Quartz glass tube as a semi-finished product for an optical component

Abstract

A quartz glass tube as a semi-finished product for an optical component that has an inner bore extending along a tube center axis for the acceptance of a core rod and a tube wall limited by an inner casing surface and an outer casing surface is already known; within said tube wall an inner region made of a first quartz glass and an outer region made of a second quartz glass with a different index of refraction surrounding the inner region contact one another at a contact surface which runs around the center axis. In order to provide a quartz glass on this basis that facilitates the production of optical components for special applications such as laser-activated optical components in wand or fiber form, the invention states that the contact surface has a non-round course in the radial cross-section and the inner casing surface has a circular course.

Claims

1. A quartz glass tube comprising: a tube wall having an inner surface and an outer surface, with said inner surface defining an inner bore extending along a tube center axis, and said tube wall comprising an inner region of a first quartz glass surrounding the inner bore and an outer region of a second quartz glass surrounding the inner region, wherein said inner region and said outer region are adjacent to each other on a contact surface that extends around the center axis, wherein said first quartz glass and said second quartz glass have refractive indices that differ from each other, and wherein in a radial cross-section the contact surface has a non-round profile and the inner surface has a circular profile.

2. The quartz glass tube according to claim 1, wherein the contact surface in the radial cross-section has at least one corner.

3. The quartz glass tube according to claim 1, wherein the contact surface in the radial cross-section has a polygonal profile.

4. The quartz glass tube according to claim 1, wherein the contact surface in the radial cross-section has a profile with alternatingly straight and curved longitudinal sections or with longitudinal sections alternatingly curved to left and right sides thereof.

5. The quartz glass tube according to claim 1, wherein the contact surface between the inner region and the outer region is a first contact surface surrounded within the tube wall by a second contact surface, the second contact surface also defining regions of quartz glass with different refractive indices and having a profile that is non-circular in the radial cross-section.

6. The quartz glass tube according to claim 1, wherein the outer surface has a circular profile.

7. The quartz glass tube according to claim 1, wherein the inner bore extends concentrically relative to the center axis.

8. The quartz glass tube according to claim 1, wherein the quartz glass of the outer region is doped with fluorine.

9. The quartz glass tube according to claim 1, wherein the inner region consists of undoped quartz glass.

10. The quartz glass tube according to claim 1, wherein the tube wall comprises an inner layer that extends between the inner bore and the inner region, and that consists of quartz glass having a higher refractive index than undoped quartz glass.

Description

EMBODIMENT

(1) The invention shall now be explained in more detail with reference to embodiments and a drawing. In detail, in a schematic representation,

(2) FIG. 1 shows a first embodiment of a cladding tube with a non-round contact surface with round outer cladding surface in a radial cross-section;

(3) FIG. 2 shows a second embodiment of a cladding tube with a non-round contact surface with polygonal outer cladding surface;

(4) FIG. 3 shows a third embodiment of a cladding tube with a non-round contact surface which does not concentrically extend around the inner bore;

(5) FIG. 4 shows a fourth embodiment of a cladding tube with a non-round contact surface with a cross-sectional geometry that is D-shaped four times, in the cladding region;

(6) FIG. 5 shows a fifth embodiment of a cladding tube with a non-round contact surface with additional bores laterally offset relative to the center axis;

(7) FIG. 6 shows a sixth embodiment of a cladding tube with a non-round contact surface with multiple additional bores distributed around the central inner bore;

(8) FIG. 7 shows a seventh embodiment of a cladding tube with a non-round contact surface with two concentric contact surfaces;

(9) FIG. 8 shows an eighth embodiment of a cladding tube with a non-round contact surface and a refractive index profile with so-called pedestal design;

(10) FIG. 9 shows a preform produced by using the quartz glass tube according to FIG. 1 for a laser fiber with a rectangular core zone, in radial cross-section;

(11) FIG. 10 shows a flow diagram with method steps for producing a preform according to the prior art; and

(12) FIG. 11 shows a flow diagram with method steps for producing a preform according to the invention.

(13) In the quartz glass tube which is schematically shown in FIG. 1, an inner cladding surface 1 of circular cross-section and an outer cladding surface 2 which is also circular define a tube wall 3 which is composed of an inner cladding region 4 and an outer cladding region 5. The inner bore is designated with reference numeral 7. The contact surface 6 between inner and outer cladding region is octagonal in the radial cross-section. The cladding structure of the quartz glass tube has an eight-fold symmetry relative to a rotation about the center axis 17, which extends in coaxial fashion relative to the inner bore 7.

(14) To the extent that identical reference numerals are used in FIGS. 2 to 9 as in FIG. 1, this designates the same or equivalent components and constituents of the cladding tube as in FIG. 1.

(15) The embodiment of FIG. 2 also shows an octagonal contact surface 6 between inner cladding region 4 and outer cladding region 5. This is a particularly thin-walled tube also having an eightfold symmetry of the cladding structure. In contrast to the embodiment of FIG. 1, the outer cladding surface 8 is also of an octagonal type, so that the outer cladding region 5, viewed both in longitudinal direction and in radial direction, has an approximately constant thickness.

(16) The essential characteristic in the embodiment of FIG. 3 as compared with that of FIG. 1 is that the inner cladding region 4, which is also made octagonal, has a center of gravity which is laterally offset relative to the inner bore 7. The contact surface 6 extends in parallel with the center axis 17, but not coaxial thereto. This cladding structure only shows mirror symmetry around a mirror plane extending through the center axis 17. The width of the outer cladding region 5 varies when viewed in radial direction.

(17) FIG. 4 schematically shows another cross-sectional geometry of the inner cladding region which differs from the circular form, namely a contour with a so-called fourfold D shape. The contact surface 6 has four bent longitudinal sections 10, one in each quadrant, which are interconnected by straight or slightly inwardly curved longitudinal sections 11. This cladding structure has fourfold symmetry.

(18) In the embodiments described with reference to FIGS. 1 to 4, the inner cladding region 4 consists each time of undoped quartz glass and the outer cladding region 5 of a fluorine-doped quartz glass with a refractive index which is reduced by comparison with the inner cladding region 4.

(19) In the cross-sectional representation of FIG. 5 one can see that two additional bores 28 are provided in the cladding region, laterally offset relative to the inner bore 7 and at both sides of the center axis 17. These serve to accommodate so-called stress rods for the production of a preform for polarization-maintaining optical fibers.

(20) In the embodiment shown in FIG. 6, six additional bores 30 are provided in the cladding region 4; these are evenly distributed around the inner bore 7 and the center axis 17. These serve e.g. to accommodate laser active quartz glass.

(21) The embodiment shown schematically in FIG. 7 of a cladding tube with non-round contact surface differs from that of FIG. 2 in that the outer cladding region 5 is outwardly followed by two further cladding layers. Two cladding regions 4, 14 of undoped quartz glass and two cladding regions 5, 15 of fluorine-doped quartz glass are provided, with regions of undoped and doped quartz glass alternating in radial direction from the inside to the outside. The cladding regions 4, 14 of undoped quartz glass have each an inner boundary which is circular in cross section and an octagonal outer boundary, and the exact opposite can be observed in the case of the cladding regions 5, 15 of fluorine-doped quartz glass. As a result, two of the contact surfaces 6, 16, 26 are octagonal in cross section (6, 26); the other is circular (16).

(22) FIG. 8 schematically shows an embodiment of the cladding tube according to the invention with a non-round contact surface in which the innermost cladding region 18 adjoining the inner bore 7 has a circular contact surface 19 with respect to the next cladding region 4. Here, the cladding region 4 represents the inner cladding region within the meaning of the invention, for it possesses the non-circular contact surface 6 with respect to the outwardly adjoining outer cladding region 5. The innermost cladding region 18 which adjoins the inner bore 7 consists of quartz glass which due to doping with germanium oxide has a refractive index which is adapted to that in the outer region of the core rod to be inserted into the inner bore (not shown in the figure). This embodiment can be designated as a pedestal design.

(23) When the cladding tube with non-round contact surface is used as a pump light cladding for a cladding-pumped laser, the contours of the contact surfaces 6, 26 which differ from the circular geometry and are within the tube wall reduce the formation of undesired helix modes. The efficiency of the pump light is thereby improved. Since these contours are provided in a quartz glass tube having a circular inner bore, which can be used as a quartz glass tube for component production, namely as an overcladding tube for overcladding a circular core rod, the manufacturing process is simplified and the risk of material losses is reduced.

(24) The preform which is schematically shown in cross section in FIG. 9 is produced by using a cladding tube 31 with a non-round contact surface. The cladding tube 31 provides the outer cladding region 5 of fluorine-doped quartz glass and the inner cladding region 4 of undoped quartz glass. The cladding regions 4 and 5 surround a core rod 32 which is composed of a central core zone 33 with square cross-section and of a cladding layer 34. The cladding layer 34 surrounds the core zone 33 of a square cross-section such that, when viewed in cross section, a round profile is obtained having an outer diameter adapted to the diameter of the inner bore of the cladding tube 31. The core zone 33 consists of quartz glass which is doped with laser active substances. The cladding zone 34 consists of undoped quartz glass.

(25) For the production of the preform a glass cladding of undoped quartz glass is produced by means of a POD method on a rod of a square cross-sectional area which consists of laser active quartz glass, and said glass cladding is ground to become round. The core rod 32 obtained in this way is overclad with the cladding tube 31 while forming the preform. A laser fiber which is distinguished by a square beam profile is drawn from the preform.

(26) The manufacture of the cladding tube with non-round contact surface is now explained in more detail with reference to FIG. 1.

(27) A thick-walled hollow cylinder of undoped quartz glass with an inner diameter of 10 mm and an outer diameter of 100 mm is provided. The outer cladding surface is given an octagonal shape by way of grinding. After mechanical treatment the quartz glass surface is etched with fluoric acid and cleaned with ethanol to remove grinding residues. The treated surface is subsequently vitrified and sealed by hot polishing.

(28) The quartz glass cylinder obtained thereby with an octagonal cross-sectional area is provided by means of plasma outside deposition method (POD method) with an outer cladding consisting of a fluorine-doped quartz glass.

(29) The outer contour of the quartz glass layer deposited thereby substantially assumes the octagonal shape of the quartz glass cylinder. To achieve a round tube geometry, the outer glass cladding is ground to become round and is subsequently cleaned with hydrofluoric acid to remove grinding traces, and the ground surface is fire-polished to seal the surface.

(30) The inner wall of the quartz glass tube obtained in this way is subjected to a hot-gas etching process using SF.sub.6 to clean the surface.

(31) The mother tube obtained thereby is elongated in a drawing process at a draw ratio of 30 without any tool to an outer diameter of 20 mm. During the elongation process the inner bore of the mother tube or of the drawn-off tubular strand is flushed with nitrogen. The quality of the inner surface is further improved, and plural structured cladding tubes are made from one batch, which reduces the manufacturing costs.

(32) After the grinding process the mother tube has a geometric exact polygon shape. During etching and especially due to the elongation process the corners of the polygon may become round to a certain extent. Moreover, the quartz glass of the mother tube is softened in the elongation process, so that the straight surfaces of the polygon are also deformed and, depending on the drawing conditions, may be curved inwards or outwards in the finished quartz glass tube.

(33) In a modification of this procedure, a solid cylinder of undoped quartz glass is used as the starting material instead of the tube. After the grinding operation and the POD process a central inner bore is produced in the solid cylinder by core drilling. In this variant, a very exact concentricity of core hole and center axis can be realized more easily.

(34) By subdividing the manufacturing steps of the preform into cladding tube production on the one hand and core rod production on the other hand, the components are combined only in the last step, the overcladding step, to form the preform. This reduces the number of the process steps in which the core rod is involved, and lowers the risk that the core rod gets destroyed. This is also illustrated by the comparison of the flow diagrams of FIG. 10 and FIG. 11, which name the method steps for producing a preform, in which the core rod is involved (the frame is marked with double lines where necessary). FIG. 10 specifies the method steps needed for this purpose in the known method mentioned at the outset, and FIG. 11 shows the single method step, namely overcladding the core rod, in the method according to the invention.

(35) As an alternative to this, the overcladding of the core rod with the quartz glass tube according to the invention may also be carried out in the fiber drawing process in that an assembly consisting of the cladding tube and the core rod is supplied to a heating zone, softened therein zone by zone and thereby fused and directly drawn into the fiber. The use of the cladding tube with non-round contact surface is especially recommended in the case of very expensive core rods or in the case of core rods which are particularly sensitive mechanically, thermally or to UV radiation. The costs per unit can be lowered by producing a large structured cladding tube batch in a single elongation process. Moreover, the production of several preforms of constant quality is easier because process fluctuations arising in the individual production do not occur. Moreover, a large cladding tube is equipped with a plurality of different core materials and thereby tested and qualified more easily. This is particularly helpful in saving time and costs in the development of new core materials.