MICROSTRUCTURED OPTICAL FIBER AND PREFORM FOR SAME

Abstract

The invention relates to microstructured optical fibers that are drawn through hollow channels and have a core region, which extends along a fiber longitudinal axis, and a jacket region surrounding the core region. The aim of the invention is to reduce a damping increase due to corrosion and to reduce the emission of chlorine on the basis of the microstructured optical fibers. This is achieved in that at least some of the hollow channels are delimited by a wall material made of synthetic quartz glass which has a chlorine concentration of less than 300 wt. ppm and oxygen deficiency centers in a concentration of at least 21015 cm-3.

Claims

1. A microstructured optical fiber which has a core region extending along a longitudinal axis of the fiber and an inner cladding region surrounding the core region, and through which hollow channels run, of which at least some are bordered by a wall material made of synthetic quartz glass which has a chlorine concentration of less than 300 ppm by weight, wherein the synthetic quartz glass of the wall material has oxygen deficiency centers at a concentration of at least 210.sup.15 cm.sup.3.

2. The optical fiber according to claim 1, wherein the quartz glass has a concentration of oxygen deficiency centers of at least 510.sup.15 cm.sup.3, better 110.sup.16 cm.sup.3, and preferably at least 510.sup.16 cm.sup.3.

3. The optical fiber according to claim 1, wherein the quartz glass contains oxygen deficiency centers at a concentration of at most 210.sup.20 cm.sup.3, preferably at most 510.sup.19 cm.sup.3, at most 110.sup.19 cm.sup.3, at most 510.sup.18 cm.sup.3, at most 210.sup.18 cm.sup.3, at most 110.sup.18 cm.sup.3, at most 510.sup.17 cm.sup.3, at most 210.sup.17 cm.sup.3 and particularly preferably at most 110.sup.17 cm.sup.3.

4. The optical fiber according to claim 1, wherein the quartz glass has a hydroxyl group content of less than 10 ppm by weight, preferably less than 1 ppm by weight.

5. The optical fiber according to claim 4, wherein the quartz glass has a halogen content of less than 20 ppm by weight, preferably less than 10 ppm by weight.

6. The optical fiber according to claim 1, wherein the quartz glass contains foreign substances, except for carbon and nitrogen, at an overall concentration of less than 30 ppm by weight, preferably less than 3 ppm by weight.

7. The optical fiber according to claim 1, wherein the quartz glass has a viscosity at a temperature of 1200 C. of at least 10.sup.13 dPa s.

8. The optical fiber according to one of the preceding claim 1, wherein the core region has at least one hollow core, and in that the inner cladding region comprises a microstructured regular arrangement of hollow channels extending in the longitudinal direction, and in that the inner cladding region is made of the wall material.

9. The optical fiber according to claim 1, wherein the core region has at least one hollow core, and the inner cladding region comprises hollow structural elements extending in the direction of the longitudinal axis of the fiber which are arranged annularly on the inner lateral surface of an inner cladding, and which guide the light by an anti-resonance effect along the hollow core.

10. The optical fiber according to claim 9, wherein the hollow structural elements each comprise an ARE outer capillary and at least one nested NE inner capillary connected to an inner lateral surface of the ARE outer capillary, and in that the inner cladding, the ARE outer capillary and/or the NE inner capillaries are made from the wall material.

11. The optical fiber according to claim 9, wherein it comprises an outer cladding region surrounding the inner cladding region, which outer cladding region is made of the wall material.

12. A preform for producing a microstructure optical fiber according to claim 1, wherein the preform has a core region extending along a preform longitudinal axis and an inner cladding region surrounding the core region, and through which hollow channels run, of which at least some are bordered by a wall material made of synthetically produced quartz glass, which has a chlorine concentration of less than 300 ppm by weight and oxygen deficiency centers at a concentration of at least 210.sup.15 cm.sup.3.

Description

EXEMPLARY EMBODIMENT

[0090] The invention is explained in more detail below with reference to an exemplary embodiment and a drawing. The following are shown in detail:

[0091] FIG. 1 a diagram with transmission spectra of various quartz glass grades,

[0092] FIG. 2 a primary preform with a cladding tube and anti-resonance element preforms positioned and fastened therein for producing a preform for a hollow-core fiber with reference a view of the radial cross-section, and

[0093] FIG. 3 a device for use in the tool-free production of ARE outer tubes and ARE inner tubes by means of a vertical drawing process.

[0094] Production of Synthetic Quartz Glass with Oxygen Deficiency Centers

[0095] The quartz glass with oxygen deficiency defects is produced according to the method described in EP 1 580 170 A1. In this process, a porous SiO.sub.2 soot body with an outer diameter of 100 mm and a weight of 1 kg is first prepared by flame hydrolysis of SiCl.sub.4 in the usual way. This is introduced into a treatment furnace, and the furnace chamber is evacuated and heated to a temperature of 500 C. After a holding time of 60 min, hexamethyldisilazane is introduced into the furnace chamber with nitrogen as carrier gas at a rate of 1 mol/h, and the soot body is treated in this atmosphere for 3 hours.

[0096] Subsequently, the soot body treated in this way is introduced into a vacuum furnace and is first heated therein under a vacuum (0.001 mmHg) to 800 C. and after one hour to a temperature of 1600 C. and sintered to form quartz glass. In the course of the temperature treatment, a large part of the hydrogen escapes that was previously introduced into the SiO.sub.2 solid body due to the manufacturing process.

[0097] A transparent, synthetic quartz glass of high purity is obtained which has a hydroxyl group content below 1 ppm by weight, a chlorine content of less than 30 ppm by weight, a carbon content of 100 ppm by weight, a nitrogen content of 80 ppm by weight, and a hydrogen content of less than 510.sup.16 molecules/cm.sup.3. The concentration of Li is less than 2 ppb by weight, and the concentration of the following metallic impurities is less than 5 ppb by weight in each case: Na, K, Mg, Al, K, Ca, Ti, Cr, Fe, Ni, Cu, Mo, W, V, and Zn. The total concentration of metallic impurities is less than 100 ppb by weight. At a temperature of 1,280 C., the quartz glass has a viscosity of 12.4 (log ).

[0098] The treatment of the porous SiO.sub.2 soot body in a reducing atmosphere-due to the action of the organic, silicon-containing compound hexamethyldisilazane-leads to different oxygen deficiency centers in the synthetic quartz glass. This is shown by the fact that the quartz glass treated in this way has a high absorption at a wavelength of approximately 247 nm (see FIG. 1).

[0099] The concentration of the ODC oxygen deficiency centers that absorb at this wavelength is determined by spectroscopy and is 9.1410.sup.16 cm.sup.3.

[0100] The ODC oxygen deficiency centers of the quartz glass cause absorption in the UV wavelength range, such as the transmission spectra of FIG. 1. On the y-axis, the measured transmission T (in %) is plotted against the wavelength (in nm) over the wavelength range of 150 to approximately 400 nm. The measured transmission T includes reflection losses on surfaces and differs in this respect from the so-called internal transmission (or pure transmission) with a sample thickness of 1 cm.

[0101] The diagram contains the transmission profile of the quartz glass, the production of which is explained above (name: L570). For comparison, additional transmission profiles of other commercially available quartz glass grades are recorded. The designations HLQ270 and HIx-LA are quartz types which have been melted from naturally occurring quartz crystal. F310 is a synthetically produced quartz glass with a low chlorine content (<0.2 ppm by weight) and a hydroxyl group content of 200 ppm by weight. And F300 is another synthetically produced quartz glass with a low hydroxyl group content (<0.2 ppm by weight) and a chlorine content within a range of 800 to 2000 ppm by weight. The oxygen deficiency centers of the sample L570 are indicated by a pronounced transmission minimum within the wavelength range of approximately 245 to 255 nm. The two quartz glass grades of synthetic quartz glass do not exhibit absorption within this wavelength range and are accordingly unsuitable for use as a wall material containing oxygen deficiency centers within the meaning of this invention. Although the relative transmission minimum of quartz glass samples melted from natural raw material is indicative of a certain amount of oxygen deficiency centers, these quartz glass grades are likewise unsuitable for this use due to the intrinsic impurities.

[0102] The quartz glass grade produced in this manner and loaded with oxygen deficiency centers is used to produce a preform for an anti-resonance hollow-core fiber. In the following, the production of the preform is explained with reference to an example.

[0103] FIG. 2 schematically shows a primary preform 23 with a cladding tube 21 having a cladding tube wall 22, on the inner surface of which are fastened equidistantly spaced anti-resonance element preforms 24 at previously defined azimuthal positions; in the exemplary embodiment, there are six preforms 24; in another preferred embodiment (not shown), there is an odd number of preforms.

[0104] The inner cladding tube 21 consists of quartz glass loaded with oxygen deficiency centers and has a length of 1000 mm, an outer diameter of 27 mm and an inner diameter of 20 mm. The anti-resonance element preforms 24 are present as an ensemble of nested structural elements consisting of an ARE outer tube 24a and an ARE inner tube 24b. The ARE outer tube 24a has an outer diameter of 6.2 mm and the ARE inner tube 24b has an outer diameter of 2.5 mm. The wall thickness of both structural elements (24a; 24b) is the same and is 0.3 mm. The diameter ratio in the ARE outer tube 24a is therefore 1.107 and in the ARE inner tube it is 1.315. The lengths of ARE outer tube 24a and ARE inner tube 24b correspond to the length of the cladding tube. The ARE inner tube 24b and the ARE outer tube 24a also consist of the quartz glass loaded with oxygen deficiency centers.

[0105] The anti-resonance element preforms 24 are fixed to the inner wall of the cladding tube 21 by thermal spot-bonding using a torch. The connection points are denoted by reference numeral 25.

[0106] The anti-resonance element preforms 24 are placed using a positioning template with a structurally predetermined star-shaped arrangement of holding arms for the individual antiresonance element preforms 24. In this case, the positioning template is limited to the region around the two end-face ends of the cladding tube.

[0107] This method creates a precise and reproducible connection between the cladding tube 21 and the anti-resonance element preforms 24.

[0108] The primary preform 23 is covered with a buffer tube made of quartz glass, wherein the buffer tube collapses onto the cladding tube 21, and at the same time, the tube ensemble is elongated to form a secondary preform. The buffer tube has an outer diameter of 63.4 mm and a wall thickness of 17 mm. The buffer tube can consist of a commercially available synthetic quartz glass with no measurable concentration of oxygen deficiency centers, as in the exemplary embodiment of the synthetic quartz glass commonly used under the trade name F300.

[0109] In the collapse and elongation process, the coaxial arrangement of the cladding tube 21 and the buffer tube 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 arrangement.

[0110] The heating zone is kept at a desired temperature of 1600 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.

[0111] 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. The maximum wall thickness variation (greatest value minus smallest value) of the anti-resonance element preforms is less than 4 m.

[0112] The following table lists the drawing parameters for different outer diameters before (BEFORE) and after (AFTER) the forming process (collapsing and elongation).

TABLE-US-00001 TABLE 1 Outer diameter Outer diameter Cladding tube Feed rate Drawing BEFORE [mm] AFTER [mm] length [mm] [mm/min] [mm/min] 90 70 1000 15 9.80 80 70 1000 15 4.59 40 20 1000 5 15 25 20 1000 10 5.63

[0113] The heating zone has a length of 100 mm. The maximum deviation of the wall thickness of the anti-resonance element preforms in the preform is about 4 m in all exemplary embodiments. Anti-resonant hollow core fibers having an outer diameter of 200 m or 230 mm, respectively, were drawn from the secondary preforms formed in this way in the collapse and elongation process, and the wall thicknesses of the antiresonance elements were determined.

[0114] 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. The heating zone is kept at a desired temperature of approximately 2100 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. 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.

[0115] By means of the fiber drawing process guided in this way, an anti-resonant hollow core fiber with antiresonance elements embedded therein is obtained. After the fiber-drawing process, the fiber end is clean and free of deposits. This is attributed to the use of the quartz glass grade loaded with oxygen deficiency centers.

[0116] The device shown in FIG. 3 serves for the tool-free elongation of a starting cylinder 4 of undoped quartz glass loaded with oxygen deficiency centers to form an intermediate cylinder.

[0117] The outer wall of the starting cylinder 4 is coarsely ground by means of a peripheral grinder equipped with a #80 grinding stone, whereby the predetermined desired outer diameter is essentially obtained. The outer surface is then finely ground by means of an NC peripheral grinder. The inner surface of the tube thus obtained is honed as a whole by means of a honing machine equipped with a #80 honing stone, wherein the degree of smoothing is continuously refined, and final treatment is carried out with a #800 honing stone. The starting cylinder 4 is then briefly etched in a 30% hydrofluoric acid etching solution. In this way, a starting cylinder 4 with an outer diameter of 200 mm and an inner diameter of 70 mm is produced. This is then elongated in a device according to FIG. 2 to form an intermediate cylinder 12.

[0118] The device comprises a vertically oriented resistance heating tube 1 made of graphite, which encloses a heating chamber 3 that is circular in cross-section. The heating tube 1 consists of an annular element with an inner diameter of 240 mm, an outer diameter of 260 mm and a length of 200 mm. The heating tube 1 surrounds the actual heating zone. At each end it is extended by means of 55 mm wide extension pieces 5 made of graphite tubing, which have an inner diameter of 250 mm and an outer diameter of 280 mm. The internal volume of the heating zone Vc is approximately 8140 mm.sup.3.

[0119] pyrometer 6, which detects the surface temperature of the starting cylinder 1, is arranged at the level of an upper detection plane E1 (at the upper edge of the upper extension piece 5). A further pyrometer 7, which detects the surface temperature of the elongated drawn tube 12, is arranged at the level of an lower detection plane E2 (at the lower edge of the lower extension piece 5). The temperature measurement values of the pyrometers 6 and 7 and the temperature of the heating tube 1 measured by the pyrometer 16 are each fed to a computer 8.

[0120] The upper end of the starting cylinder 4 is connected via a welded connection 9 to a quartz-glass holding tube 10, by means of which it can be shifted in the horizontal and vertical directions.

[0121] In the vertically oriented heating tube 1, the quartz glass starting cylinder 4 with an outer diameter of 200 mm and an inner diameter of 75 mm is adjusted in such a way that its longitudinal axis runs coaxially with the center axis 2 of the heating tube 1. The starting cylinder 4 is heated in the heating zone 3 to a temperature above 2200 C. and discharged at a predetermined rate of advance. From the forming drawing cone 11, the quartz glass drawn tube 12 is drawn at a regulated drawing speed to a nominal outer diameter of 27 mm and an inner diameter of 20 mm (wall thickness: 3.5 mm) as an intermediate cylinder. The continuous inner bore of the starting cylinder 4 and intermediate-cylinder drawn tube 12 has reference number 13. The tube drawing rate is detected by means of a discharge 15 and adjusted via the computer 8. The radial dimensions of the intermediate cylinder drawn tube correspond to those of the cladding tube 21 of FIG. 2.

[0122] The intermediate cylinder drawn tube 12 shows a smooth molten and particle-free surface. The cladding tube 21 is cut to length from it and, moreover, it is further processed into the ARE outer tube 24a. A drawn tube from which the ARE inner tube 24b is cut to length is drawn in the same way.

[0123] To accomplish this, in a second elongation step, it is used in a second drawing system as a starting cylinder for the production of ARE outer tubes or ARE inner tubes. The second drawing system used for this purpose is the same as the one in FIG. 3; differing essentially in the length and the inner diameter of its heating zone. The heating zone (the heating tube) has an inner diameter of 120 mm, an outer diameter of 140 mm and a length of 100 mm.