapor phase) by using this method and the possibility of controlling the Er concentration (up to about 1000 ppm) are considered.As the evaluation of amplification characteristics, experimental results of the dependencies upon the pumping wavelength and Er concentration are presented. In particular, in regard to the dependency upon the Er concentration, which is quite important in highly efficient amplifiers, it is shown that there is a serious degradation in the amplification characteristics when the concentration is increased, even at the low doping level of 500 ppm. The deteriorative mechanisms also are discussed.

Temperature measurements in a vapor axial deposition flame by spontaneous Raman spectroscopy

Spontaneous Raman spectra of oxygen and nitrogen have been used to determine temperatures in flames produced by a vapor axial deposition (VAD) burner, both with and without silicon tetrachloride added to form silica particles. VAD flames in the absence of silicon tetrachloride contain steep temperature gradients in both the axial and radial directions. The addition of silicon tetrachloride produces substantial changes in flame structure, as characterized by the movement of an axial thermal front to a location higher in the flame. This indicates that the effects of silicon tetrachloride upon flame properties cannot be neglected in computational models designed to predict silica deposition rates. These results also show that spontaneous Raman scattering measurements are feasible in silica-laden VAD flames and can yield insight into the structure of such flames. Because of the presence of large thermal gradients, the high spatial resolution obtained in these measurements (0.6 mm of the laser beam was imaged) represents the minimum required to obtain accurate measurements of temperature and species concentrations in these complex diffusion flames.

The VAD preform stretching process: A numerical model for evaluation of internal distortion

Making an optical preform by the Vapor Axial Deposition (VAD) method requires stretching the consolidated boule into a rod so that additional cladding may be added. The effect of such stretching on a preform, other than to reduce its diameter, has not been determined, although distortions of the layer structure within the doped glass have been observed. In an effort to identify the effect of processing parameters, the stretching of pure silica blanks is examined using a finite-element fluid flow model modified to incorporate radiative heat transfer. The model, which predicts the measured neckdown profile well, also calculates the full two-dimensional flow field within the glass. Using this flow field and a tracking routine, the relative displacements of points in the glass during stretching can be determined. With this capability to study the effects of material and processing parameters, it is found that distortion is very sensitive to the initial shape of the layer structure, moderately sensitive to the width of the hot zone, and insensitive to the maximum temperature.