JP2002533295A - Tantalum-doped waveguide and method of manufacturing the same - Google Patents

Abstract

(57) [Summary] The present invention relates to a low-loss optical waveguide doped with tantalum and a method for manufacturing the same, wherein SiO ₂ soot is doped with Ta ₂ O ₅ and is suitable for preventing crystallization in an optical fiber. Form a consolidated soot blank under the conditions. The resulting cane is drawn into an optical fiber or overcladding and subsequently into an optical fiber. In a gas atmosphere or a vacuum atmosphere, the soot blank before drawing is sintered and vitrified by high-temperature consolidate to produce a low-loss optical waveguide fiber.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD OF THE INVENTION

The present invention relates to an optical waveguide glass having a high refractive index and a method for producing such an optical waveguide glass, more particularly, adding Ta ₂ O ₅ to the optical waveguide glass,
A method of manufacturing an optical waveguide fiber having substantially no crystallinity. While the present invention may be practiced using multiple soot recovery and addition techniques, it is particularly suited for use with an external method (OVD), and such methods are described below.

[0002]

[Prior art]

With the rapid spread of the telecommunications field, there is a strong demand for a system that can carry a larger amount of data in a shorter time. Therefore, there is a continuing need in the optoelectronics field for new optical waveguide systems to meet the demands of these systems, and for new optical waveguides and new optical waveguide components used therein.

[0003] Optical waveguide fibers typically include a core surrounded by a cladding material having an index of refraction less than that of the core. This type of optical waveguide fiber is usually made of silica to which a dopant such as germanium is selectively added. Germanium is the most common and most widely used dopant, but other dopants, such as, for example, phosphorus, fluorine, boron and erbium, are only slightly used, to name a few. . Germania is most commonly used because of its low melting point and high refractive index for silica.

[0004] All dopants, including germania, have the disadvantage that their usefulness is limited to specific applications. Thus, as technology demands and demands for new applications increase, the requirements for new optical waveguide fibers that can meet the demands of such applications simultaneously increase.
Such demands provide incentives to consider the application of new dopants to meet these demands and new methods of adding to optical waveguide fibers. In addition, competition is constantly pushing researchers to develop optical waveguide fibers at lower cost. Germania is about 1,000 per kilogram
Because of the dollar and cost, other dopants that are less expensive and can provide a higher refractive index than germania in lesser amounts are ideal.

[0005] A known dopant having such a high refractive index is tantalum. In fact, Ta_TwoO _Five The thin film consists of a thin film waveguide lens for silicon wafer solar cells and a non-reflective coating.
Widely used for Ta as an optical integrated device thin film_TwoO_FiveIs attractive but
Therefore, many researchers are conducting research activities in this area. Ta_TwoO_FiveLight collection containing
The device thin film is generally manufactured using a sputtering technique and has a thickness of about 0.4 dB / cm.
Results in a measurable loss of In the field of thin films, these high losses are affected.
An additional factor is believed to be the heat treatment of the thin film following sputter.
It is known that heat treatment changes a thin film from amorphous to crystalline
. After being formed in an optical waveguide fiber, these types of defects can cause the operating characteristics of the optical waveguide fiber to fail.
Performance will be adversely affected, and in optical waveguide fiber systems,
Will not function as a fiber.

[0006] Furthermore, planar devices have been fabricated using Ta ₂ O ₅ . The Ta ₂ O ₅ —SiO ₂ core glass for such devices has been laid down using electron beam evaporation. However, even the lowest loss measured with such a device was about 0.15 dB / cm or 15,000 dB / km. About 1 in optical waveguide fiber
Loss less than dB / km is the goal. Therefore, both thin films and planar optical devices
It does not suggest the usefulness of tantalum-doped silica for optical waveguide fibers.

In view of the above, there is a need for a dopant that can provide a high core index of refraction to an optical waveguide fiber in a limited amount. In addition, there is a need for dopants that have good non-linear properties, but do not significantly affect the mechanical properties of the optical waveguide fiber, yet exhibit effective amplification properties. Further, there is a need for a preferred method of adding dopants to optical waveguide fibers that does not significantly depart from current optical fiber manufacturing techniques and is therefore economically feasible. The low cost of tantalum compared to germania, along with the high refractive index of tantalum, make it a promising dopant for such species.

[0008]

Summary of the Invention

 One feature of the present invention is a method of manufacturing a low loss optical waveguide having a high index core.
And Ta_TwoO_FiveAnd SiO_TwoForming a soot blank containing the soot;
The blank is consolidated under appropriate conditions to form a cane and contain glass-containing Ta _Two O_Five-SiO_TwoPreventing crystallization of the cane and drawing the cane into an optical fiber
Step.

In another aspect, the invention provides a step of providing a soot blank comprising at least Ta ₂ O ₅ and SiO ₂ , wherein the soot blank is suitable for preventing crystallization of a glass comprising Ta ₂ O ₅ —SiO _2. Consolidating the soot blank under conditions to form a cane;
Drawing the cane into an optical fiber.

[0010] A further feature of the present invention relates to an optical fiber having a high-purity glass cladding and a high-refractive-index glass core bounded by the cladding. 2 to 5 glass cores
With Ta ₂ O ₅ in the weight percent range, the optical attenuation of the optical fiber is less than about 1.8 dB / km at 1550 nm. Yet another feature of the present invention relates to the glass used in the core of the optical waveguide comprising SiO ₂ and about 2 to 5% amorphous Ta ₂ O ₅ by consolidation, based on the oxide weight.

[0011] The glasses and methods of the present invention result in many advantages over other known glasses and methods. One of the most attractive features of using tantalum in the glass according to the invention is its high refractive index, which is reported to be 2.2 at 632.8 nm. Therefore, in the glass of the present invention, even with the same refractive index change,
It can be achieved by adding a much smaller amount of Ta ₂ O ₅ than is achieved with GeO ₂ . In addition, since tantalum is much cheaper than germania, choosing tantalum as a dopant significantly reduces cost.

Another effect is the high viscosity of Ta ₂ O ₅ —SiO ₂ glass, which is a function of the high melting point of tantalum. The melting point of Ta ₂ O ₅ is 1887 ° C., and the melting points of SiO ₂ and GeO ₂ are 1715 ° C. and 1116 ° C., respectively. Therefore, the high viscosity of tantalum silicate glass is effective in terms of viscosity as the glass of the present invention. In an additional advantage of the present invention, the tantalum oxide is chemically stable, does not dissolve in water, and the thermal expansion of the glass containing tantalum is lower than that of the glass containing germania. Also, the method of the present invention substantially prevents crystallization in glass containing Ta ₂ O ₅ —SiO ₂ during the fabrication of the optical waveguide. This latter effect results in improved optical properties.

[0013] Additional features and advantages of the invention will be set forth in the detailed description that follows. Also, some will be readily apparent to one skilled in the art from the detailed description or will be recognized by practicing the invention as set forth in the detailed description and appended claims, along with the accompanying drawings. There will be. The foregoing description and the following detailed description are merely exemplary of the present invention and are not to be construed as providing an overview or a framework for understanding the nature and characteristics of the present invention, such as the claims. It will be understood that the purpose is.

[0014] The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiments of the invention and, together with the description, serve to explain the principles and operation of the invention.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention also specifically takes into account the manufacture of single mode optical fiber, multimode optical fiber and planar waveguide, irrespective of the detailed description, drawings or examples herein. In addition, but not limited to, external methods (OVD), improved chemical vapor deposition (MCVD), axial methods (VAD), plasma enhanced chemical vapor deposition, to name a few. It is envisioned that the method can be implemented in conjunction with any of the known optical waveguide processing methods, such as the method (PCVD) and the sol-gel method. However, for the purposes of this specification, the tantalum silicate soots and blanks described herein and shown in the accompanying drawings are described as being manufactured using the OVD method.

Reference is made to details of a preferred embodiment of the invention, an embodiment of the invention being illustrated in the drawings. Wherever possible, the same reference numbers will be used throughout the drawings for the same or like parts. An exemplary embodiment of the optical waveguide of the present invention is shown in FIG. 1 and uses reference numeral 20 throughout.

According to the present invention, the optical waveguide fiber 20 includes a high purity glass cladding 22 and a high refractive glass core 24 bordering the cladding 22. As embodied herein and shown in FIGS. 1 and 2, the high purity glass cladding 22 is predominantly silica and the core 24 is comprised of silica with the desired proportions of tantalum. The optical waveguide fiber 20 having about 2 to 5% by weight of amorphous Ta ₂ O ₅ after consolidation has a 1550n
It exhibits less than about 1.8 dB / km loss at m. In a preferred embodiment, the optical waveguide fiber 2
Optical attenuation of 0 is less than 0.25 dB / km at 1550 nm.

A preferred embodiment of a method of manufacturing a low loss optical waveguide having a high refractive index core includes forming a soot blank including Ta ₂ O ₅ and SiO ₂ and preventing crystallization of Ta ₂ O _5. Consolidating the soot blank under suitable conditions to form a cane, and drawing the cane into an optical fiber. Ta ₂ O ₅ may be provided by known techniques such as chemical vapor deposition or liquid supply. SiO ₂ is
It can likewise be provided by known techniques such as chemical vapor deposition or liquid supply.

An exemplary embodiment of a reactor used for a chemical vapor deposition method is shown in FIG. The reactor 26 includes a diffuser 28, a preheating zone 30, and a reaction zone 32. During the operation,
The tantalum fragments 34 are introduced into the preheating zone 30 of the reactor 26, and the chlorine (Cl ₂ )
Gas is flowed by a diffuser 28 over a piece 34 of tantalum in a reactor 26. Reactor 26 includes two separate heater coils (not shown) for preheating zone 30 and reaction zone 32. When the temperature of the reaction zone is 350 ° C. or higher, a sufficient amount of TaCl ₅ gas is generated in the reactor 26 to provide the soot with the desired amount of Ta ₂ O ₅ .

As schematically shown in FIG. 4, TaCl ₅ gas is transported from a vapor transport system 36 to a burner assembly 38. TaCl ₅ is converted to Ta ₂ O ₅ by burner flame 40 according to the following reaction formula. 4TaCl ₅ (g) + 5O ₂ (g) = 2Ta ₂ O ₅ + 10Cl ₂ (g) The finely divided amorphous Ta ₂ O ₅ containing soot 42 is then released from the flame for recovery (collection) Is processed. In the preferred embodiment, the soot 42 is collected on a rotating mandrel 46 to form a soot blank 44. Soot blank 44
The amount of Ta ₂ O ₅ recovered along with the flow rate of Cl ₂ flowing through the reactor 26 is determined by the number of lateral passes made by the burner assembly 38 in the longitudinal direction of the soot blank 44.

The consolidation furnace used to consolidate germania silicate blanks formed using the general OVD method provides temperatures in the range of 1000 ° C. to 1450 ° C. Experiments have shown that such furnaces cannot provide the heat required to perform the consolidate step without crystallizing the glass containing Ta ₂ O ₅ —SiO ₂ required for the present invention. Therefore, there is a need for an improved consolidation furnace that can achieve temperatures above 1450 ° C. for the present invention. A preferred embodiment of such a consolidation furnace is shown in FIGS.

FIG. 5 shows a first preferred embodiment of the consolidate step in the method of manufacturing a low-loss optical waveguide having a high refractive index core. The soot blank 44 is held in a consolidation furnace 48 while being exposed to a gas 50. A gas of, but not limited to, chlorine, helium, oxygen, or a combination thereof is conveyed to consolidation furnace 48 to form atmosphere 52. The temperature in the consolidation furnace 48 is preferably 1600 ° C.
A suitable gas, such as helium, is flowed throughout the soot blank 44 until it rises, or even higher. The temperature of the Ta ₂ O ₅ —SiO ₂ core glass is maintained at 1600 ° C. or higher for a time sufficient to sinter the glass to clear vitrification while these conditions are maintained within the consolidation furnace 48. Is maintained in. The cane obtained according to additional processing steps known to those skilled in the art of optical fiber manufacture is drawn into optical fiber. An optical fiber made from a soot blank consisting of SiO ₂ containing about 2 to about 5% by weight of Ta ₂ O ₅ heat treated at 1600 ° C. or higher in a helium atmosphere flow has about 0.25 dB / s at 1550 nm. Expected to exhibit less than km attenuation. In a preferred embodiment, the temperature range is from about 1600 ° C to 1700 ° C.

FIG. 6 illustrates a second preferred embodiment of a consolidating furnace 48 showing support for a soot blank 44. In an embodiment according to the present invention, the soot blank 44 is heated in a vacuum atmosphere. The term "vacuum atmosphere" used here is
Means an atmosphere below atmospheric pressure. As shown in FIG. 6, a pump 56 or other barometric pressure reducing device removes air from the consolidating furnace 48 to reduce barometric pressure. As a result, the soot blank 44 is heat-treated at a temperature lower than 1600 ° C., and the soot blank 44 is sintered into glass. Generally, soot blank 44 is heated to a temperature in the range of 1600 ° C. from 1500 ° C. in a vacuum atmosphere, Ta ₂ O ₅ -SiO ₂ core glass temperature is 1600 ° C. from 1500 ° C. for a sufficient time And a transparent glass substantially containing no crystals is obtained. In the preferred embodiment, the vacuum atmosphere 54 in the consolidation furnace 48 exhibits a pressure of less than about 10 ^-4 Torr. The resulting cane is drawn into an optical fiber according to additional processing steps known to those skilled in the art of optical fiber manufacture. A soot blank 4 made of SiO ₂ containing about 2 to about 5% by weight of Ta ₂ O ₅ heat-treated at a temperature in the range of 1500 ° C. to 1600 ° C. in a vacuum atmosphere at a pressure of less than 10 ⁻⁴ Torr
The optical fiber made from 4 exhibits an attenuation of less than about 0.25 dB / km at 1550 nm.

An advantageous feature of the method of the present invention is a consolidate that does not produce crystals of a soot blank containing Ta ₂ O ₅ . The following examples illustrate the effect of the method of the present invention.

[0025]

Example 1 The core blank was prepared by adding 1 Ta ₂ O ₅ —SiO ₂ containing 5.55% Ta ₂ O ₅ by chemical weight% analyzed.
00 passes is deposited, subsequently, was formed by the SiO ₂ 177 pass deposition. The resulting soot preform sample was cut into slices with a cross section of about 25 mm in length and about 50 to 60 mm in diameter. The sample was heated to a temperature of 1450 ° C. in a stream of helium, as shown in FIGS. The core material (Figs. 7 and 8) and the core material under the core-cladding interface (
The scanning electron microscope (SEM) photograph of FIG. 9) shows that the glass containing Ta ₂ O ₅ —SiO ₂ has crystalline. As clearly shown in the fiber cross section 60 of FIG. 9, when the cladding 62 is consolidated into a transparent amorphous glass, the silica cladding 62 is easily distinguished from the core 64 comprising Ta ₂ O ₅ —SiO ₂ . Core-cladding interface area 6
6 is clearly observed between the cladding 62 and the core 64.

[0026]

Example 2 For Example 1, an additional slice of the soot preform sample described above was heated to 1550 ° C. in a helium atmosphere flow. The results of such experiments are shown in FIGS. SEM observation again showed that the core glass containing Ta ₂ O ₅ shown in FIGS. 10 and 11 contained many crystals. In fact, crystallization is very widespread, and increasing the temperature by about 100 ° C. does not reduce the number of crystals compared to Example 1.

[0027]

Example 3 An additional slice from the soot preform sample described in Example 1 was heat treated in a helium atmosphere flow at a temperature of 1650 ° C. As shown by the SEM observation results in FIG. 12, the core sample was consolidated into a transparent glass having no clear crystals.

[0028]

Example 4 An additional slice of the soot preform sample described in Example 1 was obtained at 1450 ° C.
Heated at 1550 ° C. and 1650 ° C. in a 1 × 10 ^-4 Torr vacuum atmosphere. As shown in FIG. 13, the SEM observation results show that after heat treatment at 1450 ° C., crystals are formed in the core glass containing Ta ₂ O ₅ . However, at the processing temperatures of 1550 ° C. and 1650 ° C., no crystal is formed in the Ta ₂ O ₅ —SiO ₂ core glass as shown in the results of the SEM observation in FIGS.

In another test, a single mode step index optical fiber was drawn from another core blank prepared in a manner substantially similar to that described above for Examples 1-4. The% Δ and attenuation for fibers containing different amounts of Ta ₂ O ₅ expressed in weight percent are as shown in Table 1.

[0030]

[Table 1]

The consolidation furnace used to heat treat optical fibers listed in Table 1 is a standard furnace commonly used for consolidating GeO ₂ —SiO ₂ optical fiber preforms. . Therefore, the maximum temperature available for consolidation was 1450 ° C. Therefore, a maximum temperature of 1450 ° C. was used to consolidate each of the core blanks listed in Table 1. The minimum loss achieved is 2.
9 a core blank having a of Ta ₂ O ₅ which has a weight percent, at 1550 nm, attenuation was 1.73dB / km. These results confirm the importance of using consolidate temperatures higher than 1450 ° C. in optical fibers containing Ta ₂ O ₅ —SiO ₂ . Based on the information and experimental results described in Examples 1-4 above, when the soot blanks corresponding to these fibers were consolidated in a consolidation furnace capable of maintaining temperatures above 1500 ° C., Ta ₂ O an optical fiber comprising ₅ -SiO ₂ is about 0.25 dB / k at 1550nm
exhibit a loss of less than m.

It will be apparent to those skilled in the art that modifications and variations can be made by the present invention without departing from the spirit or scope of the invention. Therefore, it is intended that the present invention cover such modifications, and variations of the present invention fall within the scope of the appended claims and their equivalents.

[Brief description of the drawings]

FIG. 1 is a perspective view of an optical fiber manufactured according to the present invention.

FIG. 2 is a sectional view of the optical fiber taken along line 2-2 of FIG.

FIG. 3 is a sectional view of a Cl ₂ reactor according to the present invention.

FIG. 4 is a diagram of a material supply system in a soot blank forming step according to the present invention.

FIG. 5 is a sectional view showing a first preferred embodiment of a consolidating furnace according to the present invention.

FIG. 6 is a sectional view showing a second preferred embodiment of the consolidating furnace according to the present invention.

FIG. 7 is a photomicrograph of Ta ₂ O ₅ -doped core glass consolidated in a helium atmosphere at 1450 ° C.

FIG. 8 is a photomicrograph of Ta ₂ O ₅ -added core glass consolidated in a helium atmosphere at 1450 ° C.

FIG. 9 is a micrograph of a core-cladding interface of a Ta ₂ O ₅ -added core glass consolidated in a helium atmosphere at 1450 ° C.

FIG. 10 is a photomicrograph of Ta ₂ O ₅ -added core glass consolidated in a helium atmosphere at 1550 ° C.

FIG. 11 is a photomicrograph of Ta ₂ O ₅ -added core glass consolidated in a helium atmosphere at 1550 ° C.

FIG. 12 is a photomicrograph of Ta ₂ O ₅ -added core glass consolidated in a helium atmosphere at 1550 ° C.

FIG. 13 is a micrograph of Ta ₂ O ₅ -added core glass consolidated in a vacuum atmosphere at 1450 ° C.

FIG. 14 is a photomicrograph of a Ta ₂ O ₅ -added core glass consolidated in a vacuum atmosphere at 1550 ° C.

FIG. 15 is a photomicrograph of Ta ₂ O ₅ -added core glass consolidated in a vacuum atmosphere at 1650 ° C.

[Explanation of symbols]

 20 Optical waveguide fiber 22 Clad 24 core 26 Reactor 28 Diffuser 30 Preheating zone 32 Reaction zone 34 Tantalum fragment 36 Steam transport system 38 Burner assembly 40 Burner flame 42 Soot 44 Soot blank 46 Rotating mandrel 48 Consolidating furnace

[Procedure amendment]

[Submission date] July 19, 2001 (July 19, 2001)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Claims

[Correction method] Change

[Contents of correction]

[Claims]

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat ゛ (Reference) G02B 6/00 376 G02B 6/00 376A (81) Designated country EP (AT, BE, CH, CY, DE, DK, ES, FI, FR, GB, GR, IE, IT, LU, MC, NL, PT, SE), AE, AL, AM, AT, AU, AZ, BA, BB, BG, BR, BY , CA, CH, CN, CU, CZ, DE, DK, EE, ES, FI, GB, GD, GE, GH, GM, HR, HU, ID, IL, IN, IS, JP, KE, KG, KP, KR, KZ, LC, LK, LR, LS, LT, LU, LV, MD, MG, MK, MN, MW, MX, NO, NZ, PL, PT , RO, RU, SD, SE, SG, SI, SK, SL, TJ, TM, TR, TT, UA, UG, US, UZ, VN, YU, ZA, ZW (72) Inventor Pearson Michelle D. United States of America New York 14870 Painted Post Beartown Road 678 (72) Inventor Tennant Christine El. United States of America New York 14821 Campbell Clauson Drive 4748 F-term (reference) 2H050 AB03Z AB04Y AB18X AC09 AC71 4G014 AH15 AH21 4G021 CA13 EA03 EB18 4G062 AA06 AA07 BB02 CC07 LA10 LB01 MM04 NN02

Claims (23)

[Claims] 1. A soot forming step of forming a soot blank containing Ta ₂ O ₅ and SiO ₂ , and forming a cane by consolidating the soot blank under predetermined conditions for preventing crystallization of the soot blank. A method for manufacturing a low-loss optical waveguide having a high-refractive-index core, comprising: a consolidating step; and a step of drawing the blank into an optical fiber. 2. The consolidating step includes: exposing the soot blank to a gas atmosphere containing helium; and heating the soot blank to a temperature higher than 1550 ° C. The method of claim 1. 3. The method according to claim 1, wherein the consolidating step comprises: exposing the soot blank to a vacuum atmosphere; and heating the soot blank to a temperature higher than 1450 ° C. Method. 4. The method of claim 3, wherein said vacuum atmosphere is at a pressure less than about 10 ^-4 Torr. 5. The method of claim 2, wherein said gaseous atmosphere includes helium and oxygen. 6. The method of claim 1, wherein said soot forming step comprises adding from about 2.5% to about 3.5% by weight of Ta ₂ O ₅ to said soot blank. 7. The method of claim 1, wherein the soot forming step and the consolidating step include a parameter selecting step of adjusting parameters so that an obtained optical fiber has an optical fiber loss of less than about 1.8 dB / km at 1550 nm. The method of claim 1, wherein: 8. The method of claim 1, wherein the soot forming step and the consolidating step include a parameter selecting step of adjusting parameters such that an obtained optical fiber has an optical fiber loss of less than about 0.25 dB / km at 1550 nm. The method of claim 1, wherein: 9. The method of claim 8, wherein the consolidating step comprises: exposing the soot blank to a helium atmosphere; and heating the soot blank to a temperature greater than 1550 ° C. Method. 10. The method according to claim 8, wherein the consolidating step includes: exposing the soot blank to a vacuum atmosphere; and heating the soot blank to a temperature higher than 1450 ° C. Method. 11. The method of claim 1, further comprising overcladding the blank to form a cladding made of SiO ₂ . 12. The soot blank forming step includes: flowing Cl ₂ gas on Ta in a Cl ₂ reactor at a temperature higher than 350 ° C. to form TaCl ₅ ; and sending the TaCl ₅ to an OVD burner. The method of claim 1, comprising: forming a soot comprising Ta ₂ O ₅ by heating; and depositing the soot on a rotating mandrel to form a soot blank. 13. An optical fiber manufactured by the method according to claim 1. 14. An optical fiber comprising a high purity glass cladding and a glass core bounded by said cladding, wherein said glass core has a higher refractive index than said cladding and from about 2 after consolidation. Ta ₂ O ₅ in the range of 5% by weight
Wherein the optical attenuation of the optical fiber is less than about 1.8 dB / km at 1550 nm. 15. The optical fiber of claim 14, wherein said glass core further comprises SiO ₂ and said optical fiber is substantially free of crystals. 16. The optical fiber of claim 15, wherein the optical fiber has an optical attenuation of about 0.25 dB / km at 1550 nm. 17. A glass for use in an optical waveguide core comprising SiO ₂ and amorphous Ta ₂ O ₅ in a range of about 2 to 5% by weight based on the oxide after consolidation. . 18. The core glass of claim 17, wherein the core glass is consolidated in a helium atmosphere at a temperature in a range from about 1600 ° C to about 2000 ° C. 19. The core glass of claim 18, wherein the core glass is consolidated in a helium atmosphere at a temperature in a range from about 1600 ° C. to about 1800 ° C. 20. The core glass of claim 19, wherein the core glass is consolidated in a helium atmosphere at a temperature in a range from about 1600 ° C to about 1650 ° C. 21. The core glass according to claim 17, wherein the core glass is consolidated at a temperature higher than 1450 ° C. in a vacuum atmosphere. 22. An optical fiber in which the core glass is bounded by a cladding made of SiO ₂ to constitute an optical fiber, and the optical attenuation of the optical fiber is 1.8 dB / km at 1550 nm.
18. The core glass according to claim 17, which is less than. 23. The core glass according to claim 22, wherein the optical fiber has an optical attenuation of less than 0.25 dB / km at 1550 nm.