WO2000048027A1

WO2000048027A1 - Method and apparatus for thermal control of bragg grating devices

Info

Publication number: WO2000048027A1
Application number: PCT/CA2000/000109
Authority: WO
Inventors: Robert Maaskant
Original assignee: Jds Uniphase Corporation
Priority date: 1999-02-12
Filing date: 2000-02-11
Publication date: 2000-08-17
Also published as: AU2528200A

Abstract

The present invention provides a method and apparatus for controlling the thermal response of guided wave Bragg grating devices (10) in such a way as to accentuate or annul the thermal response as desired or to otherwise control its properties. In one embodiment the method involves the imposition of a thermally dependent distribution of strains along the length of a Bragg grating in order to render its properties temperature insensitive or as nearly so as is the requirement. The method and apparatus can be used to control both the Bragg grating's inherent thermally sensitive properties as well as those thermal sensitivities produced by external tuning devices or packaging devices. The method of the present invention relies on the imposition of bending strains onto the element (32) to which the Bragg grating (10) is attached in response to a change in temperature. This can be achieved by elements (12) integral to, or mechanically or otherwise attached to the element (32) to which the Bragg grating (10) is attached. The configuration of the device and the material of which the elements are made provide for a wide range of thermal responses which allows for control of various Bragg grating properties.

Description

METHOD AND APPARATUS FOR THERMAL CONTROL OF BRAGG GRATING DEVICES

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for controlling the thermal response of guided wave Bragg grating devices in such a way as to accentuate, to annul or to otherwise control the thermal response as desired.

BACKGROUND OF THE INVENTION

The characteristic optical spectral properties of fiber Bragg gratings and the available techniques to tailor these properties both during and after fabrication allows them to form the basis of a wide variety of devices. These devices include strain, pressure and temperature sensors, spectral filters, and chromatic dispersion compensation devices among others. For example, fiber gratings have been configured to fulfill the optical filtering requirements of high capacity WDM optical transmission systems where its spectral filtering capability is used to isolate optical carriers for the purpose of de-multiplexing and adding or dropping carriers. Specially chirped fiber gratings are also used for the purpose of correcting the chromatic dispersion which exists in many optical transmission systems, particularly in the case of high capacity systems where high speed TDM and long repeater-less links are employed. In sensing applications the sensitivity of the fiber grating spectral characteristics to external influence from physical strain is used to provide a measure of the deformation of various structures such as bridge components or ship hulls.

In all of these applications, the environmental stability of the fiber grating device is essential to its successful operation in practical deployment. In most circumstances, with a notable exception being the thermal sensor application, the grating's inherent thermal sensitivity must be reduced to acceptable levels by external means.

A number of temperature stabilization schemes have been put forth which utilize the principle of applying a thermally dependent tension on a grating which is attached at its ends to compensating members of various constructions and materials (e.g. US Patent Number 5,042,898 to Morey, W.W. and Glomb, W.I.). These members impose on the grating the necessary tension adjustment to cancel the inherent thermal effects on the grating center wavelength. These techniques are generally applicable to grating devices in which the grating center wavelength is held at a fixed value or is tuned by an additional tension adjustment of the grating.

The technique described above is not applicable to those grating devices which, because of additional requirements, cannot be held in simple tension. An example is a tunable device which requires intra-grating strain control as is embodied in US Patent Number 5,694,501 issued to Alavie et al. In such a device, the grating must be bonded or attached to a body which is able to impose the requisite strain distribution along its length. It would be advantageous to provide a method and device for thermal control of the grating properties in conjunction with such devices.

SUMMARY OF THE INVENTION

The present invention relates to a device which will allow thermal control of the Bragg grating. It can be used to annul its inherent thermal behavior or to control its properties including accentuating its thermal response.

In one aspect of the invention there is provided a thermal control device for use in association with a Bragg grating. The thermal control device includes a structural element which is attached to a portion of a Bragg grating. The structural element has a predictable thermally induced change in curvature. The portion of a Bragg grating is attached to the structural element along its length thereof such that the change in curvature in the structural element induces a predictable strain response in the Bragg grating thereby modifying the thermally induced changes in the properties of the Bragg grating.

In this aspect of the invention the material may comprise at least two materials each having a different linear coefficient of expansion formed in a bi-layer structure with the Bragg grating attached along a length of one of the materials.

In another aspect of the invention there is provided a method for thermal control of Bragg gratings. The method includes the steps of providing a structural element, attaching a portion of a Bragg grating to the structural element and inducing a predictable strain response in the Bragg grating. The structural element has a predictable thermally induced change in curvature. The thermally induced change in curvature of the structural element is transmitted to the portion of the Bragg grating thereby modifying the thermally induced changes in the properties of the Bragg grating.

Further features of the invention will be described or will become apparent in the course of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS The method and apparatus for controlling the thermal response of guided wave Bragg grating devices forming the present invention will now be described, by example only, with reference being had to the accompanying drawings:

Figure 1 is a side view of a thermal control device for a Bragg grating constructed in accordance with the present invention using a bi-layer element;

Figure 2 is a side view of a thermal control device for a Bragg grating similar to that shown in figure 1 but with the Bragg grating attached to the opposed side of the bi-layer element; Figure 3 is a side view of an alternate embodiment of a thermal control device for a Bragg grating using a bi-layer element and a homogeneous element;

Figure 4 is a side view of a second alternate embodiment of a thermal control device for a Bragg grating using a rigid riser and a linear element; Figure 5 is a side view of a third alternate embodiment of a thermal control device for a Bragg grating using an arrangement similar to that shown in figure 3 but one end of the homogeneous is rigidly attached to a bi-layer element and the other end is attached in only the transverse direction;

Figure 6 is a side view of a fourth alternate embodiment of a thermal control device for a Bragg grating using a bi-layer element and a homogeneous element similar to that shown in figure 3 but showing an alternate method of attachment; and

Figure 7 is a graph showing temperature versus the center wavelength of a Bragg grating attached to a thermal control device shown in figure 6 and of a Bragg grating not attached to a thermal control device.

DETAILED DESCRIPTION OF THE INVENTION

The thermal control device of the present invention is for use in association with a guided wave Bragg grating which may be used in a wide variety of fields. In particular fiber optic intra-core Bragg gratings (hereinafter referred to as Bragg gratings) have been applied to fields as diverse as sensing and optical communications. The present invention provides a method and apparatus for controlling the thermal response of guided wave Bragg grating devices either passively, in such a way as to accentuate or annul its inherent thermal response as desired, or actively, in such a way as to control some desired optical characteristic of the Bragg grating.

The thermal control device relies upon the thermal and strain dependence of the characteristic properties of the Bragg grating. This includes the temperature and strain dependence of the characteristic center wavelength of the Bragg grating and the ability to control other various properties of the Bragg grating through the temperature and strain distribution along the length of the grating. For example, typical Bragg grating properties are such that a positive change in temperature or a tensile strain produces a positive shift in the characteristic grating center wavelength in the amount of approximately 0.010 nanometers per °C and 0.001 nanometers per microstrain. Also, a linear strain distribution imposed on the Bragg grating along its length can produce a chirped grating which has useful optical group delay properties for dispersion compensation applications.

The present invention involves the imposition of a thermally dependent distribution of strains along the length of a Bragg grating to control the thermally sensitive properties. The thermal control device of the present invention may be used to render the Bragg grating properties temperature insensitive or nearly so, as is the requirement. Alternatively the thermal control device may be used to control both the Bragg grating's inherent thermally sensitive properties as well as those thermal sensitivities produced by external tuning devices or devices required for mechanical packaging of the Bragg grating. In addition the thermal control device provides a means by which the characteristics of the Bragg grating can be controlled by controlling the temperature of a material or structure which in turn imposes a certain distribution of strains into the Bragg grating. The method of the present invention relies on the imposition of bending or tensing strains onto the element to which the Bragg grating is attached in response to a change in temperature. This can be achieved by elements integral to, or mechanically or otherwise attached to the structural element to which the Bragg grating is attached. The configuration of the device and the material of which the structural elements are made provide for a wide range of thermal responses which will compensate for or control the Bragg grating properties as desired.

Referring to figure 1 , in a first embodiment is shown generally at 11 , a Bragg grating 10 is attached to a non-homogeneous bi-layer material element 12 which, by virtue of its material properties and construction, produces a change in curvature for a change in temperature. Such constructions exist in the form of bi-material strips or layered materials for which differences exist in the linear expansivity of the layers. The material layer 16 possesses a thermal expansivity which is larger than the thermal expansivity of layer 14 such that an increase in temperature produces bending of the bi-layer element as shown in figure 1. The longitudinal axis for element 12 is straight as shown at 18 before thermally induced curvature and curved 20 after thermally induced curvature.

The Bragg grating 10 is so attached to the bi-layer material element 12 that it experiences the compressive strains produced by the thermally induced curvature at the grating attachment site. The material element and the grating attachment are so designed that this compressive strain compensates for the inherent temperature induced shift of the characteristic grating center wavelength thus annulling the thermal response of the grating as desired. This example is illustrated for an increase in temperature, the same principle being valid for a decrease in temperature but with deformation in the opposite sense being induced. The material layers 14 and 16 can be mechanically fastened by means of screws or bonded using adhesives.

In an alternatively constructed element 12 could be made of a plurality of layers of homogeneous materials, a multi-layered composite material or any other construction which produces a thermally induced curvature. Such composite materials may be metal matrix composites and fiber reinforced polymer composites.

In Figure 2 an alternate embodiment is shown generally at 21 wherein the same thermal control device shown in Figure 1 is used to accentuate the thermal response of the grating by attaching the grating to the opposite side of the bi-layer element 12. By positioning the Bragg grating 10 on the opposite side of the bi-layer element 12, it experiences tensile strains as a result of the thermally induced curvature of the bi-layer element 12. In this case the thermally induced wavelength shift in the Bragg grating 10 is enhanced by the effect of the tensile strain. Referring to Figure 3, a third embodiment shown generally at 30 comprises two elements mechanically attached to each other. Specifically the bi- layer element 12 is attached to a homogeneous bar 32 which has the Bragg grating 10 attached thereto. The homogeneous bar 32 onto which Bragg grating 10 is attached is mechanically linked to bi-material element 12 in such a way that the thermally induced curvature in the bi-material bar 12 imposes itself on the homogeneous bar 32. As above, the longitudinal axis for element 12 is straight 18 before thermally induced curvature and curved 20 after thermally induced curvature. This can be done, as illustrated in figure 3, by rigidly fixing the bars to each other at their ends so as to require common end rotation. The second element 32 then experiences a compressive strain along the site of attachment of the grating 10, creating a compensatory strain in the grating and thereby annulling the grating's inherent thermal response.

Figure 4 illustrates a fourth embodiment of a thermal control device shown generally at 40 which is somewhat similar to the embodiment shown in figure 3. Thermal control device 40 includes a homogeneous bar 42 onto which a Bragg grating 10 is attached. The homogeneous bar 42 includes rigid risers 44 at each end thereof. A linear element 46 capable of supporting a tension or compression force is attached to the risers 44 in such a way as to prevent or minimize rotational restraint of the ends of bar 42. Thermal expansion of the linear element 46 then produces bending in the homogeneous bar 42 as illustrated in Figure 3 such that compressive bending strains at the site of the attachment of the grating 10 are transferred into the grating creating a compensatory strain in the grating and thereby annulling the grating's inherent thermal response.

A fifth embodiment of a thermal control device is shown generally at 49 in figure 5. Thermal control device 49 illustrates an example of thermal control of a non-uniform strain distribution imposed on a Bragg grating 10. Thermal control device 49 comprises a bi-layer element 12 and a homogeneous element 50 mechanically linked to each other. Homogeneous element 50 is rigidly fixed to one end 52 of the bi-layer element 12 such that both elements experience a common displacement and rotation at the point of fixation. At the other end 54 the homogeneous element 50 and the bi-layer element 12 are attached such that both elements experience a common displacement in the longitudinal or y-direction but are not constrained in any other direction and are not constrained ro+ationally. Effectively then, homogeneous element 50 deforms as a cantilever bean with linearly varying strain distribution along its length and at the site of the attachment of the Bragg grating 10 thereto. As above, the longitudinal axis for element 12 is straight 18 before thermally induced curvature and curved 20 after thermally induced curvature. The homogeneous element 50 is mechanically linked to the bi-layer element 12 in such a way that the thermally induced curvature in the bi-layer element 12 imposes itself on the homogeneous element 50. Mechanical linkage of the bi-layer element 12 to the homogeneous element 50 is configured in such a way as to produce a linearly varying strain along the length of the homogeneous element 50.

Bragg grating 10 when attached to temperature control device 49 will then experience a strain distribution along its length with an average compressive strain and a strain gradient both of which vary with temperature. The material characteristics, dimensions and the location of the grating attachment can be so chosen as to produce a thermal dependence of the strain distribution imposed upon the grating 10 such that the average strain annuls the grating's inherent thermal response and the strain gradient can be independently controlled so as to produce a desired optical response from the grating. The temperature can be controlled by placing an element such as for example a resistive heading pad or a thermo-electric cooler on or near the device (not shown).

Referring to figure 6, an alternative embodiment is shown generally at 60 which is similar to thermal control device 30 shown in figure 3. Thermal control device 60 includes a homogeneous bar 62 and a by-layer element 12 which are attached together at each end by means of a tie-rod 65 and a pin support 66. Each pin support 66 is spaced from the region in which the Bragg grating 10 is bonded to the homogeneous bar. Each tie rod 62 is spaced from pin support 66 on the side opposed to the region in which the Bragg grating 10 is bonded.

A typical center wavelength versus temperature response of the Bragg grating 10 is shown in figure 7. Curve 70 shows the response of a Bragg grating when it is not attached to the thermal control device of the present invention. Curve 72 shows the response of the Bragg grating 10 when it is attached to a thermal control device of the present invention and specifically the embodiment shown in figure 6. In the example shown herein, homogeneous bar 62 was made from invar which has a linear expansion coefficient of approximately 1 x 10^"6/°C. The bi-layer 12 included a layer 14 of invar and a layer 16 of aluminum the latter of which has a linear expansivity coefficient of approximately 23 x 10^"6/°C. The tie-rods 65 were pretensioned such that they remain in tension throughout the temperature range of interest, shown herein as 0 - 50 °C. The pretensioning of the tie-rods 65 provide a means of tuning the Bragg grating 10 attached to homogeneous bar 62. The data shown in figure 7 illustrates the reduction or near annulment of the thermally induced changes in the properties of the Bragg grating.

The foregoing description of the preferred embodiments of the invention has been presented to illustrate the principles of the invention and not to limit the invention to the particular embodiment illustrated. It is intended that the scope of the invention be defined by all of the embodiments encompassed within the following claims and their equivalents. Many variations on the invention will be obvious to those skilled in the art and such obvious variations are within the scope of the invention as described herein whether or not expressly described.

Claims

WHAT IS CLAIMED AS THE INVENTION IS:

1. A thermal control device for use in association with a Bragg grating, comprising a structural element having a predictable thermally induced change in curvature; and a portion of a Bragg grating attached to the structural element along the length thereof such that the change in curvature in the structural element induces a predictable strain response in the Bragg grating thereby modifying the thermally induced changes in the properties of the Bragg grating.

2. A thermal control device as claimed in claim 1 wherein the Bragg grating is attached along the length thereof.

3. A thermal control device as claimed in claims 1 or 2 wherein the Bragg grating is attached to the structural element so that the change in curvature of the structural element causes compressive strain of the Bragg grating thereby reducing the thermally induced changes in the properties of the Bragg grating.

4. A thermal control device as claimed in claims 1 or 2 wherein the Bragg grating is attached to the structural element so that the change in curvature of the structural element causes tensile strain of the Bragg grating thereby accentuating the thermally induced changes in the properties of the Bragg grating.

5. A thermal control device as claimed in claims 1 or 2 wherein the structural element includes at least two homogeneous layers attached together each having a different linear coefficient of thermal expansion.

6. A thermal control device as claimed in claim 5 wherein the structural element includes a first layer having linear coefficient of thermal expansion attached to a second layer having linear coefficient of thermal expansion that is larger than the linear coefficient of thermal expansion of the first layer.

7. A thermal control device as claimed in claim 6 wherein the Bragg grating is attached to the first layer.

8. A thermal control device as claimed in claim 6 wherein the Bragg grating is attached to the second layer.

9. A thermal control device as claimed in claim 6 wherein the structural element further includes a third homogeneous bar attached at each end thereof to one of the first and second element and wherein the Bragg grating is attached to the third homogeneous bar.

10. A thermal control device as claimed in claim 9 wherein the third homogeneous bar being rigidly attached at one end thereof to one of the first and second element and at the other end thereof the third homogeneous bar being attached in the transverse direction and movable in the translational direction and rotationally

11. A thermal control device as claimed in claim 9 wherein the third homogeneous bar is attached at each end thereof with a tie-rod and a pin wherein the tie rod is spaced from one end of the portion of the Bragg grating and the pin is spaced between the tie rod and one end of the portion of the Bragg grating.

12. A thermal control device as claimed in any of claims 1 - 4 wherein the structural element comprises a homogeneous bar having a first and second riser one at each end of the homogeneous bar and a linear element attached between the first and second risers and the Bragg grating attached to the homogeneous bar.

13. A thermal control device as claimed in any of claims 1 - 4 wherein the structural element is a multilayered composite.

14. A method for thermal control of Bragg gratings, comprising the steps of: providing a structural element having a predictable thermally induced change in curvature; attaching a portion of a Bragg grating to structural element; and inducing a predictable strain response by transmitting the strains resulting from the predictable thermally induced change in curvature to the portion of the Bragg grating and thereby modifying the thermally induced changes in the properties of the Bragg grating.

15. The method according to claim 14 wherein the predictable strain response is a compressive strain.

16. The method according to claim 14 wherein the predictable stain response is a tensile strain.

17. The method according to claims 15 or 16 further including the step of inducing a predictable strain gradient response.

18. The method according to any of claims 14 - 17 wherein the structural element includes at least two layers attached together each having a different linear coefficient of thermal expansion.

19. The method according to any of claims 14 -17 wherein the structural element is a multilayered composite.

20. The method according to any of claim 18 wherein the structural element wherein the structural element further includes a third homogeneous bar attached at each end thereof to the at least two layers attached together each having a different linear coefficient of thermal expansion.