WO2007122171A1

WO2007122171A1 - Method for producing a microstructured optical fiber and fiber obtained according to the method

Info

Publication number: WO2007122171A1
Application number: PCT/EP2007/053817
Authority: WO
Inventors: Jan Vydra; Clemens Schmitt
Original assignee: Heraeus Quarzglas GmbH and Co KG
Current assignee: Heraeus Quarzglas GmbH and Co KG
Priority date: 2006-04-24
Filing date: 2007-04-19
Publication date: 2007-11-01
Anticipated expiration: 2008-10-24
Also published as: DE102006019333A1

Abstract

Method and preform for producing a microstructured optical fiber and fiber obtained according to the method The present invention relates to a method for producing a microstructured optical fiber, wherein a plurality of glass capillaries are arranged inside the cladding-tube inner bore around a core region such that the capillary longitudinal axes extend in parallel with the cladding-tube longitudinal axis, and the assembly of cladding tube and capillaries is elongated into the microstructured optical fiber or into a preform from which the microstructured optical fiber is subsequently drawn. Starting from the fabrication of microstructured optical fibers with a low content of impurities, particularly to permit a low content of hydroxyl groups, it is suggested according to the invention that prior to elongation the assembly should be subjected to a cleaning treatment comprising gas phase etching.

Description

Method for producing a microstructured optical fiber and fiber obtained according to the method

The present invention relates to a method for producing a microstructured optical fiber, the method comprising the following steps:

(a) providing a cladding tube which comprises a cladding-tube longitudinal axis and a cladding-tube inner bore and consists of an Si0₂-containing glass,

(b) providing a plurality of glass capillaries, each having a capillary longitudinal axis, a capillary inner bore, and a capillary outer wall,

(c) forming an assembly consisting of the cladding tube and the plurality of glass capillaries by arranging the same inside the cladding-tube inner bore around a core region in such a manner that the capillary longitudinal axes extend in parallel with the longitudinal axis of the cladding tube, and

(d) elongating the assembly into the microstructured optical fiber or into a preform from which the microstructured optical fiber is subsequently drawn.

Moreover, the present invention relates to a microstructured optical fiber comprising a core region and a jacket region which surrounds said core region and has a plurality of capillary cavities running through it.

Microstructured optical fibers (photonic crystal fibers (PCF), also named ,,holey fibers" or ..optical hollow fibers") represent a special form of optical standard fibers, as are used in many fields regarding telecommunications, material treatment, or medical and analytical engineering.

Standard fibers consist of a core region made from a transparent material with a higher refractive index that is surrounded by a jacket having a lower refractive index. Light is guided in such fibers on the basis of the total reflection on the optical jacket of the light radiated into the core region. By comparison, the light guided in a microstructured optical fiber is influenced by cavities that are running lengthwise through the fibers and are disposed in a specific geometrical arrangement around the core region. The microstructured optical fiber has either a solid core region or a hollow core region, each surrounded by a jacket region having cavities running therethrough. If the cavities are arranged in symmetry around the core region, it is possible to talk about a ..photonic crystal", which explains the name for these fibers.

A particularly interesting embodiment of the photonic crystal fibers are the so-called ,,air-clad fibers". These are microstructured fibers having a solid quartz glass core surrounded by an air ring structure in the form of one or several parallel longitudinal bores and by a jacket made of plastics. The design of the air ring structure has an influence on both the numerical aperture and the thermal power dissipation behavior between the optically active core and the plastic jacket and offers many possibilities of producing optical functional components for high power densities.

The microstructured optical fibers are normally drawn from preforms. The cavity structure of the jacket region is predetermined in the preform by a component assembly in which in the inner bore of a cladding tube with polygonal inner cross- section a plurality of capillaries are tightly stacked and arranged with their longitudinal axes in parallel with one another. To form a hollow fiber with a solid core region, one or more capillaries of the capillary stack is/are replaced by a solid rod. To produce a microstructured optical fiber having a hollow core region, the core region is formed by a tube or a capillary having an increased inner diameter. Normally, the assembly consisting of cladding tube and capillaries is first elongated into a preform and is subsequently drawn in a fiber drawing process into the microstructured optical fiber.

The assembly of cladding tube and capillaries or rods has a large inner surface that can only be kept free from impurities by taking considerable efforts. Such impurities may arise during formation of the assembly and its further processing into preform and fiber. Particular mention should here be made of a moist film which develops in air and which during fiber drawing may lead to the incorporation of OH groups into the glass network and thus to absorption in the range of the working wavelength of the fiber and to increased attenuation.

To avoid impurities, JP-2005-247620 A suggests a method for producing a microstructured optical fiber according to the above-mentioned type, in which an assembly is arranged, consisting of an inner core rod and small capillary tubes arranged around said inner core rod, which are sealed at a joint end. Prior to fiber drawing the air inside the small capillary tubes is replaced by argon or nitrogen, and the microstructured optical fiber is then drawn from the assembly, starting with the open side.

A complete gas exchange by introducing an inert gas is however extremely tedious, if not altogether impossible, especially in the case of capillaries with a blind hole due to their narrow inner bore as compared with length, and against capillary action. Therefore, it has been found that said measure is not adequate to avoid impurities of the assembly while it is being formed and processed, i.e., particularly during fiber drawing.

It is therefore the object of the present invention to provide a method which permits the fabrication of microstructured optical fibers with a low content of impurities, particularly a low content of hydroxyl groups. Furthermore, it is the object of the present invention to provide a preform from which low-attenuation microstructured optical fibers can be drawn, and to indicate such an optical fiber.

As for the method, this object, starting from the method set forth above, is achieved according to the invention in that prior to elongation according to method step (d) the assembly is subjected to a cleaning treatment that includes gas phase etching.

According to the invention the assembly consisting of capillaries inside the cladding tube is formed and the freely accessible surfaces of the assembly are thereafter subjected to an etching gas which is introduced into the inner bore of the cladding tube. The etching gas is an etchant chemically reacting with SiO₂, for instance SF₆ or C₂F₆.

This treatment effects a superficial removal of all SiO₂ surfaces accessible to the etching gas inside the cladding-tube inner bore, particularly of the outer walls of the capillaries, so that contaminated surface layers are removed and adhering contamination clusters are also infiltrated by the etching gas and removed and discharged by means of the etchant flow. During gas phase etching the risk is small that residues remain inside the assembly, and this facilitates, in particular, the elimination of hydroxyl groups from a near-surface layer of the quartz glass.

Preferably, gas phase etching is carried out at a temperature above 1400°C.

This hot etching process yields an increased solubility of impurities; to be more specific, hydroxyl groups can only be removed by this from deep layers within economically acceptable treatment periods.

In addition to the treatment of the assembly by gas phase etching, it has turned out to be useful when after formation of the assembly according to method step (c) the capillary outer wall is subjected to a drying treatment in which a halogen-containing drying gas is passed through the cladding-tube inner bore.

This treatment aims at an elimination of hydroxyl groups that have been introduced by air or by a previous treatment process into the components of the assembly.

According to a further advantageous variant of the method, glass capillaries are provided for the formation of the assembly with the capillary inner bore being sealed at both sides.

Prior to their use for forming the assembly the capillaries are sealed at both of their ends. This guarantees that during fabrication of the assembly and its further processing into the microstructured optical fiber impurities do not pass from the environment into the inner bore of the capillaries, which impurities might have a disadvantageous effect in a later hot process, nor are possible impurities discharged from the inner bore of the crystallization into the other areas of the assembly where they might have a particularly harmful effect. A complicated and complete gas exchange by introducing an inert gas into the blind-hole inner bore of the capillaries, as suggested in the prior art, is thus not needed. Moreover, the gas pressure of the inner bore of the capillaries sealed at both sides reduces the risk of complete collapsing. The described measure regards all capillaries of the assembly or at least the capillaries directly arranged around the core region. In this respect it has turned out to be advantageous that the capillaries are produced in a drawing process and the capillary inner bore is sealed during or directly after the drawing process.

The sealing operation during the drawing process turns out to be particularly simple if the capillaries are cut to length from a capillary strand and if the capillary inner bore is simultaneously sealed during the cutting to length. This avoids the input of impurities from the environment into the capillary inner bore.

Sealing and simultaneous cutting to length are e.g. carried out by local heating in the area of the capillary ends with simultaneous squeezing off, turning off or pulling off.

Moreover, it is advantageous to clean the inner wall defining the inner bore of the capillaries.

This cleaning action takes place during the drawing process or thereafter as long as the capillary inner bore is not sealed. It comprises the above-described detailed measures, particularly gas phase etching or a drying treatment using a halogen- containing drying gas. The inner bore of the capillaries can then be sealed directly after completion of the cleaning process.

The capillaries can be sealed by means of plugs, or the like. Preferably, however, the capillary inner bore is sealed by collapsing the capillary ends.

The capillary ends are e.g. collapsed by short-term local heating by means of a burner with simultaneous squeezing off, twisting off or drawing off of the capillaries. Due to collapsing (fusion) of the capillary inner bore, the input of impurities into the inner bore is largely avoided and a reliable seal is ensured in a simple way.

The microstructured optical fiber is obtained according to the invention by elongating a preform fabricated according to the method of the invention or an assembly obtained thereafter.

The invention shall now be explained in more detail with reference to embodiments and a drawing. There is shown in detail in a schematic illustration in: Figure 1 an embodiment of an assembly consisting of cladding tube, core rod and capillaries, in a side view;

Figure 2 the assembly according to Fig. 1 , in a top view on the front side;

Figure 3 a further embodiment of the assembly for fabricating a preform according to the invention in radial cross-section; and

Figure 4 a preform obtained from the assembly according to Fig. 3 by elongation and used for drawing a microstructured optical fiber, with an illustration of its cavity structure, in radial cross-section.

Example 1

Fig. 1 is a side view showing an assembly consisting of a cladding tube 2 of synthetic quartz glass, in the inner bore 4 of which a plurality of capillaries 3 of synthetic quartz glass and a core rod 6 (see Fig. 2) of quartz glass are arranged such that they are as close packed as possible.

The capillaries 3 have an inner diameter of 1 mm, an outer diameter of 3 mm, and a length of 600 mm. They are drawn from a quartz glass tube of high purity. During the drawing process adsorbed moisture will evaporate on the quartz glass surfaces due to the high temperature prevailing during the drawing process (about 2000⁰C), so that the formation of a water film on the inner surface of the drawn-off capillary strand is prevented. During the drawing process the drawn-off capillary strand is cut to length by fusion of the capillaries 3, with the inner bore of the capillary strand collapsing at the same time, so that both ends of the capillaries 3 will be sealed after the cutting to length, and also the lower end of the inner bore of the capillary strand.

The mean hydroxyl group content of the quartz glass of the capillaries 3 is about 0.1 wt ppm.

It is evident from the illustration of Fig. 2 that the cladding tube 2 of quartz glass has a round outer jacket and an inner bore 4 with hexagonal inner cross-section. The cladding tube has an outer diameter of 50 mm, a length of 700 mm, the width across flats of the hexagon being 24 mm. The cladding tube 2 is a commercial quartz glass tube of the company Heraeus Tenevo GmbH, Hanau.

An assembly 1 , as is shown in Figs. 1 and 2, is fabricated from the cladding tube 2 and a plurality of the capillaries 3 sealed at both sides. In this process the capillaries 3 will completely fill the inner bore 4 of the cladding tube 2, the capillaries being as close packed as possible, and will be arranged such that the capillary longitudinal axis 8 runs in parallel with the longitudinal axis 7 of the heating tube. The core region of the optical preform to be fabricated is formed by the core rod 6 of quartz glass that has the same outer diameter as the capillaries 3 and is surrounded by said capillaries in symmetry and evenly.

The assembly 1 is elongated into a preform. Directly prior to this elongation process the assembly 1 is subjected to gas phase etching. An etching gas in the form of SF₆ is here introduced through the inner bore of the heating tube 2 at a temperature of 1450°C. The accessible quartz glass surfaces will thereby be removed and cleaned superficially.

The draw ratio in the elongation process is about 2, with the ratio of the radial cross- sectional dimensions relative to one another being maintained. The resulting preform is drawn in a standard fiber drawing process into a microstructured optical fiber.

Example 2

An assembly 31 is fabricated, as shown in Fig. 3 by way of a top view. The assembly 31 comprises a cladding tube 32 consisting of synthetic quartz glass, in the inner bore 34 of which a plurality of capillaries 33 of synthetic quartz glass and a core rod 36 of quartz glass are arranged to be as close packed as possible.

The capillaries 33 have an inner diameter of 1 mm, an outer diameter of 3 mm, and a length of 600 mm. They are drawn from a quartz glass tube of high purity. During the drawing process adsorbed moisture will evaporate on the quartz glass surfaces due to the high temperature prevailing during the drawing process (about 2000 °C), so that the formation of a water film on the inner surface of the drawn-off capillary strand is prevented. The mean hydroxyl group content of the quartz glass of the capillaries 33 is about 0.1 wt ppm.

The quartz glass cladding tube 32 has a round outer jacket and an inner bore with a hexagonal inner cross-section. It has an outer diameter of 50 mm and a length of 700 mm, the width across flats of the hexagon being 24 mm. The cladding tube 32 is a commercial quartz glass tube of the company Heraeus Tenevo GmbH, Hanau.

An assembly 31 , as is shown in Fig. 3, is fabricated from the cladding tube 32 and a plurality of the capillaries 33 sealed at both sides. In this process the capillaries 33 will completely fill the inner bore 34 of the cladding tube 32, the capillaries being as close packed as possible, and will be arranged such that the capillary longitudinal axis 38 runs in parallel with the longitudinal axis 37 of the heating tube. The core region of the optical preform 40 to be fabricated (see Fig. 4) is formed by the core rod 36 of quartz glass which has the same outer diameter as the capillaries 33 and which is surrounded by said capillaries in symmetry and evenly.

The assembly 31 is surrounded with further jacket material in the form of a quartz glass tube 37 and is elongated into a preform. Directly prior to this elongation process the assembly 31 which is enlarged by the quartz glass tube 37 is subjected to gas phase etching. An etching gas in the form of SF₆ is here passed through the inner bore of the heating tube 32 at a temperature of 1450°C. The accessible quartz glass surfaces, including the inner bores 35 of the capillaries 33, will thereby be removed superficially.

The draw ratio in the elongation process is 2, the ratio of the radial cross-sectional dimensions relative to one another being maintained. The resulting preform 40 is schematically shown in radial cross-section in Fig. 4. The cavity structure 41 produced according to the method of the invention can be clearly seen.

The resulting preform 40 is drawn in a standard fiber drawing process into a microstructured optical fiber. Except for the size, the radial fiber cross-section is identical with the radial preform cross-section schematically shown in Fig. 4. The microstructured optical fiber obtained after drawing is particularly distinguished by a low hydroxyl group content and a correspondingly low attenuation in the wavelength range of the OH group absorption.

Claims

Patent claims

1. A method for producing a microstructured optical fiber, the method comprising the following steps:

(a) providing a cladding tube (2) which comprises a cladding-tube longitudinal axis (7) and a cladding-tube inner bore (4) and consists of an SiO₂- containing glass,

(b) providing a plurality of glass capillaries (3), each comprising a capillary longitudinal axis (8), a capillary inner bore (5) and a capillary outer wall,

(c) forming an assembly (1 ) consisting of the cladding tube (2) and the plurality of glass capillaries (3) by arranging the same inside the cladding-tube inner bore (4) around a core region (6) in such a manner that the capillary longitudinal axes (8) extend in parallel with the longitudinal axis (7) of the cladding tube, and

(d) elongating the assembly (1 ) into the microstructured optical fiber or into a preform from which the microstructured optical fiber is then drawn,

characterized in that prior to elongation according to method step (d) the assembly (1 ) is subjected to a cleaning treatment that includes gas phase etching.

2. The method according to claim 1 , characterized in that gas phase etching is carried out at a temperature above 1400°C.

3. The method according to any one of claims 1 or 2, characterized in that after formation of the assembly (1 ) according to method step (c) the capillary outer wall is subjected to a drying treatment in which a halogen-containing drying gas is passed through the cladding-tube inner bore (4).

4. The method according to any one of the preceding claims, characterized in that for the formation of the assembly glass capillaries (3) are provided with the capillary inner bore (5) being sealed at both sides.

5. The method according to claim 4, characterized in that the capillaries (3) are produced in a drawing process, and that the capillary inner bore (5) is sealed during or directly after the drawing process.

6. The method according to claim 5, characterized in that the capillaries (3) are cut to length from a capillary strand and during the cutting to length the capillary inner bore (5) is sealed at the same time.

7. The method according to any one of the preceding claims, characterized in that the inner wall which defines the inner bore (5) of the capillaries (3) is cleaned.

8. The method according to claim 7, characterized in that cleaning comprises gas phase etching of the inner wall or a drying treatment using a halogen- containing drying gas.

9. The method according to any one of the preceding claims, characterized in that the capillary inner bore (5) is sealed by collapsing the capillary ends.

10. A microstructured optical fiber comprising a core region and a jacket region which surrounds the core region and has a plurality of capillary cavities extending through it, characterized in that the fiber is obtained according to a method according to claims 1 to 9.