Optical fiber preform and method for manufacturing such optical fiber preform from a primary preform

Abstract

The invention relates to an optical fiber preform (20) comprising a primary preform (21) and one or more purified silica-based overclad layers (22) surrounding said primary preform (21), the purified silica-based overclad layers (22) comprising lithium and aluminium, and having a ratio between lithium concentration [Li] and aluminium concentration [Al] satisfying the following inequality:
1×10.sup.−3≤[Li]/[Al]≤20×10.sup.−3.

Claims

1. An optical fiber preform comprising a primary preform and at least one purified silica-based overclad layer surrounding the primary preform, the at least one purified silica-based overclad layer comprising lithium and aluminium, wherein the at least one purified silica-based overclad layer has a ratio between lithium concentration [Li] and aluminium concentration [Al] satisfying the following inequality:
4×10.sup.−3≤[Li]/[Al]≤10×10.sup.−3.

2. The optical fiber preform according to claim 1, wherein the ratio between lithium concentration [Li] and aluminium concentration [Al] satisfies the following inequality:
4×10.sup.−3≤[Li]/[Al]≤6×10.sup.−3.

3. An optical fiber made from the optical fiber preform according to claim 1.

Description

5. LIST OF FIGURES

(1) Other features and advantages of embodiments of the invention shall appear from the following description, given by way of an indicative and non-exhaustive examples and from the appended drawings, of which:

(2) FIG. 1 provides a schematic illustration of an example of an optical fiber preform that has manufactured in application of the invention;

(3) FIG. 2 provides a schema illustrating the implementation of a method of manufacturing an optical fiber preform according to a particular embodiment of the invention;

(4) FIG. 3 graphically depicts a purification yield for alkali elements as a function of purifying compound flow rate;

(5) FIG. 4 graphically depicts the ageing at a wavelength of 1385 nm of an optical fiber after being exposed under a pressure of 10 bar of pure hydrogen during 72 hours and at a temperature of 70° C., as a function of [Li]/[Al] ratio in an overclad material for G652d single-mode fiber.

6. DETAILED DESCRIPTION

(6) In all of the figures of the present document, identical elements and steps are designated by the same numerical reference sign.

(7) With reference to FIG. 1, there can be seen an example of an optical fiber preform 20 that has been overcladded in application of the invention. The optical fiber preform 20 illustrated here is ready to be draw into a fiber.

(8) The optical fiber preform 20 comprises a primary preform 21 of rod-shaped and an overclad layer 22 surrounding said primary preform 21. The overclad layer 22 is a purified silica-based layer obtained by means of a method of purifying synthetic or natural silica implementing during the silica deposition operation on the primary preform 21. The principle of such a method is explained in greater detail below in relation with FIG. 2.

(9) The deposition operation, also known as overcladding operation, serves to increase the diameter of the primary preform 21, to enable a fiber to be drawn therefrom that is several hundreds of kilometers long. The optical fiber preform 20 can comprise one or several overclad layers 22 (illustrated in dashed lines on FIG. 1).

(10) The method of purifying silica according to the invention makes possible to deposit one or more layers of silica that contain an optimized amount of impurities (where the prior art only aims at minimizing amounts of alkali elements), so as to confer on the future optical fiber an adequate trade-off between fiber attenuation and good durability against hydrogen ageing.

(11) The optical fiber preform 20 according to the invention comprises a purified silica-based overclad layer 22 having a ratio between lithium concentration [Li] and aluminium concentration [Al] comprised between 1×10.sup.−3 and 20×10.sup.−3. Concentrations are expressed in weight ppm. For example, the purified silica-based overclad layer 22 exhibits a purification ratio between lithium concentration and aluminium concentration of about 5×10.sup.−3.

(12) The inventors have discovered that, by depositing silica-based overclad layers 22 exhibiting a ratio between lithium concentration [Li] and aluminium concentration [Al] which is comprised between 1×10.sup.−3 and 20×10.sup.−3, more particularly between 4×10.sup.−3 and 10×10.sup.−3, and even more particularly between 4×10.sup.−3 and 6×10.sup.−3, the optical fiber preform thus obtained allows the future fiber (after drawing) to exhibit a good trade-off between attenuation at wavelength of 1550 nm and durability against hydrogen ageing and therefore to meet the required fiber specifications.

(13) With reference to FIG. 2, there can be seen an example of implementation of the method of manufacturing according to the invention. The method aims at manufacturing an optical fiber preform 20 from a primary preform 21.

(14) The method comprises a step of depositing several silica-based overclad layers 22 on the primary preform 21. This step consists in injecting a powder of synthetic or natural silica 11 into a plasma 4 provided by a plasma torch 3. The primary preform 21 extends along a longitudinal axis (referenced as L) and is set into rotation around said longitudinal axis L in the direction indicated by arrow 7. The preform is moving in a back-and-forth motion along said longitudinal axis in front of the plasma source 3 that provides the plasma 4 in front of the primary preform 21 substantially perpendicular to said longitudinal axis L. The step of depositing is carried out by means of an injection duct 9 which delivers grains of silica 11 into the plasma 4. These grains are the result, for example, of grinding up coarse blocks of natural quartz or of the extraction of quartz grain from granite stone using the proper purification process. The injection is here performed merely by gravity. A valve (not shown) cooperates with the injection duct 9 to allow the injection rate to be adjusted.

(15) The method further comprises a step of purifying of the silica deposited on the primary preform 21. It consists in injecting, into the plasma 4, a purifying gas 15 containing, fluorine or chlorine element to neutralize alkali elements contained in the powder of silica which is depositing on the primary preform 21. This step is carried out by means of an injection duct 13 which feeds the plasma with the purifying gas 15. The purifying gas 15 is, for example, sulfur hexafluoride SF.sub.6.

(16) The deposition and purifying steps are carried out as the primary 21 preform is rotating and moving in front of the plasma plume.

(17) The method further comprises a step of adjusting the purifying gas flow such that the silica-based overclad layer 22 which are depositing on the primary preform 21 have a ratio between lithium concentration [Li] and aluminium concentration [Al] satisfying the following inequality:
1×10.sup.−3≤[Li]/[Al]≤20×10.sup.−3  (1)

(18) In the present example, the purified silica-based overclad layer 2 exhibits a purification ratio of about 5×10.sup.−3.

(19) A valve (not shown) is connected to a gas supply (not shown) cooperating with the injection duct 13 to adjust the purifying gas flow rate. It defines the SF.sub.6 fluorine flow rate.

(20) By way of example, the silica flow rate is set between 0.5 and 6 Kg/hour, with grain average size between 50 and 400 μm. The Plasma power is set between 60 and 140 KW. The SF6 flow rate is set between 0 and 1000 sccm. The core rod is translated at a rate of 5 to 80 mm/min in front of the plasma flame.

(21) In the plasma, the chemical reactions between the silica grains and the fluorine SF.sub.6 occur. The temperature of the plasma lies in the range 5 000° C. to 10 000° C., causing the silica grains to melt.

(22) The fluorine SF.sub.6 reacts with the alkali elements that are present in the natural silica so as to neutralize so that the overclad layers meeting to the above criteria.

(23) For a given reaction temperature and a given silica flow rate, the SF.sub.6 flow rate can be adjusted to obtain the desired lithium and aluminium concentration in the overclad layers (i.e. after purification), which depends content of lithium and aluminium contained in the raw silica batch (before i.e. purification).

(24) It is possible to tune one of these purifying gas injection parameters or a combination of these parameters to satisfy the aforesaid inequation (1).

(25) Overclad's contamination in alkali elements can be expressed by that [alkali].sub.after purification=α.[alkali].sub.before purification, with α a parameter which is function of the purifying gas flow rate.

(26) FIG. 3 graphically depicts the purification yield (1−[Alkali].sub.after purification)/[Alkali].sub.before purification as a function of SF.sub.6 flow rate, for given APVD conditions. The curve shows that, when SF6 flow rate is chosen between 0 and 800 sccm (“standard cubic centimeters per minute”), purification yield ranges between 0% and 85%. Lithium contamination in preform's overclad can be set at the required target by choosing the right flow. More generally, SF6 flow rate can be set between 0 and 5000 sccm.

(27) TABLE-US-00001 Gas Attenua- flow [Li]/[Al] tion at Al Li rate in 1550 nm Ageing* (ppm) (ppm) (sccm) overclad (dB/km) (dB/km) Trial1 14 0.4 800 0.004 0.194 0.30 Trial2 14 0.4 200 0.008 0.190 0.43 Trial3 7 0.2 250 0.007 0.190 0.40 Trial4 7 0.04 800 0.003 0.200 0.26 *Ageing measured at wavelength of 1385 nm, temperature of 70° C., during 72 H for a pressre of 10 bar.

(28) FIG. 4 depicts the ageing at a wavelength of 1385 nm measured after an exposure of an optical fiber under a pressure of 10 bar of pure hydrogen during 72 hours and at a temperature of 70° C., as a function of [Li]/[Al] ratio in the overclad material for G652d single-mode fiber.

(29) From that table, it should be noticed that the claimed ranges of purification ratio guarantee that a good trade-off between attenuation and hydrogen ageing. Indeed, if the ratio is chosen between 0.004 and 0.010, then attenuation at a wavelength of 1550 nm, for G652d preform type with usual draw configuration, is lower than 0.19 dB/km while keeping a reasonable resistance of fibers when exposed to an hydrogen atmosphere (between 0.2 and 0.6). The degradation under a pressure of 10 bar of pure hydrogen at a temperature of 70° C. will not exceed 0.6 dB/km at 1385 nm.

(30) If the ratio is chosen between 0.004 and 0.010, attenuation at wavelengths of 1310 nm and 1550 nm, for G652d preform type with usual draw configuration, are lower than 0.32 and 0.19 dB/km respectively while keeping a reasonable resistance of fibers when exposed to an hydrogen atmosphere. The degradation under a pressure of 10 bar of pure hydrogen at a temperature of 70′C will not exceed 0.6 dB/km at 1385 nm after 72 hours.

(31) In a variant of embodiment, the quantity of aluminum can be adjusted in the raw natural silica prior deposition, using an adequate procedure, in order to satisfy the targeted range of lithium and aluminum concentrations. This variant is of particular interest in case of highly contaminated silica that can't be purified during the overclad step down to satisfying purities. In this variant a purifying gas injected in the plasma through duct 13 could also be used.

(32) According to a particular embodiment, the method further comprises a step of controlling lithium concentration [Li] and aluminium concentration [Al] in real time of the silica-based overclad layers 22 deposited on the primary preform 21. Then, the step of adjusting the carrier gas injection parameters is carried out as a function of the result of said controlling step.

(33) Although the present disclosure has been described with reference to one or more examples, workers skilled in the art will recognize that changes may be made in form and detail without departing from the scope of the disclosure and/or the appended claims.