DE102011014915B4 - Process for the production of an optical waveguide with an optimizable macrobending loss and preform for the production of an optical waveguide - Google Patents

Abstract

Method for producing an optical waveguide with an optimizable macrobending loss using the method steps
Preparing a primary preform (1),
- producing thin rods (4) each having a refractive index-reducing or refractive index-increasing doped thin-rod core (5),
Surrounding the primary preform (1) with the thin rods (4) to form at least one single-layer thin rod covering,
Twisting the single-layer thin rod covering around the longitudinal axis of the primary preform (1),
Fixing the twisted thin rod covering on the primary preform (1),
Producing a secondary preform by inserting the entirety of the primary preform (1) and the twisted thin rod covering into a jacketing tube (10),
- Applying a vacuum to the gap between the primary preform and the Jacketingrohr (10) for removing air-filled cavities and pulling the optical waveguide.

Description

The invention relates to a method for producing an optical waveguide according to claim 1 and to a preform for producing an optical waveguide according to claim 8.

Fiber optic cables are widely used both for signal transmission by means of light and in the field of non-electrical stress, strain and bending sensors. The optical waveguides are designed, for example, in the form of gradient index multimode fibers. These enable a dispersion-poor light conduction of different vibration modes and polarization states of the coupled-in light.

The multimode fibers generally consist of a multimode core and one or more cladding regions. In order to reduce the differences in the propagation velocities of modes of different orders in the multimode fiber, it is known that for the core and / or cladding, a graded radial refractive index profile, e.g. a parabolic refractive index profile can be used in which the refractive index has a continuous course as a function of the fiber radius. Such optical waveguides are also referred to as gradient index fibers or gradient index multimode fibers.

A particularly important property, which is crucial for the quality and uses of such multimode fibers, is the so-called macrobend sensitivity, especially the macrobending loss of the fiber. The macrobending loss indicates the extent to which the fiber attenuation is affected by bending, bending, buckling and the like. Various embodiments and designs for multimode fibers have hitherto been known with which the curvature sensitivity of the fibers can be reduced or selectively influenced:

A first possibility consists essentially of increasing the refractive index difference between the fiber core and the cladding, or the so-called core NA calculated therefrom. However, this reduces the transmission bandwidth of the optical waveguide, because the achievable bandwidth for the optical waveguide depends indirectly proportionally on the square of the core NA.

A second possibility is to form the optical waveguide such that it contains a homogeneously doped refractive index trench which directly adjoins the fiber core or which is separated from the core by a cladding intermediate layer. However, the production of such an optical waveguide is usually very complex.

Furthermore, the fibers may be formed with a hole structure in an undoped or fluorine doped annular cladding region.

The publication JP 2004-102281 A teaches a photonic crystal fiber array. There is disclosed the use of hollow rods or capillaries. After warping the arrangement shown there, which consists of a core material, capillaries and a jacketing material, a capillary fiber is produced there in the sequence. This consists of cavities which are arranged around a core.

The publication US 4,360,372 A teaches a fiber construction that should reduce or eliminate so-called speckle and mode noise effects. The document proposes to use as a fiber optic element, a bundle of a plurality of wave-guiding individual components or filaments. According to the teaching of the document, the bundle is either enclosed in a cladding or fused together without cladding.

The publication US 2006/0024008 A1 teaches a fiber assembly constructed of two different waveguides. According to the document, a central core is provided as a so-called large mode area. The core designated there is helically surrounded by a side core. The central nucleus is supposed to guide light modes along its axis, while the helical side nucleus decouples certain modes.

The publication JP H02-120703 A teaches a light guide in which an optical image is to be transmitted by a plurality of individual fibers. The document teaches a fiber-length-dependent twisting of a recorded image.

In the US script US2008 / 0166094A1 describes a gradient index multimode fiber characterized by low sensitivity to macrobending. The improved curvature behavior over standard gradient index fibers is achieved by joining to the gradient index core a cladding region comprising an annular region with a reduced effective refractive index. The circular ring region of reduced refractive index does not surround the core region directly, but surrounds an inner cladding region. The reduced refractive index refers to the refractive index of the remaining cladding region. The reduced effective refractive index is generated by a plurality of periodically or non-periodically arranged holes or fluorine-doped glass or fluorine-doped glass with holes.

The disadvantages of in the US2008 / 0166094A1 The solution described is that problems can arise when splicing fibers with holes. These are mainly due to the fact that in the holes moisture or other contaminants can penetrate, thus limiting the life of the fiber. In addition, the production of defined hole structures requires a special technology, which is otherwise usually not required in the manufacture of compact fiber structures and therefore can not be readily incorporated into a given manufacturing process.

The font EP 2 166 386 A1 also discloses embodiments for limiting macrobending losses in optical waveguide in gradient index multimode fibers. According to the teaching disclosed therein, by introducing a fluorine-doped refractive index trench having predetermined dimensions and a radial position between the inner and outer cladding of a gradient index multimode fiber is achieved that the macrobending losses compared to a conventional Gradientenindexfaser be significantly reduced. At this time, the transmission capacity of the fiber described by the bandwidth or DMD is not significantly deteriorated. Of particular importance for curvature-related losses and DMD broadening is the radial location of the refractive index trench and the width of the trench. The trench width should be greater than 2.5 μm, preferably 10 to 13 μm. However, the production of such an optical waveguide also requires the use of special technologies and a considerable effort to realize these corresponding trench structures.

From the foregoing, therefore, the object underlying the invention to provide an optical waveguide with an optimizable macrobending loss and a method for their preparation. For the optical fiber in particular an attenuation of less than 0.1 dB under the condition of a light wavelength of 850 nm with 2 turns and a radius of curvature of 15 mm should be achievable. On the other hand, should the optical waveguide have a sufficiently clearly detectable and clearly correlated with its bending macrobending loss, if necessary, which ensures their application in the optical sensor. The method for producing the optical waveguide should be executable with the least possible effort and cost.

The object is achieved with a method for producing an optical waveguide having the features of claim 1 and with respect to its device aspect with a preform for producing an optical waveguide with the features of claim 8. The respective subclaims include expedient and / or advantageous embodiments of the claimed method or the claimed preform.

The method according to the invention for producing an optical waveguide comprises the following method steps:

The process begins with making a primary preform. Parallel to this time, thin rods are produced, each with a thin-rod core doped with a reduction in the number of refractive indices or with an increase in the refractive index. Subsequently, the primary preform is surrounded with the thin rods and there is a terminal fixation of the thin rods to the primary preform. A secondary preform is produced from the totality of primary preform and surrounding thin rods produced in this way. This is done by inserting the entirety of the primary preform and the fixed thin rods into a jacketing tube. The jacketing tube surrounds the primary preform and the thin rods from the outside. The composite formed in this way serves as a secondary and actual preform for the resulting production of the optical waveguide. This is done by applying a vacuum to the secondary preform and pulling the secondary preform, thereby producing the optical fiber.

In an expedient embodiment, the primary preform is made with a primary preform core and a primary preform cladding.

In an expedient embodiment, the production of each of a thin rod is carried out in a deposition process and by pulling a thicker starting rod in a thermal and low-moisture process. In the manufacture of the starting rod, in a first embodiment, a starting rod or a drawn thin rod having a fluorine and / or boron core doping and an undoped silicon dioxide jacket is produced. Due to the undoped regions of the SiO ₂ jacket necessary for optimal mode mixing interruptions in the doped cladding region of the subsequent optical waveguide allows and thus already created in the designs of thin rods. At the same time, the SiO ₂ cladding in the finished optical waveguide leads via mechanical and thermal stresses and, due to its viscosity, to refractive index influences of the fiber core and an improvement in the mode mixture.

Alternatively, a starting rod or drawn thin rod having a germanium and / or phosphorus core doping and an undoped silicon dioxide jacket may be produced.

The terminal fixation of the thin rods with the primary preform can be done in various ways. Conveniently, the thin rods are fixed by a fusion or gluing. Also possible is the tying of the thin rods by means of a thermally resistant wire.

The preform according to the invention for producing an optical waveguide with an optimizable macrobending loss is composed as follows:

The preform contains a primary preform inside. Arranged around the primary preform is a set of thin rods which are mounted without any spacing and fixed terminally to the primary preform. The ensemble of primary preform and thin rods thus formed is surrounded by a jacketing tube. The thin rods each have a refractive index-reducing or refractive index-increasing doped thin-rod core ( 5 ) on.

In one embodiment, the primary preform has a primary preform core and a primary preform cladding. The primary preform core may be increased in refractive index.

In one embodiment, the longitudinal axis of each respective thin rod and the longitudinal axis of the primary preform include a finite angle. In such a case, the thin rods thus form an arrangement wound around the primary preform, the angle indicating the amount of distortion.

In one embodiment, the thin rods each have a doped thin rod core and a silicon dioxide cladding. In this way, an interrupted doping structure advantageously promoting the mode mixture can be realized in the finished optical waveguide and predefined within the preform.

In an expedient embodiment, the jacketing tube has a inside diameter in the range of 0.1 to 2 mm, preferably 0.5 to 1.5 mm, larger than the outside diameter of the entirety of the primary preform and the surrounding thin rods. By such a design, a superficial scratching and concomitant damage between the inner surface of the Jacketingrohrs and the Dünnstabummantelung is avoided when joining the preform, especially when inserting the entirety of primary preform and sheathing thin rods. The thin rods are significantly thinner than the primary preform. The diameter of each thin rod is about 1/5 to 1/100, preferably 1/8 to 1/20 of the diameter of the primary preform.

The respective thin rod has a ratio between a rod diameter and a rod core diameter of 1.005 to 1.2, preferably from 1.02 to 1.08. The thickness of the undoped region is thus small compared to the doped thin rod core.

The manufacturing method for the optical waveguide and the preform used therefor will be explained in more detail below with reference to exemplary embodiments. To clarify serve the 1 to 10 , The same reference numbers are used for identical or equivalent parts.

• 1 eine Darstellung einer primären Preform im Querschnitt,
• 2 eine Darstellung mehrerer Dünnstäbe und eines beispielhaften Dünnstabquerschnittes,
• 3 eine Darstellung einer von Dünnstäben ummantelten primären Preform im Querschnitt,
• 4 eine Darstellung des Fertigungsschritts des endständigen Fixierens der Dünnstäbe an der primären Preform,
• 5 eine Darstellung des Fertigungsschritts des Vereinigens zwischen ummantelter primärer Preform und Jacketingrohr,
• 6 eine Darstellung einer Dünnstabummantelung der primären Preform mit verdrillten Dünnstäben,
• 7 eine Darstellung der fertigen Preform aus primärer Preform, Dünnstabummantelung und Jacketingrohr im Querschnitt,
• 8 eine schematische Darstellung des Ziehens des Lichtwellenleiters aus der in 7 gezeigten Preform,
• 9 eine schematische Darstellung der Struktur eines fertigen Lichtwellenleiters im Querschnitt.
• 10 ein beispielhaftes Brechzahlprofil eines aus der Preform gefertigten Lichtwellenleiters.

It shows

• 1 a representation of a primary preform in cross section,
• 2 a representation of several thin rods and an exemplary thin rod cross-section,
• 3 a representation of a sheathed by thin rods primary preform in cross section,
• 4 an illustration of the manufacturing step of the terminal fixing of the thin rods to the primary preform,
• 5 a representation of the manufacturing step of uniting between jacketed primary preform and jacketing tube,
• 6 a representation of a thin rod sheath of the primary preform with twisted thin rods,
• 7 a representation of the finished preform of primary preform, thin rod sheath and Jacketingrohr in cross section,
• 8th a schematic representation of the pulling of the optical waveguide from the in 7 shown preform,
• 9 a schematic representation of the structure of a finished optical waveguide in cross section.
• 10 an exemplary refractive index profile of a fabricated from the preform optical fiber.

1 shows a primary preform 1 in cross section. The primary preform contains a preform core in this example 2 and a preformcladding 3 , The primary preform can also be designed as a bare core or rod without cladding. In the case of a cladding-reinforced preform, the diameter of the preform core is typically 10 to 30 mm, the diameter of the entire primary preform is about 15 to 35 mm. Depending on the specific manufacturing requirements but also smaller or larger dimensions can be selected. For the preparation of the primary preform, the usual methods, For example, the MCVD, the OVD or the PCVD method, are used.

The refractive index profile in the preform core is determined during the production of the primary preform. In particular, it can have a graduated, radius-dependent profile. The refractive index of Preformcladding the Primärpreform is usually smaller than the refractive index of the corresponding Preformkerns.

2 shows a series of thin rods 4 , The thin rods typically have a circular cross-section. However, it is in principle possible to use other cross-sectional shapes. In particular, rectangular, in particular square, trapezoidal, semicircular, triangular or hexagonal shapes are considered here.

Each of the thin rods has a thin rod core 5 and a thin rod coat 6 on. The thin rod core has a fluorine and / or boron doping, optionally a germanium and / or phosphorus doping. In order to reduce the macrobending losses of the later optical fiber in the form of the multimode fiber, refractive index reducing dopants such as fluorine or boron must be used in the core region of the rods. If the macrobending loss is to be increased, refractive index-increasing dopants such as germanium or phosphorus must be used. Such multimode fibers with increased macrobend sensitivity are used, for example, in sensor technology. On the design of the thin rods can thus make a targeted adjustment of the subsequent properties of the optical waveguide in a simple manner.

The thin rod coat 6 consists of undoped silicon dioxide SiO ₂ . This undoped region around the doped thin-rod cores is required to have low azimuthal voltage changes and thus refractive index changes after subsequent fusion to the bulk fiber due to the different textures, particularly in refractive indices and viscosities, of the doped glass in the thin rod core and the undoped glass in the thin rod core to produce in the core region, and so to effect an increased mode mixing and thus a reduction of mode delay differences in the later than multimode fiber formed optical waveguide. In addition, a bubble-free fusion of the thin rods with the undoped quartz glass of Primärpreformcladding or Jacketingrohres is ensured by the undoped Dünnstabschichtes.

The production of the thin rods described is done by pulling a thicker output rod using a heat source. The cross-section of the thin rods, in particular their diameter in a circular cross-sectional shape, is only a fraction of the cross-sectional size of the primary preform. The diameter of the doped thin rod core is for example about 2 to 2.4 mm, while the diameter of the entire thin rod is about 2.2 to 2.6 mm. The coated thin rod is thus only slightly thicker than its doped core. The ratio of the overall diameter of the thin rod to the diameter of the core is generally about 1.005 to 1.2.

3 and 4 show the subsequent production steps. The in their core with fluorine / boron or germanium / phosphorus doped thin rods 4 be as a bundle-shaped cladding around the primary preform 1 essentially complete and include these as the cross-sectional representation 3 shows.

Subsequently, a terminal fixing of the thin rods with the primary preform, as in 4 shown. The terminal fixing can be done, for example, by fusing over an external heat supply or by gluing using a high-temperature glass adhesive. It is also possible to fasten the thin rods by means of a temperature-resistant wire, which is wound around the bundle of primary preform and thin-rod sheath.

At the in 4 The example shown is a heat source 8th , in particular an annular burner assembly, provided the ends of the thin rods and the primary preform at a hot spot 7 melts into a melt or a viscous ring. Upon cooling, the thin rods are fixed to the bar ends with the primary preform. The result is a solid composite of primary preform and sheathing thin rods.

When wrapping the bundle with wire, a revolving wire roll is used. This contains the temperature-resistant wire and wraps it around the bundle. The wrapping can be done zigzag so that the winding stabilizes itself. The wire is cut off after reaching a predetermined number of turns.

5 shows the subsequent production step. The over a terminal fixation 9 with the thin rods 4 occupied primary preform 1 is now in the opening of a Jacketingrohres 10 inserted, in particular inserted. The Jacketingrohr consists in the simplest case of an undoped quartz glass matrix, but can also be doped and / or structured if necessary.

For a smooth insertion process is the inner diameter of the Jacketingrohres slightly, for example, 0.5 to 2 mm, larger than the outer diameter of the occupied with the thin rods primary preform. This avoids scratching the inner jacket tubing surface during threading of the barred primary preform.

The outer cross section and the outer shape of the Jacketingrohres are arbitrary ansich. Only its inner diameter and inner shape is determined by the cross section of the sheathed primary preform. For example, the diameter of the primary preform with the thin-rod sheath is about 30 mm. The necessary inner diameter of the Jacketingrohrs is then at least about 30.5 mm, wherein the outer diameter of the Jacketingrohres is chosen so that the resulting fiber has the desired core / sheath ratio.

6 shows a further embodiment of the jacketed primary preform 1 , In the present variant close the longitudinal axes of the thin rods 4 in the longitudinal axis of the primary preform 1 a finite twist angle α one. This angle corresponds to a helical turn of the thin rods around the primary preform, which depends on the size of the angle α is narrower or wider.

The twist of the thin rods is either at the coverage of the primary preform or together with the formation of the terminal fixation 9 executed. In the latter case, a first end of the thin rods is first fixed to the primary preform. Subsequently, the other end of the thin-fiber bundle is detected by a gripping device. The gripping device is then rotated by a fixed predetermined angle of rotation about the longitudinal axis of the primary preform, this turns the twist angle α one. Thereafter, the other end of the thin rods is fixed to the primary preform and detached the gripping device.

7 shows the secondary preform produced by the preceding process steps in cross section. This serves as a semi-finished and intermediate product for the subsequent production of the optical waveguide. The interior of the secondary preform is through the preform core 2 and the preformcladding 3 educated. This interior area is from the Dünstabstabummantelung from the previously described thin rods 4 surround. The jacketing tube 10 forms the outer shell of the secondary preform.

8th shows the final fabrication of the optical waveguide using the previously discussed secondary preform. Basically, this is based on the known drawing method using a schematically indicated pulling device 11 resorted. Depending on the size of the preform used and the desired diameter of the optical waveguide to be produced, the preform is drawn into a fiber whose outer diameter is, for example, 125 μm and which has a core diameter of 50 μm. For a solid and free of voids and inclusions connection between the Jacketingrohr and the Dünstabstabummantelung is via a vacuum device 12 built a negative pressure between Primärpreform surface and inner Jacketingrohroberfläche, whereby the Jacketingrohr is pressed during the drawing process on the Dünstabstabummantelung. The thin rods press on the core of the preform and at the same time assemble into a structured trench running around the core. The resulting fiber is thus free of holes and inclusions.

9 shows a schematic cross section through the fiber structure of a finished optical waveguide. The jacketing tube of the preform now forms an outer jacket zone in the fiber structure 13 of the optical fiber, which can also be referred to as external cladding. The outer cladding, like the jacketing tube, consists essentially of a pure SiO ₂ matrix.

The core of the fiber optic cable consists of a core 14a and an inner cladding 14b , This section is from the primary preform. The inner cladding 14b is from an azimuthally structured trench 15 surround. This consists of the formed in the drawing process thin rods and has a sectoral subdivision of trench sections 16 on, through fine bridges 17 of SiO ₂ are separated from each other and also to the outside to the outer cladding and inwardly toward the outer core. The trench sections correspond to the thin-rod cores, the webs to the thin-rod shells within the preform.

The webs cause an entry of mechanical, thermal or viscous stresses in the trench structure and especially in the direction of the fiber core, which cause an influence on the refractive index and ensure optimum mode mixing of the guided light in the fiber. 10 shows an exemplary refractive index profile of an manufacturable from the preform optical waveguide. The refractive index profile represents the normalized to a reference value refractive index difference .DELTA.n as a function of the radius R of the optical waveguide. The optical waveguide has in the core of a continuously increasing radius, in particular bell-shaped decreasing refractive index difference. This opens into a step with the width a one. The step is followed by a single trench of width b at. Following the trench, the refractive index difference Δn is constant. The flank of the trench in the direction of the core forms in its course a continuation of the refractive index profile given in the core.

During manufacture of the optical waveguide, the refractive index profile of the core emerges from the primary preform. The trench substantially corresponds to the above-described Dünstabstabummantelung. The course of the refractive index profile at the flank of the trench located in the direction of the core is obtained by the interaction of the coated or uncoated thin rods with the primary preform cladding. The rest of the refractive index profile outward for larger radii is evident from the jacketing tube of the above-explained preform. The step width a is about 0.5 microns to 10 microns, the trench width b moves around in the same area.

The invention has been explained with reference to exemplary embodiments. In the context of professional action further designs are possible. These arise in particular from the dependent claims.

LIST OF REFERENCE NUMBERS

1

primary preform

2

Preformkern

3

inner preformcladding

4

thin rod

5

Thin rod core

6

Thin rod cladding

7

hotspot

8th

heat source

9

fixation

10

Jacketing tube

11

Drawing device, schematic

12

Vacuum device, schematic

13

outer cladding

14a

Preformkern

14b

inner preformcladding

15

structured trench

16

grave sections

17

grave webs

α

twist

a

Tread width

b

grave depth

Claims (15)

Method for producing an optical waveguide with an optimizable macrobending loss using the method steps Preparing a primary preform (1), - producing thin rods (4) each having a refractive index-reducing or refractive index-increasing doped thin-rod core (5), Surrounding the primary preform (1) with the thin rods (4) to form at least one single-layer thin rod covering, Twisting the single-layer thin rod covering around the longitudinal axis of the primary preform (1), Fixing the twisted thin rod covering on the primary preform (1), Producing a secondary preform by inserting the entirety of the primary preform (1) and the twisted thin rod covering into a jacketing tube (10), - Applying a vacuum to the gap between the primary preform and the Jacketingrohr (10) for removing air-filled cavities and pulling the optical waveguide. Method according to Claim 1 , characterized in that in the manufacture of the primary preform (1), a manufacturing of a primary preform core (2) and a primary preform cladding (3) takes place. Method according to Claim 1 or 2 , characterized in that each of a thin rod (4) is produced in a deposition process and by pulling a thicker starting rod in a thermal and low-moisture process. Method according to one of Claims 1 to 3 , characterized in that a starting rod or a drawn thin rod (4) with a fluorine and / or boron core doping and an undoped silicon oxide jacket (6) is produced. Method according to one of Claims 1 to 3 , characterized in that a starting rod or a drawn thin rod (4) with a germanium and / or phosphorus core doping and an undoped silicon oxide jacket (6) is produced. Method according to one of the preceding claims, characterized in that in the twisting of the single-layer Dünnstabbedeckung about the longitudinal axis of the primary preform (1) a twisting angle (a) is set, the tangent of which corresponds to at least the ratio between half of the thin rod diameter to Dünnstablänge. Method according to one of the preceding claims, characterized in that the terminal fixing of the thin rods (4) with the primary preform (1) by fusion, bonding or by a fixed bonding by means of a thermally stable wire. A preform for producing an optical waveguide with an optimizable macrobending loss, comprising a primary preform (1), an at least single-layer thin-rod cover fixed around the primary preform (1) without a gap, fixed to the primary preform (1) at the end and around the longitudinal axis of the primary preform (1). 1) twisted thin rods (4) and a jacketing tube (10) surrounding the entirety of the primary preform and thin rods, the thin rods each having a thin-rod core (5) doped with a reduced number of refractions or increased in terms of refractive index. Preform after Claim 8 , characterized in that the primary preform (1) has a primary preform core (2) and a primary preform cladding (3). Preform after Claim 8 or 9 , characterized in that the longitudinal axis of each respective thin rod (4) and the longitudinal axis of the primary preform (1) include a finite twist angle (a). Preform after one of Claims 8 to 10 , characterized in that the twisting angle (a) has an amount whose tangent at least equal to the ratio between the half of the thin rod diameter and the Dünnstablänge. Preform according to one of the preceding claims, characterized in that the thin rods (4) each have a doped thin rod core (5) and a silicon oxide jacket (6). Preform after one of Claims 8 to 11 , characterized in that the Jacketingrohr (10) has a larger in the range of 0.1 to 2 mm inner diameter than the outer diameter of the entirety of the primary preform (1) and surrounding thin rods (4). Preform after one of Claims 8 to 13 , characterized in that the diameter of each of a thin rod (4) is 1/5 to 1/100 of the diameter of the primary preform (1). Preform after one of Claims 8 to 14 , characterized in that the respective thin rod (4) has a ratio between a rod diameter and a rod core diameter of 1.005 to 1.2.

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