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 further processing the primary preform according to step (c), an external layer cylinder is used which has a radial viscosity profile such that the viscosity increases towards the interior of the external layer cylinder.

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 a plurality of anti-resonance elements, comprising the method steps of: (a) providing a primary preform (1) for a hollow-core fiber which comprises at least one cladding tube (3) having a cladding tube inner bore and a cladding tube longitudinal axis 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 in desired positions of the cladding tube wall, and (c) further processing the primary preform (1) to form a secondary preform from which the hollow-core fiber is drawn, the further processing comprising a single or repeated collapse of additional sheath material in the form of a collecting cylinder (2), the collecting cylinder (2) having a collecting cylinder inner side facing the cladding tube (3), a collecting cylinder outer side and a collecting cylinder wall therebetween, characterized in that, for further processing of the primary preform (1) according to method step (c), a collecting cylinder (2) is used which has a radial viscosity profile in which viscosity increases toward the collecting cylinder inner side.

2. Method according to claim 1, characterized in that the viscosity in the region of the collecting cylinder inner side has a value η(Z), and in that the viscosity in the region of the cladding tube outer side has a viscosity η(M), wherein, at a measuring temperature of 1250° C., the following applies for the viscosity values (when viscosity is specified as a logarithmic value in dPa.Math.s): η(M)=η(Z)±0.5 dPa.Math.s, preferably η(M)=η(Z)±0.3 dPa.Math.s.

3. Method according to claim 1 or 2, characterized in that the viscosity profile in the collecting cylinder wall increases from a viscosity minimum η(Z.sub.min) toward the collecting cylinder inner side.

4. Method according to claim 3, characterized in that the following applies for the viscosity minimum η(Z.sub.min) at a measuring temperature of 1250° C. (when the viscosity is specified as a logarithmic value): η(M)−η(Z.sub.min)>0.8 dPa.Math.s, preferably >1 dPa.Math.s.

5. Method according to any one of the preceding claims, characterized in that the cladding tube (3) has a radial viscosity profile between its cladding tube outer side and its cladding tube inner side in which the viscosity gradually increases toward the inner side.

6. Method according to claim 5, characterized in that the cladding tube (3) has a viscosity profile with a viscosity maximum η(M.sub.max) and in that in method step (b) at least some of the anti-resonance element preforms (4) or of the precursors for anti-resonance elements are present as tubular anti-resonance element preforms (4), which are preferably composed of a plurality of nested structural elements, comprising an ARE outer tube (4a) and an ARE inner tube (4b) inserted therein, and in that the anti-resonance element preforms (4) are made of quartz glass which, at a measuring temperature of 1250° C., has a viscosity at least 0.4 dPa.Math.s higher than the maximum viscosity η(M.sub.max) of the quartz glass of the cladding tube (3), preferably a viscosity at least 0.5 dPa.Math.s higher (when the viscosity is given as a logarithmic value).

7. Method according to any one of the preceding claims, characterized in that the collecting cylinder (2) is made of quartz glass and in that the viscosity profile of the collecting cylinder (2) is produced by adding at least one dopant lowering the viscosity of quartz glass, the dopant preferably containing fluorine, chlorine and/or hydroxyl groups.

8. Method according to any one of the preceding claims, characterized in that the cladding tube (3) is made of quartz glass and in that the viscosity profile of the cladding tube (3) is produced by adding at least one dopant lowering the viscosity of quartz glass, the dopant preferably containing fluorine, chlorine and/or hydroxyl groups.

9. Method according to claims 7 and 8, characterized in that the quartz glass of the collecting cylinder (2) and of the cladding tube (3) contains fluorine as dopant in a concentration of between 500 and 8000 ppm by weight.

10. Method according to any one of the preceding claims, characterized in that the cladding tube (3) is produced in a vertical drawing method without a molding tool with a two-stage elongation process, in the first stage, a hollow starting cylinder made of glass being mechanically processed to set the final dimensions of the hollow starting cylinder, the starting cylinder in a first elongation process with a vertically oriented longitudinal axis being continuously fed into a first heating zone with a first heating zone length, being softened therein in certain regions and an intermediate cylinder being drawn from the softened region, the intermediate cylinder with a vertically oriented longitudinal axis being fed in a second elongation process into a second heating zone with a second, shorter heating zone length, being softened therein in certain regions and a continuous tube being drawn from the softened region, and the cladding tube (3) being obtained from the continuous tube by cutting it to length.

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 a primary preform (1) 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 preforms for anti-resonance elements according to method step (b) comprises arranging the anti-resonance element preforms (4) at desired positions on the inner side of the cladding tube wall, a positioning template being used for the arrangement which has holding elements for positioning the anti-resonance element preforms (4) in the desired positions.

13. 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 a plurality of anti-resonance elements, comprising the method steps of: (a) providing a primary preform (1) for a hollow-core fiber which comprises at least one cladding tube (3) having a cladding tube inner bore and a cladding tube longitudinal axis 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 on the cladding tube wall, and (c) further processing the primary preform (1) to form a secondary preform for the hollow-core fiber, the further processing comprising a single or repeated collapse of additional sheathing material in the form of a collecting cylinder (2), the collecting cylinder (2) having a collecting cylinder (3) inner side facing the cladding tube, a collecting cylinder outer side and a collecting cylinder wall therebetween, characterized in that, for further processing of the primary preform (1) according to method step (c), a collecting cylinder (2) is used which has a radial viscosity profile in which viscosity increases toward the collecting cylinder inner side.

Description

EXEMPLARY EMBODIMENT

[0077] 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:

[0078] FIG. 1 a coaxial tube arrangement of a collecting cylinder and a primary preform which is composed of a cladding tube and anti-resonance element preforms positioned and fastened therein based on a view of the radial cross-section,

[0079] FIG. 2 a diagram for the radial course of fluorine concentration and viscosity in the collecting cylinder and in the cladding tube, and

[0080] FIG. 3 a sketch to explain an ideal radial concentration or viscosity profile of a preform for a hollow-core fiber.

[0081] In the production of the hollow-core fiber or the preform for the hollow-core fiber, a plurality of components is to be connected together. 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 silica 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.

[0082] FIG. 1 schematically shows the coaxial tube arrangement 1 with a collecting 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.

[0083] 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.

[0084] The anti-resonance element preforms 4 are fastened 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 lateral surface of the cladding tube in the region of the 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 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 fastening, so that an intense heating of the surrounding regions and thus a deformation of anti-resonance element preforms 4 is avoided.

[0085] The primary preform thus obtained is overlaid by the collecting cylinder 2 made of quartz glass. The collecting cylinder 2 has an outer diameter of 63.4 mm and a wall thickness of 17 mm. When the collecting 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 collecting cylinder 2 coming from below in a vertically oriented longitudinal axis is fed into 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.

[0086] 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 faces upward during the fiber drawing process. This end face is joined to a holding tube made of quartz glass which simultaneously serves as a gas connection. The holder is fastened to the collecting cylinder 2 and to the cladding tube 3 by means of the sealing or bonding compound.

[0087] In the fiber-drawing process, the secondary preform is in the case of a vertically oriented longitudinal axis fed from above into 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 established 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.

[0088] By drawing the preform into the hollow-core fiber, the absolute geometry error present is scaled down, so that the anti-resonance elements obtained from the anti-resonance element preforms in the hollow-core fiber have a maximum deviation of less than 3.5% in wall thickness (with respect to an average wall thickness).

[0089] 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 absolute geometry deviations that are present 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 collecting cylinder 2 and the cladding tube 3 consist of quartz glass doped with fluorine. In the coaxial arrangement 1, these components represent the components with the greatest surface area and were instrumental in determining the processing temperature. As a result of the fluorine doping of the components with the greatest surface area of the secondary preform, the necessary processing temperature can be reduced, and the relative stiffness and thermal stability of the anti-resonance element preforms 4 lying further inward can thus be indirectly improved by exposing them to a lower temperature in the hot-forming process. 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 numeral in FIG. 1 Name/function Material 2 Collecting cylinder Flzuorine-doped quartz glass 10 000 ppm by weight 3 Cladding tube Fluorine-doped quartz glass 2700 ppm by weight 4a ARE outer tube Undoped quartz glass 4b ARE inner tube Undoped quartz glass

[0090] The quartz glass tubes (2; 3) doped with fluorine have a fluorine concentration profile with a maximum fluorine concentration in the center of the tube wall. The data regarding the fluorine concentration of the quartz glass that are given in the “Material” column of Table 1 are mean values.

[0091] 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 a collecting cylinder C.sub.F(Z)″, as well as viscosity profiles n (in log dPa.Math.s) along the radial spatial coordinate (position P (in mm)) calculated from the concentration profiles for a temperature of 1250° C.

[0092] The fluorine concentration curve in quartz glass is determined by infrared spectroscopy. Viscosity scales with the fluorine concentration for a given temperature and is calculated starting from a basic value for undoped quartz glass (q=11.8 dPa.Math.s (corresponding to 100%)) using the following formula:

decrease in viscosity at 1250° C.: 12% (±2%) wt. % fluorine.

[0093] Table 2 shows viscosity values for fluorine concentrations of commercially available quartz glass grades (for a measuring temperature of 1250° C.).

TABLE-US-00002 TABLE 2 Fluorine content [wt. ppm] log η @ 1250° C. [dPa*s]    0 11.80   4800 11.00 10 000 10.50 13 000 9.80

[0094] The diagram in FIG. 2 shows that the viscosity of the collecting 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 in the cladding tube and for the collecting tube is approximately 10.sup.10.65 dPa.Math.s. 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.15 dPa.Math.s) and the viscosity minimum for the capture cylinder is approximately 0.85 (in log dPa.Math.s).

[0095] In the preform, the outer lateral surface of the cladding tube and the inner lateral surface of the collecting cylinder form a common contact surface. The spatial 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 collecting cylinder and the cladding tube:

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

[0096] The viscosity difference in the region of the contact surface is thus approximately 0.35 (in log dPa.Math.s).

[0097] 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.

[0098] The diagram in FIG. 3 shows in idealized form the radial concentration curve for dopant over the wall of the secondary preform. On the y-axis, the fluorine concentration C.sub.F (in relative units) is plotted against the spatial coordinate P (in relative units). At the contact surface “K,” the dopant concentration C.sub.F(Z) of the fluorine-doped quartz glass originating from the collecting 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 collecting cylinder thus exhibits the same viscosity at the contact surface K on both sides.