WO2007009450A1

WO2007009450A1 - Method for producing glass-fibre preforms with a large core diameter

Info

Publication number: WO2007009450A1
Application number: PCT/DE2006/001272
Authority: WO
Inventors: Lothar Brehm; Christian Genz; Wolfgang HÄMMERLE; Jörg KÖTZING; Michael Schewe
Original assignee: J-Fiber Gmbh
Priority date: 2005-07-20
Filing date: 2006-07-19
Publication date: 2007-01-25
Also published as: DE102005034594A1; DE102005034594B4

Abstract

The invention relates to a method for producing glass-fibre preforms with a large core diameter. The object of the invention, that of providing a method for producing glass-fibre preforms with a large core diameter which avoids the disadvantages of the prior art and provides glass-fibre preforms that have a larger core diameter than previously, is achieved by the method comprising the following steps: a) production of a primary preform, b) working of this primary preform to a smaller core diameter, c) production of an inner-coated substrate tube, provided with cladding and core layers, by means of MCVD processes, d) subsequent pre-collapsing of the inner-coated tube to a capillary diameter which is slightly greater than the diameter of the worked primary preform, e) inserting the worked primary preform into the inner-coated and pre-collapsed tube, f) and subsequent fusing of the pre-collapsed tube onto the worked primary preform, it being possible for these individual steps a to f to be repeated a number of times, in that the glass-fibre preform obtained in the last method step is used in place of the primary preform of the method step a for the method steps which follow.

Description

Process for producing glass fiber preforms with a large core diameter

The present invention relates to a process for producing glass fiber preforms having a large core diameter.

The production of a glass fiber takes place in such a way that a Glasfaserpreform heated in a vertical orientation on one side at the lower end and a fiber is withdrawn at a defined speed. For better handleability, the fiber is provided with a protective layer which is generally polymer, but also other materials such as e.g. Metal or graphite may consist, coated. The coating layer protects the fiber against environmental influences.

The Glasfaserpreform represents a solid cylindrical rod with a specific radial refractive index profile profile and derived therefrom core / shell diameter ratio. For the production of conventional multimode fibers for

For communication applications, the core sheath ratio is 50μm / 125μm = 0.40 and 62.5μm / 125μm = 0.50, respectively. Standard monomode fibers have a core-sheath-to-diameter ratio of typically 9μm / 125μm = 0.072.

During fiber drawing, the refractive index profile profile and the core / sheath ratio remain virtually unchanged.

The diameter of the glass fiber preform is several orders of magnitude greater than the diameter of the drawn fiber. The length of fiber that can be pulled out of the glass fiber preform is directly proportional to the preform mass. This in turn is proportional to the preform core diameter. The use of glass semifinished products with a large mass or a large core diameter results in substantial cost savings in fiber drawing with the same semifinished product length.

Several competing processes for the production of glass fiber preforms are known. The most common method is the so-called MCVD process (Modified Chemical Vapor Deposition).

In this method, the desired radial refractive index profile or the desired core / sheath diameter ratio is produced by depositing glassy soot on the inner surface of a rotating glass tube and melting it. The glass boat consists of SiO ₂ and one or more dopants affecting the refractive index. The refractive index increase (for example by germanium) in the deposited core material and / or the reduction in refractive index (for example by fluorine) generated by special dopants in a deposited cladding material surrounding the core material is necessary to enable a desired waveguiding in the core region.

The size of the core diameter of the glass preforms in the production by the MCVD method is limited by several factors. This is on the one hand, the limited cross-sectional area of the tube interior for the deposition of Glassootes on the inner tube wall and the other with increasing thickness of the tube wall (substrate tube + deposited glass layers) more difficult entry of heat of the outer heat source in the interior of the substrate tube. For the chemical reaction in Rohrinnem and for the transparent melting of the deposited Glassootes a sufficiently high temperature (> 1300 ⁰ C) is required. In the production of Monomodepreformen the deposited core layers to produce the required typical refractive index difference significantly less doped of about 4.5 * 10 ^'3 with GeO ₂ as it (typical maximum for the deposition of core layers for Multimodepreformen refractive index differences core / mantle is 13 ... 26 * 10 ^{^"3)} of the case. the lower doping concentration of the core layers increases with single-mode preforms the required reflow of the deposited soot layers. However, a higher reflow temperature requires for a radially decreasing temperature profile in the tube wall a higher substrate tube external temperature However, at higher substrate temperature during core deposition, the pipe tends to increase Collapse. During collapse, the pipe wall thickness increases and the heat input into the interior of the pipe is made even more difficult.

In the MCVD method, an external one generally moves

Heat source along the substrate tube and heats the tube locally partially on the outer surface.

During each Heizerdurchlaufes inside the tube is a thin

Glass layer deposited and melted. In the production of

Monomode glass semi-finished products are first produced by cladding layers and then core layers. The cladding layers serve as

Diffusion barrier for OH ions and other impurities from the

Substrate tube. In addition, the deposited allows

Claddingbereich by its participation in the waveguide low fiber attenuation. The size of the cladding area therefore very much depends on the quality of the substrate tubes used

(Purity of the substrate tube material and inner substrate tube interface).

In the production of multimode glass semi-finished products, the deposition of cladding layers can generally be dispensed with.

At the end of the deposition process, the temperature on the

Substrate tube outside locally partially further increased by a directed

Collapse of the internally coated substrate tube to obtain a core rod.

The core diameter of the thus produced core rod adds

Monomode glass semi-finished products have a typical diameter of 4 to

6 mm, for multimode glass semi-finished products a core diameter of approx.

10 to 20 mm.

In the subsequent glass fiber preforms, further tubes can be made by means of the so-called rod-in

Tube technique to be melted. By this procedure, it is possible to supply a large part of the glass in the entire manufacturing process through the cladding tubes used.

The efficiency of fiber production depends very much on the size of the preform used. Currently, the fiber yield of monomode preforms is up to about 5200 km (KHChang et al., Optical Fiber Communication Conference 2005, Next generation fiber manufacturing for highest performing conventional single-mode fiber, JWA5). Such large preforms are made by the so-called Rod-in-Cylinder (RCI) process. A core rod is melted into a thick-walled cylinder. The use of cylinders with a typical cross-sectional area (CSA)> 10,000 mm ² is less expensive than the use of previously used jacketing tubes with a CSA up to about 5000 mm ² , because the Jacketingrohrherstellungsprozeß elimination and larger preforms allow higher pulling speeds.

However, the use of the RIC process requires primary preforms or core rods with a larger cladding or core diameter. With the mentioned fiber yield of 5200 km, the preform has a diameter of approx. 170 mm. At the core-shell diameter ratio of 0.072, the preform core must have a diameter of 12.2 mm. Such a large core diameter can hitherto only be produced using the VAD method (K.H.Chang et al., Next Generation Fiber Manufacturing for the Highest Performing Conventional Single-Mode Fiber, Lecture JWA5, OFC '05). The maximum achievable single-mode preform core diameter when using substrate tube-based primary preform production methods (MCVD method, PCVD method) is currently around 6 mm. A combination of the rod-in-cylinder process with the overclad-during-draw process leads to a further increase in the effectiveness of the fiber manufacturing process (OFC 05, Next generation fiber manufacturing for the highest performing conventional single-mode fiber, JWA5).

The document DE 699 00 958 T2 discloses a method for producing a single-mode optical fiber with the following steps:

Selecting a first cylindrical tube having a length and a central axis and consisting of a glass having a hydroxyl ion (OH) content which is less than 0.5 parts by weight per million (ppm). Arranging the first cylindrical tube in a horizontal axis and rotating the tube about its central axis;

Depositing successive layers of cladding material on the inner surface of the first tube over its length;

Depositing successive layers of a core material on the cladding material over its length;

Exposing the first tube to a heat source moving along the length of the tube, the heat from the source causing inward collapse of the tube to form a core rod, the deposited core material having a diameter greater than about 5 mm and the deposited cladding material has a diameter less than about 15 mm;

Selecting a second cylindrical tube having an inner diameter slightly larger than the outer diameter of the core rod, the second tube being a glass having a hydroxyl ion (OH) content less than 1.0 ppm by weight;

Placing a substantial portion of the core rod in the second tube;

Exposing the second tube to a heat source causing it to collapse inwardly onto the core rod to produce a first preform, characterized by the steps of:

Monitoring the outer diameter of the first tube and varying the gas pressure within the first tube to control its outer diameter during the deposition of the core and jacket materials; and

• Monitoring the straightness of the first tube and varying the rotational speed of the first tube depending on its angular position during the deposition of the core and cladding materials to control its straightness during the deposition of the core and cladding materials. The disadvantage of this method is that only Glasfaserpreformen can be achieved with a core diameter of up to 6 mm.

The invention is therefore based on the object to provide a method for producing Glasfaserpreformen with a large core diameter, which avoids the disadvantages of the prior art and Glasfaserpreformen with a larger core diameter than before provides.

According to the invention, this object is solved by the features of the first claim and advantageously designed by the features of the subclaims.

Method for producing a multimode optical fiber / monomode fiber comprises the following steps:

- Choosing a first substrate tube with the lowest possible OH content, good geometric parameters (Siding, Bow, longitudinal uniformity), inexpensive glass quality

- Installation of the tube in a glass lathe and rotation of the substrate tube

Monomode preform: deposition of successive layers of a cladding material on the inner surface of a first tube over its length, the number of necessary Claddingschichten depends on the OH content of the substrate tube used

Depositing successive layers of a core material onto the cladding material along its length

Collapsing the first internally coated substrate tube by locally increasing the substrate tube temperature locally with the aid of an external heat source moving along the substrate tube

- The deposited core material has a diameter of about 5 mm, the deposited cladding material has a diameter between 7 and 12 mm

- The substrate tube material and the deposited Gladdingglas is by a wet chemical reaction (eg HF, NH ₄ HF ₂ ), gas phase etching / plasma etching or completely removed by evaporation over the entire Preformlänge

Selecting a second cylindrical substrate tube and depositing successive layers of a cladding material on the inner surface of a second substrate tube over its length

Depositing successive layers of a core material onto the cladding material over the length of the cladding material

- Collapse of the inner coated substrate tube by locally partial increase of the substrate tube temperature until the inner diameter is about 2 mm above the outer diameter of the exposed core rod

- Inserting the core rod into the internally coated substrate tube and fixation of the rod in the tube

Partial local heating of the second tube and collapse of the tube on the core rod to produce a second preform, fusion of the core region of the core rod with the core region of the internally coated substrate tube

- The second preform can be further processed by repeated removal of the substrate tube or the Claddingglases and jacketing with internally coated substrate tubes

- Warping the preform manufactured according to the above claims to an optical

fiber

The invention will be explained in more detail below with reference to the exemplary embodiments.

Embodiment 1

Single-mode preform production First, the thickest possible primary preform consisting of 20 cladding layers and 9 core layers is prepared according to the existing temperature conditions and taking into account a minimum core / cladding diameter ratio of about 2 (eg a primary preform with a core diameter of about 5 mm and an outside diameter of approx 19.2 mm). Such a primary preform of length 1 m, after jacketing with a Jacketingrohr on a fiber yield of about 300 km.

According to the invention, the outer glass of the Primärpreform is then removed to the core of about 5 mm diameter by wet etching in dilute HF (hydrofluoric acid) or plasma etching. In addition, a substrate tube up to a total wall thickness of about 5.5 mm only with, for example, 30 cladding layers and then coated with, for example, 10 core layers inside. However, this coated tube is not collapsed into a massive rod, but precollutated only to a capillary diameter of about 6-7 mm. Subsequently, the acidified rod is sheathed by pre-collapsed tube. In this way one obtains a primary preform which achieves a core cross-sectional area which is at least twice as large as the core cross-section of the first primary preform.

Now, this enlarged primary preform can also be etched back to the core and the resulting rod serves as the starting material for further cladding with tubes that have a deposited core and cladding region. Such a large primary preform can be encased again by means of a RIC (Rod in Cylinder) process. The fiber yield in this way reaches values of more than 1000 km per preform. If the process of removing the cladding layer from this core rod is repeated and melted into another substrate tube provided with core and cladding layers, primary preforms with extreme fiber yield are formed until well over 5000 km.

For the production of Primärpreformen, for further removal of the substrate tube, taking into account the OH diffusion during the hot temperature steps and a suitably chosen deposited Claddingdicke can be a cheaper Substrate tube with an increased OH content (up to about 10 ppm) can be used.

The production of a single-mode preform takes place in detail as follows:

Preparation of a primary preform with a core diameter of approximately 5.0 mm, a core / clad diameter ratio (CCR) of approximately 2 (cladding diameter is 2 × 5.0 mm = 10.0 mm) and a primary preform diameter of approximately 18 , 2 mm,

• This Primärpreform is acidified in 35% hydrofluoric acid up to the core diameter of 4.7 to 4.9) to mm (etch rate about 0.1 mm / h). The resulting core rod is now provided on one side with a rod-shaped handle end,

• A substrate tube is by MCVD method inside with a dependent on the quality of the substrate tube number of cladding layers, for example, 30 and additionally dependent on the limited heat input into the interior of the coated substrate tube number of cladding and core layers, in addition to the 30 cladding still 10 example Core layers, coated,

• The tube is then pre-collapsed in the MCVD system to a capillary diameter of approx. 6.5 mm,

• In the MCVD plant, the soot discharge pipe is now melted off the substrate pipe so that the substrate pipe has an opening of approx. 5 mm at this end of the pipe. Subsequently, the pre-collapsed pipe is removed from the plant with the still attached front approach pipe,

• The pre-collapsed pipe is now clamped in a vertical jacket with the melted end facing down. At the pre-collapsed tube, an extender tube with a small capillary diameter of approx. 4 mm is now set under the purging with dry oxygen at the lower melting point. At the upper end of the tube is clamped on the non-melted in the MCVD plant approach tube, • The preform acidified down to the core, fitted with an attachment rod, is inserted in the vertical jacket from above into the pre-collapsed tube until it is placed on the 4 mm capillary diameter tubing,

• After connecting a rotary feedthrough, the core rod with the jacket tube, starting from above or from below, is fused with the heat from a graphite furnace using a vacuum. The vacuum of <100 mbar is generated via the connected rotary feedthrough in the space between the core rod and the sheath tube and supports the fusion of sheath tube and core rod.

• When the graphite furnace reaches its reversal point after fusing, the furnace stops and the associated tailstock moves away from the fused preform. In this way, a clean Abschmelzende is generated.

The following process conditions are selected:

Temperature in the hot zone: about 1570

⁰ C

Oven capacity: approx. 30 kW

Fusion rate of rod and tube: 35 mm / min

Length of the travel parallel to the tube axis: approx. 800 mm

Exhaustion Example 2

Graded-index multimode preform

In the case of multimode primary preform production, the method according to exemplary embodiment 1 is modified as follows: Depending on how large the primary preform is to be, at first only a limited section of the inner overall core profile is deposited. This core area is in turn exposed by removing the outer cladding glass. In the next tube is in accordance with the technological possibilities in terms of max. achievable pipe wall thickness in turn deposited a certain annulus area of the overall profile. The tube is fused with the existing core rod. The resulting rod is now freed again up to the core area of the glass and serves as a core rod for further cladding with Profilkreisringbereichen that were deposited in other substrate tubes. In this way, both multimode step index and multimode gradient index primary preforming can be created.

All features described in the description and the following claims may be essential to the invention both individually and in any combination with one another.

Claims

claims

1. A process for the production of Glasfaserpreformen with a large core diameter comprising the following steps: a) production of Primärpreform, b) processing this Primärpreform to a smaller core diameter, c) preparation of an internally coated, provided with cladding and core layers substrate tube by MCVD method d) subsequently precollating the internally coated tube to a capillary diameter slightly greater than the diameter of the processed primary preform; e) introducing the processed primary preform into the internally coated and precollated tube; f) and finally melting the precollated tube onto the machined primary preform; these individual steps a to f can be repeated several times, in which the glass fiber preform produced in the last method step is used instead of the primary preform of method step a for the following method steps.

2. A process for the preparation of Glasfaserpreformen according to claim 1, characterized in that the substrate tube is provided with up to 30 cladding and up to 10 core layers.

3. A process for the preparation of Glasfaserpreformen according to claim 1, characterized in that the processing is carried out by removing the cladding and Gladdingglases.

4. A process for the preparation of Glasfaserpreformen according to spoke 3, characterized in that the removal is carried out by acidification.

5. A process for the preparation of Glasfaserpreformen according to claim 1, characterized in that the core diameter of the Primärpreform is set to up to 6 mm.

6. A process for the preparation of Glasfaserpreformen according to one or more of the preceding claims, characterized in that the Primärpreform is exposed to the core diameter by wet chemical etching with hydrofluoric acid or ammonium bifluorides, by plasma etching or by mechanical abrasion and the resulting core rod on one side with a stabformigen Handieende is provided.

7. A process for the preparation of Glasfaserpreformen according to claim 6, characterized in that the hydrofluoric acid is 35%.

8. A process for the preparation of Glasfaserpreformen according to spoke 1, characterized in that the substrate tube is pre-collapsed in a MCVD system to a capillary diameter, which is about 2 mm above the machined core diameter of the Primärpreform.

9. A process for the preparation of Glasfaserpreformen according to spoke

8, characterized in that of the precollated tube, the Rußaustragsrohr is melted so that an opening of at least 4 mm at the pipe end remains open and the pipe is then removed with front approach pipe from the MCVD system.

I O.Procedure for the production of Glasfaserpreformen according to spoke

9, characterized in that is attached to the precollated tube with rinsing in a vertical jacket plant at the bottom of a neck tube with a smaller capillary diameter of about 4 mm.

I 1.Procedure for the production of glass fiber preforms in accordance with

10, characterized in that the acidified to the core preform in the vertical sheathing plant in the lower Leakage pipe from purged pre-collapsed pipe is used until it is placed on the lower neck pipe.

12. A process for producing Glasfaserpreformen according to claim 7, characterized in that the central position of the core rod is fixed within the pre-collapsed pipe at the upper end of the tube and then the core rod is fused with this tube from above or from below.

13. A process for the preparation of Glasfaserpreformen according to claim

12, characterized in that a vacuum is applied to the upper or lower handler tube via a vacuum suitable rotary feedthrough to assist in the fusion of the core rod with the inside of the sheath tube

H.Verfahren for the production of Glasfaserpreformen according to claim

13, characterized in that when the furnace is annealed at the upper or lower preform end, the furnace stops and the upper and lower tailstock moves up and down, respectively, to produce a conical melting end.