WO2002039160A1

WO2002039160A1 - Platform with controlled thermal expansion coefficient

Info

Publication number: WO2002039160A1
Application number: PCT/GB2001/004955
Authority: WO
Inventors: William John Clegg; Anthony Kelly; Robert James Stearn; Luc Jean Marie Vandeperre
Original assignee: Cambridge University Technical Services Ltd.
Priority date: 2000-11-09
Filing date: 2001-11-07
Publication date: 2002-05-16
Also published as: GB2386436B; GB2386436A; GB0312996D0; AU2002223790A1

Abstract

A platform comprising a support providing a controlled thermal expansion coefficient in a given direction. The support is formed of a material having a first thermal expansion coefficient and is connected to a body having a second thermal expansion coefficient. The first thermal expansion coefficient is different from the second thermal expansion coefficient, such that a change in temperature causes a relative change in the dimensions of the support and the body in a direction perpendicular to the said given direction, thereby resulting in the variation of a force applied to the support by the body in the direction perpendicular to the said given direction to control the resultant expansion coefficient in the said direction. The use of the platform for mounting optical elements such as fibre Bragg gratings, Fabry-Perot cavities or telescopes is disclosed.

Description

PLATFORM WITH CONTROLLED THERMAL EXPANSION COEFFICIENT

It is well known that changes in temperature will cause changes in the dimensions of objects. In general, an increase in temperature will result in the expansion of objects, and a decrease in temperature will result in their contraction.

It is often desirable to provide objects that respond in a controlled manner to changes in temperature. For example, it may be desirable that the size of an object remains unchanged or even decreases despite an increase in the temperature, or that the expansion of the object with an increase in temperature is controlled. Examples of objects or devices for which it is desirable to control the temperature response include optical devices such as telescopes, Fabry-Perot cavities and Bragg fibre gratings, sensors and other measuring devices . In the case of a telescope, and in the case of many measuring devices, it is required that there are no dimensional changes as the temperature fluctuates. However in devices such as the Bragg fibre grating, there is a need to impose a controlled contraction on the device as the temperature is increased to give the required response independent of temperature. In Bragg fibre gratings, light interacts with a series of spaced grating elements. The frequency of the light that interacts is dependent upon the refractive index of the material from which the grating is formed, and the spacing of the grating elements. The application of a compressive or a tensile stress to the fibre, or the change in ambient temperature, will result in the spacing of the grating elements varying. Further, variations in temperature will vary the refractive index. Therefore, the frequency of the interacting light will change. In many cases, this change in the frequency of the interacting light is desirable, since this change can be measured to provide an indication of a change in the ambient temperature or the applied force. However, in other applications, this change may not be desired.

One known approach to overcome the problems associated with variations in temperature is to compensate for the change in the spacing of the grating by the variation in the refractive index with temperature. Incorporating materials, such as water or methanol, whose refractive index decreases on heating, can do this. Such an approach has been proposed in US-A-5938811. Whilst useful in some applications, it is not possible to incorporate sufficient material to obtain full thermal compensation of a Bragg fibre grating.

Alternatively a strain may be imposed on the grating, which is sufficient to overcome the change in the refractive index. For an increase in the ambient temperature, a compressive strain must be applied. However the magnitude of this strain must be greater than that due simply to the thermal expansion that was associated with the temperature change, so that the spacing of the grating must be reduced to below that of its original .

It has been proposed to place actuators along the optic fibre to apply either a tensile or compressive strain depending on the variation in temperature. However such an approach is unreliable, particularly over the very long periods (up to 30 years) over which a Bragg fibre grating may have to operate. It has also been proposed to fix the fibre to a material whose thermal expansion coefficient is sufficiently negative. The magnitude required is approximately -9xl0^"6 K^"1. There are materials with these properties, such as β-eucryptite or zirconium tungstate, although some form of internal cracking is often required, and their use has been considered unsuitable for Bragg fibre gratings . Furthermore they tend to be sensitive to water and their properties are unproven over the lifetimes envisaged for such gratings. The use of substrates of different materials has also been suggested, using the difference in thermal expansion coefficients, both of which may be positive, to exert a compressive strain upon the fibre. Such devices require that the fibre be fixed at two points along its length. Because the fibre would buckle if it were to be compressed, it is instead held under an overall tension, typically of the order of 500 MPa, and this stress is either relaxed or increased as the temperature changes. However as the optic fibre is a brittle material, in which cracks can grow with time when the fibre is under load, this approach has significant disadvantages, especially when considering the long lifetimes (of the order of 30 years) that are envisaged for such fibres. In this type of device the strain applied to the fibre per unit change of temperature is dependent on the relative lengths of the two components, provided they are massive compared to the fibre. This means extreme accuracy is required with regard to the placing of the fixing points if the behaviour of the Bragg fibre grating is to be sufficiently independent of temperature.

One example of such a system is disclosed in US-A- 5,042,898 in which the fibre is supported between two spaced members. The two members are pretensioned to apply a stress to the optical fibre. The two members have different thermal expansion coefficients, such that an increase in temperature causes the spacing between the two support members to decrease, thereby relaxing the tension applied to the fibre. A potential problem with such an arrangement is that, even at normal operating temperatures, the fibre is subjected to a force and this may reduce the lifetime of the fibre. This is especially the case where the temperature is not. constant, and therefore the force applied to the fibre is frequently varied.

Alternatively, it is known how to provide a platform made of materials of two different expansion coefficients, so that when the temperature changes, the platform bends. Provided the fibre is sufficiently far from the neutral axis, the grating in the fibre is essentially in pure tension, although the platform itself is in bending.

It has therefore been proposed to mount the fibre grating on a support platform which is arranged such that an increase in temperature relaxes a force applied to the fibre, and thereby compensates for the effect of the increase in temperature on the fibre.

An example of such an arrangement is disclosed in US-A- 5,841,920. In this case, the fibre is fixed to one side of a support made from a material having a first thermal expansion coefficient. The support is mounted within a frame having a second, different, thermal expansion coefficient . A change in temperature will cause a differential change in the size of the support and frame which results in the bending of the support . This bending causes a change in the length of the fibre attached to the support, which compensates for the change in temperature. A problem with this arrangement is that the repeated bending of the fibre may weaken the fibre, causing its eventual failure. Further, the bent fibre may not transmit light in the same way as a straight fibre.

The use of differential thermal expansion as in either US-A-5042898 or as in US-A-5841920 gives a support where the dimension parallel to that of the optic fibre when fixed to the support varies approximately linearly with temperature. Non-linear behaviour is possible provided materials whose expansion coefficient also varies with temperature in exactly the manner required can be found. Ideally in certain applications it is desirable to be able to control the degree of non-linearity precisely and an alternative, more flexible, means of doing this would be preferred.

According to a first aspect of the present invention, a platform comprises a support providing a controlled thermal expansion coefficient in a given direction, the support being formed of a material having a first thermal expansion coefficient and being connected to a body having a second thermal expansion coefficient, the first thermal expansion coefficient being different from the second thermal expansion coefficient, such that a change in temperature causes a relative change in the dimensions of the support and the body in a direction perpendicular to the said given direction, thereby resulting in the variation of a force applied to the support by the body in the direction perpendicular to the said given direction to control the resultant expansion coefficient in the said direction.

With the arrangement of the present invention, a support is provided that, alone, will respond normally to changes in temperature. In particular, where there is an increase in temperature, the support will naturally increase in size in all directions. However, due to the differential expansion of the support and the body, a force acting on the support by the body will vary. This force will be in a direction dependent on the connection between the support and the body. By virtue of the Poisson effect, the force applied to the support in a particular direction will moderate the dimensional change due to the variation in temperature in a direction perpendicular to the direction of the force. For example, where the body has a greater thermal expansion coefficient than the support, an increase in temperature will increase the force applied to the support. This increase in force will elastically strain the support. Where the Poisson ratio is positive, this will cause a contraction, of the support in a direction perpendicular to the direction of the force applied to the support. This will at least partially counteract the increase in size in the direction perpendicular to the direction of the force due to the increase in temperature. Therefore, in the given direction, the dimension will be less than that which would be predicted based on the increase in temperature alone, but greater than that predicted for the lateral contraction of the support when strained longitudinally. Some materials may show a negative Poisson ratio in a specific direction corresponding to a lateral expansion when subject to a longitudinal extension.

For some materials, the thermal expansion coefficient of the material may be different in different directions. In this case, the first and second thermal expansion coefficients may be the thermal expansion coefficients in the direction perpendicular to the said given direction.

The thermal expansion coefficient of the strip may be positive, negative or neutral.

It is preferred that the support remains generally flat or planar in the said given direction. This is of particular advantage where an optical element, such as an optical fibre, is supported by the support, since this avoids bending of the fibre. Bending of the fibre may result in undesirable bending forces being applied to the fibre.

Preferably, the force between the support and the body acts in a plurality of directions, each perpendicular to the said given direction. More preferably, the force is applied in two directions, each perpendicular to the said given direction. By applying the forces in more than one direction perpendicular to the said given direction, the magnitude of the thermal expansion coefficient in the said given direction is varied.

The support may be in the form of a strip or sheet, and in this case it is preferred that the body in the form of a frame connected to at least two opposite ends of the strip or sheet . Alternatively, the support may be in the form of a core, and the body is in the form of a sheath surrounding the core. In this case, the forces will be applied to the core in all radially extending planes including the axis of the core.

The support may be connected to the body by any suitable means, including an adhesive or bonding, or a mechanical fixing, for example by a screw. Many materials may be used for the support and the body providing these have the required thermal expansion coefficients and Poisson value. Suitable materials for the support or body include Invar (Trade Mark) , Kovar (Trade Mark) , Elvinar (Trade Mark) , β-eucryptite, zirconium tungstate and iron. Suitable materials for the other of the support and the body include iron, aluminium, glass reinforced plastic or zinc.

To provide for variable, yet controlled and known, thermal expansion of the support in the said given direction, the relative cross-sectional area of the support with respect to the overall cross-sectional area of the support and body may be different at different longitudinal positions of the support. In this way, the support will have a different thermal expansion coefficient at different longitudinal positions.

According to a second aspect of the present invention, a Bragg fibre grating is mounted on a platform according to the first aspect of the present invention. As discussed above, in many applications it is desirable for the Bragg fibre grating to respond in a particular manner to changes in temperature, in particular to control the frequency response of the grating. This often requires a net contraction or compression of the grating with an increase in temperature. Since the platform according to the first aspect of the present invention can provide a support having a negative thermal expansion coefficient in a given, lateral, direction, the mounting of a Bragg fibre grating on such a support can achieve this compression with an increase in temperature.

The Bragg fibre grating may be attached to the support by any suitable means, including an adhesive, solder or low melting point glass material. In this case, the Bragg fibre grating may either be attached to the support at a number of discrete positions, or along substantially its entire length. Alternatively, the Bragg fibre grating may be formed integrally with the support. A means may be provided to apply a force to the support and/or the body to apply a strain or a compression to the Bragg fibre grating. This may further enhance the control of the grating in response to temperature variations. The support may be a loop of material with a rod attached across the loop. The support may be provided by beams pivotally attached to one another to define a three or more sided shape with the body being formed from the rod attached to opposing corners of the shape. The support may also comprise connecting members attached to the beam onto which a component for which temperature compensation is to be provided can be attached in use .

The present invention overcomes difficulties in some systems associated with the need for any device to be made with great precision, in particular with regard to the lengths of the parts, and can be used to give controlled thermal expansion in more than one direction by using more than one platform.

The present invention will now be described, by way of example, with reference the accompanying drawings, in which:

Figure 1 shows a first example of a platform according to the present invention;

Figure 1A shows a cross-sectional view of the platform of Figure 1 according to the present invention, taken through the plane A-A.

Figure 2 shows a second example of a platform according to the present invention;

Figure 3 shows a third example of a platform according to the present invention;

Figure 4 shows a fourth example of a platform according to the present invention;

Figure 5 shows a fifth example of a platform according to the present invention; Figure 6 shows a graph of the variations in dimensions of a strip with temperature, showing how the thermal expansion of an iron strip can be moderated in a controlled fashion with the solid line giving the predicted dimension change in dimensions;

Figure 7 shows a graph of the variations in dimensions of a strip with temperature; showing how the width of the Invar strip varies as the temperature is increased from 25 to 150 °C, where the Invar is constrained by an aluminium frame; the solid line giving the predicted change in dimensions; Figure 8 show the results of example 3 of the invention;

Figure 9 is a schematic view of a further example platform in accordance with the present invention;

Figure 10 is a plan view of the example of figure 9; Figure 11 showing a non-linear design according to the invention, in which a controlled non-linear overall expansion coefficient is also obtained;

Figure 12 is a graph showing the characteristics of an example of the device of figures 9 and 10; Figure 13 is a graph showing the changing coefficient of thermal expansion as the angle of the framework, to 2β is varied;

Figure 14 is a plan view of an alternative example of the device of figure 9; Figure 15 is a graph showing change in dimensions for the example of figure 14 ; and

Figure 16 is a graph showing changes in dimensions on heating and cooling for a framework with an example angle, β of 15°. One particular use of the platform according to the present invention is for use with a Bragg grating. The following description of preferred examples of the invention will be made with reference to such a grating, although it will be appreciated that the platform may support other items, in particular optical systems and measuring devices . The Bragg grating has a modulated change in the refractive index along an optic fibre, which will interact with a specific wavelength of light travelling along the fibre. Such gratings may be used to make basic optical components such as mirrors and wavelength filters.

The magnitude of the wavelength, λ_B/ that interacts with the grating is determined by the expression: Λ_B=2nΛ where n is the refractive index of the optical fibre and Λ is the spacing of the grating.

A change in the external temperature causes both a change in the refractive index and a change in the spacing of the grating within the fibre, causing a change in the wavelength of the signal that will interact with the grating: leading to a loss of wavelength discrimination of the grating. In many circumstances, this is undesirable.

It is therefore desirable for the fibre to be supported in such a way that the length of the fibre, and in particular the spacing between the grating elements, is controlled such that there is no change in the reflected wavelength of light despite variations in the temperature. It will be appreciated that this requires a compensation to prevent the normal expansion of the fibre due to an increase in temperature (or to prevent the normal contraction of the fibre due to a decrease in temperature) , and to compensate for the change in refractive index due to a change in the temperature. In general, this will require that the support have a negative temperature expansion coefficient. It is also desirable that the fibre is not subjected to unnecessary strains or other forces that may reduce its life, and that the fibre is not bent, which may also reduce its life and cause loss of light. It is also important that the fibre can be readily attached to the support. The present invention provides a platform or support that can provide the required thermal properties . An example of a support according to the present invention is shown in Figure 1. A strip of a material S, with a low thermal expansion coefficient, α_s, is fixed at its ends to a body or frame F made of a material with a high thermal expansion coefficient, α_F. The strip S is made of Invar, Kovar, Elvinar (all trade names) , β-eucryptite, zirconium tungstate or other material of a low expansion coefficient, whilst the body or frame F is made of iron, aluminium, glass reinforced plastic, zinc or other material of a high expansion coefficient. The support S is fixed to the frame F by any suitable means, for example by an adhesive. The strip S is arranged to lie on the neutral axis of the platform. The use of material is such as Invar and aluminium, which are already in widespread use in optic fibre applications, avoids the difficulties associated with the degradation of materials such as zirconium tungstate in humid environments.

As can be seen from Figure 1, the cross-sectional area of the strip S in the direction perpendicular to the direction of the connection between the strip and the frame is small compared to the overall cross-section area of the device (the frame and the strip) in the same direction. This is best seen in the cross-sectional drawing of Figure 1A. In this case, the area of the strip is A_s and the area of the frame as A_F. As shown, the area of the strip A_s is much less than the overall area of the device (A_S+A_F) in this plane.

If both the strip and the frame have a positive temperature expansion coefficient when the temperature increases, both the strip S and the frame F will naturally expand. However, the frame, being formed from a material with a higher thermal expansion coefficient will increase in length more than the strip S that has a lower expansion coefficient Because the frame F is joined to the strip S, the strip S with the lower thermal expansion coefficient will be placed in tension by an amount depending on the difference in the expansion coefficients of the two members, provided that the geometry of the frame is such that bending cannot occur. This elastic tension will normally cause the strip S to contract laterally (known as the Poisson effect) since the Poisson ratio is normally positive. However for some cases v may be negative. Because the temperature has been raised it will also expand laterally.

As the area of the strip A_s is very much less than that of the frame A_F, the net effective lateral coefficient of thermal expansion of the material of lower coefficient, α_eff, will then be given by the difference between the expansion due to the rise in temperature (α_s) and the contraction due to the elastic Poisson contraction, v, of the material from which the strip is made. The contraction due to the elastic Poisson contraction (assuming v is positive) is the difference between the thermal expansion coefficients, times the Poisson ratio, times the temperature difference. Therefore,

α_eff=-vα_F+ (l+v)α_s (1) and the strip will contract when the temperature is raised if:

α_F≥il±vlc-_s (2) v By choosing the properties of the two materials, in particular the thermal expansion coefficients of both the frame F and the strip S and the Poisson ratio of the strip S, it is possible to' vary the effective lateral thermal expansion coefficient of the material of lower expansion coefficient at will so as to obtain a net lateral coefficient which may be positive, negative or zero over a wide temperature range. In particular the expansion coefficient, α_eff, may be less than the smaller value where o-_s< _F, or greater than the larger value where α_F<α_s. A Bragg fibre grating to be supported by the platform can be bonded to the strip S either continuously along its length or at two points, one on either side of the grating, using existing materials and techniques, such as organic adhesives, including epoxy resins, metal solders including Au-Sn alloys, and low melting point glasses. As the strip to which the fibre is bonded is likely to be stressed, the bonding material should be sufficiently resistant to any long-term deformation of the bond.

Where the effective thermal expansion coefficient of the strip is negative, then at the bonding temperature, e.g. the freezing point of a low melting point glass used to bond the fibre to the support, the optical fibre, and hence also the joint between the fibre and the strip, will be unstressed. However on cooling to ambient temperature, the strip will expand in the lateral direction and the fibre will be placed in tension. It is therefore important to ensure that the spacing of the grating at ambient (or some other fixed) temperature is equal to that required.

This might be achieved, for instance, by applying a compressive force to the fibre during bonding.

With the arrangement according to this example of the present invention, the frame F and the strip S are joined so that the differences in their thermal expansion coefficients induce only tensile (or compressive) stresses on the strip, avoiding bending stresses.

An alternative example of the present invention is shown in Figure 2. Unlike the example shown in Figure 1, the strip S is not placed on the neutral axis of the device. Instead, the frame or body F is made sufficiently massive (that is A_F»A_S) that the tensile or compressive stresses in the strip cannot be relaxed by bending of the frame or the strip. This allows the strip to be simply fixed by its ends to the surface of the body. Example 1 To show that a controlled expansion coefficient could be produced, a strip of iron, with an expansion coefficient of 14.1xl0^~6 K^-1, which was 21 mm wide and 0.5 mm thick was fixed to an aluminium block, with an expansion coefficient of 24.0xl0^~6 K^"1, and dimensions 31.5mm wide 34mm high and 75mm long. The strip was fixed by screws through the strip into the block and placed a distance of 50.5mm apart. The platform was placed on an electrical heater and heated from room temperature (approximately 25°C) up to approximately 130°C. The changes in the lateral dimension of the platform was measured using a scanning laser extensometer and the temperature of the iron was measured using a Type K thermocouple.

As shown in Figure 6, the iron strip undergoes a lateral expansion on heating and contracts on cooling. The effective expansion coefficient, _eff, is measured to be 10.8xl0^"6 K^"1 which is less than the value of 14.1xl0^"6 K^"1 which would be measured if the iron strip were able to expand freely on heating. To see if Poisson effects were important, the

Poisson ratio was determined by measuring the lateral ^■ contraction of the strip when the strip was uniaxially strained at a constant temperature. The value obtained using this approach was 0.33. Using this value together with the measured expansion coefficients substituted into equation 1 gives the solid line shown in Figure 6, in good agreement with the experimental measurements, indicating that Poisson effects were significant and that the expansion coefficient of the strip in the lateral direction can be carefully controlled'.

Example 2

To show that an expansion coefficient could be produced which was both negative and lay outside the range in between the expansion coefficients of the strip and the frame, a strip of Invar sheet (grade Standard IY, Imphy Ugine Precision UK) 21 mm wide, and 0.5 mm thick was fixed to an aluminium block of dimensions 31.5 mm wide 34 mm high and 75 mm long with a 50.5 mm distance between the points fixing the strip. The platform was placed on an electrical heater and heated from room temperature (approximately 25°C) up to approximately 130°C. The changes in the lateral dimension of the Invar sheet was measured using a scanning laser extensometer and the temperature of the Invar was measured using a Type K thermocouple.

From Figure 7 it can be seen that when the platform is heated the strip undergoes a lateral contraction. The effective expansion coefficient in this case is measured to be -5.2xl0^"6 K^"1, which is negative and less than the expansion coefficient of both the Invar and the aluminium. To investigate whether Poisson effects were important the coefficients of thermal expansion of both the strip and the frame material were measured using dilatometry and were found to be of 0.86xl0^~6 K^"1 and of 24.0xl0^-6 K^"1 respectively.

The Poisson ration was measured as in the first example to be 0.26. Substitution of these values into equation 1, gives an expansion coefficient of -5.4xl0^~6 K^-1, consistent with that predicted by equation (1) , indicating that Poisson effects are important and again that the expansion coefficient of the strip in the lateral direction can be carefully controlled. Example 3

To show that such controlled expansion behaviour, where the expansion coefficient lies outside the range in between the expansion coefficients of the strip and the frame, could be repeated on the same sample, a strip of glass (Zerodur, Schott) 20 mm wide and 1 mm thick was fixed to an aluminium block of dimensions 25 mm square section and 80 mm long with a 60 mm distance between the points fixing the strip to the frame. The platform was then heated from room temperature (approximately 25 °C) to approximately 90 °C. The changes in dimensions and temperature were measured as given in previous examples . As shown in Figure 8, the glass strip undergoes a lateral contraction on heating and then the same lateral expansion on cooling, measured as in the previous examples.

In the example shown in Figure 3 , the support S is in the form of a sheet fixed to the frame F at both its ends and sides. When the temperature is increased, the support S is placed in a state of biaxial, rather than uniaxial tension, so that the support extends along these two sides, causing a Poisson contraction in the direction normal to the plane of the support S. In the arrangement shown in Figure 3, the direction' will be into and out of the paper.

In this case it can be shown that the effective expansion coefficient in the direction normal to the plane of the strip, _eff is given by

(1-v)

For values of v less than 1, this gives a contraction greater in magnitude than in the uniaxial case by a factor of 2 (1-v) or about three times for most materials.

The structure of the frame and strip above could be made as a series of concentric tubes, as shown in figure 4. The outermost tube would be the high expansion material corresponding to the frame F containing a support S in the form of an inner tube with a lower thermal expansion coefficient, with a hole along the centre line through which is passed the optic fibre. Alternatively the material S may be the optic fibre itself.

It is often found that there is a need for adjusting the package or tuning it either during assembly or during connection of the device. Such fine-tuning may be considered to be of two types. The first is to compensate for discrepancies between the actual grating spacing and that required. The second is to compensate for inaccuracies in the production of the temperature-compensating package, so as to ensure that the change in dimensions with changing temperature is precisely that required.

Adjusting the frequency of the signal that interacts with the grating may be carried out by imposing an initial strain on the optic fibre. This can be done mechanically, using a screw mechanism to load the platform after the fibre has been fixed to the platform, or by applying a load to the fibre whilst fixing it to the platform. Alternatively part of the platform could be made of a piezoelectric material, so that dimensional changes might be induced by applying a voltage or of a material that change shape under the influence of an applied magnetic field, such as Invar. It is most likely that piezoelectric, e.g. quartz, or magnetostrictive, e.g. Invar, parts would make up the low expansion element of the platform.

For a given material system, the magnitude of α_eff is dependent on the relative cross-sectional area of the frame and the strip. Adjusting o-_eff requires that the relative area of the strip and the frame be changed. This is most easily done by fixing extra plates of either the strip or frame material to the appropriate part of the device .

Alternatively this might be achieved by having a strip with a varying cross section, for instance by continuously varying the width of the strip from one end to the other, as shown In Figure 5. This would give a variation in o-_eff along the length of the strip. An appropriate value of α_eff could then be chosen by fixing the fibre to that section of the strip with the appropriate cross-section.

In the example of • figure 9, a frame F is coupled to a rod R, with each of the two being made from materials with differing thermal expansion co-efficients. In principle, when the temperature changes, the frame F and rod R, if they were separate from one another, would change dimension by an amount related to the change in temperature, ΔT and the thermal expansion co-efficient of the materials from which they are formed. With the frame F and the rod R joined together, the difference in thermal expansion coefficient causes a change in the shape of the frame F so that the frame F elongates in one direction and contracts in another. This changes the spacing between supports S attached to the frame F and hence to any components attached to the supports S. The example of figure 9 has a frame F formed from four beams pivotally attached to one another. The beams may be formed from a low expansion coefficient material, such has Invar (trademark) . The rod R may then be made from a material such as aluminium, which has a higher expansion co-efficient. A rise in temperature causes the elongation of the rod R, contracting the frame F in the direction perpendicular to the rod. Appropriate selection of the material and the physical dimensions of the frame F and rod R in order to select an appropriate half angle β (figure 10) enables control of the platform so that in the support direction it has an effective expansion co-efficient, α eff described by the formula:

Ceff —

sin J3ΔT

It should be noted that other configurations of this example are possible. For example, the frame F could be formed from a ring of material with the rod R attached across the diameter of the ring.

So far it has been assumed that the effective expansion coefficient that the platform applies to the optical fibre or other device does not vary with temperature. However, the expansion coefficient varies with the angle β and as the temperature increases (or decreases) the angle will decrease (or increase) . As this occurs the expansion coefficient will also change very slightly by an amount dependent on the angle β. If β is larger than approximately 45° the change is extremely small and the effective expansion coefficient is essentially linear. However, as β becomes smaller the degree of non-linearity increases .

Achieving the required degree of non-linearity required for the thermal compensation of Bragg fibre gratings requires that the effective expansion coefficient of the frame is much more negative than is required. To overcome this problem the framework device might be modified by providing an extension bar, E.

The effective expansion coefficient of the new platform, . _eff, now becomes

L eff, frame + ^ E a 'e_ff =

(L+ E)

where o-_θff, _frame is given by equation above, α_E is the coefficient of thermal expansion of the extension bar E, and L and E are defined in figure 11. The frame can be set so that α_{effι frame} varies with temperature or not by varying the angle β and the materials from which the frame F and the rod R are constructed.

Examples of these further aspects will now be described, and the characteristics of examples of these are shown in figures 12 and 13.

Example 4

To show that a controlled thermal expansion coefficient can be obtained from a framework device . The segments of the frame were made from the strips of an Invar alloy (grade standard IY, Imphy Ugine Precision, UK) . These were 75 mm long, 1.5 mm thick and 15 mm wide and were bent through an angle of 90° to increase their stiffness. These were joined together using rods which ran through holes drilled into the sheet. The rod was made from aluminium and was constructed in three parts which allowed the length of the rod to be changed. The thermal expansion coefficients of the aluminium and the Invar (Trademark) were measured using dilatometry and were 24X10^"6K^-1 and 0.86X10^"6K^-1 respectively. The framework was placed on an electrical heater and heated from room temperature (approximately 25°C) up to around 100°C. The change in the dimensions of the framework in the direction perpendicular to. the rod was measured using a linear voltage displacement transducer (LVDT) . The length of the rod was fixed so that the measured valve of β was 43.

Figure 3 shows the change in dimensions, plotted as microstrain of the perpendicular dimension. It can be seen that the measured dimensional changes give good agreement with those predicted and correspond to an effective expansion coefficient, α_eff, of approximately -20X10^~6K^_1. This is outside the range of the values that lie between α_F and α_R.

Example 5

To show that changing the angle, β, gives rise to a change in the effective expansion coefficient and that this angle change might be achieved using a screw device in the rod. The framework above was used, with the length of the aluminium rod being varied in order to change the angle β . The change in the dimensions of the frame on heating or cooling in the perpendicular direction was measured using either an LVDT or a scanning laser extensometer. The changes in dimensions, .which are reversible on heating and cooling, are plotted as an effective expansion coefficient, α_eff. These vary with^' β as predicted by eqn 1 as shown in figure 4, with the values of α_eff varying between -20X10^"6K^_1 and -140X10^"6K^_1. Example 6

In example 4, the .joints between the members of the framework were made using rods which ran through holes drilled in the members of the framework. Alternatively the frame may be cut out of a single piece of metal and, if appropriate, the cross-sectional area of the joint can be reduced by introducing a cut into the framework as shown in figure 14.

Frameworks were made from Invar sheet 0.5 mm thick with ligaments of width 2 mm and length 26.5 mm. The angle, 2β, between the arms, as shown in Figure 11, is approximately 68°. The frame was fastened with two screws as shown in figure 14 to the aluminium block, which was sufficiently thick that it could not bend. The measured and predicted changes in dimensions perpendicular to the aluminium cross- piece for a device of this type is shown in Figure 15. The average coefficient of thermal expansion was -46 10^"16 K^"1. This shows that the effective expansion coefficient of the frame can be controlled and that it lies outside the range of expansion coefficients between Invar and aluminium, as described in the previous examples . Example 7 Frameworks were made from Invar sheet 0.5 mm thick with ligaments of width 2 mm and length 22.6 mm. The angle, 2β, between the arms, as shown in Figure 14, is approximately 30°. This single piece frame was fastened with two screws in to the aluminium block as above . The measured and predicted changes in dimensions plotted as microstrain in the direction perpendicular to the aluminium cross-piece as previously are shown in Figure 16. It can be seen that the measured dimensional changes are non-linear and are also close to those that can be predicted (« _rf=-32ixιo^-6-57Xio"⁹(τ-τ₀)-κr¹) , showing that precisely controlled non-linear changes of dimensions can be introduced and that the changes of dimensions are the same after repeated heating and cooling.

Claims

1. A platform comprising a support providing a controlled thermal expansion coefficient in a given direction, the support being formed of a material having a first thermal expansion coefficient and being connected to a body having a second thermal expansion coefficient, the first thermal expansion coefficient being different from the second thermal expansion coefficient, such that a change in temperature causes a relative change in the dimensions of the support and the body in a direction perpendicular to the said given direction, thereby resulting in the variation of a force applied to the support by the body in the direction perpendicular to the said given direction to control the resultant expansion coefficient in the said direction.

2. A platform according to Claim 1, in which the support remains generally planar in the said given direction irrespective of changes in temperature.

3. A platform according to Claim 1 or Claim 2, in which the support has a negative thermal expansion coefficient in the said given direction.

4. A platform according to Claim 1 or Claim 2, in which the support has a positive thermal expansion coefficient in the said given direction.

5. A platform according to Claim 1 or Claim 2, in which the support has a zero thermal expansion coefficient in the said given direction. •

6. A platform according to any one of the preceding claims, in which the force between the support and the body acts in a plurality of directions, each perpendicular to the said given direction.

7. A platform according to Claim 6, in which the force between the support and the body acts in two directions, each perpendicular to the said given direction.

8. A platform according to any one of the preceding claims, in which the support is in the form of a strip or sheet .

9. A platform according to Claim 8, in which the body is in the form of a frame connected to at least two opposite ends of the strip or sheet .

10. A platform according to any one of Claims 1 to 7, in which the support is in the form of a core,- and the body is in the form of a sheath surrounding the core.

11. A platform according to any one of the preceding claims, in which the support is connected to the body by an adhesive fixing or by bonding.

12. A platform according to any one of Claims 1 to 10, in which the support is connected to the body by a mechanical fixing, for example by a screw.

13. A platform according to claim 1, wherein the support is a loop of materials with a rod attached across the loop.

14. A platform according to claim 13 , wherein the support is provided by beams pivotally attached to one another to define a three or more sided shape with the body being formed from the rod attached to opposing corners of the shape .

15. A platform according to claim 13 or 14, wherein the support also comprises connecting members attached to the beam onto which a component for which temperature compensation is to be provided can be attached in use.

16. A platform according to any one of the preceding claims, in which the support or the body is formed from one or more of Invar, Kovar, Elvinar, β-eucryptite, zirconium tungstate, iron, aluminium, glass reinforced plastic or zinc.

17. A platform according to Claim 16, in which the other of the support and the body is formed from one or more of iron, aluminium, glass reinforced plastic or zinc.

18. A platform according to any one of the preceding claims, in which the cross-sectional area of the support is small compared to the overall cross-sectional area of the support and the frame in the same plane .

19. A platform according to any one of the preceding claims, in which the relative cross-sectional area of the support with respect to the overall cross-sectional area of the support and body is different at different longitudinal positions of the support, and therefore the support has a different thermal expansion coefficient at different longitudinal positions:

20. A platform according to any one of the preceding claims, in which an extension bar attached to the framework to provide a non-linear degree of thermal compensation.

21. A platform according to any one of the preceding claims, in which the support supports an optical element, such as a telescope, a Fabray-Perot cavity or a Bragg fibre grating.

22. A Bragg fibre grating mounted on a platform according to any one of Claims l to 20.

23. A Bragg fibre grating according to Claim 22, in which the Bragg fibre grating is attached to the support by an adhesive, solder or low melting point glass material.

24. A Bragg fibre grating according to Claim 22 or Claim 23, in which the Bragg fibre grating is attached to the support at a number of discrete positions.

25. A Bragg fibre grating according to Claim 22 or Claim 23, in which the Bragg fibre grating is attached to the support along substantially its entire length.

26. A Bragg fibre grating according to Claim 22, in which the Bragg fibre grating is formed integrally with the support.

27. A Bragg fibre grating according to any one of Claims 22 to 26, in which the fibre is pre-stressed or compressed when this is attached to the support such that the fibre will be compressed or stressed to a predetermined degree at ambient temperature at which the grating is intended to operate .

28. A Bragg fibre grating according to any one of Claims 22 to 26, including a means to apply a force to the support and/or the body to apply a strain or a compression to the fibre .