MXPA98008973A - Method for manufacturing alma segment ondasoptic guidance preforms - Google Patents

Abstract

A method for manufacturing a segmented core optical waveguide preform to make a fiber that is resistant to the attenuation increments due to hydrogen and heat aging is disclosed. A first core region comprising a silica glass rod containing at least one first dopant is inserted into a central opening of a second core region comprising silica soot containing a second dopant. The first core region and the second core region are consolidated together to form a segmented core region, and a coating is deposited on the outer surface of the segmented core region.

Description

METHOD FOR MANUFACTURING PREFORMS OF OPTICAL WAVE GUIDES OF SEGMENTED SOUL BACKGROUND OF THE INVENTION This invention relates to a method for manufacturing an optical waveguide preform. More specifically, the method of the present invention is useful for manufacturing low cost optical waveguides, especially waveguide fibers having a segmented core profile. Optical fibers having refractive index profiles such as W profiles, segmented core profiles and the like, possess desirable dispersion characteristics. See patents of E.U.A. 4,715,679 and 5,031,131 for teachings of various types of modified dispersion optical fibers. Fibers having these types of refractive index profiles have commonly been made by chemical vapor deposition (CVD) methods such as plasma CVD processes that are capable of forming monomode glass fibers whose webs include layers of different refractive indexes . These procedures produce relatively small prefaces. It is advantageous to form modified dispersion optical fiber preforms by external vapor deposition (OVD) processes that produce relatively large preforms or stretch pieces to decrease the cost of fiber manufacture.

A typical OVD process for forming such fibers is described in the U.S.A. No. 4,629,485. According to that patent, a silica bar impregnated with gerania is formed and stretched to decrease its diameter. A piece of the bar is used as a mandrel on which particles of pure glass or soot are deposited. The resulting mixed structure is heated in a consolidation furnace (drying and sintering) through which a fluorine-containing gas flows. The soot is therefore doped with fluorine and sintered on the bar. One or more additional layers of glass are formed on the outer surface of the fluorinated dope silica layer to form a part from which a fiber can be stretched. When the soot is sintered according to the aforementioned method, whereby fluorine is supplied to the porous preform solely by means of the muffle gas containing fluorine, the concentration of fluorine (measured by the? Of the fluorine-containing layer) it is not enough to provide certain desirable optical characteristics. The typical fluorine concentration achieved with the muffle gas impurification provides -0.4%? When CF is the fluorine-containing constituent. The maximum delta value for CF produced by the procedure described above is -0.5% ?. As used herein, the term? .a_ (j, the relative difference in the refractive index between two materials with refractive index na and n ^, is defined as To simplify the expression, is it commonly expressed in percent, that is, a hundred times? In this description, na is the refractive index of fluoride-contaminated glass and nb is refractive index of silica.When a fluorine-contaminated silica tube is crushed on a silica-contaminated silica bar, or when A tube of silica impregnated with germania is crushed on a silica bar doped with fluorine, it is extremely difficult to achieve a satisfactory interface between these two members.This is because the interface typically contains many small bubbles, and much of the preform or piece The resultant produces an unusable optical waveguide.This formation of small bubbles is less frequent when members formed of other composition It is made of glass such as silica tube impurified with fluorine and a pure silica bar are fused to form a preform. The patent of E.U.A. No. 4,675,040, discloses the insertion of a core glass rod made of pure silica into a soot tube of coating material made of pure silica impregnated with fluorine, and the sintering of the core / shell structure to fuse the coating onto the core. pure silica soul. The patent of E.U.A. 4, 668,263 discloses a method for crushing a silica tube having an inner layer impurified with fluorine on the surface of a silica rod. According to that patent, the crushing step is achieved by turning the tube and heating it with the flame of a longitudinal travel burner. This technique can not be used to fabricate modified dispersion fiber designs of the type using the complete fluorinated doped tube, including the outer surface, as part of the core region or region of light propagation of the fiber. The reason for this is that, because the flame moistens the glass, that is, introduces hydroxyl contamination, the resulting fiber would be unsuitable for operation at wavelengths in which the attenuation due to the hydroxyl ions is large. An additional disadvantage of this method refers to the temperature of the flame, which is not less than 1900 ° C. At such high temperatures, control of the procedure becomes difficult. The axis of the preform can become non-linear or arch. If the soul bar is a soft glass such as glass doped with germania, the bar can become softer than the tube; and this may result in a non-round soul or a soul that is not concentric with the outer surface of the resulting fiber. The patent of E.U.A. No. 4,846,867, describes a method for crushing a fluorinated doped silica tube on the surface of a silica rod. Prior to the crushing step of the tube, a gas phase attack reagent is flowed through the space between the rod and the tube while the tube is heated by a flame. In the specific examples, in which SF-j_ is the attack reagent, a gaseous mixture of SF6, Cl2 oxygen (ratio 1: 1: 6 by volume) is introduced through a space between the rod and the tube. Said gaseous mixture removes the glass from the treated surfaces of the bar and the tube, thereby forming new surfaces in the bar / tube interface. The chlorine is present in an amount sufficient to remove the water generated by the attack reagent that contains fluorine. The outer surface of the resulting preform is subsequently coated with silica soot particles which are dried, fluorinated and then sintered to form a part from which an optical fiber is stretched. The flame that was directed onto the tube during the gas phase attack step introduces water to the outer surface of the tube. The attenuation of the resulting fiber from that water is high. The attenuation at 1380 nm for an example is 30 dB / km which is attributed to the contact of the oxyhydrogen flame with the preform. The co-pending patent application of E.U.A. No. 08 / 795,687, filed on February 5, 1997 and entitled "Method of Making Optical Fiber Having Depressed Index Core Region," describes a method for inserting a glass rod of doped silica with germania into a glass tube of doped silica with fluoride to form an assembly and consolidate the assembly to form a gap free of small bubbles. The tube can be overcoated with a coating material such as pure silica. It has been found that although this method avoids a free interplay of small bubbles between the consolidated interface of silica impregnated with silica and silica impurified with fluorine, it is difficult to control the attenuation increases due to aging by hydrogen and heat in the fibers stretched from preforms made by this method. As used herein, the term "hydrogen aging" refers to an attenuation increase in an optical waveguide that has been exposed to an atmosphere containing hydrogen at a certain concentration, pressure and temperature. The term "heat aging" refers to an increase in attenuation exhibited by an optical waveguide that has been exposed to heat. In view of the disadvantages mentioned above, it would be desirable to provide a method for producing a segmented core optical waveguide preform that would allow the entire active region of light from the waveguide preform part to be dried from inside the piece. . In addition, it would be particularly advantageous to provide a modified dispersion optical waveguide having low attenuation and exhibiting little or no attenuation increase due to heat or hydrogen aging.

BRIEF DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing an optical waveguide preform having a segmented core region. The method comprises providing a first core region comprising a glass rod, preferably a silica glass rod, the first core region containing at least one first purifier, preferably a dopant to lower the refractive index of the core. of silica glass, such as fluorine. The method further comprises depositing silica soot containing a second dopant on a mandrel and removing the mandrel to provide a piece of soot having a central opening therethrough to provide a second core region. The second impurifier contained in the silica soot is preferably an index increase dopant such as germania. The method also comprises inserting the first core region into the central opening of the second core region in an oven to provide the segmented core region of the waveguide preform. The method further includes the step of depositing a coating comprising silica soot on the outer surface of the segmented core region of the waveguide preform. In one embodiment of the invention, the step of providing the first core region can further comprise inserting the glass rod into a silica glass tube containing the first dopant to provide an assembly, inserting the assembly into an oven, heating the assemble and crush the tube on the bar in the oven. Preferably, a gas selected from the group consisting of 100% chlorine and chlorine mixed with diluent gas is flowed at the first end of the tube, between the tube and the rod, and towards the second end of the tube before the tube is crushed on the bar. The rod is preferably a silica glass rod containing a dopant such as germania to increase the refractive index of the glass. The crushing step of the tube can be carried out in the same furnace in which the chlorine gas flow step occurs. Advantageously, when the adjacent surfaces of the bar and tube are cleaned by the gas while the assembly is in an oven, the outer surface of the tube is not contaminated by the water that would be present if a flame were used to heat the assembly. during the gas flow step. This method is especially suitable for forming an optical fiber having a core including an annular region of depressed refractive index, as described in the copending U.S. patent application. entitled "Managed Optical aveguide Fiber Dispersion", presented the same date as the present application, fibers that have W profiles and segmented soul fibers. A particular advantage of the method of the present invention is that the entire active region of light of a waveguide preform piece can be dried from the interior of the piece. The Applicant has discovered that modified dispersion optical waveguide fibers made by the method of the present invention exhibit little or no increase in attenuation due to aging by heat or hydrogen. The additional features and advantages of the invention will be set forth in the following description. It should be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are designed to provide a further explanation of the claimed invention.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates an exemplary refractive index profile of an optical fiber that can be produced by the method of this invention; Figure 2 illustrates an exemplary refractive index profile of an optical fiber having a depressed index core region that can be produced by the method of this invention; Figure 3 illustrates the formation of a porous glass preform on a mandrel; and Figure 4 illustrates the sintering of a porous glass preform.

DETAILED DESCRIPTION The method of this invention can be used to produce an optical waveguide preform having a segmented core index profile. Generally, this method comprises (a) providing a first core region containing at least one first dopant, (b) providing a second core region made by depositing silica soot containing a second dopant on a mandrel and removing the core for providing a piece of soot having a central opening therethrough, (c) inserting the first core region into the opening of the second core region and consolidating the first core region and the second core region together in an oven for providing the segmented core region of the waveguide preform and (d) depositing on the outer surface of the core region a coating comprising silica soot. The core of the resultant fiber drawn from the waveguide preform includes the inner core region and an outer core region, each core region optionally including additional annular core regions. As used herein, the term "soul" refers to the active area of light of the waveguide, that is, the region of the waveguide through which light is transmitted. Steps (a) to (d) are not necessarily carried out in the order mentioned. In one embodiment, a silica glass rod containing a dopant to lower the refractive index of the glass is the first core region. In an alternative embodiment, the step of providing the first core region comprises crushing a glass tube on a glass rod, the rod and tube assembly being initially exposed to a chlorine gas at a temperature sufficient to achieve the cleaning of the core. the surface of the bar and the inner surface of the tube. After cleaning with chlorine, the temperature is then increased to crush and fuse the tube to the bar in a drying oven. One embodiment of the invention includes producing an optical waveguide fiber having a refractive index profile in which a core region of the core has a lower refractive index than an annular region of the core surrounding the central region. Figure 1 shows an exemplary refractive index profile of a waveguide fiber produced by the method of the present invention. In this embodiment, the first core region comprises a glass rod, preferably a silica glass rod containing a dopant. The dopant preferably decreases the refractive index of the silica glass. The preferred dopant is fluorine, since the attenuation by B2O3 limits the use of the fiber at wavelengths of less than about 1200 nm. The first core region can be made by any suitable method for manufacturing a waveguide core such as OVD, VAD, etc. For example, a bar doped with fluorine can be manufactured by depositing a piece of pure silica on an alumina mandrel 635 cm long or more. The mandrel is removed to provide a central opening along a central region of the piece, and the piece can be consolidated with approximately 70 cc of CF ^ to impurify the piece with fluorine, 66 ccm (cubic centimeters per minute) of chlorine and 1 liter of helium along the central opening. The piece can then be exposed to a temperature of at least about 1900 ° C, preferably around 2050 ° C and stretched to form a solid bar doped with fluorine. The diameter of the bar will depend on the desired index profile of the fiber that is formed from the preform piece. For example, the silica glass rod impurified with fluorine can be stretched to a diameter of about 8 mm. As shown in Figure 3, a second core region of the waveguide preform is formed by depositing silica soot containing a second dopant on a mandrel 10 of relatively large diameter and removing the mandrel to provide a piece of soot that It has a central opening through it. Before the deposition step, the mandrel 10 is inserted through the tubular sleeve 11. While the mandrel 10 rotates, it is also subjected to the translational movement with respect to the soot generation burner 13, whereby a porous glass preform 12 is formed on the mandrel 12. it can serve as the second soul region. The mandrel 10 has a diameter large enough to produce a tube structure having an internal diameter large enough to be useful in later steps of the method. For example, an alumina mandrel having a diameter of 6.35 cm or more is sufficient. The mandrel may be in the form of a rod or tube. The patent of E.U.A. No. 5,180,410, the content of which is incorporated herein by reference, includes a detailed description of the formation of porous preforms on tubular mandrels, which may be useful to carry out the step of providing a tubular porous preform that can be impurified during consolidation according to the method of the present invention. As mentioned above, during the deposition of the second core region, the mandrel rotates and also undergoes translational movement with respect to a soot generation burner to create a soot preform on the core. The second dopant is preferably a dopant for increasing the refractive index of the silica, such as germania. The amount of dopant in the second web region will depend on the desired refractive index profile of the waveguide formed from the waveguide preform. As shown in Figure 4, after the mandrel 10 has been removed from the second core region 12 to provide a second core region having a central opening 18 therethrough, a sleeve 14 can be fixed to one end of the second core region 12 to allow the piece of soot with the second core region to be suspended in a consolidation furnace. Preferably, the sleeve 14 is a normal ball socket 14 which is fused to the sleeve 11, and the assembly including a second core region 12 is suspended in the consolidation furnace 15 by that sleeve. The first core region comprising the glass rod impurified with fluorine is inserted into the central opening 18 through the second core region. The bar can be suspended 'within the piece of soot with the second core region by any suitable method, such as by making a small normal ball at one end of the bar and suspending the ball inside the sleeve at the end of the foot of the bar. soot with the second soul region (not shown). The first core region and the second core region can be placed together in an oven at a temperature of about 1000 ° C to about 1100 ° C, flowing helium to approximately 1 liter per minute and approximately 60 cm3 / min of chlorine between the central opening in the second core region and the first core region for about 1 hour in the direction of the arrow 16. Muffle gas (which preferably contains helium) in the furnace as indicated by the arrows 17. The end of the second core region 12 may optionally contain a capillary tube 19. The first core region and the second core region are then consolidated together for about one hour going down in increments the assembly of the first core region and the second core region at a rate of approximately 5 mm per minute within the zone of an oven at a temperature of at least about 14Q0 ° C, preferably around 1500 ° C . After the consolidation of the first and second core regions to provide the segmented core region of the preform, a normal end cap is fused, and a coating material comprising silica can be deposited on the surface outside of the segmented soul region. Before depositing the coating material, the segmented core region can be heated to a temperature of at least about 2050 ° C, preferably about 2050 ° C (sic) and stretched to a suitable diameter for the overcoating step. In an alternative embodiment, the step of providing the first core region may include additional steps. A modified dispersion fiber having a more complicated refraction profile may require additional processing steps to achieve the more complicated index profile. In Figure 2 an exemplary refractive index profile for a dispersion compensation optical waveguide fiber is shown.

The index profile shown in Figure 2 can be provided by the method of the present invention. In this embodiment, the step of providing the first core region includes inserting a silica glass rod into a silica glass tube containing at least one first dopant to provide an assembly. Preferably, the silica glass rod contains a dopant such as germania, similar P2 ° 5 ° to increase the refractive index of the silica glass rod. The rod can be formed by any of the various known techniques such as modified chemical vapor deposition (MCVD), axial vapor deposition (VAD) and external vapor deposition (OVD), depending on its desired refractive index profile. The at least one first dopant contained in the tube is preferably a dopant such as fluorine to lower the refractive index of the tube. The tube / bar assembly is inserted in an oven at a temperature of about 1000 ° C to about 1100 ° C. Drying gas selected from the group consisting of 100% chlorine and chlorine mixed with a diluent gas such as helium, is flowed through one end of the tube, between the tube and the rod, and towards the second end of the tube during one hour to clean the outer surface of the bar and the inner surface of the tube. The drying gas conventionally comprises a mixture of chlorine and an inert gas such as helium. Although the flowing gas stream may contain a diluent such as helium, 100% chlorine is preferred for cleaning purposes. The gas streams consist of dry gases, so that no water is present in the vicinity of the assembly during the heat treatment. The gases can be purchased dry; moreover, the helium used for the muffle gas is also made to run through a dryer. Advantageously, the diameter of the bar is slightly smaller than the inner diameter of the tube, allowing the chlorine to flow down around the entire periphery of the bar. Chlorine acts as a hot chemical cleaning agent. The step of cleaning with chlorine is more effective at high temperatures. It is preferred that the temperature of the cleaning step be at least about 1000 ° C to about 15 ° C., since at lower temperatures the duration of the passage would be so long that the step would be undesirable for commercial purposes. Obviously, lower temperatures can be used if processing time is not an important factor. The flow of hot chlorine between the tube and the bar is very beneficial because it allows the surfaces of the two members to be joined to each other without the formation of small bubbles in their interface. Small bubbles include defects such as bubbles and impurities that can cause attenuation in the resulting optical fiber. After flowing the drying / cleaning gas for about one hour, one end of the tube / bar assembly is lowered to increments in an area of an oven to at least about 1900 ° C, preferably around 2050 ° C, and The tube is crushed on the bar and stretched to a suitable diameter to be inserted into the central opening through the second core region. The upper end of the bar may be provided with an elongate end that is suspended from a narrow region in or near the tube sleeve. A vacuum source is connected to the sleeve. The lower tip of the tube / bar assembly is heated in the furnace zone to a temperature of approximately 2050 ° C. As the tip of the assembly passes through the area of the furnace, the diameter of the assembly decreases and the tube collapses on the bar and the space between the two members is evacuated. The assembly can be stretched to elongate it within a first region of core in which the tube is fused to the bar. The first core region can then be inserted into the central opening of the second core region as described above with respect to the previous embodiment. The remaining processing steps of providing a second core region, inserting the first core region in a central opening through the second core region and providing a coating layer comprising silica, are similar to the steps previously described in FIG. modality described above.

A fluoride-contaminated glass tube used in the step of manufacturing the first core region can be made by inserting a mandrel through a tubular sleeve. The mandrel has a relatively large diameter to produce a tube having an internal diameter large enough to be useful for receiving the silica glass rod. While the mandrel rotates, it also undergoes translational movement with respect to a soot generation burner, whereby a porous glass preform is formed on the mandrel. The mandrel is removed from the porous glass soot preform to provide a tubular preform having a central opening therethrough. A normal ball sleeve is fused to the tubular sleeve, and the preform is suspended in the consolidating furnace by the normal ball sleeve. The sintering is carried out in an atmosphere including an axial fluorine-containing gas such as SIF, CF4, C2F1 or the like. IFS tends to give higher levels of fluoride doping (typically producing -0.7%? And occasionally producing a delta of about -0.8%), but that doping causes high levels of water in the resulting gas. Such high levels of water in the fluorine-containing glass can be tolerated if the core of the fiber has a value? relatively high with respect to the silica coating, whereby little energy is propagated in the annular region of the fluorine-containing fiber. CF4 results in a drier glass but does not give the high levels of dopant that can be obtained by using IFF4. High concentrations of fluorine can be used in this process because the porous soot preform is formed of pure silica, i.e., there is no dopant such as germania that could be diffusely spread within the part. The resulting sintered tube contains a relatively high concentration of fluorine, since the fluorine-containing gas is flowed into the central opening of the tube and out through the pores of the porous glass preform thereby achieving a contact maximum with the full body of porous glass. The muffle gas preferably contains a diluent gas such as helium and a sufficient amount of chlorine to dry the preform. The central flow gas also preferably contains one or more diluent gases such as helium and chlorine. The flow of chlorine can be discontinued after the desired water content has been achieved and before the porous preform sinters. The resulting fluorinated doped tube can be stretched or re-stretched to decrease the internal diameter to the desired size. If the tube is stretched, it can then be cut to lengths suitable for deposition of soot on it. A tube contaminated with boron is simpler to manufacture than a tube contaminated with fluorine. For example, a porous preform of SÍO2-B2O3 can be formed on a mandrel as described above with respect to the tube impurified with fluorine, with BCI3 being fed into the burner together with SÍCI4. The mandrel is removed, leaving a longitudinal central opening, and the preform is placed in a consolidation furnace. A muffle gas of 40 normal liters per minute (slp) of helium flows upwards through the muffle of the furnace and gases of 1 slp of helium and 75 cm3 normal per minute (sccm) of chlorine flow into the central opening. After the preform is dried, it is sintered. The resulting tube can be stretched as described above. A more detailed description of the manufacture of porous preforms, the formation of core bars, the impurification of porous preforms and the attachment of sleeves to the preforms can be found in the co-pending patent application of E.U.A. No. 08 / 795,687, filed on February 5, 1997, entitled "Method of Making Optical Fiber Having Depressed Index Core Region", whose contents are incorporated herein by way of reference. The waveguide fibers produced by the method of the present invention exhibit low attenuation as a result of the low count of small bubbles in the interface between the first core region and the second core region. The attenuation in the water peak of at about 1380 nm for fibers made by the method of the present invention is low, because the tube is not heated by a flame. The fibers produced by the method of this invention exhibit an excess loss of about 1 dB / km at the water peak of about 1380 nm. The fibers made by the method of the present invention also exhibit low heat and hydrogen aging. Heat aging was measured by exposing the fibers produced according to the present invention at 200 ° C for 24 hours, and the fibers exhibited an attenuation of less than about 0.02 dB / km. Aging by hydrogen was measured by exposing fibers made by the method of the present invention at 85 ° C for one week to about 1% hydrogen, and the fibers exhibited an attenuation of less than about 0.03 dB / km. In this way, the fibers produced according to the present invention exhibit excellent resistance to aging by hydrogen and heat. An advantage of the fibers produced according to the present invention is that they do not require a hermetic coating to avoid aging by hydrogen and heat. It will be apparent to those skilled in the art that various modifications and variations may be made to the method of the present invention without departing from the spirit or scope of the invention. In this way, it is intended that the present invention cover the modifications and variations of this invention, as long as they are within the scope of the appended claims and their equivalents.

Claims (18)

NOVELTY OF THE INVENTION CLAIMS

1. - A method for manufacturing an optical waveguide preform having a segmented core region, comprising the steps of: providing a first core region comprising a glass rod, the first core region containing at least a first impurifier; depositing silica soot containing a second dopant on a mandrel, removing the mandrel to provide a piece of soot having a central opening therethrough to provide a second core region; inserting the first core region into the central opening of the second core region and consolidating together the first web region and the second core region in a furnace to provide the segmented core region of the waveguide preform; Segmented soul region has an outer surface; and depositing a coating comprising silica soot on the outer surface of the segmented core region.

2. The method according to claim 1, further characterized in that the glass rod is a silica glass rod, and the at least first dopant decreases the refractive index of the silica bar.

3. - The method according to claim 2, further characterized in that the at least first dopant comprises fluorine.

4. The method according to claim 3, further characterized in that the second impurifier increases the refractive index of the second core region.

5. The method according to claim 4, further characterized in that the second dopant comprises germania.

6. The method according to claim 5, further characterized in that the optical waveguide preform is a preform of optical waveguide fiber.

7. The method according to claim 1, further characterized in that the step of providing the first core region further comprises the steps of: inserting the glass rod into a glass tube containing the at least first dopant to provide an assembly, insert the assembly in an oven and heat the assembly by flowing a gas selected from the group consisting of 100% chlorine and chlorine mixed with a diluent gas at the first end of the tube, between the tube and the bar, and the second end of the tube, and crush the tube on the bar in the oven.

8. The method according to claim 7, further characterized in that the glass tube is silica glass and the at least first dopant decreases the refractive index of the silica glass.

9. - The method according to claim 8, further characterized in that the first dopant comprises fluorine.

10. The method according to claim 9, further characterized in that the second dopant increases the refractive index of the second core region.

11. The method according to the claim 10, further characterized because the second dopant comprises germania.

12. The method according to the claim 11, further characterized in that the glass rod is a silica glass rod and contains dopant to increase the refractive index of the silica glass. 13.- The method according to the claim 12, further characterized in that the impurifier contained in the bar comprises germania. 14. The method according to the claim 13, further characterized in that the waveguide preform is an optical waveguide fiber preform. 15. A method for manufacturing a modified dispersion optical waveguide preform having a segmented core region, comprising the steps of: inserting a silica glass rod containing a dopant to increase the refractive index of the silica bar in a silica glass tube containing a dopant to lower the refractive index of the silica glass to provide an assembly, insert the assembly in an oven, heating the assembly to a temperature of at least about 1000 ° C and flowing a gas selected from the group consisting of 100% chlorine and chlorine mixed with a diluent gas at the first end of the assembly. tube, between the tube and the bar, and towards the second end of the tube, and lowering the assembly in an area of the furnace at a temperature of at least about 1900 ° C in order to crush the tube on the bar and provide a first region of nt soul; depositing silica soot containing a dopant to increase the refractive index of the silica on a mandrel, removing the mandrel to provide a piece of soot having a central opening through it for 15 provide a second soul region; inserting the first core region into the central opening of the second core region and consolidating together the first web region and the second core region in a furnace to provide the segmented core region of the waveguide preform; region of soul 20 segmented has an outer surface; and depositing a coating comprising silica on the outer surface of the segmented core region. 16. The method according to claim 15, further characterized in that the impurifier for Increasing the refractive index of the silica glass rod and the dopant to increase the refractive index of the silica soot comprise germania. 17.- The method according to the claim 16, further characterized in that the dopant for decreasing the refractive index of the silica glass tube comprises fluorine. 18.- The method of compliance with the claim 17, further characterized in that the optical waveguide preform is an optical waveguide fiber preform.