FR2828938A1 - Fibre optic support for Bragg filter has beam with temperature compensating insert - Google Patents

Abstract

A fibre optic support (1) is an H, C or U beam section with a different thermal expansion coefficient insert (52) exerting a force through the supports (40, 42) parallel to the fibre (10) axis when the temperature varies.

Description

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ATHERMIC FIBER OPTICAL DEVICE WITH CONTROLLED FLEXION
The present invention relates to the field of optical fibers. It relates more particularly to the field of optical fibers comprising an integrated component.

 The present invention applies in particular to fiber optic devices comprising an integrated Bragg grating. In this context, it aims to propose a device for stabilizing the temperature and / or adjusting the Bragg wavelength of the photoinscribed networks in the optical fibers.

 Bragg gratings are periodic structures of the optical index, which have the particularity of reflecting a signal of well-defined wavelength, called the Bragg wavelength of the network. Bragg's network-based systems have already done great service and have given rise to an abundance of literature.

 Optical components incorporating such Bragg gratings are used for example to manufacture chromatic dispersion compensating filters (CDC), gain equalizing filters (FEG), or Insertion / Extraction Multiplexers (MIE) components.

 However, it turns out that optical fibers provided with an integrated component, in particular a Bragg grating, are sensitive to temperature. In particular, the properties of the Bragg gratings, and in particular the Bragg wavelength of the gratings vary: - as a function of the temperature by thermo-optical effects due to the expansion or compression of the fiber, or to the change index of the Bragg grating, - as a function of the traction or the tension to which the Bragg grating is subjected in the fiber.

 It follows that the Bragg wavelength is: - an increasing function of the temperature, and - an increasing function of the traction applied to the fiber.

 Means are known for limiting the temperature drift of devices using Bragg gratings. The most used means of

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 nowadays are known as table-top or half-table-top assembly. A description of examples of implementation of these means can be found in documents [1] and [2].

 Figures 1 and 2 attached respectively represent table-top and half-table-top structures according to the state of the art.

 Table-top or half-table-top assemblies consist of a beam 20 made of material with a low coefficient of expansion, Invar, ceramic ... and one or two studs 30, 32 made of material with a high coefficient of expansion, aluminum for example. The fibers 10 comprising a Bragg grating, are mounted tensioned between the two pads 30, 32 for the case of a table-top, or between the pad 30 and the opposite end of the beam 20 for the case of a half table-top, depending on the type of assembly. The fiber attachment points 10 are referenced 12 and 14.

 The distance between these two points 12,14 is reduced with increasing temperature, thus the tension applied to the fiber decreases and tends to make decrease the wavelength of Bragg, while the temperature inducing the opposite effect, it there is a balance between the two phenomena.

 These arrangements make it possible to compensate for the effects of temperature by assuming as a first approximation that the Bragg wavelength is a linear function increasing with temperature and decreasing with mechanical stress exerted on the network. This linear approximation of the compensation remains valid within a certain temperature range. In reality, the curve of the Bragg wavelength as a function of the temperature has an upward concavity so that at low temperatures, these arrangements tend to over-compensate for the effect of the temperature while at at high temperatures, they tend to under-compensate for this effect.

 As a result, in particular, the fiber 10 is subjected to high mechanical stresses at low temperatures (below -10 ° C.). In reality, at these temperatures, the mounting should not be compensated. High tensions may shorten the life of the fiber or break it.

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 The aim of the present invention is to compensate for the Bragg wavelength of the grating either linearly or by following an expansion function which varies with temperature.

 To this end, the invention provides a fiber optic device comprising at least one component integrated into the fiber and a support on which the fiber is fixed at two points located on either side of the integrated component, the device further comprising at least one element having a coefficient of thermal expansion different from that of the support, characterized in that during a temperature variation, said element is capable of exerting by contact on the support a tensile or compressive stress parallel to the fiber axis.

 This arrangement advantageously makes it possible to impose a bending on the support which causes the two fixing points of the fiber by bringing them closer when the temperature increases and by moving them away when the temperature decreases. An assembly is thus produced which reduces the stress on the fiber when the temperature increases and mechanically compensates for the thermo-optical effects of the fiber.

 Compared to the table-top and half-table-top type assemblies of FIGS. 1 and 2, the length of the assembly of the invention can be reduced. Indeed, the length of a table-top or half-table-top assembly is at least equal to the length of the fiber to be compensated, plus the length of the compensation pad. In the case of the invention, the compensation element is integrated into the length of the support, which considerably reduces the size of the device.

 The device has the advantage that the fiber can be fixed on a monobloc support, that is to say having no connecting surfaces. Indeed, such surfaces tend to corrode and disturb the traction of the fiber. With the device of the invention, the reliability of the assembly is increased.

 In addition, the device of the invention does not require either machining or complex assembly. Apart from fixing the fiber which can be done by methods known to those skilled in the art, the mounting of the elements does not require any specific developments.

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 Furthermore, the support may have a coefficient of expansion very close to the fiber. Thus the fixing points behave like fiber.

 Other characteristics and advantages of the present invention will emerge from the description which follows, which is purely illustrative and not limiting and should be read with reference to the appended figures, among which: - Figures 1 and 2 already commented on schematically represent conforming devices in the prior art, - Figure 3 shows schematically a first alternative embodiment of the device of the invention using a compensation element, - Figure 4 shows schematically a device similar to that of Figure 3 highlighting works several compensation elements, - Figure 5 schematically shows a second alternative embodiment of the device of the invention using a compensation element, - Figure 6 schematically shows a device similar to that of Figure 5 using several compensation elements, - figure 7 represents s schematically a device combining the characteristics of the devices shown in Figures 3,4, 5 and 6, - Figure 8 shows in more detail an example of a conical compensation element, - Figures 9 to 12 show examples of arrangements and of possible forms of compensation elements, - Figure 13 shows in perspective an example of a device according to the invention and comprising a device for calibrating the Bragg wavelength, - Figure 14 shows schematically a device putting using compensation elements positioned in series.

 FIG. 3 represents a first variant of the device of the invention. This device comprises a one-piece support 1. The support 1 has the general shape of an H. It includes a longitudinal beam 20,

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generally parallel to the axis of the fiber 10. The beam 20 is provided at each of its ends with two opposite protuberances 30, 32 and 34,
36. An optical fiber 10 is fixed to the support 1 at two fixing points 40 and 42 located respectively on the pads 30 and 32. This fiber
10 comprises an integrated component such as a Bragg grating positioned between the two fixing points 40 and 42. The beam 20 extends essentially parallel to the fiber 10.

 The support 1 has in its lower longitudinal part, between the protrusions 34, 36, a transverse groove 50 forming a housing in which is inserted (optionally with play) an element 52 having for example a general shape of parallelepiped and having a coefficient of expansion greater than the coefficient of expansion of the support 1. The support can for example be made of glass or silica and the element 52 is made of aluminum or steel. The fact of carrying the support 1 in optically transparent material makes it possible to carry out a photoinscription in the fiber 10 when the latter is already mounted on the support, directly through the latter.

 During a rise in temperature, the support 1 and the element 52 expand, the expansion of the element 52 being greater than that of the support 1. From a certain temperature threshold, the element 52 comes in contact with the edges 51 and 53 of the groove 50 located opposite one another. If the temperature increases further, the element 52 exerts on the edges 51 and 53 a longitudinal stress which deforms the support 1 and imposes a bending on it. This bending tends to bring the fixing points 40 and 42 closer to the fiber 10, hence a shift in the Bragg wavelength on the component.

 The opposite effect is obtained when the temperature drops due to the elasticity of the materials.

 In the case of small displacements, it is considered that the bringing together of the two points 40 and 42 is a quasi-linear function of the temperature.

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 One can choose to weld the element 52 in its housing 50. In this case, the element 52 is still either in compression or in tension.

 One can also choose to leave the element 52 free in its housing 50, so that below the temperature threshold, the compensating element has a length equal to or less than the dimensions of the housing 50. Thus, when the temperature drops below the threshold, the element contracts and no longer exerts stress on the body 1. It therefore no longer stretches the fiber 10. This characteristic makes it possible to avoid the risks of fatigue or breakage of the fiber at low temperatures .

 Thanks to the device of the invention, it is advantageously possible to directly adjust the initial tension of the fiber by judiciously choosing the length of the compensating element which is inserted into the support.

 As illustrated in FIG. 8, it is possible, for example, to use a compensating element 52 of generally conical shape which is more or less inserted into a housing 50 of the beam 20, in order to impose a more or less significant flexion on beam. This characteristic makes it possible to precisely regulate the initial traction of the fiber 10 and consequently the wavelength of the Bragg grating.

 In Figure 4, there is shown a device using several compensating elements 52 and 54 placed in the groove 50 As in Figure 3, this device comprises a one-piece support 1 formed of a beam 20 and two protuberances 30 , 32 and 34,36 located at the ends of the beam 20. An optical fiber 10 comprising an integrated component is fixed on the support 1 at two fixing points 40 and 42 located respectively on the protuberances 30 and 32. The beam 20 extends essentially parallel to the fiber 10.

 The beam 20 has in its lower longitudinal part a transverse groove 50 forming, between the two protrusions 34, 36, a housing in which the elements 52 and 54 superimposed are inserted with play. These elements are formed from materials with different coefficients of expansion. For example, the coefficient of expansion of element 54 is greater than the coefficient of expansion of element 52 which

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 itself has a coefficient of expansion greater than that of the support 1. The element 54 is mounted in the groove 50 with a greater clearance than the clearance with which the element 52 is mounted.

 When the temperature rises, the support 1 and the elements 52 and 54 expand. When a first predetermined temperature threshold has been reached, the element 52 comes into contact with the edges 51 and 53 of the groove 50 and exerts a stress in the lower longitudinal part of the beam 20 which causes it to flex. This bending tends to bring the fixing points 40 and 42 closer to the fiber 10, hence a shift in the Bragg wavelength on the component.

 Up to a second predetermined temperature threshold, the element 54 expands freely in its housing. When this second temperature threshold is reached, the element 54 comes into contact with the edges 51 and 53 of the groove 50 and takes over from the element 52, in turn imposing a stress on the lower longitudinal part of the beam 20 which causes it to flex.

compensation à pas variable . Chaque élément 52 et 54 impose une loi de déformation du support 1 dans une gamme de température donnée, ce qui permet avantageusement d'obtenir une compensation acceptable sur une plus grande plage de température. By appropriately choosing the materials constituting the support 1 and the compensation elements 52 and 54, a

variable pitch compensation. Each element 52 and 54 imposes a law of deformation of the support 1 in a given temperature range, which advantageously makes it possible to obtain acceptable compensation over a larger temperature range.

 In the case of small displacements, it is considered that the approximation of the two points 40 and 42 is a quasi-linear function of the temperature in each temperature range corresponding to the action of one of the elements 52 or 54 of compensation.

 It is of course possible to produce similar devices comprising as many compensation elements as necessary according to the compensation law that it is desired to implement. The temperature thresholds from which the elements come into play are determined by the different sets with which they are inserted in the housing formed in the support. The laws of deformation are determined by the dimensions of the different elements inserted in the support.

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 It is also possible to modify the position of the elements. One can for example, place them side by side as illustrated in Figure 9, superimpose them as illustrated in Figure 10 or insert them in different housings.

 The compensation elements can have any geometric shape which makes it possible to limit the size of the fiber device. They can for example be in the form of half-cylinders as illustrated in FIG. 12.

 Compensation elements can also be used in the form of hollow cylinders of different diameters. In FIG. 11, the different cylindrical elements 52 and 54 are nested one inside the other concentrically.

 FIG. 5 represents a second variant of the device of the invention. This device comprises a one-piece support 1 in the general shape of an H comprising a beam 20 and two protrusions 30 and 32 located at the ends of the beam 20, the protrusions 30 and 32 being located in the upper longitudinal part of the beam 20. An optical fiber 10 is fixed to the support 1 at two fixing points 40 and 42 located respectively on the protrusions 50 and 52. This fiber 10 comprises an integrated component such as a Bragg grating positioned between the two fixing points 40 and 42 The beam 20 extends essentially parallel to the fiber 10.

 The support 1 is positioned (possibly with play) between the branches 60 and 62 of an element 56 in the general shape of C. The ends of the branches 60 and 62 of the element 56 are located opposite the protuberances 30 and 32 of the support 1. The element 56 is made of a material having a coefficient of expansion lower than that of the support 1.

 During an increase in temperature, the support 1 and the element 56 expand, the expansion of the element 56 being less significant than that of the support 1. From a certain increase in temperature, the support 1 comes in contact with the element 56 at the external faces 55 and 57 of the protuberances 30 and 32. If the temperature increases further, the element

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56 exerts on the pads 30 and 32 a longitudinal compression stress which deforms the beam 20 and imposes a bending on it. This bending tends to bring the fixing points 40 and 42 closer to the fiber 10, hence a shift in the Bragg wavelength on the component.

As illustrated in FIG. 6, one can envisage positioning the support 1 in several elements 56 and 58 in C having different expansion coefficients or not, with different clearances or not, or even different stiffnesses or not. By appropriately choosing the materials constituting the support 1 and the compensating elements 56 and
58, it performs in the same way as with the device of Figure 5, a variable pitch compensation.

 It is of course possible to produce similar devices comprising as many compensation elements as necessary according to the compensation law that it is desired to implement.

 In the same way as the elements 52 and 54 of FIG. 4, it is also possible to modify the position of the elements 56 and 58. Instead of superimposing them, they can be placed side by side, in parallel, in series or the insert in different housings.

 In addition, as illustrated in FIG. 7, it is possible to combine the compensation elements 52 and 54 of the first variant and the elements of the second variant 56 and 58 on the same device. This combination makes it possible to use a large number of elements and to have a greater choice of materials.

 One can adjust the athermicity of each of the described assemblies by modifying the distance of the elements of compensations to the neutral line of the beam 20. In fact, the more an element is distant from the neutral line of the beam, the more the bending imposes on it is reduced.

 In the case of an assembly comprising several compensating elements, these elements can be made of materials having the same coefficient of expansion, these elements being positioned at different distances from the neutral line of the beam 20. In fact, placed at different distances, these elements do not exert the same moment on the beam 20. We can mount these elements with

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 different clearances in the support so that, during a temperature variation, they successively exert a stress by contact with the support.

 Finally, to obtain a finer adjustment of the Bragg wavelength, it may be advantageous to supplement the device of the invention with a device for lateral clamping of the beam such as those described in document [3] for example or a conventional adjustment screw device as illustrated in FIG. 13 or any other compensation device.

 In FIG. 13, the support 1 of the optical fiber device comprises a longitudinal beam 20, generally parallel to the axis of the fiber 10. The beam 20 is provided at each of its ends with opposite protuberances 30 and 32. The beam 20 has a transverse groove 50 in which a compensation element 52 of generally parallelepiped shape has been inserted. The beam 20 further comprises a longitudinal slot 60 separating the beam 20 into two branches 61 and 62. A screw 63 passes through the two branches 61 and 62. In known manner, it is possible by pivoting the screw 63 to bring together or move aside the branches 61 and 62 of the beam 20. The bringing together of the branches 61 and 62 brings about the bringing together of the points 40 and 42 for fixing the fiber 10. The spacing of the branches 61 and 62 brings about the distance of the points 40 and 42 fixing the fiber. This screw device allows fine adjustment of the initial tension in the fiber 10 to calibrate the fiber optic device.

 In Figure 14, there is shown a variant of the devices of Figures 3 and 4 in which the compensating elements 52 and 54 are positioned in series. In this variant, the elements 52 and 54 are inserted in transverse grooves 50 and 59 positioned successively along the support 1. One of the elements 52 has a greater length than the other element 54. The element 52 constitutes l main compensation element while the smaller element 54 makes it possible to finely adjust the stress imposed on the fiber 10.

 In this variant, the device has two grooves 50 and 59 separated by a protuberance 38. It is also possible not to make

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 only one groove in the support 1 and to position the compensation elements 52 and 54 in series in this groove.

 Of course the present invention is not limited to the particular embodiments which have just been described, but extends to any variant in accordance with its spirit.

 For example, each support 1 can carry several fibers 10.

 The contact zones defined between the support 1 and the elements 52, 54, 56, 58 may be planar and perpendicular to the axis of the fiber or inclined with respect to this axis. More precisely in the latter case, the contact between the support 1 and the elements 52, 54, 56, 58 is preferably defined by an edge between a surface generally perpendicular to the axis of the fiber 10 and a surface inclined with respect to this axis.

 For reasons of cost or size, the support 20 can have a generally rectangular or cylindrical shape or any other shape.

mv. FLEURY Stabilisation thermique des réseaux de Bragg photoinscrits sur fibre optique Rapport de Stage de DESS INGENERIE LASER à l'Université des Sciences et Technologies de Lille. BIBLIOGRAPHY:

mv. FLEURY Thermal stabilization of Bragg gratings photoinscribed on optical fiber Internship report of DESS LASER ENGINEERING at the University of Sciences and Technologies of Lille.

 [2J J. Rioublanc et al.

  Optimization of a passive stabilization system of the drift in temperature of the tuning wavelength of the Bragg gratings JNOG96, paper n 85.

[3] Patent application in France n FR / 00 07004000 filed on May 31, 2001

Claims (16)

1. Optical fiber device comprising at least one component integrated in the fiber (10) and a support (1) on which the fiber (10) is fixed at two points (40,42) located on either side of the component integrated, the device further comprising at least one element (52; 54; 56; 58) having a coefficient of thermal expansion different from that of the support (1), characterized in that during a temperature variation, said element ( 52; 54; 56; 58) is able to exert by contact on the support (1) a tensile or compressive stress parallel to the axis of the fiber (10).  2. Device according to claim 1, characterized in that the tensile or compressive stress causes a bending of the support (1) which generates a modification of the distance between the fixing points (40, 42) of the fiber (10).  3. Device according to one of the preceding claims, characterized in that the support (1) is in one piece 4. Device according to one of the preceding claims, characterized in that the support (1) has a coefficient of expansion close to that of the fiber (10).  5. Device according to one of the preceding claims, characterized in that it comprises at least two elements (52; 54; 56; 58) having different coefficients of thermal expansion and coming successively to exert a stress by contact with the support ( 1) during a temperature variation.  6. Device according to one of the preceding claims, characterized in that it comprises at least two elements (52; 54; 56; 58) having similar coefficients of thermal expansion, said elements (52; 54; 56; 58) being positioned with different clearances relative to the support (1) and at different distances from the neutral axis of the support (1), and successively exerting a stress by contact with the support (1) during a temperature variation . <Desc / Clms Page number 13>  7. Device according to one of the preceding claims, characterized in that one of the elements (52; 54) is capable of exerting a stress at the level of two zones (51; 53) of contact with the support (1) arranged in look at each other.  8. Device according to one of the preceding claims, characterized in that at least one element (52; 54) has a coefficient of expansion greater than that of the support (1) and in that it is capable of exerting a stress generating a bending of the support (1) and a bringing together of the two points (40; 42) for fixing the fiber (10).  9. Device according to one of the preceding claims, characterized in that at least one element (52) has a generally parallelepiped shape.  10. Device according to one of the preceding claims, characterized in that at least one element (52) has a semi-cylindrical shape.  11. Device according to one of the preceding claims, characterized in that it comprises at least two elements (52; 54) in the general form of hollow cylinder and arranged nested one inside the other concentrically.  12. Device according to one of the preceding claims, characterized in that at least one element (56,58) is capable of exerting a stress at two contact zones (55,57) external to the support (1).  13. Device according to one of the preceding claims, characterized in that at least one element (56; 58) has a coefficient of expansion lower than that of the support and in that it is capable of exerting a compressive stress on the support (1) generating a bringing together of the two points (40; 42) for fixing the fiber (10).  14. Device according to one of the preceding claims, characterized in that the support (1) has the shape of an H comprising a central beam (20) provided at each end with two protuberances (30, 32,34, 36) , the fiber (10) being fixed between two protuberances (30,32) <Desc / Clms Page number 14>  situated respectively at each end of the beam (20) and at least one element (52,54) being placed between the two other protuberances (34,36).  15. Device according to one of claims 1 to 13, characterized in that the support (1) has a U shape comprising a beam (20) provided with two protuberances (30,32) on which the fiber (10) is fixed ) and in that the device comprises a C-shaped element (56; 58) whose branches can rest on the protuberances of the support (1).  16. Device according to one of the preceding claims, characterized in that the support (1) comprises a beam (20) comprising a longitudinal slot (60) separating said beam (20) into two branches (61,62), a screw (63) passing through the two branches (61,62), the rotation of the screw (63) causing the branches (61,62) to approach or move apart and the points (40,42) of the fiber fixation.