METHODS FOR PRODUCING A HOLLOW-CORE FIBER AND FOR PRODUCING A PREFORM FOR A HOLLOW-CORE FIBER

Abstract

Methods are known for producing an anti-resonant hollow-core fiber which has a hollow core extending along a fiber longitudinal axis and an inner jacket region that surrounds the hollow core, said jacket region comprising multiple anti-resonant elements. The known methods have the steps of: providing a cladding tube that has a cladding tube inner bore and a cladding tube longitudinal axis along which a cladding tube wall extends that is delimited by an interior and an exterior; providing a number of tubular anti-resonant element preforms; arranging the anti-resonant element preforms at target positions of the interior of the cladding tube wall, thereby forming a primary preform which has a hollow core region and an inner jacket region; and elongating the primary preform in order to form the hollow-core fiber or further processing the primary preform in order to form a secondary preform. The aim of the invention is to achieve a high degree of precision and an exact positioning of the anti-resonant elements in a sufficiently stable and reproducible manner on the basis of the aforementioned methods. This is achieved in that while carrying out a process according to step (c), components of the primary preform made of quartz glass and/or parts surrounding the primary preform made of quartz glass are heated and softened together, wherein the quartz glass of at least one of the preform components and/or the quartz glass of at least one of the parts surrounding the preform contains at least one dopant which decreases or increases the viscosity of quartz glass.

Claims

1. Method for producing an anti-resonant hollow-core fiber comprising a hollow core extending along a longitudinal axis of the fiber and an inner sheath region surrounding the hollow core, which sheath region comprises several anti-resonance elements, comprising the method steps of: (a) providing a primary preform (1) for the hollow-core fiber which comprises at least one cladding tube (3) having an inner bore of the cladding tube and a longitudinal axis of the cladding tube along which a cladding tube wall delimited by an inner side and an outer side extends, (b) forming a number of precursors or preforms (4) for anti-resonance elements at desired positions of the cladding tube wall, and (c) elongating the primary preform (1) to form the hollow-core fiber or further processing the primary preform (1) into a secondary preform from which the hollow-core fiber is drawn, wherein the further processing comprises a single or repeated performance of one or more of the following hot-forming processes: (i) elongation, (ii) collapse, (iii) collapse and simultaneous elongation, (iv) collapse of additional sheath material, (v) collapse of additional sheath material and subsequent elongation, (vi) collapse of additional sheath material and simultaneous elongation, characterized in that, when performing a process in accordance with method step (c), components (2; 3) of the primary preform (1) made of quartz glass and/or components (4) made of quartz glass surrounding the primary preform (1) are heated and softened together, wherein the quartz glass of at least one of the preform components (2; 3) and/or the quartz glass of at least one of the components (4) surrounding the preform contains at least one dopant that lowers or increases the viscosity of quartz glass.

2. Method according to claim 1, characterized in that the dopant lowering the viscosity of quartz glass comprises fluorine, chlorine, and/or hydroxyl groups, and that the dopant increasing the viscosity comprises Al.sub.2O.sub.3 and/or nitrogen.

3. Method according to claim 1 or 2, characterized in that the optional further processing of the primary preform (1) comprises collapsing additional sheath material (4), and that the additional sheath material (4) consists of quartz glass containing a dopant lowering the viscosity of quartz glass.

4. Method according to claim 3, characterized in that the quartz glass of the additional sheath material (4) contains fluorine as dopant in a concentration between 500 and 14,500 ppm by weight, preferably between 2000 and 10,000 ppm by weight.

5. Method according to any one of claims 2 to 4, characterized in that, at a measured temperature of 1250° C., the quartz glass of the cladding tube (3) has a viscosity higher by at least 0.5 dPa.Math.s, preferably a viscosity higher by at least 0.6 dPa.Math.s, than the quartz glass of the additional sheath material (4).

6. Method according to any one of the preceding claims, characterized in that only the additional sheath material (4) contains a dopant and consists of quartz glass containing fluorine.

7. Method according to any one of the preceding claims, characterized in that at least a portion of the precursors for anti-resonance elements is present as tubular anti-resonance element preforms (4), which are preferably composed of multiple nested structural elements, comprising an ARE outer tube (4a) and an ARE inner tube (4b) inserted therein, and that the anti-resonance element preforms (4) consist of quartz glass which at a measured temperature of 1250° C. has a viscosity higher by at least 0.4 dPa.Math.s, preferably a viscosity higher by at least 0.5 dPa.Math.s, than the quartz glass of the cladding tube (3).

8. Method according to claim 8, characterized in that the cladding tube (3) consists of quartz glass, which contains a dopant lowering the viscosity of quartz glass.

9. Method according to claim 7 or 8, characterized in that in the case of nested structural elements, at least a portion of the ARE inner tubes (4b) consists of quartz glass which at a measured temperature of 1250° C. has a viscosity higher by at least 0.4 dPa.Math.s, preferably a viscosity higher by at least 0.5 dPa.Math.s, than the quartz glass of the ARE outer tube (4a).

10. Method according to any one of claims 7 to 9, characterized in that the cladding tube (3), the ARE outer tube (4a), the ARE inner tube (4b) and/or an overlay cylinder (2) for collapsing additional sheath material are produced on the basis of a vertical drawing process without a molding tool.

11. Method according to any one of the preceding claims, characterized in that a secondary preform is formed which has an outer diameter in the range of 30 to 90 mm, and/or that a primary preform is formed which has an outer diameter in the range of 20 mm to 70 mm.

12. Method according to any one of the preceding claims, characterized in that the formation of anti-resonance element preforms (4) in accordance with method step (b) comprises arranging the anti-resonance element preforms (4) at desired positions on the inner side of the cladding tube wall, wherein a positioning template that has holding elements for positioning the anti-resonance element preforms at the desired positions is used for arranging.

13. Method according to claim 12, characterized in that a positioning template is inserted with a shaft projecting into the inner bore of the cladding tube, which shaft is provided with holding elements in the form of several holding arms pointing radially outward.

14. Method according to any one of the preceding claims, characterized in that the inner side of the cladding tube is produced by machining, in particular by drilling, milling, grinding, honing, and/or polishing.

15. Method for producing a preform for an anti-resonant hollow-core fiber comprising a hollow core extending along a longitudinal axis of the fiber and an inner sheath region surrounding the hollow core, which sheath region comprises several anti-resonance elements, comprising the method steps of: (a) providing a primary preform (1) for the hollow-core fiber which comprises at least one cladding tube (3) having an inner bore of the cladding tube and a longitudinal axis of the cladding tube along which a cladding tube wall de-limited by an inner side and an outer side extends, (b) forming a number of precursors or preforms (4) for anti-resonance elements at desired positions of the cladding tube wall, and (c) further processing the primary preform (1) to form a secondary preform for the hollow-core fiber, wherein the further processing comprises a single or repeated performance of one or more of the following hot-forming processes: (i) elongation, (ii) collapse, (iii) collapse and simultaneous elongation, (iv) collapse of additional sheath material, (v) collapse of additional sheath material and subsequent elongation, (vi) collapse of additional sheath material and simultaneous elongation, characterized in that, when performing a process in accordance with method step (c), components of the primary preform (1) made of quartz glass and/or components (2) made of quartz glass surrounding the primary preform (1) are heated and softened together, wherein the quartz glass of at least one of the preform components (1; 2, 3) and/or the quartz glass of at least one of the components (4) surrounding the preform (1; 2, 3) contains at least one dopant that lowers or increases the viscosity of quartz glass.

Description

Exemplary Embodiment

[0093] The invention is explained in more detail below with reference to an exemplary embodiment and a drawing. The following are shown in detail in schematic representation:

[0094] FIG. 1 a coaxial tube arrangement consisting of an overlay cylinder and a primary preform, which is composed of a cladding tube and anti-resonance element preforms positioned and fixed therein based on a view of the cross-section,

[0095] FIG. 2 a diagram for the radial progression of the fluorine concentration and viscosity in the overlay cylinder and in the cladding tube, and

[0096] FIG. 3 a sketch to explain an ideal radial concentration or viscosity profile of a preform for a hollow-core fiber.

[0097] In the production of the hollow-core fiber or the preform for the hollow-core fiber, a plurality of components is to be connected to one another. In addition, it can be helpful to seal existing gaps or channels of the preform when carrying out hot-forming processes. For bonding or sealing, a sealing or bonding compound based on SiO.sub.2 and as disclosed in DE 10 2004 054 392 A1 is used. In this case, an aqueous slip containing amorphous SiO.sub.2 particles having a particle size distribution characterized by a D.sub.50 value of about 5 μm and by a D.sub.90 value of about 23 μm is produced by wet milling quartz glass grain. Further amorphous SiO.sub.2 grains with an average grain size of about 5 μm are mixed with the base slip. The slip used as a bonding compound has a solid content of 90%, which consists of at least 99.9 wt. % SiO.sub.2.

[0098] FIG. 1 schematically shows the coaxial tube arrangement 1 with an overlay cylinder 2, a cladding tube 3 having a cladding tube wall, on the inner side of which are fixed, at a uniform distance, anti-resonance element preforms 4 at previously defined azimuthal positions; in the exemplary embodiment, there are six preforms 4; in another preferred embodiment (not shown), there is an odd number of preforms.

[0099] The cladding tube 3 has an outer diameter of 27 mm and an inner diameter of 20 mm. The anti-resonance element preforms 4 are present as an ensemble of nested structural elements consisting of an ARE outer tube 4a and an ARE inner tube 4b. The ARE outer tube 4a has an outer diameter of 6.2 mm and the ARE inner tube 4b has an outer diameter of 2.5 mm. The wall thickness of both structural elements (4a; 4b) is equal and is 0.3 mm. All tubular components 2, 3, 4a, 4b have a length of 700 mm.

[0100] The anti-resonance element preforms 4 are fixed to the inner wall of the cladding tube 3 by means of the bonding compound based on SiO.sub.2. The bonding compound is applied locally to the inner sheath surface of the cladding tube in the region of the face ends, and the anti-resonance element preforms 4 are placed thereon using a positioning template with a structurally predetermined star-shaped arrangement of holding arms for the individual anti-resonance element preforms 4. In this case, the effect of the positioning template is limited to the region around the two face ends of the cladding tube. This method creates a precise and reproducible connection between the cladding tube 3 and the anti-resonance element preforms 4. Solidification of the bonding compound at a low temperature below 300° C. is sufficient for fixing so that an intense heating of the surrounding regions and thus a deformation of anti-resonance element preforms 4 is avoided.

[0101] The primary preform thus obtained is overlaid by the overlay cylinder 2 made of quartz glass. The overlay cylinder 2 has an outer diameter of 63.4 mm and a wall thickness of 17 mm. When the overlay cylinder 2 collapses onto the cladding tube 3, the coaxial tube arrangement is simultaneously elongated. For this purpose, the coaxial tube arrangement of the cladding tube 3 and the overlay cylinder 2 with a vertically oriented longitudinal axis is supplied from below to a temperature-controlled heating zone and softens therein zone by zone starting with the upper end of the tube arrangement. The heating zone is kept at a desired temperature of 1580° C. with a control accuracy of +/−0.1° C. Temperature fluctuations in the hot-forming process can thereby be limited to less than +/−0.5° C.

[0102] The secondary preform formed in the collapse and elongation process has an outer diameter of approximately 50 mm and a sheath wall thickness of 16.6 mm composed of an outer sheath and an inner sheath. It is subsequently drawn into the anti-resonant hollow-core fiber. All anti-resonance element preforms are sealed beforehand with the sealing or bonding compound. The sealing compound is applied only to the end face of the anti-resonance element preforms that points upward during the fiber-drawing process. This end face is connected to a holding tube made of quartz glass, which simultaneously serves as a gas connection. The holder is fixed to the overlay cylinder 2 and to the cladding tube 3 by means of the sealing or bonding compound.

[0103] In the fiber-drawing process, the secondary preform with a vertically oriented longitudinal axis is supplied from above to a temperature-controlled heating zone and softens therein zone by zone starting at the lower end. At the same time, gas is supplied to the core region (hollow core) so that an internal pressure of 4 mbar is adjusted in the core region. The heating zone is kept at a desired temperature of approximately 2080° C. with a control accuracy of +/−0.1° C. Temperature fluctuations in the hot-forming process can thereby be limited to less than +/−0.5° C.

[0104] By drawing the preform toward the hollow-core fiber, the existing absolute geometry error is scaled down so that in the hollow-core fiber, the anti-resonance elements obtained from the anti-resonance element preforms have a maximum deviation of less than 3.5% in the wall thickness (with respect to an average wall thickness).

[0105] The slight error in wall thickness is attributed, on the one hand, to the use of the comparatively large secondary preform and the accompanying scaling down of the original existing absolute geometry deviations and, on the other hand, to comparatively low processing temperatures during the hot-forming processes (elongating and collapsing, fiber drawing). The lower processing temperatures are in turn attributable to the fact that the overlay cylinder 2 and the cladding tube 3 consist of quartz glass doped with fluorine. In the coaxial arrangement 1, these components represent the components having the greatest surface area and were definitive in determining the processing temperature. As a result of the fluorine doping of the components having the greatest surface area of the secondary preform, the necessary processing temperature can be reduced, and the relative stiffness and thermal stability of the further inner anti-resonance element preforms 4 can thus be indirectly improved by exposing them to a lower temperature in the hot-forming process.

[0106] Table 1 below summarizes details of the materials of the components of the coaxial arrangement or of the secondary preform.

TABLE-US-00001 TABLE 1 Reference number in FIG. 1 Name/Function Material 2 Overlay cylinder Fluorine-doped quartz glass 10,000 ppm by weight 3 Cladding tube Fluorine-doped quartz glass 2,700 ppm by weight 4a ARE outer tube Undoped quartz glass 4b ARE inner tube Undoped quartz glass

[0107] The quartz glass tubes (2; 3) doped with fluorine have a fluorine concentration profile with a maximum of the fluorine concentration in the center of the tube wall. The data regarding the fluorine concentration of the quartz glass that are mentioned in the “Material” column of Table 1 are mean values.

[0108] The diagram in FIG. 2 shows measured fluorine concentration profiles C (in ppm by weight) for a cladding tube C.sub.F(M) and in the case of an overlay cylinder C.sub.F(Z), as well as viscosity profiles η (in log dPa.Math.s) along the radial coordinate (position P (in mm)) and calculated from the concentration profiles for a temperature of 1250° C.

[0109] The fluorine concentration curve in quartz glass is determined by infrared spectroscopy. The viscosity scales with the fluorine concentration for a given temperature and is calculated starting from a base value for undoped quartz glass (η=11.8 dPa.Math.s (corresponding to 100%)) using the following formula:

[0110] Decrease in viscosity at 1250° C.: 12% (±2%) per wt. % fluorine.

[0111] Table 2 shows viscosity values for fluorine contents of commercially available quartz glass grades (for a measured temperature of 1250° C.).

TABLE-US-00002 TABLE 2 Fluorine content log η @ 1250° C. [ppm by weight] [dPa * s] 0 11.80 4,800 11.00 10,000 10.50 13,000 9.80

[0112] The diagram in FIG. 2 shows that the viscosity of the overlay cylinder η(Z) is lower than that of the cladding tube η(M). In both quartz glass tubes, the viscosity at the center of the tube has a minimum, which is approximately 10.sup.11.45 dPa.Math.s for the cladding tube and approximately 10.sup.10.65 dPa.Math.s for the overlay cylinder. The difference in viscosity of the minima (in log dPa.Math.s) is thus approximately 0.80 dPa.Math.s. The difference between the viscosity of the cladding tube in the region of the cladding tube outer side (approximately 10.sup.11.5 dPa.Math.s) and the viscosity minimum in the overlay cylinder is approximately 0.85 (in log dPa.Math.s).

[0113] In the preform, the outer sheath surface of the cladding tube and the inner sheath surface of the overlay cylinder form a common contact surface. The locational position of the contact surface—transferred to the viscosity profiles— is indicated in the diagram by the two rectangles “K”. The following values result at these positions for the viscosities of the overlay cylinder and the cladding tube:

TABLE-US-00003 Cladding tube: approximately 11.5 log(dPa .Math. s) Overlay cylinder: approximately 11.15 log(dPa .Math. s)

[0114] The viscosity difference in the region of the contact surface is thus approximately 0.35 (in log dPa.Math.s).

[0115] The structural elements (4a; 4b) of the anti-resonance element preforms (4) consist of undoped quartz glass and have a viscosity of about 10.sup.11.8 dPa.Math.s.

[0116] The diagram in FIG. 3 shows the radial dopant concentration curve over the wall of the secondary preform in idealized form. On the y-axis, the fluorine concentration C.sub.F (in relative unit) is plotted against the coordinate P (in relative unit). At the contact surface “K,” the dopant concentration C.sub.F(Z) of the fluorine-doped quartz glass originating from the overlay cylinder is ideally as high as the concentration C.sub.F(M) of the fluorine-doped quartz glass originating from the cladding tube. The corresponding viscosity profile of the viscosities of the cladding tube and the overlay cylinder thus shows the same viscosity at the contact surface K on both sides.