Flame Hydrolysis Deposition Process Research Articles

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.

High-birefringence direct UV-written waveguides for use as heralded single-photon sources at telecommunication wavelengths.

Direct UV-written waveguides are fabricated in silica-on-silicon with birefringence of (4.9 ± 0.2) × 10-4, much greater than previously reported in this platform. We show that these waveguides are suitable for the generation of heralded single photons at telecommunication wavelengths by spontaneous four-wave mixing. A pulsed pump field at 1060 nm generates pairs of photons in highly detuned, spectrally uncorrelated modes near 1550 nm and 800 nm. Waveguide-to-fiber coupling efficiencies of 78-91 % are achieved for all fields. Waveguide birefringence is controlled through dopant concentration of GeCl4 and BCl3 using the flame hydrolysis deposition process. The technology provides a route towards the scalability of silica-on-silicon integrated components for photonic quantum experiments.

Leaky mode integrated optical fibre refractometer.

A route to monitor external refractive indices greater than the core index of the waveguide is presented. Initial application utilizes an integrated optical fibre (IOF) platform due to its potential for use in harsh environment sensing. IOF is fabricated using a bespoke flame hydrolysis deposition process to fuse an optical fibre to a planar substrate achieving an optical quality, ruggedized glass layer between the fibre and substrate was fabricated. The presented refractometer is created by direct UV writing of multiple fibre Bragg gratings into an etched (22 μm diameter) optical fibre post fabrication. Linear regression analysis is applied to quantify propagation loss by monitoring each FBG's back reflected power. The device operates with a sensitivity of approximately 350 dB/cm/RIU at a refractive index of 1.451 at 1550 nm. Numerical simulations using a transfer matrix method are presented and potential routes for development are discussed.

Fundamental Understanding of Processes Involved in Optical Fiber Manufacturing Using Outside Vapor Deposition Method

Optical fibers are often made using a flame hydrolysis deposition process, in which silica and doped silica particles from a soot‐laden flame are deposited on a target rod. The porous preforms thus formed are subsequently sintered into glass blanks and then drawn into optical fiber. We review here various tools that have been developed for better understanding of different processes involved in the making of optical fibers using the outside vapor deposition technique. Fundamental understanding of the different unit operations involved in the optical fiber manufacturing is critical for process optimization and improved product attributes.

Experimental and theoretical studies of flame hydrolysis deposition process for making glasses for optical planar devices

We have here presented experimental and theoretical studies of a flame hydrolysis deposition (FHD) process for making glasses for optical planar devices. FHD involves deposition of soot particles generated in the flame on a planar surface to form a porous layer. In order to function as a waveguide core, the porous soot deposit must be sintered at high temperatures to form a dense glass. However, these temperatures are high enough to cause the dopants to volatilize at the surfaces of the deposit. As a result, dopant concentration gradients and compositional inhomogeneities are created in the densified glass layer which result in inferior optical properties. If the layers could be deposited pre-sintered during laydown, these problems could be minimized or avoided. To understand the FHD process and its effect on the morphology of the resulting deposit, a number of models have been developed. We present models to predict the structure of the soot-laden flame along with the methods to estimate soot number density and mean size at different locations in the flame. A simple method is presented to predict morphology of the suspended soot and soot deposit formed during deposition. Motivated by the predictions of the model, process changes were made which resulted in the morphology of the core layers to change from sooty to pre-sintered during deposition. These process changes resulted in core glasses with significantly improved core roughness, index uniformity and thickness uniformity which helped reduce the straight waveguide losses from 0.2–0.3 to 0.02–0.05 dB/cm. Details of underclad and overclad glasses which resulted in low warpage and polarization sensitivity of 0.1 nm for phasar or phasar-arrayed waveguide devices are also presented.

Fabrication of low-cost planar wavelength-selective optical add-drop multiplexer by employing UV photosensitivity

A 2×2 optical add-drop multiplexer has been fabricated by exclusive use of the UV photosensitivity effect in defining the waveguide patterns and Bragg pattern without using any etching process on the multilayered slab structure prepared by a single session of flame hydrolysis deposition process. That is, the waveguide patterns were written by direct writing of a focused UV beam and, subsequently, the periodic grating was imprinted on the plate by a broad UV exposure over a phasemask. The add and drop efficiencies measured at the Bragg wavelength were 25 and 20 dB, respectively.

Surface chemistry of SiO 2 and TiO 2–SiO 2 glasses as determined by titration of soot particles

Surface charge is a critical issue for various types of glass processing including surface finishing (polishing) and cleaning. Unlike monomeric silicic acid with a p K a of 9.8, glass surfaces can have variable p K a values as determined by extent of Si–O–Si bonding, composition, and structure. The purpose of this study was to determine the surface charge, point of zero charge, and p K as for high purity fused SiO 2 and TiO 2–SiO 2 (∼7 wt% TiO 2) glasses. To achieve necessary surface areas for titration, colloidal soot particles were used in this study rather than glass monolith. Soot particles have the same inherent physical and chemical properties as the product glasses that are made by the same chemical vapor flame hydrolysis deposition process. Titration experiments performed in 10 −1–10 −5 M NaCl solutions revealed dissociation constants (i.e., intrinsic p K a1 and p K a2 values) of 0.0±0.2 and 7.0±0.1, respectively for the fused SiO 2 particles, and 0.0±0.1 and 5.0±0.2 , respectively for the TiO 2–SiO 2 particles. Points of zero charge for each material were calculated as 3.5±0.1 and 2.5±0.1 for the fused SiO 2 and TiO 2–SiO 2 particles, respectively. The role of TiO 2 in lowering point of zero charge and p K a2 values is hypothesized to be the result of four-fold coordination as compared to the six-fold coordinated TiO 2 used in surface chemistry studies reviewed by this paper.

Measurements of Temperature and OH Radical Distributions in Flame Hydrolysis Deposition Process

The effects of SiCladdition on flame structures have been studied in flame hydrolysis deposition (FHD) processes using Coherent anti-Stokes Raman spectroscopy (CARS) and planar laser induced fluorescence (PLIF) to measure temperatures and OH concentrations, respectively. The results demonstrate that even a small amount of SiCl addition can change thermal and chemical structures of H/O diffusion flames. When SiCl is added to a flame temperature decreases in non-reacting zone due to the increases in both specific heat and density of the gas mixture, while flame temperature increase in particle formation zone due to the heat release through hydrolysis and oxidation reactions of SiCl. It is also found that OH concentration decreases dramatically in particle formation zone where temperatures increase. This can be attributed to consumption of oxidative species and generation of HCl during silica formation.

Fabrication of integrated microanalytical chambers and channels for biological assays using flame hydrolysis deposition glass

The aim of this study was to develop and optimise a process to etch deep, high aspect ratio microstructures in glasses. To this end, we examined the use of flame hydrolysis deposition (FHD) to form integrated lab-on-a-chip structures involving silica-on-silicon. The FHD process allows buried optical waveguide structures to be produced, which are isolated from the surface using a glass overlayer, or cladding. This arrangement allows sensing regions, such as microanalytical chambers and capillary channels for microfluidics, to be defined by etching through the upper cladding. In this paper we also demonstrate a simple fluorescence assay measurement, to illustrate the potential application of this technology.

Integrated-optic Nd:glass laser fabricated by flame hydrolysis deposition using chelates

We have demonstrated an Nd:glass waveguide laser deposited on a silica substrate by flame hydrolysis deposition using chelates. Continuous-wave lasing at lambda = 1062 nm was verified with a threshold of 143 mW and a slope efficiency of 2.1% when the laser was pumped at 807 nm. The flame hydrolysis deposition process using rare-earth chelates produces low-loss films with high Nd concentration and is suitable for multiple rare-earth doping.