The present invention relates to a reaction chamber, in particular for fabricating a preform for double index optical fibers.
It is known that double index optical fibers can be obtained in particular from a preform manufactured beforehand. This preform consists of two glasses: a core glass and a cladding glass, the refractive index of which is less than that of the core glass.
One of the difficulties in the production of double index preforms is that contamination of each of the glasses by the other glass or by external contaminants should be avoided.
To do so, it would be necessary to produce the preform in a sealed evacuated chamber after introducing the two glasses into the chamber, while operating so that they cannot be in contact with each other except at the time when the preform is actually being produced.
There are several methods for obtaining double index preforms. For example, a rotational casting method could be adopted: a molten glass is poured into a cylindrical mold which is rotated rapidly in order to produce the cladding tube, into which the core glass is then poured.
According to the so-called build-in casting method, a molten cladding glass is poured into a cylindrical mold which is inverted rapidly so as to obtain a tube, into which the molten core glass is poured. The cladding glass can also be poured around a rod of core glass (the so-called cladding over core method). Other techniques operate by suction at the lower end of the mold. It is also possible to use a technique of injecting the core glass (CIT standing for core injection technique) which consists in fabricating a cladding glass by rotational casting then immersing the lower part of the tube in a bath containing the molten core glass, while applying a pressure difference between the surface of the bath and the interior of the tube.
The methods described above make it possible to obtain preforms which can be used in the preparation of multimode fibers (the core diameter of which is relatively large).
In order to obtain single-mode fibers (the core diameter of which is relatively small, for example less than one fifth of the cladding diameter), it is possible to start with preforms obtained either by chemical vapor deposition (CVD) methods or the modified CVD method, or by the so-called rod in tube method (a rod of core glass is put inside a cladding tube, and together they are then drawn into a fiber), or alternatively by the drawing-sleeving technique (a prefabricated preform is drawn then sleeved in a cladding glass tube, which is then shrunk either before the fiber pulling or during the fiber pulling).
All these methods therefore require the prefabrication of preforms.
Among all the methods of fabricating preforms which have been explained above, none is carried out in a sealed evacuated chamber containing the two glasses of different indices. Conversely, the present invention allows such production by virtue of a chamber with a special design.
The present invention therefore relates to a reaction chamber comprising:
In particular embodiments, the reaction chamber of the invention may also have the following features, taken individually or optionally in combination:
The invention relates in particular to a reaction chamber as defined above, in which said containers and said tubes and conduit form a closed assembly containing a first glass in the first container and a second glass, of different refractive index, in the second container.
The reaction chamber of the invention may be made of any material which is compatible with the cladding and core glasses being used, and which has a melting temperature higher than the softening temperature of said glasses. The material must be sufficiently inert in chemical terms so that it does not contaminate the cladding and core glasses beyond what is acceptable. The reaction chamber of the invention may, for example, be made of silica or pyrex glass.
Silica or pyrex glass have the advantage that they behave like normal glass, and that they allow the content of the chamber to be observed directly.
The invention also relates to a method for preparing a preform for optical fibers with a cladding glass and a core glass of different indices, or a corresponding optical fiber, with the aid of a reaction chamber as defined above. This method has, in particular, the following features:
The outer second tube as described above may, for example, be used in order to introduce the cladding glass into the first container. This outer tube is then sealed at a sufficient distance from the first orifice, and can then be used as a mold for the preform.
A first tube as described above may be used in order to introduce the core glass into the second container. This tube may then be sealed in an outer region close to the wall of the first container.
The cladding and core glasses may be introduced in the form of solid particles, for example.
The method of the invention is, in particular, a method in which:
Particular embodiments of the invention will now be described in more detail with reference to the appended drawings, in which:
As can be seen in
The second container 4 communicates via an orifice 5 with one end of a conduit 6, the other end 7 of which is an open end. The end 7 is in the form of a spout oriented so that the unsolidified cladding glass coming from the centre of the tube 3 cannot obstruct or contaminate the conduit through which the core glass will be poured. A second conduit 8 communicates with the conduit 6 via one of its ends, and its other end located outside the chamber is an open end.
The reaction chamber of the invention is represented in
The cladding glass is introduced in the form of solid particles through the orifice 3b, optionally while inclining the chamber slightly by counterclockwise rotation about an axis perpendicular to the plane of the drawing, so as to prevent the cladding glass from being introduced into the conduit 6 through the orifice 7. By increasing the angle of rotation, the end 8b of the tube 8 is raised and the core glass can then be introduced through the end 8b, whereupon it falls under gravity into the second container 4. The tube 8 is provided at 8b with a tap (not shown in the drawing). This tap is closed. The chamber is evacuated by connecting the tube 3 to a vacuum pump, after which the tube 8 is sealed at 8a then the tube 3 is sealed at the level 3a, as represented in
It is of course possible to evacuate by means of another outer tube (not shown) which, for example, opens into the container 1, then seal the tubes 3 and 8 and lastly said other outer tube.
The second container 4/conduit 6 assembly may also be stiffened inside the first container 1 with the aid of stiffeners such as welded silica rods, for example joining the containers 1 and 4 or the conduit 6 and the container 1.
If such a rod extends outside the container 1, it may also be used as a handle.
In
This second position can be reached from the first position after rotation through 180° about an axis perpendicular to the plane of the drawing.
As indicated above, the configuration of the chamber is such that when moving from the first position to the second position, a liquid present in the second container will either remain confined or alternatively flow in the conduit 6, depending on the direction of rotation. This is principally due to the upper position (in the first position of
It is not necessary for the conduit 6 to have bends. Nevertheless, the presence of bends does lengthen time taken for the liquid core glass to flow to the open end 7 of the conduit 6, which allows this flow to be controlled better.
In order to prepare a preform, the cladding and core glasses are therefore introduced in a solid form, the vacuum is created, and sealing is carried out as indicated above.
The chamber is then put in an oven in the first position, which is that represented in
The tube 3 is then cooled, for example by immersing the tube in water, for a time such that only a peripheral part of the cladding glass is solidified, while the central part is still liquid. The cooling time depends on the respective thicknesses intended for the cladding and for the core part of the preform. This time will be determined beforehand by simple routine experiments. If thin cladding is desired, it may simply be left to cool for a sufficient time in air at room temperature.
Another rotation of about 180° is then carried out in order to return the chamber to the first position, this rotation preferably being carried out in the clockwise direction (although it may also be carried out in the counterclockwise direction owing to the viscosity of liquid glass, which makes it flow relatively slowly). In this new position, the unsolidified cladding glass coming from the centre of the tube 3 will flow along the walls of the container 1 and collect in the bottom of said container.
Another rotation of 180° is then carried out, this time in the clockwise direction, so that the core glass in the container 4 flows through the conduit 6, reaches the orifice 7 and then flows under the influence of the forces of gravity into the hollow part of the cladding formed in the tube 3.
During this last rotation, the excess cladding glass which has cooled during its stay in the tube 3 and during its return into the container 1 has a greatly increased viscosity, and this glass remains substantially stuck to the wall of the container 1 during the last rotation so that there is no risk that it could flow into the core of the preform.
Once the preform has been produced in this way, the entire assembly can be reintroduced into an oven for annealing, at a temperature close to the glass transition temperature of whichever out of the cladding and core glasses has the lower transition temperature, in order to reduce the internal stresses.
Single-mode or multimode optical fibers can then be prepared from the preforms obtained according to the invention, by using the known methods which will be summarized below. Particular examples of glasses which can be used as cladding or core glasses are:
It can be seen in the appended drawings that the axis of the conduit 6 lies in the plane of the drawing, which is meant to be vertical. The conduit may have a bend close to the orifice 5, for example, so that the axis of the tube is in a plane perpendicular to the plane of the drawing, for example, after this bend. It is readily apparent that the second predetermined direction of rotation as mentioned above then corresponds to rotation about a horizontal axis lying in the plane of the drawing, while the first predetermined direction of rotation may correspond to rotation about a horizontal axis perpendicular to the plane of the drawing.
Certain glasses, such as Te—As—Se glasses, can be distilled in a vacuum. In order to introduce these glasses into their respective containers, it is thus possible to operate by vacuum distillation in the tubes 3 and 8. For example, the tube 3 sealed at 3b contains at the level 3c a closure cap (not shown) through which a small distillation tube extends, the solid cladding glass being contained in the part between 3b and 3c. This part is introduced into an oven, after having evacuated the chamber. The distillate collected in the lower part of the tube 3 (
An example of producing a preform will now be given.
A silica reaction chamber similar to that in the
The container 1 has a diameter of 60 mm and a height of 60 mm. The tube 3 has a height of 190 mm, and 90 mm after sealing at the level 3a. The container 4 has a diameter of 15 mm. The outer part of the tube 8 has a length of 100 mm.
The cladding glass (25 g) has the following composition (expressed in atoms): Te2As3Se5.
The cladding glass (12 g) has the following composition: Te2.5As3Se4.5.
After introducing the glasses into their respective containers, the tube 8 is sealed at 8a, the vacuum is established via the tube 3, and the tube 3 is sealed at 3a. The chamber is heated to 500° C. in the oven for about 1 hour. The chamber is removed from the oven in the first position. At time t0, a rotation of 180° is carried out in the counterclockwise direction. The cladding glass fills the tube 3. At time t0+15 s, the tube 3 is immersed in water. The tube is removed from the water at time t0+25 s and the chamber is subjected to another rotation of 180°, in the clockwise direction. The centre of the tube 3 empties, but a solidified glass cladding remains at the inner periphery.
Another rotation of 180° is carried out at time t0+45 s, in the clockwise direction. The liquid core glass flows in the conduit 6, reaches the spout 7 and fills the hollow part of the tube 3. All of the molding is then annealed at a temperature lower than Tg (glass transition temperature). The end of the tube 3 is broken and the preform is extracted. Studying the composition along a diameter of a cross section by electron microscopy shows a constant arsenic content, with tellurium increasing and selenium decreasing in the core of the preform.
Number | Date | Country | Kind |
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0207732 | Jun 2002 | FR | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP03/50223 | 6/12/2003 | WO | 6/23/2005 |