AU725267B2 - Temperature stable bragg grating package with post tuning for accurate setting of center frequency - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Applicant(s): JOS UNIPHASE FIBRE COMPONENTS-PTY LIMITED Invention Title: TEMPERATURE STABLE BRAGG GRATING PACKAGE WITH POST TUNING FOR ACCURATE SETTING OF CENTER FREQUENCY The following statement is a full description of this invention, including the best method of performing it known to me/us: Temperature Stable Bragg Grating Package with Post Tuning for Accurate Setting of Centre Frequency Field of the Invention The present invention relates to the construction of a package for an optical fibre Bragg grating which facilitates accurate setting of the centre frequency (ie tuning) of the grating. In a preferred embodiment of the invention the package provides for stability over a range of temperatures.

10 Background of the Invention In-fibre Bragg gratings are well known and a typical such grating normally comprises a repeating pattern of refractive index variation that is written into a photosensitive optical fibre by a UV light source or the like. The modulating pattern is normally inherently highly sensitive to the effects of strain including temperature induced strain. For this reason, it is a typical requirement to mount the in-fibre grating in a package so 0*0. as to isolate the grating region from external strain or temperature effects. Two prior art mounting arrangements are shown in Figs 1 and 2 of the accompanying drawings.

In Fig. 1, there is shown a simple grating package 1 which mounts a pre-tensioned (tuned) fibre 2 at two ends of a package mount 3 by means of epoxy resin anchors 4. With this arrangement expansion of the package 3 with temperature relative to the fibre 2 will result in a change in tension on the fibre 2.

In Fig. 2, there is shown a slightly more complex arrangement 10 which includes a temperature compensating package which uses a combination of materials with differing coefficients of thermal expansion. A first material 11 is utilised to expand at a first low rate while a second material comprising end portions 12, 13 expands with temperature at a substantially higher rate. The combined expansions of the two forms of material 11, 12 and S:36335 13 produces a more stable arrangement whereby the fibre fixing points 15, 16 are maintained with a substantially constant spaling, thereby maintaining the overall strain on the fibre 18 in the intermediate region.

In some applications, the temperature characteristic may not require specific compensating. However, it may be the case, such as when utilising the arrangement of Fig. 1, that the fibre is still required to be packaged under strain so as to achieve a greater wavelength accuracy than 10 that which can be achieved in the fibre writing process.

In the majority of prior art packaged Bragg fibre gratings, the fibre is positioned and held under strain.

When fixing the fibre to the package, a number of techniques may be employed. These include the use of epoxy adhesive or glass "fritting". These processes often require heat and as a result, they can have a further ~effect on the wavelength of the grating making it difficult to distinguish between the effects of strain or heat when setting the fibre in the package. Further, prior art methods of fixing the fibre to the package may involve some shrinkage effects as either the epoxy cures or the glass **oo So frit sets. The shrinkage can further introduce unwanted changes to the strain on the fibre and result in an alteration of the wavelength setting.

Further, the use of epoxy resin or the like to affix the fibre to the package can result in creepage of the adhesive material relative to the package or to the fibre.

Also, epoxy can be undesirably humidity sensitive.

Summary of the Invention The present invention provides a method of tuning a Bragg grating that is embodied in an optical fibre, the method comprising the steps of: positioning the optical fibre within a metal, R temperature compensating package with the grating- S\ containing region of the fibre located between spaced- S:36335 28/07 '00 FRI 10:54 FAX 01 2 9957 3582GRFIHHC j04 GRIFFITH HACK IM 004 4 apart fibre anchoring points within the package, seciaring axially spaced Portions of the fibre to the anchoring points without subjecting the package to tensile strain and, thereafter, progressively elongating the package and the contained fibre to an extent sufficient to effect permanent plastic deformation of the package and to induce tensile strain in the grating region of the fibre and so tune the grating to a required centre wavelength- Brief Description of the Drawings Notwithstanding any other forms which may fall within the scope of the present invention, preferred forms of the invention will now be described, by way of example only, 15 with reference to the accompanying drawings in which: Fig. 1 illustrates a simple grating package as utilised in the prior art; Fig. 2 illustrates a temperature compensation package 0 as known in the prior art; and Fig. 3(a) and Fig. 3(b) illustrate schematically the 20 structure of the preferred embodiment.

Description of the Preferred and Other Embodiments 000In accordance with the preferred embodiment of the present invention there is provided an apparatus for, providing a tunable Bragg grating with the apparatus 25 separating the problem of setting the centre wavelength from that of fixing or mounting the fibre to the package.

This is achieved by first fixing the fibre to the package and then applying a post fixing "tune" so as to tune the wavelength of the grating in accordance with requirements.

Further, through the utilisation of an intermediate material which has a coefficient of thermal expansion similar to the glass fibre and a supporting platform and is also compatible for the use of glass or metal solder the problem of the difficulty in utilising epoxy or polymer adhesives, especially in their sensitivity to humidity, is S:36335 28/07 '00 FRI 16:53 [TX/RX NO 5093] alleviated and an improved package results.

Turning now to Fig. 3(a) and Fig. there is illustrated one form of the preferred embodiment with Fig.3(a) showing a side sectional view and Fig. 3(b) showing a top sectional view. The preferred embodiment is designed to maintain a previously written grating 21 on a fibre 22 in a highly stable environment such that the grating frequency can be readily set. The package includes two end portions 23, 24 made of a first material, such as stainless steel, having a high coefficient of thermal expansion. A second cylindrical member 25 is provided and, importantly, includes a thinned section 26 which is provided for post tuning of the wavelength.

*Each of the members 23, 24 include a cylindrical hole in which is mounted a Kovar platform e.g. 27, 28. The Kovar platform can be slotted into a hole formed in the :corresponding member 23, 24. The Kovar platform e.g. 27, 28 provides for a material matching capability between the Kovar platform and a solder material 29, 30 which is utilised to affix the fibre 22 to the package. In this way, the necessity to utilise a polymer adhesive can be dispensed with providing for longer term stability.

Of course, many different materials can be utilised for the platform 27, 28. For example, Kovar or invar may be utilised, zirconia ceramic and some other silica materials could be utilised. Ideally, each member e.g. 26, 23, 24 and 27 and 28 are laser welded together where laser welding is suitable.

The arrangement 20 thereby provides for the utilisation of a glass or metal solder (frit) and eliminates the need to utilise any polymer adhesive thereby reducing the effects of long term creep and providing for higher long term stability.

The first step of the preferred embodiment therefore 4 T comprises manufacturing the package arrangement and fixing S:36335 the fibre 22 in place. The wavelength setting at this stage is not critical but is preferably lower than that desired in the final device.

After the fibre has been fixed and any post fixing shrinking has occurred, the package 20 can be expanded in an axial direction. The stretching can be achieved by a controlled force around a thinned portion 26 so as to permanently elongate the package. Preferably, the package is constructed from such materials as steel, kovar or invar which can be plastically deformed. The deformation achieves a permanent change in length which in turn alters the strain on the optical fibre Bragg grating in middle area 21 thereby varying the centre wavelength.

It has been found in practice that the range of increase in length required to provide a full range of tuning for most applications is typically less than The post tuning operation is preferably carried out at the temperature of operation of the device thereby providing a resulting package which provides an accurate wavelength at a given temperature irrespective of the residual characteristic of the device across a temperature range.

It would be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiment without departing from the spirit or scope of the invention as broadly described. The present embodiment is, therefore, to be considered in all respects to be illustrative and not restrictive.

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Claims (5)

1. A met hod of tuning a Bragg grating that is embodied in an Optical fibre and which comprises the steps of Positioning the optical fibre within a metal, temperature compensating package with the grating- containing region of the fibre located between spaced- apart fibre anchoring points within the package, securing axially spaced portions of the fibre to the anchoring points without subjecting the package to tensile strain and, thereafter, progressively elongating the package and the contained fibre to an extent sufficient to effect permanent plastic deformation of the package and to induce tensile strain in the grating region of the fibre and so tune *.the grating to a required centre wavelength.

2- The method as claimed in claim 1 wherein the package into which the fibre is positioned comprises a first ,**metal member extending parallel to the fibre and two second metal members secured to the first member and extending parallel to the first member, the second members having respective inner ends which provide the anchoring points for the fibr-e and outer ends that are secured to end regions of the first member, the first member being formed from a metal having a first coefficient of thermal expansion and the second member being formed from a metal having a coefficient of thermal expansion that is greater than that of the f irst member.

3. The method as claimed in claim 1 wherein the fibre is secured to the respective anchoring points by glass solder. (ST F:? S:3 6335 28/07 '00 FRI 16:53 [TX/RX NO 5093] 8

4. The method as claimed in claim 1 wherein the fibre is secured to the respective anchoring points by metal solder.

5. The method as claimed in claim 1 wherein the fibre is secured to the respective anchoring points by an epoxy resin. DATED this 2 4 th day of DECEMBER 1999 S0 UNIPHASE FIBRE COMPONENTS PTY LIMITED By their Patent Attorneys GRIFFITH HACK *00 00 00 0 o o 0 S:36335