FR2551884A1 - Process for manufacturing a monomode optical fibre having linear-polarisation constancy and implementation device - Google Patents

Abstract

The invention relates to processes for manufacturing optical fibre having linear-polarisation constancy for a guided lightwave, of the type comprising a step for forming a preform by necking-down (diameter reduction) from a tube 1 inside which are deposited silica and dopants intended to form core and cladding regions of the optical fibre after drawing down the preform. According to the invention, before necking down (diameter reduction), a partial vacuum is created inside the tube 1, with the aid of a member 26 for generating a partial vacuum using the Venturi effect, and the dopants are selected so that the expansion coefficients and softening temperatures, respectively of the core and claddings, are in predetermined ratios so as to obtain core and cladding sections of various shapes.

Description

PROCESS FOR PRODUCING A FIBER
OPTICAL MONOMODE HAVING LINEAR POLARIZATION
AND DEVICE FOR IMPLEMENTING.

 The present invention relates to a method of manufacturing monomode optical fibers with linear polarization maintenance as well as to the device allowing the implementation of such a method.

 A process for manufacturing optical fibers is known by the abbreviations "MCVI: 3" and the English expression "modified chemical vapor deposition" which can be translated by a modified method of chemical deposition by gaseous means). This method consists in circulating inside a quartz tube or other equivalent refractive material a gaseous mixture comprising, in particular, oxygen and silicon compounds of germanium or boron, the latter being introduced into the inside the tube according to predefined sequences. The tube is heated using a heat source that is moved in a longitudinal direction to this tube. The cycle is repeated several times. At each passage in the heated zones there is carried out a redox reaction of the components in the presence and deposition of silicon oxides, boron or germanium on the inner wall. At each passage of the tube in front of the heat source, a new layer is deposited. The last phase of the process consists in heating very strongly the quartz tube, it follows a contraction or shrinkage and one obtains a preform. The product obtained by this process is also in the form of a bar diameter of a little smaller than the internal diameter of the silica tube. This preform then undergoes a subsequent drawing phase and at the end of this phase, the actual optical fiber is obtained.

 This method makes it possible to obtain very high performance fibers, especially monomode fibers, used for the digital transmission of high rate information or for the manufacture of optical devices such as ring interferometers.

 Such fibers have a low attenuation (typically less than 0.5 dB / km for a wavelength centered on 1.3 micrometers) and a large bandwidth (typically greater than 1 GHz.km).

 Among the opto-geometric parameters associated with these fibers, an important parameter is the maintenance of the linear polarization of a polarized light wave during its propagation along the optical fiber. The quality of polarization maintenance is reflected, for example, by a measurable optogeometric parameter associated with optical fibers called "beat length".

 It is known that monomode fibers of cylindrical symmetry structure propagate two "degenerate" orthogonal linear polarization states. For some applications such as, for example, that the use of these optical fibers in the production of sensors or ring interferometers, it is necessary to lift this degeneracy by creating a difference in propagation speed between these two polarization states.

 This can be achieved by introducing nonhomogeneous effects of geometry or various constraints. Usually, in the known art, techniques are used such as bar material supply, chemical etching, flame hydrolysis or machining of the preform.

 These techniques are difficult to implement, require additional manipulations of the preform and the progress of the steps of the manufacturing process departs significantly from that of the conventional method "MCVD1, which has just been briefly recalled.

 The aim of the invention is to overcome the drawbacks of the prior art and proposes a process for manufacturing optical fibers with linear polarization maintenance that is simple to implement, inexpensive as regards the apparatus, because it does not require significant modifications of conventional equipment, as it relates to the process itself, because it does not require disassembly and manipulation of the preform, etching or doping materials outside conventional ranges.

 In order to obtain the preservation of the linear polarization, non-homogeneous stresses in a plane of section orthogonal to the axis of propagation of the guided waves are created, according to the method of the invention, ie axis of symmetry of the core region of the optical fiber.

 To do this, the respective geometries of the core and optical cladding sections are modified in a controlled manner.

The subject of the invention is therefore a method for manufacturing a monomode optical fiber with linear polarization conservation of a light wave guided by this optical fiber, comprising:
a first step of depositing oxides inside a quartz tube with inner and outer circularly rotating sections along which is moved a heat source and in which are sent halides and oxygen, the tube being heated to a first temperature by this heat source; this step comprising two phases, a phase of deposition of a sheath and a deposition phase of a core;
a second step of shrinking of the tube obtained by increasing the temperature of the tube through the heat source until a bar or preform is obtained;
a third stretching step of this preform to obtain a fiber of homothetic dimension from those of the preform comprising a central core region surrounded by at least one region forming a sheath of different dopings;
characterized in that it comprises an additional step performed between the first and the second step during which one end of the tube is closed a depression relative to the pressure prevailing in the medium outside the tube is established within this tube and in that the dopings of the materials constituting the sheath and core regions are selected so that the softening temperatures and expansion coefficients of these materials are in pre-established ratios and, in addition, the amplitude of said depressing is adjusted to a calibrated value, so as to obtain in the third step an optical fiber whose respective sections of core and sheath have determined geometries.

 It also relates to a device for implementing such a method.

The invention will be better understood and other characteristics will appear by means of the description which follows, with reference to the figures which accompany it, among which:
- Figures 1 to 3 illustrate various aspects of the method and apparatus of the prior art.

 - Figure 4 schematically illustrates a device for implementing the method of the invention.

 FIG. 5 illustrates an element of the device of the invention.

 FIGS. 6 to 9 show different variants of optical fibers obtained by the method of the invention.

 First of all, it is useful to recall the main characteristics of the "MCVD" technique used in the manufacture of the most common optical fibers.

 The "MCVD" technique is schematically illustrated in FIG.

A quartz tube 1 or similar material is mounted between the two mandrels of a glass lathe (not shown). Along this rotating tube 1 moves the flame of an oxyhydrogenated torch at a sufficient temperature (typically 1600 ° C to 1900 ° C) to allow the reaction of halides such as silicon tetrachloride (SiC 14) with oxytrichloride. phosphorus oxide (POC 13) or silicon tetrafluoride (5iF4) for the cladding layer; and silicon tetrachloride (SiC 14) and germanium tetrachloride (GeC14) for the core layer, with oxygen to give oxides of silicon (SiO 2), germanium (GeO 2), phosphorus (P 2 O 5) or boron (B203).

 These oxides are deposited on the inner surface of the tube to form a transparent glass layer. The different layers are deposited at each passage of the torch. The index of refraction is controlled by acting on the concentration of the dopants with each pass of the torch When the thickness of the desired deposit is obtained on the inner surface of the tube, the temperature of the torch is greatly increased (typically 2200tC) to shrink the tube and close it to give a bar which is called a preform shown schematically in FIG. 2 with a core 6 and an optical cladding 5.

 Instead of the torch any other suitable source of heat may be used.

 The preform is then drawn into a high purity graphite furnace to obtain a fiber which is wound on a drum. Since the length of the fiber is related to the geometrical parameters of the preform and to the outside diameter of the desired fiber, lengths of one to several kilometers can be obtained.

 A glass lathe is illustrated schematically in FIG. 3. It comprises a support plate 10 for the torch 2 which can move along a merle screw 14, this displacement between two stops 12 and 13 being obtained by a motor 15.

 The rotating quartz tube 1 is held by two mandrels 16, 17 in rotation.

 The method of the invention comprises most of the steps of the conventional "MCVD" method. However, during the shrinkage step and according to one of the essential characteristics, it is created a damplitude depression calibrated in the tube. In addition, the doping materials are selected so that, by one pc t, the ratio of the refractive indices of the core and optical cladding regions are such that the guiding conditions are satisfied, which is common to the known art, but also, and according to a second essential aspect of the invention that the ratios between the coefficients of expansion, on the one hand, and the softening temperatures of these regions, on the other hand, are equal to pre-established values.

As a function of these choices and the amplitude of the depression, the sections of the core and optical cladding regions of the optical fibers obtained after drawing are going to follow various geometrical and indiidually controllable shapes as will be described in detail below.
The amplitude depression calibrated during the preform necking step is created according to another aspect of the invention using the effect known as the Venturi effect.

 Figure 4 schematically illustrates a device for carrying out the method of the invention.

 This device comprises a device 24 for implementing the method 9'MCVn'4 as illustrated in FIG. 3 It further comprises a manometer 21, a 2D flowmeter, three valves EV1, EV2, F to enable or prohibit the gas circulation.

 It comprises a first channelization 27 for introducing the gases into the quartz tube comprising a first valve EV2, a second oxygen circulation pipe 28, for example, comprising a second valve EV1 which is connected to the first pipe 27 at its first end by which are introduced the gases. A third pipe 29 comprising a third valve EV3 is connected to these first two pipes at their second ends. A flow meter 20 makes it possible to know the flow of gas in the second pipe. A pressure gauge 21 makes it possible to know the pressure at the point P downstream of the junction between the pipes 28 and 29, ie the calibrated amplitude of the depression created before necking.

According to one of the important points of the invention, a depressurizing member 26 of the Venturi effect type is inserted on the pipe 28 upstream of the point
P, and forms the junction between the pipes 28 and 29.

 The Venturi effect is relative to the flow of a gas for example in a pipe of variable section whose axis is horizontal. In this case, the pressure of the gas is lower where the section of the tube is the smallest and where the speed is the largest, which allows to obtain the desired depression.

 An example of a deprimogenic member structure 26 is shown in FIG.

 The flow of gas flowing through the second duct 28 enters a "tee" via a funnel-shaped duct 42 which makes it possible to create a vacuum in the tubes 29 and 1 by suction 31 of the gases internal to this tube by the flow The large opening 43 of the funnel is disposed on the junction side with the pipe 28 and the small 44 at the junctions of the pipes 29 and 32, preferably arranged at right angles.

 The gas flowing in this second pipe may be oxygen but other gases may be used. One plays only on the pressure in this pipe to create a depression, via the tube 29, in the tube 1, which was previously closed at its other end.

 The resulting gas stream 25 is ejected by a third pipe 32.

 The process of the invention repeats the steps of the MCVD process.

 Thus, firstly there is deposition of oxides inside a tube 1 of quartz by external heating with the aid of a torch 2 which moves along this tube 1. At each passage a layer typical thickness of 10 microns is deposited.

 The temperature obtained at the surface of the tube is of the order of 16000 ° C. to 1900 ° C. There is successively a phase of deposition of sheath and then a phase of deposition of the heart.There is then a step of shrinking which consists of bringing back the tube in the form of a bar by heating at a temperature of the order of 22000C with the aid of the torch which moves along this tube.

There is thus a decrease in the thickness of the tube due to heating.

 According to one of the features of the method of the invention, before complete shrinkage of the tube, it is closed at one end and a depression inside this closed tube is created at the other end.

 During the deposition phase in the tube 1, the valve EV2 is open, and the valves EV1 and EV3 are closed. There is then circulation of gas and oxygen inside the quartz tube. At the moment of shrinkage under vacuum, EV2 is closed, the valves EV1 and EV3 are open, which causes the suction of the gases contained in the tube 1 via the tube 29 by the pressure of an oxygen stream 31.

 According to the desired shapes of the sections of the core and sheath regions of the optical fiber obtained after the final drawing step, the magnitude of the depression is adjusted to a predetermined value as will be described in the following.

 It also depends on the geometry of the starting tube, the exact composition of the glass and the speed of movement of the torch.

 This value can be determined by calculation or experimentation.

 Examples of embodiments of optical fibers of various configurations will now be detailed in connection with FIGS. 6 to 9.

 Figures 6 and 7 show optical fibers whose hearts have sections, respectively elliptical and approximately rectangular.

 To obtain c @ this type of optical fiber geometries, it is necessary that the coefficients of expansion and the softening temperatures of the core regions are respectively higher and lower than those of the cladding regions.

 In addition, the depression created inside the tube 1 before necking is of low amplitude to obtain a core section having the general shape of an ellipse (FIG. 6) and of greater amplitude to obtain a more flat heart shape. , ie of approximately rectangular shape of a ribbon according to Paxe propagation of guided waves (Figure 7).

To fix ideas, the heart regions (respectively Fig.6: 66 and
Fig.7: 76) were obtained by doping silica (SiO 2) with germanium dioxide (Ge 02) such that the concentration (Ce 02hui 2) is greater than 0.30.

 The vacuum with respect to the ambient pressure external to the tube 1 should be of the order of one hundred Pascal in the case of Figure 6 and 1000 Pa in the case of Figure 7.

 The sheaths, respectively Fig.6: 65 and Fig.7: 75, are sparingly doped with boron oxide, fluorine oxide or with a mixture of fluorine and phosphorus oxides or boron and phosphorus , so that the refractive index of the sheaths is less than 3.10-3.

 In addition, it may be provided an intermediate optical sheath (Fig.7: 750) doped for example with fluorine for the transmission of higher wavelengths, ie in the infra-red range: 1.3 micrometer for example.

An optical fiber made in the manner just recalled and having an elliptical core section <FIG. 6) had the following characteristics:
- beat length measured at a wavelength of 0.63 micrometers: 14mm;
attenuation at a wavelength of 0.84 micrometers: 4 dB / km and at a wavelength of 1.06 micrometers: 1.5 dB / km;
- ellipticity of the heart section: 0.65
- differences in refractive indices between the core and sheath regions: 9.5 × 3 and between the cladding regions and the 1: 1.6 tube
If an intermediate sheath (Fig.7: 750) is provided, the materials constituting this sheath have, with respect to the softening temperature and the softening coefficient, the same characteristics as those of the core.

 Figures 8 and 9 show optical fibers in which the hearts have retained a circular or quasi-circular shape.

The softening temperature and the coefficient of expansion of the materials constituting the cladding regions (respectively Fig.8: 85 and Fig.9: 95) must in this case be, respectively, lower and higher than those of the materials constituting the regions of the cladding regions. heart (respectively
Fig.8: 86 and Fig.9: 96). This allows a depression of very low amplification
of de, typically of the order of 30 Pa. This has the consequence that the core region undergoes little deformation and its section remains circular or quasicircular after necking.

On the other hand, if one increases the amplitude of the depression, one can obtain, at the same time, a section of heart of elliptic type (Fig.4: 95) or
even flattened (not shown), and an elliptical sheath section.

To fix ideas, the heart region can be weakly doped with germanium and be surrounded by a strongly sheath region.
doped with boron dioxide or with a mixture of fluorine oxides and
phosphorus. under these conditions, the difference in refractive index between
Heart and sheath regions are typically greater than 5.10-3.

For the highest wavelengths, in the infra-red range, it is
may be necessary to provide, as in the case of Figure 7, a sheath
intermediate optical, doped with fluorine oxide for example.

A fiber made under the conditions which have just been described and of the type described with reference to FIG. 8 has the following characteristics:
beat length measured at 0.63 micro wavelength
meters: 6 mm and at a wavelength of 0.84 micrometers 8 mm;
attenuation at a wavelength of 0.84 micrometers: 6 dB / km;
- The optical sheath rellipticity was equal to 0.2G;
the coefficients of expansion of the sheath and core materials were respectively 1.4 × 10 -6 and 0.5 ×
the softening temperatures of these materials were,
respectively, 6000C and 10000C approximately.

It can therefore be seen that optical fibers can be obtained whose
sections of hearts and / or ducts are adjustable at will, this
which creates anisotropies of geometry and allows to obtain a conservation
linear polarization.

 As has been recalled, the method has the advantage of being simple to implement and inexpensive because it allows to use without significant mdification existing equipment and deviates little steps of the processes of the Known Art, in particular of the "MCVD" process.

 In addition, it does not require disassembly or special handling of the preform, which could lead to a risk of pollution.

 The nature of the gases used and the doping ranges of the materials do not differ from those known. Only particular choices must be made, also taking into account the depression made before necking, according to the basic characteristic of the invention, to obtain the desired final geometry.

 The use of the Venturi effect, according to another characteristic of the invention, to obtain the depression in the tube before shrinking, allows easy and precise control of ram plltud e, the accuracy obtained being of the order of 10 Pa.

 Finally, as experience has shown, this process allows the manufacture of optical fibers with a short beat length and low attenuation.

Claims (9)

 A method for manufacturing a monomode optical fiber with linear polarization conservation of a light wave guided by this optical fiber comprising:  a first step of depositing oxides inside a quartz tube (1) with inner and outer circularly rotating sections along which is moved a heat source (2), and in which are sent halides and oxygen, the tube being heated to a first temperature by this heat source; this step comprising two phases, a phase of deposition of a sheath and a phase of deposition of uncoeur;  - A second necking step of the tube (1) obtained by increasing the temperature of the tube (1) gracie to the heat source (2) until a bar or preform is obtained;  a third stretching step of this preform to obtain a fiber of dimension homothétiue those of the preform comprising a core central region (6) surrounded said at least a cladding region (5) of different dopings; characterized in that it comprises an additional gas step carried out between the first and the second step during which one of the end of the tube (1) is closed, a depression with respect to the pressure prevailing in the medium outside the tube ( I) is established inside this tube and in that the dopings of the materials constituting the sheath (5) and core (6) regions are selected so that the softening temperatures and the expansion coefficients of these materials are in pre-established ratios and, in addition, the amplitude of said depression is adjusted to a calibrated value, so as to obtain in the third step an optical fiber whose respective sections of core (6) and sheath (5) have certain geometries.  2. Method according to claim 1, characterized in that the softening temperature and the coefficient of expansion of the materials constituting the core region are, respectively, less than the softening temperature and greater than the coefficient of expansion of the materials constituting the sheath surrounding the core to obtain a circular cladding section optical fiber (6fui, 75) and an elliptical (66) or rectangular (76) core section.  3. Method according to claim 1 characterized in that the softening temperature and the expansion coefficient of the materials constituting the core region are, respectively, less than the softening temperature and greater than the coefficient of expansion of the materials constituting the sheath region. surrounding the core (86,96) so as to obtain an optical fiber of which at least the sheath (85,95) has an elliptical section.  4. Method according to any one of claims 1 to 3 characterized in that the core region (6) is constituted by silica doped with germanium dioxide, the concentration of germanium oxide being greater than 30%; and in that the cladding region (5) is silica doped with an oxide selected from the following: boron oxide or fluorine oxide or a mixture of fluorine and phosphorus oxides or boron and phosphorus.  5. Method according to any one of revendicatiopns 1 to 3 characterized in that said depression has an amplitude in the range 100 to 1000 Pa.  6. Device for implementing the method according to any one of claims 1 to 5, comprising:  - a lead screw (14) actuated by a motor (15)  - a torch (11) supported by this screw moving along those in one direction and then in the other between two stops (12,13)  - two mandrels (16,17) in rotation, holding the tube above the torch and rotating it,  characterized in that it further comprises a first pipe (27) for introducing the gases into the tube (1) on which is positioned a first valve (EV2), a second pipe (28) for circulating these gases on which is positioned a second valve (EV1), connected to the first pipe (27) at its first end through which the gases are introduced into the pipe (1), a third pipe (29) on which is positioned a third valve (EV3) connecting said first two pipes (27,28) at their second ends and a venturi effect inducing member (26) joining the second (28) and third (29) pipes creating said depression during said further step in the tube (1) via the third pipe (29).  7. Device according to claim 6 characterized in that said pressure-reducing member (26) consists of a three-branched "Tee" shaped end piece comprising a funnel-shaped wall element (42) having two ends, a large (43) and a small (44), disposed within a first branch of the "T" "connected to the second pipe (28), the small end (44) of the funnel being directed towards a branch "Tee" connected to the third pipe (29) and at a right angle with said first leg (28) and in that the third leg is connected to an additional suction pipe (32) for evacuating gases (25), during the additional step, via the third pipe (29), the gases contained in the tube (1) and creating said depression  8. Device according to claim G characterized in that a manf meter (21) is coupled to the third pipe (29) for measuring said depression.  9. Device according to claim 6, characterized in that a flowmeter (20) is positioned in series with the second valve (EV1) on the second pipe (28).