A computational fluid dynamics model for co-deposition of silica and germania in the MCVD process

The modified chemical vapor deposition (MCVD) process is used to create the doped silica preforms that are subsequently drawn to optical fiber. This paper reports on the extension of a computational fluid dynamics model of the MCVD process to include the simultaneous creation, transport and wall deposition of silica and germania particles. Simulations indicate the crucial role played by the interplay between chemical kinetics and equilibria along the substrate tube. Illustrative results show the likely impact of particle size on deposition patterns, along with a possible explanation for the observed high wastage of germanium in this process.     

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 AlCl₃ 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.     

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.     

Fluorine incorporation in silica glass by the MCVD process: Study of fluorine incorporation zone,evaluation of optical properties and structure of the glass

A theoretical approach was made to find out a complete fluorine incorporation zone on a ternary diagram which serves as a useful graphical representation to select the flows of the supplied reagents for incorporation of the suitable amount of fluorine into cladding glass of optical fiber preform made by the MCVD process using CCl₂F₂ as a source of fluorine under oxygen abundance, oxygen deficiency and intermediate oxygen state conditions. The possible mechanism for incorporation of fluorine into cladding glass of optical fiber is also evaluated on the basis of the thermodynamical data. The fluorine incorporation mechanism in silica glass by the MCVD process is found to be dependent on the CCl₂F₂/SiCl₄ ratio in the input gas mixture. Fluorine doping is found to be effective for removing the strained Si–O–Si bonds, which govern the optical transparency in deep ultra-violet (DUV) and vacuum ultra-violet (VUV) regions. The maximum refractive index depression of ?0.5 × 10^?3 is obtained with incorporation of fluorine into silica cladding glass by the MCVD process using CCl₂F₂ as a dopant precursor with suitable flow of SiCl₄ vapor along with O₂ through backward deposition pass. The structure of fluorine doped silica glass preform samples containing 1.70–1.79 mol% fluorine incorporated by the MCVD process based on the analyses of ¹⁹F MAS spectra done by high-resolution ¹⁹F NMR spectroscopy reveal the presence of two distinct types of fluorine environments. The majority of the fluorine environments are formed in SiO_1.5F polyhedral and less abundant species is observed to be highly unusual, yielding a fivefold coordinated silicon of the type SiO₂F polyhedral which become increased with increasing the fluorine content.     

Linear and nonlinear optical properties of the optical fiber doped with silicon nano-particles

We report for the first time the realization of the optical fiber doped with silicon nano-particles. Silica glass optical fibers incorporated with silicon nano-particles were fabricated by the modified chemical vapor deposition and the solution doping technique. We found that a broadband absorption appeared at 450–1300 nm and a photoluminescence band appeared at 600–1100 nm upon pumping with the Ar-ion laser, indicating that silicon nano-particles were successfully incorporated in the fiber core. We also estimated the third-order optical nonlinearity of the optical fibers by measuring the fringe shift obtained from the long-period gratings upon pumping with the Ar-ion laser from 477 to 512 nm. Importantly the optical nonlinearity at 1550 nm was found to be ～1.5 × 10^?15 m²/W and we believe that this large optical nonlinearity is caused by the exciton absorption of the silicon particles.     

Grain size distribution in the Kolmogorov model for nucleation and growth: Heterogeneous versus homogeneous nucleation

A theory of grain size distribution in nucleation and growth reactions described by the KOLMOGOROV model is developed. The distribution of agglomerates of growing and impinging nuclei is explicitely calculated for the two cases of heterogeneous and homogeneous nucleation. The results are compared and discussed with respect to the investigation of the nucleation mechanism in crystallization processes.     

Ultrafast modification of elements distribution and local luminescence properties in glass

The success in construction of three-dimensional micro optical components or devices inside transparent materials is highly dependent on the ability to modify materials’ local structure. Especially, the realization of space-selective manipulation of element distribution is highly desirable since most of optical parameters such as refractive index and luminescence are closely related to element distribution. Up to present, the only way to control selective element distribution is local melting of glass. Here, we reported, for the first time to our knowledge, the success in realization of space-selective manipulation of element distribution in glassy state region (i.e., un-melted region) inside glass with the irradiation of high repetition rate femtosecond laser. Confocal fluorescence spectra and micro-Raman spectra show that the luminescence distribution of Cu⁺ ions and the glass network structure can be controlled with femtosecond laser irradiation, revealing the potential applications of this technique in the fabrication of functional waveguides and integrated optical devices.     

The influence of core geometry on the crystallography of silicon optical fiber

Crystalline semiconductor core optical fibers have received growing attention as greater understanding of the underlying materials science, coupled with advances in fiber processing and fabrication, have expanded the quality and portfolio of available materials. In a continued effort to better understand the nature of the crystal formation this work studies the role of the cross-sectional geometry on the resultant core crystallography with respect to the fiber axis. More specifically, a molten-core approach was used to fabricate silicon optical fibers clad in silica tubes of either circular or square inner cross-sections. In both geometric cases, the silicon core was found to possess regions of single crystallinity where specific crystal orientations persisted along a fiber length of about 4–5 mm prior to transitioning through polycrystalline regions. However, the rotation and tilting angular combination needed to align a given crystallographic axis with the fiber axis was more constant over the single crystalline region in the case of the square-core fiber while more significant variations were observed in the round-core case. This work begins to elucidate some of the microstructural features, not present in conventional glass optical fibers, that could be important for future low-loss single crystalline semiconductor optical fibers.     

Elastic property differences in a multi-mode optical fiber and the effect on predicted mechanical lifetime

Multi-mode optical fibers are used for high energy transmission. One configuration of such fibers has a fluorine-doped silica cladding, and a pure silica core. Nanoindentation measurements of elastic modulus and hardness in the core and cladding of such a fiber are presented, and compared to those in the preform. The cladding region is softer, and has a significantly lower elastic modulus than the core in both the fiber and preform. Ultrasonic measurements made on core and clad materials extracted from the preform confirm the observed elastic property differences in the fiber. Water-immersion mandrel wrap data for time to failure of up to ~ 1 year are analyzed to obtain allowable design stresses for 30 year lifetime for the fiber. Incorporating the lower clad elastic modulus leads to design stresses that are ~ 20–25% lower than those predicted without taking the correct modulus into account.     

About the Fluorine Chemistry in MCVD: The Influence of Fluorine Doping on SiO2 Deposition

In the fabrication of quartz glass fibers for optical communication, fluorine doping gets increasing importance. In contrast to other dopants, fluorine influences the deposition of SiO₂ in Modified Chemical Vapour Deposition. An equation is derived from considerations of equilibrium chemistry which yields the deposition efficiency of SiO₂ as function of fluorine doping. It is compared with experimental results.     

Spatial distribution of piezoinduced change in the optical pathlength in lithium niobate crystals

For the first time, the “indicative” surfaces of the constants of the piezooptic effect and their stereographic projections have been constructed with due regard for crystal elasticity for lithium niobate crystals described by the symmetry class 3m. The comparative analysis of these surfaces and the corresponding surfaces of the true piezooptic effect has been performed.     

Electrical elements of the optical systems based on hydrogel - electrochromic polymer composites

Abstract

The results of studies of the electrically controlled elements of optical systems based on hydrogel - electrochromic polymer with conjugated π-electron system are shown. One important feature of conjugated polymers is the ability to change their optical characteristics under an external electric field. The electrochromic effect of these polymers may find applications in optical gating and filtering systems, optical sensors, ‘‘smart windows’’, optical memories, IR-switching, and electrochromic displays. Such electrochromic polymers were embedded in the matrix of a macroporous polymer hydrogel by the oxidative chemical polymerization method. The obtained samples based on hydrogel - polyaniline and hydrogel - polyorthotoluidine composites demonstrate the electrochromic behavior. In these composites, the electrolyte, as well as the electrochromic polymer, are located directly in the volume of hydrogel.     

Study of the possibility of interactive control of the surface profile of X-ray optical elements

We investigate the possibility of controlling the curvature parameters of parabolic mirrors that are modular elements of two types consisting of a base and thin inserts placed at the opposite side of the work surface. In the first type of modular elements, bending is controlled by the difference in the coefficients of the thermal expansion of the base and inserts. In the second type of elements, the profile is changed by the piezoelectric straining of the inserts under an electric field. A correlation is established between the parameters of modular elements and their surface curvature profile.     

About the fluorine chemistry in MCVD: The mechanism of fluorine incorporation into SiO2 layers

Fluorine incorporation during the deposition of silica layers is studied under MCVD conditions using C₂F₃Cl₃ as fluorine source. The experimental results are compared with known models of the incorporation mechanism. It is found that a gas/solid equilibrium during the consolidation step determines the final fluorine content of the layers.     

Probing the erbium ion distribution in silica optical fibers with fluorescence based measurements

Accurate determination of the rare earth dopant distribution in optical fibers enhances our understanding of the fiber manufacture process and enables further improvement in the design of fiber based products such as optical fiber lasers and amplifiers. Here a simple theoretical model consisting of an ensemble of rate equation systems, characteristic of the most likely electronic transitions that take place in the vicinity of erbium (Er³⁺) doped silica glasses, is developed and solved. Through this theoretical study it is established that information about the relative Er³⁺ ion distribution in fibers can be inferred by simply monitoring the backscattered fluorescence signal originating from the de-excitation of specific energy levels in the investigated samples. Following these theoretical studies a fluorescence intensity confocal optical microscopy (FICOM) scheme was employed to investigate the Er³⁺ ion distribution profiles in a range of silica optical fibers. The validity of the proposed theoretical model was confirmed through a comparison of the Er³⁺ ion distribution profiles acquired using the FICOM technique and those obtained from the application of a powerful analytical ion probe.     

Particle size control of PbTe quantum dots incorporated in the germano-silicate glass optical fiber by heat treatment

The effect of heat treatment of the germano-silicate glass optical fiber incorporated with PbTe quantum dots on the absorption characteristics was investigated. Incorporation of the PbTe QDs in the fiber core was confirmed by the absorption peaks that appeared at 687 nm and 1055 nm and their shift upon heat treatment. The absorption peak was found to shift linearly toward a longer wavelength after heat treatment at 700 °C, 1000 °C, and 1100 °C for 1 h. The red shift of the excitonic absorption peak was attributed to the increase in the average size of the PbTe QDs in the fiber core.     

Preparation and optical properties of titania-doped hybrid polymer via anhydrous sol-gel process

Incorporation of metal alkoxides (Ti, Zr, etc.) for tuning the optical properties of silica glasses by the sol-gel process is of significant interest for optical applications. In this paper, we report an anhydrous sol-gel process for preparation of photosensitive titania-doped hybrid glassy polymer with good homogeneity and high doping concentration (TiO₂ up to 40 mol%). The process consists of two steps: in the first step methacryloxypropyltrimethoxysilane (MPS) is hydrolyzed by boric acid through ligand exchange reaction (OH↔OR) under anhydrous conditions; and in the second step dimethyldimethoxysilane (DMDMS), diphenyldimethoxysilane (DPhDMS) and titanium ethoxide (TET) were added to condense with the silanols formed in the first step. The optical properties of the synthesized hybrid polymer were studied, and results showed that the hybrid material has low OH absorption, low optical losses (0.45 dB/cm at 1550 nm and 0.16 dB/cm at 1310 nm respectively), and good thermo-optical linearity with tuneable refractive index. The effect of TiO₂ doping in reducing the OH concentration of the hybrid material was observed, and the mechanism for this effect is discussed.     

Phase separation in highly photosensitive tin-codoped silica optical fibers and fiber preforms exposed to UV radiation

Sn-doped silicate fibers exhibit extremely high photosensitivity when exposed to 248 nm KrF excimer laser. Electron micrograph probing has shown that nanoscale phase separation occurs in fibers and fiber preforms after UV exposure. This phenomenon has been observed only in SnO₂-doped germanosilicate and phosphosilicate fibers and fiber preforms when the SnO₂ concentration is high. At low SnO₂ concentrations no phase separation has been observed. SnO₂ codoped glasses are the first optical fibers to exhibit phase separation.