WO2003019255A1

WO2003019255A1 - Optical fibre athermal device

Info

Publication number: WO2003019255A1
Application number: PCT/FR2002/002924
Authority: WO
Inventors: Stéphane RAOUL; Michel Boitel
Original assignee: Highwave Optical Technologies
Priority date: 2001-08-22
Filing date: 2002-08-22
Publication date: 2003-03-06
Also published as: FR2828940B1; FR2828940A1

Abstract

The invention relates to an optical fibre (10) device (100) comprising at least one component which is integrated in the fibre (10) and a support (1) to which the fibre (10) is fixed at two points (50, 52), said points being disposed on either side of the integrated component. The inventive device is characterised in that the support (1) forms a frame and comprises two longitudinal beams (20, 22) which are linked to crossbeams (30, 32) by means of hinges (42, 44, 46, 48). The aforementioned longitudinal beams can slide in relation to one another in the longitudinal direction thereof. Moreover, said support (1) comprises two studs (60, 62) which support the attachment points (50, 52) at the ends of the fibre (10). The inventive device also comprises a compensating element (70) of a pre-determined length having an expansion coefficient that is greater than that of the support (1). During a variation in temperature, said element (70), the ends of which are fixed to one of the beams (20, 22), can cause the beams (20, 22) to slide in relation to one another and the attachment points (50, 52) of the fibre (10) to move in relation to each other.

Description

OPTICAL FIBER ATHERMIC DEVICE

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 "Bragg wavelength of the network". Bragg network-based systems have already done great service and have given rise to an abundance of literature.

The optical components integrating such Bragg gratings are used for example to manufacture chromatic dispersion compensating filters (CDC), gain equalizing filters (FEG), or insertion / extraction multiplexer components (MIE). However, it turns out that optical filters provided with an integrated component, in particular a Bragg grating, are sensitive to temperature. In particular, the properties of Bragg gratings, and in particular the Bragg wavelength of gratings, vary:

- as a function of temperature by thermo-optical effects due to the expansion or compression of the fiber, or to the change in 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 commonly used means these days are known by the term "table-top" or "half-table-top" arrangements. A description of an example of implementation of these means can be found in documents [1] and [2].

Figures 1 and 2 attached respectively represent "half table-top" and "table-top" structures according to the state of the art. The “half table-top” or “table-top” assemblies consist of a beam 20 made of materials with a low coefficient of expansion such as Invar, ceramic, etc. and studs 30 and 32 made of material with high coefficient of expansion like aluminum for example. A fiber 10 comprising a Bragg grating is tensioned between the two pads 30 and 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 ”according to the type of assembly. The fiber attachment points 10 are referenced 12 and 14.

The distance between these two points 12 and 14 is reduced with increasing temperature, thus the tension applied to the fiber 10 decreases and tends to make lower the Bragg wavelength, while the temperature inducing the opposite effect , there is a balance between the two phenomena.

These arrangements make it possible to compensate for the effects of temperature by assuming at 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.

A drawback of these arrangements is that, by their design, they have fairly large dimensions. These assemblies have for example a length of the order of 45 mm if they are composed of Invar for the beam 20 and aluminum for the studs 30 or 32. This length depends directly on the choice of materials and on the ratio between the expansion coefficients of the materials used. At the same time, the evolution of the integration of components in optical networks increasingly requires their miniaturization. Consequently, the “table-top” and “half-table-top” arrangements appear increasingly bulky. In addition, the current recommendations in the field of telecommunications impose severe tests of temperature rise and humidity of the components. Consequently, the bonding systems are not sufficient to meet these requirements and have been replaced by holding the fiber by brazing on the assembly. Most of the time, for reasons of thermo-mechanical incompatibility between materials having very different coefficients of expansion, these solders are not sufficiently reliable fixing means. Indeed, this incompatibility induces a very slight but nevertheless harmful sliding of the brazed fiber over time on its support. An object of the present invention is to provide an athermal assembly which does not have the aforementioned drawbacks, ie an assembly which can reach reduced dimensions and which has good reliability over time.

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, characterized in that the support comprises two longitudinal beams linked to transverse beams by hinge means, the two longitudinal beams being able to slide relative to each other in their longitudinal direction and two studs supporting the fixing points of the ends of the fiber, the device further comprising a compensating element of predetermined length having a coefficient of expansion greater than that of the support, each end of which is fixed to one of the beams, said element being able, during a temperature variation, to cause the sliding of the beams relative to each other and the displacement of the fiber attachment points relative to each other. Such a device has the advantage that the fiber is fixed to the elements of the device having the lowest coefficient of expansion. This characteristic ensures good adhesion of the fixing points when they are made by brazing. Compared to the “table-top” and “half-table-top” type assembly of FIGS. 1 and 2, the length of the assembly of the invention can be reduced. In fact, 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 factor limiting the size of the device is no longer the length of the different dilating elements but the length of the integrated component. Such devices can therefore reach a length of 25 mm.

The hinge means advantageously allow the pivoting of the beams constituting the support with respect to each other and therefore facilitate the sliding of the two longitudinal beams relative to each other.

In one implementation of the invention, the longitudinal beams are parallel to each other.

In one implementation of the invention, the compensating element extends in a general direction substantially parallel to the longitudinal beams.

In another implementation of the invention, the compensating element extends substantially along a diagonal of the frame formed by the support. Advantageously, the support can be in one piece.

Advantageously, the longitudinal beams are formed from materials having different coefficients of expansion.

This characteristic makes it possible to refine the thermal compensation of the device obtained by an appropriate choice of the materials used. Advantageously, the transverse beams can be formed from a material different from that used for the longitudinal beams. This characteristic makes it possible to improve the elasticity of the frame formed by the longitudinal and transverse beams.

Advantageously, the longitudinal beams and / or the transverse beams may have different sections or shapes. This feature can also improve the elasticity of the frame.

The hinge means can be constituted by thinning of the junctions between the longitudinal and transverse beams. Each of the studs can be formed by a protuberance from one of the longitudinal beams, the protrusions being positioned near the opposite ends of the longitudinal beams and facing each other.

In an implementation of the invention, the compensating element has a general shape of S (it comprises a useful part in the form of a beam and two protuberances at the ends of the beam, the protrusions being directed in opposite directions, on either side of the beam), each end being fixed to one of the longitudinal beams. The studs being positioned near the opposite ends of the longitudinal beams, the compensating element can be fixed to each beam at its end opposite the end supporting a stud.

Other characteristics and advantages of the present invention will emerge from the description which follows which is purely illustrative and not limiting should be read with reference to the appended figures among which:

FIGS. 1 and 2 already commented on schematically represent devices conforming to the state of the art,

- Figure 3 is an example of a device according to the invention, - Figure 4 is a schematic representation of the device when it is in its "equilibrium position", FIG. 5 is a schematic representation of the device during a rise in temperature,

FIGS. 6 is a diagrammatic representation of the device during a temperature decrease, FIG. 7 represents the deformation of the structure of FIG. 3 during a temperature increase of + 50 ° C,

FIG. 8 represents the deformation of the structure of FIG. 3 during a temperature drop of -50 ° C,

- Figure 9 is an alternative embodiment of the device of Figure 1. In Figure 3, the optical fiber device 100 shown is "in equilibrium position". This fiber optic device 100 comprises a one-piece support 1 in the form of a generally rectangular frame consisting of two longitudinal beams 20 and 22 parallel and two transverse beams 30 and 32 connecting the ends of the beams 20 and 22. The support has been machined at the junctions between the beams 20, 22, 30 and 32 so as to reduce its thickness at these junctions. These thickness reductions constitute zones 42, 44, 46, 48 of lesser resistance or "hinge" which allow the beams 20, 22, 30 and 32 to slightly pivot with respect to each other. Each longitudinal beam 20 and 22 comprises near one of its ends a protuberance denoted respectively 60 and 62. The protrusions 60 and 62 of the beams 20 and 22 are each positioned at an opposite end of the support 1. An optical fiber 10 comprising an integrated component is fixed at two points 50 and 52 of the protrusions 60 and 62 so that the integrated component is located between the two fixing points. The fiber optic device further comprises a beam 70 in the general shape of an S (the beam 70 comprises a useful part and two protrusions at its ends, the protrusions being directed in opposite directions, on either side of the beam) each end of which is fixed to one of the beams 20 and 22 at the end opposite to that having a protrusion 50 or 52. The beam 70 is made of a material having a coefficient of expansion greater than that of the material constituting the support 1. For example, the beam 70 can be made of aluminum or stainless steel, while the support 1 is made of Invar or Covar . FIGS. 4 to 6 schematically illustrate the operation of the device 100 of FIG. 3. We find in these figures the various constituent elements of the device of FIG. 3: the support 1 consisting of two longitudinal beams 20 and 22 and transverse 30, 32, the hinges 42, 44, 46, 48 positioned at the junction between the beams 20, 22, 30 and 32, the protrusions 60 and 62 of the beams 20 and 22, the fixing points 50 and 52 of the fiber 10, as well as the S-shaped beam 70, the ends of which are fixed to the beams 20 and 22.

In Figure 4, the device 100 is in its "equilibrium position". The support forms a perfect rectangle. During a rise in temperature, the support 1 and the beam 70 expand, the expansion of the support 1 being less significant than that of the beam 70. The beams 20, 22 and 70 tend to elongate, the elongation of the beam 70 being greater than that of the beams 20 and 22. As illustrated in FIG. 5, the elongation of the beam 70 causes the beam 20 to slide relative to the beam 22 in their longitudinal direction in the direction indicated by the arrows. The support 1 is deformed, this deformation being authorized by the hinges 42, 44, 46 and 48. The support 1 forms a parallelogram. As a result, the fixing points 60 and 62 of the fiber 10 tend to approach, releasing the tensile stress of the fiber 10.

Conversely, during a temperature decrease, the support 1 and the beam 70 contract, the beam 70 contracting more significantly than the beams 20 and 22 of the support 1. As illustrated in FIGS. 6, the beam 70 causes the beams 20 and 22 to slide relative to each other in their longitudinal direction, in the direction indicated by the arrows. The support 1 deforms, this deformation being authorized by the hinges 42, 44, 46 and 48. In the same way as in FIG. 5, the support 1 forms a parallelogram (inverted with respect to that of FIG. 5). As a result, the fixing points 50 and 52 of the fiber 10 tend to move away, increasing the tensile stress in the fiber 10. 5 An assembly is thus produced which reduces the stress in the fiber 10 when the temperature increases and increases this stress when the temperature drops so as to mechanically compensate for the thermo-optical effects of the fiber 10.

Figures 7 and 8 show the deformation of the support 1 and the 0 beam 70 respectively during a temperature rise of + 50 ° C and during a temperature decrease of -50 ° C compared to the temperature of " equilibrium position ”of the assembly. In these figures, the deformations have been amplified 30 times.

In such a device, the fiber 10 is fixed on the low expansion material 5 forming the support 1. It is thus possible during its assembly to position the fiber on the support 1 alone and then come to fix the beam 70. In this way , setting the wavelength of the integrated component to a particular value can be performed a posteriori by shearing the support 1 then fixing the beam 70. 0 In addition, brazing of the fiber on the least expanding material ensures the good adhesion of it. Soldering on a low expansion material ensures better grip in a wide range of temperatures. This is due to compression stresses on the fiber present at high and low temperatures. The fiber can also be fixed to the studs by bonding.

In the above description, the support comprises a one-piece frame. It is of course possible to produce a support comprising beams connected to each other by welds. In this case, the longitudinal and transverse beams can be made of different materials. The materials are chosen flexible enough to allow the beams to pivot relative to each other and ensure the "hinge" effect.

For example, in an alternative embodiment of the device in FIG. 3, one of the longitudinal beams 20 or 22 is made of Invar and the other longitudinal beam is made of Covar. These two materials with low expansion coefficients have complementary thermal expansion curves, which makes it possible to obtain a flatter athermicity curve for the device. The use of different materials for the production of the support makes it possible to refine the thermal compensation of the device obtained.

Furthermore, the longitudinal and / or transverse beams can have different sections or shapes in order to further improve the athermicity obtained.

It will also be noted that in the above description, the studs 60 and 62 are fixed on the longitudinal beams 20 and 22. It is also possible to fix these studs on the transverse beams 30 and 32. In this case, the studs will be preferably located at the opposite ends of the transverse beams 30 and 32 so that the deformation of the support 1 causes the attachment points 50 and 52 located on the studs to move towards or away from each other. The fiber 10 is positioned diagonally with respect to the longitudinal beams 20 and 22 parallel.

Finally, the compensation element is not necessarily produced in the form of an S-beam such as those shown in the figures. It may, for example, be a straight beam positioned on bosses produced asymmetrically on the inner faces of the beams 20 and 22.

In an alternative embodiment shown in FIG. 9, the compensation element is a straight beam 70 extending substantially along a diagonal of the frame formed by the support 1.

For reasons of space requirement of the device 100, the direction of expansion of the compensating element 70 should preferably have a maximum component in the longitudinal direction of the device (direction along which the fiber extends 10).

Such an assembly can have a reduction of up to 45% in its length compared to the old assemblies (the longitudinal dimensions can reach 25mm). This reduction in length is in fact limited only by the size of the integrated component.

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.

BIBLIOGRAPHY

[1] V. FLEURY

“Thermal stabilization of Bragg gratings photo-registered on optical fibers”, internship report of DESS laser engineering at the University of

Sciences and Technologies of Lille. [2] J. RIOBLANC et al.

"Optimization of a passive stabilization system of the drift in temperature of the tuning wavelength of the Bragg gratings", INOG

96 Paper No. 85.

Claims

1. Fiber optic device (100) (10) comprising at least one component integrated in the fiber (10) and a support (1) on which the fiber (10) is fixed at two points (50, 52) located on the side and on the other side of the integrated component, characterized in that the support (1) forms a frame and comprises two longitudinal beams (20, 22) linked to transverse beams (30, 32) by hinge means (42, 44, 46 , 48), the two longitudinal beams being able to slide relative to each other in their longitudinal direction and two studs (60, 62) supporting the fixing points (50, 52) of the ends of the fiber (10 ), the device further comprising a compensating element (70) of predetermined length having a coefficient of expansion greater than that of the support (1), each end of which is fixed to one of the beams (20; 22), said element ( 70) being able during a temperature variation to cause the sliding of the beams (20; 22) relative to each other and the displacement of the fixing points (50, 52) of the fiber (10) relative to each other.

2. Device (100) according to claim 1, characterized in that the longitudinal beams (20, 22) are mutually parallel.

3. Device (100) according to one of claims 1 or 2, characterized in that the compensating element (70) extends in a general direction substantially parallel to the longitudinal beams (20, 22).

4. Device (100) according to one of claims 1 or 2, characterized in that the compensating element (70) extends substantially along a diagonal of the frame formed by the support (1).

5. Device (100) according to one of claims 1 to 4, characterized in that the hinge means (42, 44, 46, 48) consist of thinning of the junctions between the longitudinal (20, 22) and transverse beams (30, 32).

6. Device (100) according to one of claims 1 to 4, characterized in that the transverse beams (30, 32) are fixed to the beams longitudinal (20, 22) by welding and in that the beams are flexible enough to allow them to pivot relative to each other.

7. Device (100) according to one of the preceding claims, characterized in that the longitudinal beams (20, 22) are formed from materials having different coefficients of expansion.

8. Device (100) according to one of the preceding claims, characterized in that the longitudinal beams (20,22) and the transverse beams (30, 32) are formed from different materials.

9. Device according to one of claims 1 to 5, characterized in that the support (1) is in one piece.

10. Device (100) according to one of the preceding claims, characterized in that the longitudinal beams (20, 22) and / or the transverse beams (30, 32) have different sections or shapes.

11. Device (100) according to one of the preceding claims, characterized in that each of the studs (60, 62) consists of a protuberance of one of the longitudinal beams (20, 22), the protrusions being positioned in the vicinity opposite ends of the longitudinal beams (20, 22) and facing each other.

12. Device (100) according to one of the preceding claims, characterized in that the compensating element (70) has a general shape of S, each end of which is fixed to one of the longitudinal beams (20, 22).

13. Device (100) according to one of the preceding claims, characterized in that the studs (60, 62) being positioned near the opposite ends of the longitudinal beams (20, 22), the compensating element (70) is fixed on each beam (20; 22) at its end opposite the end supporting a stud (60; 62).

14. Device (100) according to one of the preceding claims, characterized in that one of the longitudinal beams (20; 22) is made of Invar and the other longitudinal beam is made of Covar and the compensating element (70) is made of aluminum or stainless steel.

15. Device (100) according to one of the preceding claims, characterized in that the length of the support is less than 25 mm.