Modified Chemical Vapor Deposition Research Articles

Single-mode fiber waveguides with losses below 1 dB/km

The modified chemical vapor deposition (MCVD) technology was used in fabrication of single-mode fiber waveguides from a quartz glass. The core was doped with germanium dioxide and the cladding deposited on it was doped with phosphorus oxide and with fluorine. The minimum losses were 0.6 dB/km at the wavelength of 1.3 µ and 0.35 dB/km at 1.5 µ. The excess losses were attributed to the presence of hydroxyl impurity groups and also due to variation of the fiber parameters along their length.

Germanium chemistry in the MCVD process for optical fiber fabrication

In the widely used modified chemical vapor deposition (MCVD) process for making optical fibers, germanium is the primary dopant for precisely increasing the refractive index of silica to form a guiding structure. The reactants in the process are usually GeCl 4 , SiCl 4 , and O 2 , often with POCl 3 , flowing in a silica support tube heated from the outside with an oxy-hydrogen flame. The oxide reaction products which are formed in the hot zone deposit as small particles downstream on the walls of the tube, and are consolidated into clear glass layers as the hot zone is traversed in the direction of flow. We have investigated the chemistry of germanium incorporation in the glass by infrared spectrophotometric analysis of the effluent gases, and by measurement of the properties of preforms and fibers produced by the process. It was found that SiCl 4 oxidizes completely to form the base glass, but GeCl 4 is incompletely reacted, with as much as 70 percent of the initial germanium present in the effluent as GeCl 4 . This is due to an unfavorable equilibrium in the reaction of GeCl 4 with O 2 to form GeO 2 , and we have developed a simple model for the process based on this equilibrium. At low temperatures the reactions in the hot zone are controlled by kinetics, but at the higher temperatures encountered in practice the gas concentrations and glass composition are controlled by thermodynamic equilibria. The composition of the glass produced can be quantitatively predicted by the model from such process parameters as the starting reactant concentrations, and the gas flow rates. It is necessary to include the dynamics of particle formation in the model to understand the inhomogeneity of multilayered deposits and the dependence of the final composition on the reaction zone maximum temperature. This results from the fact that the particles may grow large enough, especially with phosphorus present, to prevent diffusion of Ge from the particle interior to the equilibrating ambient at the surface. The model has been useful for predicting process parameters for the production of fibers with tightly controlled composition profiles.

Making Single-Mode Preforms by the MCVD Process

In the last few years, system technology has developed sufficiently to take advantage of the low-loss, high-transmission capability of single-mode fibers. First produced commercially in 1983, single-mode fiber now accounts for over 90 percent of all fiber made at AT&T Network Systems' Atlanta Works. Using the Modified Chemical Vapor Deposition (MCVD) process, glass rods, or preforms, of high-quality doped silica are created from which the fibers are then drawn. These fibers have losses below 0.40 dB/km and 0.22 dB/km at 1.31 μm and 1.55 μm, respectively, and transmission-rate capabilities in the gigabit-per-second range. Excellent concentricity of both the fiber core and outer diameter allows fibers to be joined into long lengths with a minimal additional loss. These characteristics allow single-mode fibers to be used in systems that operate at high bit rates with regenerator spacings of 30 km to 70 km apart.

Laser enhancement of thermophoretic deposition processes

Axial CO 2 laser radiation has been shown to have a significant influence in modifying the deposition of silicon dioxide in the modified chemical vapor deposition (MCVD) fiber preform process. Experiments on soot deposition with a stationary burner have shown a doubling of both the rate of deposition and the deposition efficiency.

A model for thermal collapse of tubes: Application to optical glass fibres

The collapsing of a glass tube by repeated traverses of a moving burner is a part of the modified chemical vapour deposition (MCVD) process of producing optical glass fibres. The collapsing process has been modelled by taking into account the forces generated due to surface tension and the reduced viscosity caused by heating. The model indicates that a significant reduction in diameter can be achieved when the temperatures at both the inner and the outer surfaces of the tube are higher than the softening point of glass, and the amount of reduction increases with the width over which such conditions prevail. Accordingly larger heat input, smaller tube wall thickness and slower burner speeds are shown to favour reduction in the diameter of the tube. The number of passes required by the burner to collapse a tube completely into a rod was calculated and it was in reasonable agreement with observations.

Laser-Induced Thermophoresis and Particulate Deposition Efficiency

The interaction of laser radiation and an absorbing aerosol in a tube flow has been considered. The aerosol is produced by external heating of reactants as in the MCVD (Modified Chemical Vapor Deposition) process to produce submicron size particles in the manufacture of optical fiber preforms. These particles are deposited by thermophoretic forces on the inner wall of the tube as they are convected by a Poiseuille velocity profile. Axial laser radiation in the tube interacts with the absorbing particles, and the laser heating of the gas induces additional thermophoretic forces that markedly increase the efficiency of particulate deposition. A particle concentration dependent absorption coefficient that appears in the energy equation couples the energy equation to the equation of particle conservation, so that a nonlinear set of coupled partial integro-differential equations must be solved. Numerical solutions for aerosol particle trajectories, and thus deposition efficiencies, have been obtained. It is shown that laser enhanced thermophoresis markedly improves the deposition efficiency.

The fabrication and performance of long lengths of silica core fiber

Three hundred kilometers of single-mode fiber exhibiting median optical losses of 0.19 dB/km at 1.57 μm have been fabricated from preforms made by a high-rate Modified Chemical Vapor Deposition (MCVD) process. A new fiber design [1] was utilized which minimizes Rayleigh scattering loss by reducing the amount of dopants in the core. Milestone systems experiments incorporating this fiber have already demonstrated 420-Mbit transmission through 203 km [2], 2-Gbit transmission through 130 km [3], 1.37 Tbit km/s using 10 wavelength division multiplexed lasers [4], 4-Gbit through 102 km using a novel electronic multiplexer/demultiplexer [5], and 4 Gbit through 117 km using a Ti:LiNbO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> external modulator [6]. Additionally, very low induced losses from hydrogen and radiation are reported.

Review of the Depressed Cladding Single-Mode Fiber Design and Performance for the SL Undersea System Application

High-capacity undersea cable systems based on optical transmission over single-mode lightguides offer tremendous economic advantage over coaxial cable technology due to their combined high-bit-rate long repeater spacing potential. However, very stringent requirements for the lightguide transmission medium properties must be met in order to realize the performance goals of such systems. This paper will review the depressed index cladding single-mode fiber design and its resultant performance. The ability to simultaneously realize excellent optical, mechanical, and dimensional properties in lightguides of this type fabricated using the Modified Chemical Vapor Deposition (MCVD) process has resulted in their use in the AT&T SL system to be used in TAT8.

Review of the depressed cladding single-mode fiber design and performance for the SL undersea system application

High-capacity undersea cable systems based on optical transmission over single-mode lightguides offer tremendous economic advantage over coaxial cable technology due to their combined high-bit-rate long repeater spacing potential. However, very stringent requirements for the lightguide transmission medium properties must be met in order to realize the performance goals of Such systems. This paper will review the depressed index cladding single-mode fiber design and its resultant performance. The ability to simultaneously realize excellent optical, mechanical, and dimensional properties in lightguides of this type fabricated using the Modified Chemical Vapor Deposition (MCVD) process has resulted in their use in the AT&T SL system to be used in TAT8.

Fundamental mode size and bend sensitivity of graded and step-index single-mode fibers with zero-dispersion near 1.55 µm

Single-mode fibers, for near 1.55-μm operating system wavelength with triangular ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\alpha = 1</tex> ), parabolic ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\alpha = 2</tex> ), and step-index ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\alpha=\infty</tex> ) profiles were fabricated by the modified chemical vapor deposition (MCVD) technique. Optical transmission losses under zero-tension and with the basket weave under 10, 40, and 70 gm tensions were measured, respectively. Fundamental mode size was obtained as a function of wavelength by using the transverse offset technique. The triangular-profile fiber shows lower loss (≃0.25 dB/km at 1.55μm) under zero-tension and a larger spot size than the other fibers. However, the basket weave test showed the triangular-profile fiber incurred higher loss with tension than the other profiles.

Modal Structure of an MCVD Optical Waveguide Fiber

In this paper we compare modal analyses of a Modified Chemical Vapor Deposition (MCVD) optical waveguide fiber and an idealized fiber. The pertinent profile parameters of the idealized fiber are obtained from the MCVD profile. We have validated the concept of an optimum α developed in our previous work. The reduction of the bandwidth of the MCVD fiber is shown to be directly related to the imperfections in the profile inherent in the MCVD manufacturing process.

An overview of the modified chemical vapor deposition (MCVD) process and performance

This paper reviews the MCVD process, with special emphasis on fiber design and material choices, understanding of mechanisms involved in the process, process improvements, and performance.

Ultraviolet absorption studies of germanium silicate glasses

The absorptions of pure and codoped germanium silicate preforms made by modified chemical vapor deposition (MCVD) have been studied from 170 to 400 nm. Three bands with maxima at 185, 242, and 325 nm have been observed. The shortest wavelength band is attributed to GeO(2), while the other bands are assigned to GeO. The 242- and 325-nm bands are correlated with a singlet-singlet and a triplet-singlet transition of GeO, respectively. The GeO content is found to increase with processing temperature, because higher temperature allows the greater dissociation of GeO(2) to GeO. The amount of GeO incorporated is determined to be of the order of 10(-2) wt. %. The 325-nm band of GeO should have no effect on fiber loss in the IR region.

Partition of Hydrogen in the Modified Chemical Vapor Deposition Process

In the modified chemical vapor deposition (MCVD) process for making glass fibers, most of the hydrogen coming into the reaction from hydrogen‐bearing impurities in the starting materials is not incorporated in the glass. It is instead mostly converted to HCl, which is not absorbed by the newly formed silica particles, and passes out the exhaust stack. The residual partial pressure of H2O formed in equilibrium with HCl in the presence of O2 and Cl2 accounts quantitatively for the OH appearing in the glass when the known solubility of H2O in silica is considered. The H2O/HCl equilibrium is quenched at a temperature below that of the reaction zone at a value for which the rate of reaction approximates the transit time across the deposition zone. Quantitative agreement is obtained for published OH concentrations produced by SiHCl3 doping.

Undistorted messages

Optical fibre communications is now a highly sophisticated art, thanks in part to production techniques such as modified chemical vapour deposition (MCVD) (for which J B MacChesney and P B O'Connor of Bell Laboratories have recently been awarded a patent) and to testing techniques that ensure that the variation of refractive index across the core optimises signal transmission (see Bell Laboratories Record 1980 58 381 and the Technology section of this issue of Physics Bulletin p85). However, manufacturing excellence cannot prevent the broadening of optical pulses as a result of dispersion: ingenuity is required to eliminate this distortion.

Investigation of the reactions of SiCl4 and O2 at elevated temperatures by infra-red spectroscopy

The modified chemical vapour deposition (MCVD) process for making fibre optic preforms uses direct oxidation of SiCl4, together with dopant chlorides to produce core glass. This reaction has been studied by direct infra-red absorption spectrophotometry of the flowing gas system. The spectrum of the SiCl4 reactant is recognizable, its decrease with increasing temperature was measured, and the absorption spectra of SiO2 and of silicon oxychloride reaction products were recognized in the flowing gas stream. The oxychlorides began to appear in the spectra at 900° C and increased with temperature to 1110° C. Their concentration decreased drastically between 1110° C and 1160° C above which only the spectrum of SiO2 was observed. The identities of the oxychloride molecular species produced are uncertain, but small amounts of Si2OCl6 and the cyclic Si4O4Cl8 were present together with several unidentified higher molecular weight compounds. The particulate matter formed in the MCVD process shows infra-red spectra varying with conditions of formation. At temperatures below 1160° C the particulate matter shows absorption bands attributable to silicon oxychlorides. At higher temperatures, the particulate matter is free of oxychloride bands and shows only the proper SiO2 spectrum. If moisture is present, these spectra are modified.

Low-loss single-mode fibers with different B_2O_3–SiO_2 compositions

Design considerations, fabrication, and transmission characteristics of single-mode optical fibers made with different borosilicate compositions are discussed. Fibers with core sizes as large as 15 microm and refractive-index differences below 0.1% were made with the modified chemical vapor deposition (MCVD) process. Minimum losses achieved with fibers up to 1-km lengths were 1.3-1.4 dB/km in the 1.02-1.1-microm range. Microbending losses were avoided by jacketing the fibers with silicone resin. Single-mode operation is maintained in moderately overmoded fibers (V ~ 3.0) because of leakage loss through a finite cladding thickness and because of bends in the fiber axis.