Tubular semifinished product for producing an optical fiber

Abstract

Methods for producing an optical fiber by elongating a silica glass blank or a coaxial group of silica glass components, on the basis of which a fiber is obtained that comprises a core zone, an inner jacket zone enclosing the core zone and a ring zone surrounding the inner jacket zone, are known. In order to provide, proceeding from this, a method, a tubular semi-finished product and a group of coaxial components for the cost-effective production of an optical fiber, which is characterized by a high quality of the boundary between the core and jacket and by low bending sensitivity, according to the invention, the silica glass of the ring zone is provided in the form of a ring zone tube made of silica glass having a mean fluorine content of at least 6000 weight ppm and the tube has an inner tube surface and an outer tube surface, wherein via the wall of the ring zone tube, a radial fluorine concentration profile is adjusted which has an inner fluorine depletion layer with a layer thickness of at least 1 m and no more than 10 m, in which the fluorine content decreases toward the inner tube surface and is no more than 3000 weight ppm in a region close to the surface which has a thickness of 1 m.

Claims

1. A tubular semifinished product for producing an optical fiber, said product comprising: a tube of quartz glass having a mean fluorine content of at least 6000 wt. ppm, said tube having a wall with an inner tube surface and an outer tube surface, a radial fluorine concentration profile being set over said wall, wherein said wall has an inner fluorine depletion layer with a layer thickness of at least 1 m and not more than 10 m, and the inner fluorine depletion layer has a near-surface region with a thickness of 1 m near the inner tube surface, and wherein in the inner fluorine depletion layer the radial fluorine concentration profile decreases towards the inner tube surface and is not more than 3000 wt. ppm in said near-surface region, and the radial fluorine concentration profile in the inner fluorine depletion layer is less than 80% of a maximum fluorine concentration in the quartz glass of the tube.

2. The tubular semifinished product according to claim 1, wherein the product has an outer fluorine depletion layer in which the radial fluorine concentration profile decreases towards the outer tube surface.

3. The tubular semifinished product according to claim 1, wherein the layer thickness of the inner fluorine depletion layer is less than 4 m.

4. The tubular semifinished product according to claim 1, wherein the inner tube surface is produced without tools in a melt flow.

5. The tubular semifinished product according to claim 1, wherein the fluorine concentration in the near-surface region is not more than 2000 wt. ppm.

6. The tubular semifinished product according to claim 1, wherein the quartz glass of the tube in the area of the inner fluorine depletion layer has a mean hydroxyl-group content of less than 1 wt. ppm.

7. The tubular semifinished product according to claim 1, wherein the quartz glass of the tube in the area of the inner fluorine depletion layer has a mean hydroxyl-group content of less than 0.5 wt. ppm.

8. A preform or coaxial assembly for producing a quartz glass fiber comprising: a tube of quartz glass; said tube having a wall extending between an inner tube surface and an outer tube surface and having a radial thickness of at least 2 mm, and having a fluorine concentration over the radial thickness that includes a maximum fluorine concentration at one or more points; said wall including an inner fluorine depletion layer with a layer thickness of at least 1 m and less than 4 m; the inner fluorine depletion layer having a near-surface region radially inwardly therein with a thickness of 1 m adjacent the inner tube surface; the wall having a mean fluorine content over the radial thickness thereof of at least 6000 wt. ppm; the inner fluorine depletion layer having a fluorine content that is less than 80% of the maximum fluorine concentration over the layer thickness and that decreases towards the inner tube surface; the near-surface region having a fluorine content that is not more than 3000 wt. ppm.

9. The tubular semifinished product according to claim 8, wherein the tube has an inner tube surface produced without tools in a melt flow.

10. The tubular semifinished product according to claim 8, wherein the fluorine content in the near-surface region is not more than 2000 wt. ppm.

11. The tubular semifinished product according to claim 8, wherein the quartz glass of the tube in the area of the fluorine depletion layer has a mean hydroxyl-group content of less than 1 wt. ppm.

12. The tubular semifinished product according to claim 8, wherein the quartz glass of the tube in the area of the fluorine depletion layer has a mean hydroxyl-group content of less than 0.5 wt. ppm.

Description

EMBODIMENT

(1) The invention will now be explained in more detail with reference to embodiments and a patent drawing, which drawing shows in detail in:

(2) FIG. 1 a diagram with radial concentration profiles of SiO.sub.2 and fluorine in the area of the outer tube surface of a ring zone tube, and

(3) FIG. 2 a diagram with radial concentration profiles of SiO.sub.2 and fluorine in the area of the inner tube surface of a ring zone tube.

(4) A soot body is produced with the help of a standard soot deposition method and is subsequently doped with fluorine.

(5) After vitrification of the soot body a quartz glass cylinder is obtained with an outer diameter of 200 mm, an inner diameter of 80 mm, resulting in a ratio of outer diameter to inner diameter of 2.5. The quartz glass has a mean hydroxyl group content of 0.1 wt. ppm and a mean fluorine content of 6100 wt. ppm, which yields a refractive index decrease as compared with undoped quartz glass.

(6) The start cylinder is drawn without tools into a ring zone tube having an outer diameter of 38 mm, an inner diameter of 25 mm, and a wall thickness of 6.5 mm.

(7) The start cylinder is here fed in a hydrogen-free nitrogen atmosphere to an electric heating zone at a temperature of at least 2200 and at a length L of 170 mm at a feed rate v of 10 mm/min, resulting in a value of 10 min for the quotient L/v.

(8) The ring zone tube obtained thereafter is distinguished by an inner tube surface which is smoothed by hot forming and has a particularly high surface quality. A fluorine-depleted surface layer with a thickness of about 5 m is formed by heating during the elongation process in the area of the inner and outer tube surfaces of the ring zone tube.

(9) The corresponding concentration profile in the area of the outer surface is shown in FIG. 1. The concentrations for fluorine (curve 2) and SiO.sub.2 (curve 1) are plotted on the ordinate in relative units (based on the respective maximum concentration of SiO.sub.2 and fluorine), and the radial position is plotted in [m] on the abscissa. The gradual rise in the respective concentration profiles and the offset from the zero point is due to the spatial resolution of the measuring method.

(10) The layer thickness that has a fluorine concentration of less than 80% of the maximum value is defined as the fluorine depletion layer. The associated position is designated with B in FIG. 1. The concentration of SiO.sub.2 represents the reference point (zero point) for the position value. The zero point for the position, which zero point pertains to the 80% concentration value, is designated with A in FIG. 1. The layer thickness for the fluorine depletion layer thus follows from the distance A-B of the concentration profiles of fluorine and SiO.sub.2 at the 80% concentration value. In the near-surface region of up to 1 m the fluorine concentration is less than 2100 wt. ppm and the thickness of the fluorine-depleted surface layer is in this case about 7 m on the whole. It is acceptable for the outer cylinder jacket of the ring zone tube, but not optimal for the inner cylinder jacket.

(11) Therefore, the inner surface of the ring zone tube, which after elongation has a similar fluorine depletion layer as the outer cladding, is etched off in that a hot gas stream of etching gas (SF.sub.6) is passed through the inner bore. The concentration profiles obtained thereafter on the inner wall are shown in FIG. 2. It follows from the comparison of the concentration profiles of SiO.sub.2 (curve 2) and fluorine (curve) that due to the removal of the inner surface the thickness of the fluorine depletion layer is adjusted to the predetermined desired value of about 1.5 m and a steeper concentration curve is simultaneously obtained in the near-surface region. Due to the etching process a clean and defect-free surface is produced. In this case, too, the layer thickness which has a fluorine concentration which is less than 80% of the maximum concentration of fluorine and which can be read by way of the positional distance A-B is defined as the fluorine depletion layer. The mean fluorine content within the fluorine depletion layer is more than 3000 wt. ppm and it is about 2800 wt. ppm within an inner near-surface region of 1 m.

(12) It is true that the mean chlorine content of the ring zone tube is 200 wt. ppm and the nominal hydroxyl group content of the quartz glass of the ring zone tube is 0.1 wt. ppm. Due to the elongation process, it is however increased near the surface to maximum values of 5 wt. ppm or more. Due to the subsequent gas-phase etching a layer with a relatively high hydroxyl group content is however removed, so that a mean hydroxyl group of not more than 0.4 wt. ppm is obtained on the surface of the fluorine depletion layer, measured over a layer thickness of 1 m.

(13) The ring zone tube obtained thereby is used for overcladding a core rod in a rod-in-tube method. To this end segments are cut to the desired lengths from the ring zone tube. The core rod has a GeO.sub.2 doped core region with a radius of 12 mm and is surrounded with an inner cladding of undoped quartz glass having a layer thickness of 5.5 mm.

(14) The core rod is inserted into the inner bore of the ring zone tube and the tube, in turn, is surrounded by a jacket tube of undoped quartz glass with the refractive index n.sub.Ma that has an outer diameter of 175 mm, an inner diameter of 40 mm, and a mean chlorine content of 1800 wt. ppm.

(15) This coaxial arrangement of components is subsequently introduced into a drawing furnace in vertical orientation and is softened therein zone by zone, starting with the lower end, and a fiber is drawn from the softened region. The outer and inner fluorine depletion layers of the ring zone tube serve as passivation layers that reduce the out-diffusion of fluorine and thereby prevent the formation of bubbles in the area of the interfaces. The fluorine depletion layers thereby contribute to a low-defect contact area and interface to the core rod and to the jacket tube.

(16) A bending-insensitive optical single-mode fiber with an outer diameter of 125 m is drawn; it is distinguished by a ring zone with a high fluorine concentration and has a distance from the outer region of the core zone. The following is true for the radial course of the refractive index of the assembly: n.sub.Ma>n.sub.F<n.sub.Mi<n.sub.K.