FR2489957A1 - Optical measuring instrument for fibre=optic - passes collimated light through fibre to determine geometrical characteristics of core and covering layer - Google Patents

Abstract

A sample of silica rod is positioned in the bottom of a tank filled with a liquid whose refractive index is close to that of the rod. The tank base is transparent and luminous rays entering keep to their path. The rod is illuminated by a collimated beam of light from an iodine lamp whose output passes through a condensing lens and diaphragm to a collimating lens. A mirror limits the size of the equipment and directs the beam through a polariser into the tank. The polariser output is linearly polarised parallel to the optical axis of an analyser. A lens projects an image onto a ground glass plate where a series of light and shaded zones are observed permitting measurement of the core and external diameters of the rod.

Description

 The present invention relates to a device for measuring the geometrical characteristics of a cylindrical bar made of refringent materials and, in particular, to the continuous measurement of the geometrical characteristics of a preform used for the manufacture of optical fibers, in order to perform sorting against pre-established standards.

 A process for manufacturing optical fibers is known by the abbreviations "MCVD" (from the English expression: modified chemical vapor-phase deposition "which can be translated by a modified chemical deposition process by gas). circulating inside a silica tube a gaseous mixture comprising in particular oxygen and silicon, germanium or boron compounds, the latter being introduced inside the tube according to predetermined sequences. tube is heated using a heat source which is moved in a longitudinal direction to this tube. The cycle is repeated several times. Each time in the heated areas, an oxide reaction is carried out reduction components in the presence and deposition of oxides of silicon, boron or germanium on the internal wall. Each time the tube passes in front of the heat source, a new layer is deposited. The last phase of the process consists in heating very strongly. nt the silica tube, it follows a contraction, we get a preform. The product obtained by this process is also in the form of a bar with a diameter slightly less 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. To hope for obtaining a very good quality fiber, it is necessary, on the one hand, that the silica tube inside which successive deposits are made to obtain the preform, which is also very homogeneous in its geometric characteristics. It is necessary to control with great precision its inside diameter, the thickness of its walls and its outside diameter in particular. A method and a device for measuring these characteristics have been proposed, in particular, in French patent application NO 80. 09 111, filed on April 23, 1980.

 I1 is necessary, on the other hand, that the geometric dimensions of the preform are known with precision and that the homogeneity of these dimensions is preserved over the entire length of the preform.

Different measurement methods have been proposed in the past. Non-limiting examples that may be mentioned include the methods based on lateral interferometry or other methods involving interference. An example of such a method is described in Partiale de IGA et al, published in "Applied Optic", Vol. 16 (1977), page 1305. The so-called "back-scattering method" (according to the English expression commonly used) can also be implemented. An example is given in the CHU article: "Nondestructive Measurement of Index Profile of an
Optical - Fiber Preform "published in the British magazine:" Electronics
Letters ", vol. 13, no 24, November 24, 1977, pages 736 to 738.

 These methods are relatively complex to implement and some require the compulsory use of electronic computers, this is notably the case of the method described in the above-mentioned article by CHU.

 The invention, on the contrary, proposes a simple device, allowing continuous measurements so as to perform a sorting with respect to pre-established tolerances and using only elements well known in the field of optics and generally inexpensive.

 The subject of the invention is a device for measuring the geometrical characteristics of a cylindrical bar comprising at least one central region surrounded by a peripheral region made of refractive materials with optical indices of different refractions; device mainly characterized in that it comprises a light energy source producing a beam of parallel rays with homogeneous energy distribution, of section at least equal to the section of the bar, illuminating a tank of elongated shape in a direction orthogonal to the direction beam propulsion and intended to receive the bar, transparent to the rays in at least predetermined zones allowing the crossing of the rays as well as the complete illumination of the bar, and filled with a liquid of optical refractive index substantially equal to that of the peripheral region of the bar and immersing it, and optical means comprising a polarizing element arranged upstream of the vessel on the optical path of the beam so as to linearly polarize it and an analyzer element disposed downstream of the vessel on the optical path of the beam whose direction of polarization is orthogonal to the direction of polarization of the rays leaving the po and in that, the rays emerging from the analyzer after crossing the bar have a non-homogeneous energy distribution due to the induced optical anisotropies, the measurement of the geometric characteristics is carried out using means making it possible to evaluate the difference between two extrema of the spatial distribution curve of the energy of the beam emerging from the analyzer element in a determined plane orthogonal to the direction of propagation of the beam.

The invention will be better understood and other advantages will appear from the following description, with reference to the appended figures:
- Figures 1 and 2 schematically illustrate two stages of the manufacturing process of a preform used for obtaining optical fibers, using a silica tube;
- Figures 3 to 5 are explanatory diagrams of the phenomena involved in the device of the invention;
- Figure 6 is an optical diagram illustrating the path of light rays in a device of the invention;
- Figure 7 is a concrete embodiment of a device according to the invention.

Before describing the measurement method according to the invention, it is useful to briefly recall the process of manufacturing an optical fiber preform using a silica tube. Figure I schematically illustrates a device for this manufacture. The device essentially comprises a silica tube 10 ′ set in relative motion relative to a heat source, for example of the blowtorch type. In this tube is introduced by one end a gas mixture G which circulates inside in a direction essentially parallel to the axis of symmetry of the tube
Z. The gas mixture emerges from the other end (not shown) of the tube. The gas mixture G comprises, on the one hand, oxygen and, on the other hand, other gaseous compounds such as silicon, boron or germanium chlorides. According to the so-called "MCVD" process which has been recalled, in the zones subjected to the heat source, redox reactions occur between the various components present in the gas mixture G. There follows a deposit in a thin layer llt on the internal face of the tube 10 'of the oxides formed. The tube 10 'is set in relative motion with respect to the heat source. Each pass creates a layer of about 5 µm. About 70 to 80 successive cycles are carried out. At the end of the process, a tube with an outside diameter equal to the inside diameter of the silica tube and a thickness equal to 10 llm m multiplied by the number of cycles is obtained. At this stage, there is already a first cause of inhomogeneity in the geometric characteristics of the preform which will be obtained in a later phase.

 The diameter of the silica tube varies near the ends and the thickness of each deposited layer also varies. It has a smaller thickness: about 7 llm m instead of 10 Um. This part should be eliminated.

On the other hand along the tube, the deposits are not carried out in a completely homogeneous manner and the resulting layer has undulations around the abovementioned average value of 10 1 lm.

 A final step is then carried out which consists in very strongly heating the silica tube. There follows a contraction (more commonly known by the Anglo-Saxon expression "collapsing"). The product obtained, illustrated in FIG. 2, is called a preform and is in the form of a bar 1, with external dimensions typically of the order of 9 mm. This preform will be used for the manufacture of the actual optical fibers, which will be obtained by drawing this preform.

After this final phase, the fiber will have an outside diameter of approximately 125 µm. As is known, an optical fiber comprises a core and an optical cladding. The core and the sheath are essentially differentiated by their optical refractive index. The evolution of this optical refraction index must be carried out according to a determined profile. To achieve this result, it is necessary to deposit on the internal face of the tube 1 ′ different materials so that the optical refractive index varies according to a profile homothetic to that desired for the optical fiber. By way of example, the average refractive index being that of silica, that is to say of the tube 1, if it is desired to increase the refractive index, germanium dioxide can be introduced in a determined percentage.

 These two regions, core and optical sheath, are represented by the region represented in FIG. 2 by the unique reference il. After drawing, the optical fiber will most often consist of these two regions, a support tube formed by the material of region 10 (after drawing) and an external protective sheath disposed around a support tube in a phase of subsequent manufacturing.

There are two types of fibers:
- optical fibers of the so-called multimode type, most often with index gradient, for which the core diameter is typically 50 μm and the external diameter of the optical cladding is 70 μm;
- Fibers of the so-called single-mode type, most often index jump, for which the core diameter is between 5 and 10 F.tm and the outside diameter of the optical cladding is approximately 40 μm.

 The outside diameter of the support tube, most often made of pure silica, is around 125 μm and the outside diameter of the protective sheath, made of silicone for example, is of the order of 300 μm.

 Figures 3 and 4 show the optical refractive index profiles n, along an axis passing through the axis of symmetry Z of preforms, of two types of preforms used respectively for the manufacture of optical fibers jumping index (Figure 3) and index gradient (Figure 4).

 In the latter case the refractive index varies according to a parabolic law in the region of the heart.

 The appearance of optical fibers made of silica / doped silica has made it possible to reduce the optical transmission losses of optical fibers in large proportions. At present fibers having losses of less than 0.5 dB per km can be produced. It is necessary, to obtain such performance, that the fibers are very homogeneous. In particular, to obtain a coupling between two fibers having low losses, the geometric characteristics of these two fibers must be very close.

It is indeed necessary that the thickness of the sheaths and the diameters of the respective hearts are identical. It is therefore necessary to control with great precision the geometric characteristics of the tube 10 '(Figure 1).

 It is also necessary to carry out a sufficient number of measurements to ensure the homogeneity of these geometrical characteristics along the silica tube. A measurement method and a device for implementing the method are described in the aforementioned French patent application NO 80.09111.

 According to the method of this patent application, the variations in the outside diameter of the tube 1 ′ are measured using two mechanical feelers, arranged on either side of the tube on an axis passing through the axis of symmetry 2 of the tube, and variations in the thickness of the tube are measured optically by focusing a laser beam using a cylindrical lens within the wall of the tube; the two beams due to the reflections, respectively on the internal face and the external face of the tube of the focused beam, are refocused on a strip of optoelectronic detectors and the distance separating the focal points of these two beams being directly function of the thickness of the wall. This thickness can be determined by counting the number of photodiodes separating these focal points using a counting circuit, and a digital-to-analog conversion. The measurements are made on a helix with constant pitch and the same axis of symmetry as the axis of symmetry Z of the tube 1 '. Other methods can be implemented.

 To achieve the desired result, it is also necessary to start from a very homogeneous preform when these geometrical characteristics and within very tight tolerances. In particular, the relationship between the diameter of the core of the preform and the outside diameter of this preform must be included within pre-established tolerances. If we refer again to Figures 3 and 4, we must know the radii of circles a and c to calculate this ratio.

 As has been recalled, various methods and devices have been proposed in the past for carrying out these measurements, these methods being for the most part very complex and time-consuming to implement, some necessarily using expensive equipment, such as electronic computers.

The invention, on the contrary, according to a preferred variant, proposes a device requiring only a simple apparatus, performing continuous measurements of the aforementioned parameters to allow sorting and rejection of the
non-standard preforms, which would a priori lead to obtaining fibers
defective after wire drawing.

To do this, the device of the invention uses a physical phenomenon
known as "photo-elastic effect". As has been recalled, for
obtain a preform, the silica tube (figure 1:10 ') is subjected to high temperatures. This results in residual elastic stresses within the material, when the preform is brought to room temperature.

The importance of these residual stresses depends in particular on the difference between the coefficients of thermal expansion of the materials constituting the core, the sheath and the support tube, as well as on the temperature differences between these regions. These constraints induce anisotropies in the optical characteristics of the materials forming different regions of the preform.

 As illustrated in FIG. 5, at any point P, inside the preform, relative to a reference trihedron O, X, Y, Z whose axis Z coincides with the axis of symmetry of the preform, these constraints are entirely determined by a set of three vectors: aQ, ar and az; O being the angle made by the axis joining point O to point P, with the reference axis X. These vectors can be obtained by calculation or measured, and depend in particular on the type of preform considered: index or index gradient In all cases, there is discontinuity in the curves representing the variations of these vectors during the passage from one region to another; the borders being constituted by the circles of radii a, b and c in FIG. 5.

 The photo-elastic effect, due to the electrical anisotropies induced by the residual stresses, manifests itself by the fact that at any point P, an incident light ray propagating along axis 1, for example parallel to the reference axis Y, is divided into two waves linearly polarized and whose polarization directions are perpendicular to the direction of incidence. If we consider a propagation of the two waves over a distance dl, there occurs between these two waves a phase delay equal to dt, this delay can be calculated from the three vectors ac->, ar and az. overall between the two waves between the point of incidence of a ray entering the preform and the point of emergence of this ray can be obtained by integration. The calculation shows that the two components due to the vectors ar and a0 cancel each other out and that only the axial stress vector #z influences the optical delay, this in the context of the example chosen: axis 1 was perpendicular to the reference axis Z. It also follows that the gradient of d has discontinuities at the borders of the different regions, like the vector #z. The invention takes advantage of these effects.

 Figure 6 schematically illustrates a device according to the invention. The device comprises a tank 3 on the bottom of which the preform 1 has been placed. The bottom 30, transparent, consists of a plate of silica or glass of optical quality. The interior of the tank is filled with an index liquid close to that of silica so that a light ray entering the fiber is not refracted but keeps its original direction. It can be, for example, glycerin or eucalyptol.

The preform is illuminated by a collimated beam of axis light
Y perpendicular to the axis of symmetry of the preform. On either side of the tank, on the beam path, are arranged respectively, a polarizer 4 and an analyzer 5. The optical axis of the polarizer is orthogonal to that of the analyzer, the collimated beam leaving the polarizer linearly polarized in a direction parallel to the optical axis of the analyzer.

For a radius of the collimated beam leaving the analyzer and entering the preform, according to what has just been recalled, its direction of propagation being parallel to the reference axis Y as in the case of FIG. 5, the magnitude of the optical delay due to the photo elastic effect depends on the distance between the axis of propagation of this ray and the reference axis
Y. If the optical intensity of this ray is lo at penetration into the preform, # we show that the intensity at emergence I becomes: I = I0 Cos (α#(2); formula in which a is the angle formed between them by the optical axes of the polarizer and the analyzer and T the optical delay due to the photoelastic effect.

It follows that the curve described by the intensity I passes through zero for all values of T satisfying the relation: (a + = (2n + 1) 2 in which n is an integer which can take the value zero. a = s i.e. if the optical axes of the polarizer and the analyzer respectively are orthogonal to each other, the intensities of the rays located at distances from the reference axis Y passing through the center of symmetry of the preform equal at the radii a, b and c are zero according to the calculation In practice the curve representing the optical intensity passes through minima All devices of the polaroid film type or prism polarizer / analyzer can be used to constitute the polarizer or the analyzer.The preform having only an outside diameter of the order of a centimeter, to better observe the phenomenon, the beam emerging from the analyzer is focused using a lens represented by the lens L2 and projected on a plane P. This objective is corrected to avoid ute distortion. As an example, a growth objective
G = 5 can be used. The axis of symmetry of the preform must be located at a distance such that it is optically conjugated with the plane P. In this plane can be placed a frosted glass 6. We can then observe by transparency a series of clear zones and dark areas, the latter having the form of a strip of small thicknesses. The differences between these dark areas of the optical axis of symmetry there are directly proportional to the radii of the circles a, b and c; the proportionality factor being equal to the magnification G.

 The light energy source 2 producing the collimated beam can consist, by way of nonlimiting example, of a conventional lamp 20, for example with iodine, a CO condenser focusing the beam emitted by the lamp 20 on a diaphragm 21 and a collimating lens L1. The opening of the diaphragm to a diameter of the order of 10-2 mm. It is also possible to use a monochromatic lamp: with mercury or sodium vapors. These devices make it possible to avoid the use of a more expensive laser type source. For practical reasons, for example to limit the size of the device, one or more deflection mirrors, such as the mirror M, can be used.

FIG. 7 shows a concrete example of embodiment of the device according to the invention. With the exception of the tank 3, the optical elements described in relation to FIG. 6 are integral with a movable carriage 7. This carriage has openings made in its sides so as to insert two sleeves 72, 73 and through the opening 70, the tank 3. The device comprises a chassis 8. The tank 3 and two rods on which the sleeves are threaded are made integral with the chassis. The mobile carriage can move between the two ends of the chassis in a direction
Z 'parallel to the axis of symmetry of the bar Z.

 In a first variant, not shown, the carriage can be moved by simple sliding on the rods 80 and 81, the drive being carried out by hand. In the variant shown a drive motor 82 is provided. This motor rotates the rod 80 which is constituted by an endless screw. The sleeve 72 has a profile complementary to that of the worm. Stops can be provided on the sides of the chassis to automatically interrupt the power supply to the motor at the end of the stroke or to reverse the direction of travel of this motor.

 The mobile carriage has a window 71 on one of these faces so as to position the frosted glass plate 6. The inclination of this face is chosen so as to facilitate the observation of the images projected on the frosted glass 6.

 This frosted glass may include a graduated scale 60 making it possible to determine the diameter of the heart (2 x a) and the outside diameter of the preform (2 x c) by observing the position of the corresponding dark lines relative to the graduated scale. This scale can be replaced by simple marks between which the dark bands must be.

For example, for the diameter of the heart, two pairs of marks are inscribed on the frosted glass representing respectively the values (2a + A a) and (2a
A a) maximum and minimum tolerances within which the value of the diameter of the core must lie over the entire length of the preform.

The outside diameter c of the preform is determined by the same method.

 The ratio (a / c) can be calculated simply if the scale 60 is a graduated scale. From the knowledge of these data, it is possible to sort the preforms before drawing and reject the preforms out of tolerance.

 The invention is not limited to only the exemplary embodiments which have been described by way of illustration. In particular, it is possible to fully automate the measurement of diameter measurements. To do this, in a variant not shown of the device of FIG. 7, the frosted glass plate in the plane 6 can be replaced by a strip of photodiodes or any other optoelectronic detectors, for example a television camera. This bar is arranged in the plane 6 along an axis perpendicular to the axis A, that is to say to the axis of symmetry of the preform.

 An electronic scan, for example, of the outputs of these detectors using appropriate electronic circuits provides signals by sampling whose amplitudes are proportional in all points to the optical intensity of the rays striking the detectors. We can deduce from the position in plane 6 of the minima of intensity detected, the diameters (2 x a) and (2 x c) sought. These signals can be transmitted in analog or digital form, either to display devices such as a cathode ray tube with memory or plotter, or to calculation devices which compare them to pre-established standards and carry out, for example, the calculation (a / c), this preferably continuously along the axis A. The collection of these different measurement points leads to a final decision to reject or accept the preform during the test.

 I1 is also possible to use other means for guiding the carriage or its drive. These means are within the reach of those skilled in the art.

 Finally, the invention can be applied to the measurement of geometric characteristics of all elements having the general shape of a bar made of refractive materials, other than the preforms used for the manufacture of optical fibers, and comprising at least two index regions. of separate optical refraction.

Claims (11)

 1. Device for measuring the geometric characteristics of a cylindrical bar (1) comprising at least one central region (11) surrounded by a peripheral region (10) made of refractive materials with optical indices of different refractions; device characterized in that it comprises a light energy source (2) producing a beam of parallel rays with homogeneous energy distribution, of section at least equal to the section of the bar (1), illuminating a shaped tank (3) elongated in a direction (z) orthogonal to the direction of propagation of the beam (Y) and intended to receive the bar (1), transparent to the rays in at least predetermined zones (30) authorizing the crossing of the rays as well as the illumination complete with the bar, and filled with a liquid (31) with an optical refractive index substantially equal to that of the peripheral region (10) of the bar (1) immersing the latter, and optical means comprising, a polarizing element ( 4) arranged upstream of the cell on the optical path of the beam so as to linearly polarize it and an analyzer element (5) - arranged downstream of the cell on the optical path of the beam whose optical axis is orthogonal to the direction polarization of ra yons coming out of the polarizer (4) and in that, the rays emerging from the analyzer (5) after crossing the bar having a non-homogeneous energy distribution due to the induced optical anisotropies, the measurement of the geometric characteristics is carried out using means making it possible to evaluate the difference between two extrema of the spatial distribution curve of the energy of the beam emerging from the analyzer element in a determined plane (P) orthogonal to the direction (Y) of propagation of the beam.  2. Device according to claim 1, characterized in that the optical means further comprise a lens (L2) projecting with magnification (G) greater than the unit disposed downstream of the analyzer (5) so that the axis of symmetry (Z) of the cylindrical bar and the determined plane (P) is the optical conjugate of this axis of symmetry.  3. Device according to any one of claims 1 to 2, characterized in that the optical means further comprise at least one mirror (M) intended to deflect the beam of parallel rays by a determined angle.  4. Device according to claim 1, characterized in that the light energy source (2) comprises a light energy emitter (20), a condenser (CO) concentrating the light energy on a diaphragm (21) provided with 'an opening of small section so as to transmit a diverging beam at the outlet and a collimating lens (L1) producing the beam of parallel rays.  5. Device according to any one of claims 1 to 4, characterized in that a plate of refractive material (6) one of the faces of which has a frosted surface is disposed in the determined plane (P).  6. Device according to claim 5, characterized in that the plate of refractive material (P) is provided with a graduated scale (60) disposed along an axis orthogonal to the axis of symmetry of the bar (Z), the graduated scale (6) constituting the means for evaluating the difference between the extremes of the spatial energy distribution curve.  7. Device according to claim 5, characterized in that the plate of refractive material (6) is provided with marks distributed in a preestablished manner on an axis orthogonal to the axis of symmetry of the bar (2), these marks constituting the means d evaluation of the difference between the minima of the spatial energy distribution curve.  8. Device according to any one of claims 1 to 4, characterized in that a bar comprising several optoelectronic detectors is arranged in the determined plane (P) in a direction orthogonal to the axis of symmetry of the bar (z), so as to convert the energy variations of the beam emerging from the analyzer element in this direction into electrical signals; this bar constituting the means for evaluating the difference between the minima of the spatial energy distribution curve.  9. Device according to any one of claims 1 to 8, characterized in that it comprises a chassis (8) supporting the tank (3), a mobile carriage (7) comprising the light energy source (2) and the optical means, provided with an opening (70) allowing the passage of the tank and being able to move in a direction parallel (z ′) to the axis of symmetry of the bar (z) and guide means (72-73 , 80-81) supporting the mobile carriage (7) and integral with the chassis (8).  10. Device according to claim 9, characterized in that it further comprises means (82) of motorized drive of the mobile carriage (7) so as to perform an automatic measurement cycle at points distributed along the bar.  11. Device according to any one of claims 1 to 10, characterized in that the bar (1) made of refractive materials is a preform intended for the manufacture of optical fibers comprising at least a first region constituting the core (11) and a second peripheral region (10); and in that the geometric characteristics measured are the diameter of the core (a) and the external diameter (c) of the preform; the diameters (a, c) being deduced from the measurement of the distances separating the minima in the determined plane (P) from the curve of the energetic variations of the beam emerging from the analyzer (5) so as to sort with respect to pre-established standards.