Abstract: A method of making a preform for drawing optical fibers includes the steps of depositing a dopant material 3 in a dopant carrier chamber 1, heating the dopant material to cause it to vaporise at a predetermined rate, depositing from a mixture of a source material (GeCl.sub.4, SiO.sub.4, O.sub.2) and said vaporised dopant a mixture of solid components 8 and fusing said solid components to form a doped glass.

Abstract: A sol-gel process for preparing dimensionally precise tubular silica glass articles wherein a solution having a specific gravity higher than the specific gravity of the sol solution is placed in a cylindrical container with the sol solution is provided. The container is rotated around its axis and the high specific gravity solution becomes aligned adjacent the inner wall of the cylindrical container due to the effect of centrifugal forces acting on the high specific gravity solution. The sol solution becomes aligned adjacent the inner surface of the high specific gravity solution. The tubular wet gel formed from the sol solution has a uniform thickness and a highly precise round cross-section. The dimensional precision of the wet gel is maintained by rotating the wet gel during drying in order to prevent warping.

Abstract: A method for producing a glass preform for an optical fiber comprising jetting SiCl.sub.4 as a glass-forming raw material and an insert gas from a first nozzle of a multi-nozzle burner and, as another glass-forming raw material, at least one hydrogen-containing silicon compound with larger oxidation or hydrolysis reaction heat than that of SiCl.sub.4 selected from the group consisting of SiHCl.sub.3, SiH.sub.2 Cl.sub.2, SiH.sub.3 Cl and SiH.sub.4 from a second nozzle surrounding the first nozzle, flame hydrolyzing or oxidizing the glass-forming raw materials to synthesize glass soot particles, depositing the glass soot particles on a starting member to form a porous soot preform with a controlled bulk density distribution and heating and sintering the soot preform to obtain a transparent glass preform, by which a large glass preform is stably produced without cracking.

Abstract: A laser initiated process generates a shock wave of sufficient strength to produce a sintered product. The sintered product is prepared from at least a fuel and an oxidizer that will sustain an explosion in a reaction vessel. The explosion is initiated by introducing a laser beam into the reaction vessel and a shock wave begins to propagate at the point where the explosion is initiated. A sintered product is prepared when the shock wave is sufficiently strong to densify the product. Although the reaction will proceed using a two-component fuel-oxidizer mixture, the reaction is especially useful for producing sintered doped or undoped silicon dioxide (SiO.sub.2) from a reaction mixture containing at least silicon tetrachloride, hydrogen and oxygen. The sintered SiO.sub.2 can be deposited on a deposition surface situated in the reaction vessel to produce a preform that can be processed to produce an optical fiber.

Abstract: A hollow optical fibre waveguide for low loss transmission at mid infra-red wavelengths, and particularly in the region of the CO.sub.2 laser wavelength of 10.6 microns, is made of a germanium dioxide based glass, preferably comprising not less than 80 wt. % GeO.sub.2 and not more than 20 wt. % PbO, and has at least the inner surface of the hollow glass fibre devitrified and/or provided with a thin germanium lining. This change at the inner surface of the hollow fibre reduces transmission loss at the wavelengths of interest, and is effected by heat treatment at a temperature of at least 400.degree. C., simultaneously with exposure of the inner surface to a reducing atmosphere if a thin germanium layer is required at the inner surface.

Abstract: Optical waveguide having a fused silica core and a fluorine doped silica cladding is made by heating a cylinder of silica in fluorine, the cylinder initially having an outer annular particulate or porous region which is dried by the fluorine and into which fluorine diffuses to lower the refractive index of the silica when fused. The cylinder is made by depositing particulate silica onto a mandrel, drying the silica in chlorine, and heating the silica to densify it. Further particulate silica is deposited and is then heated in a fluorine-containing gas to dry, fluorine diffuse, and sinter the porous outer part of the silica. The resulting tubular silica preform is heated to collapse the tubular preform into a rod from which optical waveguide is drawn, the waveguide having a fluorine doped silica cladding.

Abstract: A process for fabricating an optical fiber by inner-lining a quartz tube with synthetic glass having a refractive index higher than the quartz tube and collapsing it while wire-drawing it via a wire-drawing furnace, comprising the steps of inner-lining the synthetic glass in the quartz tube so that the value of the inner diameter/the outer diameter of the quartz tube is 0.85 or less, sealing one end of the quartz tube to form a sealed end, then holding the quartz tube in a wire-drawing furnace, and wire-drawing it from the sealed end while holding the internal pressure of the quartz tube in negative pressure. Thus, the process prevents moisture from the exterior from mixing and allows the wire-drawing and collapsing steps to be performed stably and reliably.

Abstract: The disclosed simple, economical and rapid sol-gel process for forming a silica-based glass body, termed the "vapogel" process, comprises introduction of a silicon halide-containing gas (e.g., SiCl.sub.4 +O.sub.2) into an aqueous medium. The thus formed sol can gel within minutes, resulting in a monolithic gel from which particles having a narrow size distribution can easily be formed. The thus formed particles can be used to produce a glass body such as an optical fiber. Various techniques for forming the glass body from the gel are disclosed. Among the many advantages of the vapogel method are its ability to produce very homogeneous mixed oxide glasses of composition not obtainable by prior art sol-gel processes, and the advantageous mechanical properties of the gel produced by the method. The latter makes possible, inter alia, formation of particles having a relatively narrow size distribution.

Abstract: A method for producing a glass preform for an optical fiber by means at least two burners, comprising jetting a glass raw material containing SiCl.sub.4 from the first center burner and a hydrogen-containing silicon compound with larger reaction heat than SiCl.sub.4 from at least one second burner, flame hydrolyzing the glas raw material to synthesizing fine glass particles, depositing the fine glass particles on a starting member to form a porous soot preform and heating and sintering the soot preform to obtain a transparent glass preform, by which a large preform is stably produced and a bulk density of the preform is easily controlled.

Abstract: An improved method of forming an optical fiber preform includes separately forming a rod-like inner member and a hollow tube-like outer member by a soot glass deposition technique. The inner member is inserted into the outer member, while they are both in their unconsolidated state, and the asssembled parts heated to cause the outer member to collapse about the inner member and form a preform. The resulting preform can be drawn in one or more stages to provide an optical fiber having sharp step change in its index of refraction between the core and the cladding, a superior quality core to cladding interface, and improved optical transmission qualities. The inner and outer members can, if desired, be partially consolidated but for an unconsolidated layer at their interface prior to heating the outer member to cause it to join with the inner member.

Abstract: A method for depositing glass by decomposition of glass forming compounds in the vapor phase wherein a gaseous mixture of glass forming compounds is introduced into a refractory tube, and a source of heat is applied to the tube while the tube is being rotated. The source of heat is moved parallel to the axis of the tube to provide a moving hot zone therealong and at the same time to cause a conical neck portion to develop in the tube at which deposition of the glass forming compounds takes place.

Abstract: In the PCVD method glass layers are deposited on the inner wall of a glass tube by heating the tube to a temperature between 1100.degree. and 1300.degree. C., passing a reactive gas mixture at a pressure between 1 and 30 hPa through the glass tube, and moving back and forth inside the glass tube a plasma. After the glass layers are deposited, the glass tube is collapsed to produce a solid preform from which optical fibers are drawn. The regions of nonconstant deposition geometry at the ends of the preform (taper) are reduced by moving the plasma in the area of at least one reversal point nonlinearly with time and/or by changing the longitudinal extent of the plasma as a function of time. The length of the ramp is chosen to ensure an effective reduction of the end taper.

Abstract: A method of making optical performs from which optical fibers are drawn after collapsing of the preform, which comprises depositing by a vapor-phase reaction a plurality of layers on the inner surface of a support tube and controlling the temperature profile around the perimeter of the support so that the thickness of the deposit varies between at least one maximum and at least one minimum. Upon collapse of the support this gives rise to a core having a noncircular cross section.

Abstract: In an apparatus for the production of a porous optical fiber preform in a reaction container is provided in which a bar like starting member is movably supported for rotating and reciprocally moving across a gas flow, and a burner is provided for generating glass particles. An opening is provided on one side of the reaction container for introducing a gas flow and another opening is located on the other side of the reaction container opposite to the first mentioned opening, so that the gas flow is rectified in the reaction container and the glass particles can be deposited on the starting member with a uniform density distribution.

Abstract: The method allows the production of silica and dopant with reactions among gaseous chemical compounds. The optical fibers produced do not present dip and exhibit low attenuation. Carbon dioxide is used as the oxidizer and organometallic aluminum compounds are used to obtain the dopant; silica is obtained from organometallic silicon compounds or silicon tetrachloride.

Abstract: A method for producing a glass preform for use in fabrication of an optical fiber, which comprises steps of forming a cylindrical rod of glass having a refractive index distribution with axial symmetry around the rod axis by an VAD method, stretching the rod glass to decrease its diameter, inserting the stretched rod in a glass tube having a refractive index distribution with axial symmetry around the tube axis and heating and melting a complex of the stretched rod and the tube to integrate them from the glass preform produced by the method, an optical fiber having precisely controlled refractive index distribution can be fabricated.

Abstract: A method for the manufacture of glass, in particular, for glass fibers for optical communications, is provided, wherein glass particles on a substrate rod rotating about its axis, are deposited from a flame. In order to increase the yield of the deposition, two or more substrate rods are arranged at a distance from one another such that, between them an interstice with a diameter which is preferably smaller than the diameter of the free flame remains free and the flame is conducted through the interstice. By this method it is possible for the flame to be surrounded by substrate surfaces from several sides, as a consequence of which an increase in the yield of the deposition rate results.

Abstract: In an inside tube vapor phase deposition process for the production of doped silica glass by an oxidation reaction for optical fibre manufacture, in particular fluorine doped silica, the oxidation reaction is prevented from occurring until the reactant material, for example silicon tetrachloride and a fluorinating reagent, has been heated to a temperature above that required for the oxidation reaction. This preheating of the reactant material results in the oxidation reaction producing compositions of glasses which are not allowed from thermodynamic considerations at the lower temperature usually employed for the oxidation reaction, for example silica more highly doped with fluorine than hitherto achieved.

Abstract: A method for producing an aerosol stream, particularly an aerosol stream suitable for the production of a preform for an optical fiber. At least two gas phase reactants are reacted in a reaction chamber to form the aerosol stream. Undesirable deposition of liqid or solid reaction products, particularly at the point of entry into the reaction chamber, is avoided by the addition of at least one gas phase product to at least one of the gas phase reactants.

Abstract: Optical waveguide having a fused silica core and a fluorine doped fused silica cladding is made by depositing particulate core silica onto a support tube and then drying and densifying the silica. Further particulate cladding silica is deposited and is heated in a fluorine containing gas to effect drying, fluorine diffusion into and sintering the cladding silica. The support tube is etched away and the resulting tubular preform is heated to collapse it into a rod from which waveguide is drawn, the waveguide having a fluorine doped silica cladding.

Abstract: A glass preform for an optical fiber having homogeneous distribution of bulk density in its radial direction is produced by a method comprising forming fine glass particles by jetting a gaseous glass material and a fuel gas from a burner, depositing the fine glass particles on a seed rod to form a soot rod with measuring a temperature of a part of the soot rod on which the glass particles are being deposited so as to control said temperature, and then sintering the soot rod to obtain a transparent glass preform.

Abstract: The process employs solid-state halides heated at high temperature, under pressure conditions necessary to their vaporization, and obtains their condensation to a solid state on the internal surface of a glass tube, which, after collapsing, forms the preform.

Abstract: A porous glass optical waveguide preform is formed by depositing a coating of glass particulate material on the lateral surface of a core which may be a porous glass body continuously produced by the axial deposition of glass particles. The core rotates and moves longitudinally in one direction with respect to two flame hydrolysis burners which emit streams of glass particles having different compositions. In addition, the two burners reciprocatingly move with respect to a portion of the length of the core. The speed of each burner varies as it traverses along its path of reciprocating motion. The thickness of the layer produced by a burner at a given point is inversely related to the speed of the burner as it passes that point. The layers formed by the completion of a single traverse by both burners combine to form a conically-shaped layer, the composition of which varies from the base toward the apex thereof.

Abstract: A vapor phase method for making a metal halide material useful for the manufacture of an infrared transmitting optical waveguide fiber or other optical device, and the use of the method for making such a device, wherein a halogenated beta-diketonate of a metal to be incorporated in the device is converted to the corresponding metal halide by controlled decomposition of the halogenated diketonate, are described.

Abstract: Manufacture of optical fibre preforms. A material such as Si, Ge and B.sub.2 O.sub.3 is evaporated in vacuo into a silica tube and is made to condense therein as a viscous liquid which is then oxidized. The operation may be repeated several times with different materials. In this way the tube is clad from within in such a way that the optical index of refraction increases from the periphery inwards. Upon completion of the cladding the tube is collapsed into a rod which is then used for drawing optical fibres.

Abstract: The inside of a tube of an electrically insulating material is coated with a layer of electrically insulating material by reactive deposition of the coating material from a gas mixture which is passed through the tube. The tube is heated and the deposition is activated by a plasma produced by microwaves. The plasma reciprocates in the tube. Energy consumption is reduced by heating the tube to the reaction temperature at least partly by the energy supplied by the plasma to the tube wall. Thermal losses of the tube are reflected back to the tube wall by a heat reflecting element.

Abstract: A lightguide preform is fabricated by depositing a thin carbon layer (40) on a cylindrical glass mandrel (30) and further depositing a plurality of glassy soot layers (32) thereover. The resulting composite structure is heated, in a furnace, at a low temperature to remove the carbon layer (40) and then heated at an elevated temperature to consolidate the glassy soot layers on the glass mandrel to form the lightguide preform.

Abstract: A method of fabricating a porous glass rod having a reaction vessel including an elevational passage from an exhaust port of the vessel side and the top of the vessel over the interior and a burner mounted at the end in the vessel for forming glass fine particles, and a target elevationally movable and rotatably inserted from the passage into the vessel which has injecting to accumulate glass fine particles produced through the burner to form the porous glass rod at the lower end of the target rotated at a predetermined position in the reactor, and drawing the target in response to the growing velocity of the porous glass rod, wherein downward gas stream is produced along the outer periphery of the conduit in the vessel. The method is conducted by an apparatus for fabricating the glass rod. The method and apparatus can stabilize the fluidity in a reaction vessel to stabilize the outer diameter and the refractive index distribution of the glass rod in the longitudinal direction.

Abstract: The method allows surface defect reduction in silica optical-fibres by enriching the external layer by silica bonded carbon atoms within the silica network: SiC.SiO.sub.2. Carbon is obtained from chemical reaction directly during the drawing step.Volume defects are reduced by rapidly cooling the fibre structure heated up to vitrous transition temperature.The apparatus allows the method to be carried out during drawing step.

Abstract: A method of and arrangements for manufacturing glass bodies, in which method a thixotropic suspension, being the starting material for the glass body, is used to form a porous green body which is subsequently subjected to a purification step in a heated gaseous phase and then sintered to form a glass body, the starting material being homogenized and liquefied in a closed elastic mould by subjecting it to sound or ultrasonic, after which the liquid starting material is given the shape of the glass body to be formed and then solidified.

Abstract: An optical fiber comprising a core essentially made of quartz and a cladding made of fluorine added quartz is fabricated with good productivity from a glass preform produced by a method comprising maintaining a hollow cylindrical porous glass preform in a first high temperature atmosphere comprising at least one fluorine-containing compound with jetting a cooling gas through the central hollow part of the porous glass preform to grade the amount of fluorine to be added in the radial direction of the porous glass preform and then sintering the porous glass preform in a second high temperature atmosphere kept at a temperature higher than the first high temperature atmosphere to make it transparent.

Abstract: A method of and arrangements for manufacturing glass bodies, in which the starting material for the glass body, being an anhydrous suspension having a microdispersed solids content, is used to form a porous green body which is subsequently purified and sintered, in which by separating the phases of an anhydrous suspension the green body is formed by means of electrophoresis on a deposition electrode whose shape corresponds to that of the glass body to be produced.

Abstract: In the interest of reducing the effect on tensile strength of flame processing of a silica-based optical fiber waveguide, such processing is by a method in which a significant flow of oxygen surrounds a flame produced by combustion of hydrogen, deuterium, ammonia, or deuterated ammonia. Flame processing may be for purposes such as, e.g., fiber drawing, fiber fusing for the sake of lateral coupling, refractive index modification by the diffusion of dopants, and fiber splicing in the manufacture of long lengths of fiber. Even though there is no use of chlorine, at least 80 percent of spliced fibers have a tensile strength greater than or equal to 500 kpsi (3.45 GPa) as is desirable in optical fiber cable manufacture.

Abstract: An optical fiber preform is formed by plasma chemical vapor deposition utilizing a plasma torch that includes a chamber into which a plasma starting gas, such as argon, is introduced and converted to a plasma by application of high frequency electromagnetic energy. A suitable entry port is provided in the torch chamber to allow dopant material in solid form to be introduced directly into the plasma for vaporization or ionization. The dopant material passes into a lower temperature region where gaseous silica precursor gases are introduced to undergo reaction in the presence of the vaporized dopant to form appropriately doped silica particles for deposition onto a substrate. Solid dopants of low volitility or vapor pressure may be used as dopants which could not otherwise be used in vapor deposition processes. The absence of hydrogen in the reaction dynamics results in a preform having greatly reduced hydroxyl ion content.

Abstract: Method and equipment for the manufacture of glass bodies wherein a porous green body is formed by extrusion from the starting material for the glass body in the form of a plastic mass and this green body is then purified and sintered, the plastic mass being transformed into a state of reduced viscosity by the application of mechanical forces utilizing the thixotropic effect and being extruded in this state.

Abstract: A plurality of optical waveguide preforms are simultaneously formed by depositing on a plurality of starting members layers of glass soot to build up a coating on each starting member. The soot layers are deposited by traversing a series of burners along the starting members in such a manner that a given one of the burners traverses the first starting member from a first end thereof to a second end thereof to form a layer of soot thereon. The remaining burners of the series similarly traverses the first starting member, a plurality of burners traversing the first starting member at any given time. After the first burner has traversed the first starting member it similarly traverses the second starting member. After the last of the series of burners has begun its traverse along the first starting member, the first of the series of burners completes its traverse of the second starting member and again begins traversing the first starting member immediately following the last of the series of burners.

Abstract: Method and equipment for the manufacture of glass bodies with which a porous green body is formed from the starting material for the glass body in the form of an aqueous suspension with highly disperse solids content and this green body is then purified and sintered, the green body being deposited by separation of the phases of the suspension by electrophoresis.

Abstract: Methods are provided for reducing the water content of optical waveguide fibers produced from blanks having a centerline aperture and having compositions which prevent cooling of the blank to room temperature between consolidation and drawing. In accordance with certain aspects of the invention, the water content is reduced by storing the blanks at as low a temperature as possible, preferably below about 550.degree. C. In accordance with other aspects, the water content is reduced by filling the apertures of the blanks with an inert atmosphere during storage.

Abstract: Method and equipment for the manufacture of glass bodies in which a porous green body is formed from the starting material for the glass body in the form of a thixotropic suspension, is dried, purified and then sintered, during which process the starting material is transformed into a state with minimum viscosity and homogenized by the introduction of mechanical forces, is poured in the low-viscosity state into a mould of hydrophobic material corresponding to the shape of the glass body to be manufactured and is dried in the mould until a shrinkage of approximately 10% is achieved and then, after removal from the mould, is subjected to further processing steps to produce the glass body.

Abstract: A method of and arrangements for manufacturing glass bodies, in which the starting material for the glass body, being a suspension having a microdispersed solids content, is used to form a porous green body which is subsequently purified and sintered, in which, by separating the phases of the suspension in an arrangement in which a pressure difference can be built up, the green body is deposited on a porous membrane whose shape corresponds to that of the glass body to be produced, the pores of the membrane having a diameter in the range from 1 to 500 times the average particle diameter of the solids content of the suspension.

Abstract: Method and equipment for the manufacture of glass bodies wherein a porous green body is formed by extrusion from the starting material for the glass body in the form of a thixotropic suspension and this green body is then purified and sintered, during which process the starting material is brought in a state of minimum viscosity into an extruding press closed at its outlet end, is heated in the extruding press to a temperature below the boiling point of the dispersing liquid and then, after the extruding press is opened, is extruded into a green body of dimensionally stable shape by means of the die of the extruding press.

Abstract: An improved method for forming optical fiber preforms by the chemical vapor deposition technique utilizes an elongated burner having an array of burner orifices that forms an elongated silica particle stream having a thickness generally related to the diameter dimensions of a cylindrical preform starter rod. The burner can be formed as an elongated plate having the array of gas ports that produce the desired or silica stream. As a consequence, high initial silica particle deposition rates are obtained compared to prior methods utilizing circular burners, with the high initial deposition rates maintained throughout preform fabrication to result in an increase in preform production rates with an accompanying reduction in costs.

Abstract: A method of manufacturing glass bodies, in which the starting material for the glass body, in the form of microdispersed SiO.sub.2 particles, is used to form an open-pore green body which is subjected to a cyclic purification process in which the impurities present in the green body react with a purifying gas which is heated to a temperature in the range from 600.degree. to 900.degree. C., after which the green body is sintered; in each cycle of the said process, the arrangement holding the green body to be purified being flushed and subsequently evacuated.

Abstract: Disclosed is a method for the axial building up, in a vertical arrangement, of a hollow cylindrical soot body having no internal support and consisting substantially of silicon dioxide by means of at least one flame hydrolysis build-up burner serving for the soot production. In this method the soot is deposited at the beginning of the build-up onto an auxiliary body, and during the build-up the build-up burner and the growing soot body are rotated relative to one another, and the burner is at the same time held at an unvarying distance from the growing end of the hollow cylinder and centrally above the predetermined cross section of the upwardly growing end of the cylinder wall. The penetration of soot into the interior of the cylinder is prevented by means of a directed gas stream.

Abstract: A method of fabricating an optical waveguide comprising a plane substrate made of a material having a relatively low refractive index, a plane light propagation layer made of a material having a higher refractive index than the substrate, inserted between two strips of a material having a lower refractive index than its own, and a plane light confining layer of a material having a lower refractive index than the propagation layer. At least one of the layer, meaning either the propagation or the confinement layer, is obtained by causing to circulate in a tube of pure silica the vapors of a gaseous compound of silicon transformable into silica and of a gaseous compound of a doping agent transformable into one of its own oxides and heating said vapors sufficiently to obtain the deposit of a layer of doped silica having a refractive index different from that of the pure silica.

Abstract: A method for producing a glass preform for a single mode optical fiber comprising a core and a cladding, which method comprises depositing fine glass particle of a glass raw material on a periphery of a cylindrical seed member having a smooth and clean outer surface by flame hydrolysis to form a soot of the fine glass material, pulling out the seed member from the soot, inserting a core glass rod in a hollow portion of the soot and dehydrating and sintering a complex of a core rod and a cladding to produce a glass preform from which a single mode optical fiber having low attenuation of light transmission, particularly due to absorption by hydroxyl groups can be drawn.

Abstract: An improved method of manufacturing an optical fibre by vapor phase deposition of layers of cladding and core material for an optical fibre on the inside of a tubular substrate, followed by heating of the tube to cause it to collapse into a preform and drawing of the preform to form the fibre, includes performing the collapse with the introduction of chlorine gas acting as a drying agent. An alternative to chlorine gas is a vapor which decomposes in situ to yield chlorine without producing a solid deposit.

Abstract: Optical fibres are drawn from a solid preform which consists substantially of SiO.sub.2 and doped SiO.sub.2. After leaving the heating zone, the fiber is guided through a space having a laminar gas flow to restrict the temperature drop across the fiber so that no extra stresses are incorporated in the fiber upon cooling. A device for performing this method comprises a quartz pipe through which the fiber is guided and in which a laminar gas flow is maintained.

Abstract: An optical fiber waveguide resistant to ionizing radiation having a glass core of predetermined refractive index surrounded by glass cladding having a lower predetermined refractive index. The glass core and glass cladding are each composed of high purity silica incorporating gallium as a constituent. The gallium is present in the form of Ga.sub.2 O.sub.3 in a concentration of about 0.01 to 0.15 mole percent ratio to the silica. The glass of the optical waveguide can further include phosphorus in the form of P.sub.2 O.sub.5 as an additional constituent in the amount of from about 5 to 16 mole percent ratio to the total amount of all constituents.The waveguide of the invention is preferably manufactured by using GaCl.sub.3 in combination with an internal vapor phase process to produce a silica soot (16) containing Ga.sub.2 O.sub.3 on the interior surface of a high purity silica tube (10). The soot is then consolidated and the tube collapsed to form a substantially voidless solid rod preform of high purity SiO.

Abstract: Silica-based optical fiber comprising at least a core and a cladding surrounding the core, both core and cladding material produced by a vapor phase deposition process. The core and/or the cladding comprise at least two substituents, one chosen from Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr, Ba, and the 4f-type rare earths (the "modifiers"), and the other chosen from B, Al, Ga, In, P, As, and Sb (the "homogenizers"). The maximum concentration of the substituents in the fiber is such that 3<(n.sub.1 M+n.sub.2 H)/H<20, preferably<10, where n.sub.1 and n.sub.2 are the valences of the modifier and the homogenizer, respectively, and M and H are the concentration, in mole %, of modifier and homogenizer, respectively. Fiber according to the invention comprises at least 50, frequently more than 80% by weight, SiO.sub.2, and further has a maximum modifier concentration of at least 0.2 mole %, preferably at least 1 mole %.