WO2003102650A1

WO2003102650A1 - Method of varying the chemical composition in optical fibers

Info

Publication number: WO2003102650A1
Application number: PCT/GB2003/002312
Authority: WO
Inventors: Michael Fokine
Original assignee: Optoplan As; Harding, Richard, Patrick
Priority date: 2002-05-30
Filing date: 2003-05-27
Publication date: 2003-12-11
Also published as: SE0201600D0; AU2003234033A1

Abstract

A method for forming chemical composition gratings in an optical waveguide is disclosed. The method comprises, in order, the steps of: (Step 1) loading the core of the waveguide with molecular hydrogen; (Step 2) energizing said core such that said molecular hydrogen forms hydroxyl groups in the waveguide material; (Step 3) annealing the waveguide at a predetermined annealing temperature and for a predetermined annealing time; and (Step 4) developing the grating by subjecting the waveguide to a developing temperature, which is higher than the annealing temperature and higher than a relaxation temperature for the waveguide material.

Description

METHOD OF VARYING THE CHEMICAL COMPOSITION IN OPTICAL FIBERS

The present invention relates to a method of varying the chemical composition in optical devices, more particularly to a method for forming chemical composition gratings (CCG.s) in optical fibers.

It is known in the prior art to form refractive index changes in glassy materials such as silica-type optical fibers by exposing the material to ultraviolet (UV) radiation. In particular, exposure of an optical fiber to UN radiation having spatially periodic intensity, e.g. produced by interfering two coherent beams of UN light to produce interference fringes, has been used for forming refractive index gratings in the core of the fiber.

Generally, photosensitivity of the glassy fiber material is believed to be governed by germanium defects, also known as "germanium-oxygen deficiency centers" (GODC). When the photosensitive material is exposed to radiation having a wavelength corresponding to germanium-related absorption peaks at about 195 nm or 240 nm, the refractive index of the material is changed.

However, refractive index changes obtained by the above method are erased at elevated temperatures. Typically, the visibility of the gratings fade away completely at temperatures above about 600°C. The degradation of the gratings starts at much lower temperatures around 100°C, thus preventing endured operation at such temperatures. Another limiting drawback of the above method is that radiation having one of a few specific wavelengths must be employed. Attempts have been made to overcome the above-mentioned problems of fading and special wavelength requirements. For example, methods of forming chemical composition gratings (CCG:s) in optical fibers have been described. One method of forming a CCG is disclosed in WO 98/12586, wherein a silica fiber was used that was doped with germanium (Ge) and fluorine (F) in its core. Fluorine has an index-lowering effect of the glass composition of the fiber, and the forming of refractive index changes in the fiber was based on a subsequent removal of fluorine from the fiber core. In order to remove fluorine from the fiber, hydrogen was first loaded into the glass of the fiber. The hydrogen loaded into the fiber bonded to the germanium and to the silicon in the form of hydroxyl (OH) groups. The hydroxyl groups are formed by subjecting the material to energy, e.g. by means of localized heating or electromagnetic radiation, causing the molecular hydrogen to form the said hydroxyl groups. Heating the fiber, or subjecting the same to electromagnetic radiation, caused the hydroxyl groups to react with fluorine and thus form HF. Now, HF is much more mobile in the glass matrix than fluorine bonded to germanium or silicon. Therefore, fluorine could be removed from the glass in the form of HF, thereby lowering the fluorine content in the fiber. Since fluorine has an index-lowering effect, removal of fluorine caused an increase in the refractive index. In some applications, in particular high-temperature applications, CCG:s are preferred due to their much higher temperature stability. In a CCG, the refractive index modulation is a result of a periodic change in the chemical composition of the waveguide material, rather than induced defects as is the case for conventional Bragg gratings. The stability of CCG.s is limited by diffusion of dopants at elevated temperatures, and not by any preservation of defects or deficiencies .

Although providing some quite useful improvements, the above method of forming CCG:s in optical fibers has proven to give a poor yield in the production of fiber gratings. Therefore, there is a need for improvements relating to the forming of temperature-stable CCG:s.

The present invention aims to provide an improvement of the method disclosed in the above-mentioned publication WO 98/12586.

The present invention is based on an insight into the chemical and physical processes behind the formation of chemical composition gratings. Although the term "grating" is widely used in this specification, it is to be understood that the same principles may be applied for long-period gratings and for non-varying index changes.

The inventive method is based on controlled removal of fluorine or some other refractive index modifying species from the glass composition. Taking fluorine as the example, it is removed in the form of HF, which is comparatively mobile within the glass matrix. In order for the process to be initiated, there must be a sufficient quantity of hydroxyl groups (OH groups) in the glass. How these OH groups are formed will be explained in detail later in this specification. Consider a germanium-doped optical silica fiber that contains a small amount of fluorine in its core. As known in the art, fluorine has an index lowering effect on the glass. Hence, subsequent removal of fluorine will cause an increase in the refractive index. Germanium is known to lower the activation energy for OH formation, and is therefore used for promoting OH formation. The formed OH group may be bonded to either Si or Ge.

The present invention is not restricted to any particular theoretical model of the chemical reactions taking place during the refractive index change. However, the present understanding is that either or both of the reactions below take place in the process.

≡ X - OH + F - X =<^= X - 0- X ≡ +HF (1) and/or

2 ≡ X - OH ±> H₂θA- ≡ X - 0 - X ≡ (2a) followed by

H₂0 + F -X ≡<r HF + HO- X ≡ (2b)

where X is Ge or Si, - denotes a single bond and ≡ denotes a bond to three connecting oxygen. Whichever of reactions (1) or (2) describes the "true" reaction, there is a need for diffusion/migration of OH in the glass so that the OH can react with either bonded fluorine (first case according to reaction 1) or another OH group (second case according to reactions 2) to form HF.

Hence, fluoride is removed in the form of HF, and the resulting refractive index change in the material depends on the amount of fluoride removed. The higher the temperature is during an annealing step, the faster is the production of HF by the above reactions and the faster is the diffusion of HF out from the material. However, a high temperature also increases the diffusion of OH groups in the material. Effectively, diffusion of OH takes place by formation and diffusion of molecular H₂O, governed by the equilibrium between OH and H₂O. In fact, OH per se is comparatively immobile in the glass. This diffusion may cause the concentration of OH to decrease and, more importantly, the visibility of a spatially varying OH concentration (also called "OH grating visibility") to decrease. In other words, a too high temperature will erase the pattern of OH necessary for grating forming, and said visibility will begin to gradually decrease already at comparatively low temperatures. Thus, there is a trade-off between HF-formation on the one hand, and OH pattern visibility fade out on the other hand. Therefore, it should be possible to find an appropriate annealing temperature and annealing time in order to achieve the highest possible change in the refractive index. Put differently, for any acceptable annealing time, there should be an optimal annealing temperature and vice versa.

Chemical reactions and diffusion during annealing introduce structural defects and mhomogeneities in the waveguide material. In fact, in case of a periodic formation of hydroxyl groups in a previous step, these defects and mhomogeneities actually form a grating in the waveguide. However, this kind of intermediate grating is not temperature stable. In order to achieve a "true" chemical composition grating, the annealed waveguide is processed in a final developing step, wherein the waveguide is subjected to a yet higher temperature above a glass relaxation temperature. During development, any residual refractive index variations (e.g. caused by defects and/or mhomogeneities) in the glass of the waveguide is effectively removed, and the chemical composition grating actually forms. After development, only the CCG remains in the waveguide since all periodic variations other than pure chemical variations have been removed. Hence, the intermediate grating is erased and replaced by a CCG. The above-mentioned facts, and in particular the behavior during annealing, foim a basis for the present invention.

Therefore, it is an object of preferred embodiments of the present invention to provide a method of forming chemical composition gratings in optical fibers, which method takes into account the behavior explained above, with the aim of improving the visibility and the refractive index modulation depth of the gratings formed according to the invention.

In a more general sense, it is an object of preferred embodiments of the invention to provide a method of altering the refractive index in a waveguide by means of altering the chemical composition of the waveguide material. Typically, the waveguide material is silica-based (SiO₂) and doped with index-lowering species such as fluorine. The inventive method aims at removing said index-lowering species in a controlled manner by an inventive selection of annealing temperature and annealing time. Aspects of the invention are set out in the independent claims.

Previous methods of forming chemical composition gratings in optical waveguides, such as optical fibers, have employed temperature ramping from lower to higher temperatures during some kind of combined annealing and developing of the grating, or rapidly heating the fibre to a developing temperature, thus actually eliminating the annealing and relying solely on the processes taking place at the developing temperature. For this reason, the annealing has neither been performed at the optimal temperature, nor for the optimal time. This has led to a spread in the quality of the thus formed gratings. The insight into the underlying reactions and processes in connection with formation of chemical composition gratings has led to the present invention, which teaches that annealing at a substantially constant temperature, or possibly at a few different temperature levels within a restricted temperature range, leads to a CCG of higher quality and to more reliable results. Therefore, a higher yield may be obtained by producing CCG:s in accordance with the present invention.

Different waveguide materials (i.e. glass compositions) have different characteristics. For this reason, it is not possible to know in advance exactly how a particular material will respond to the annealing and developing steps during the forming of the CCG. In many cases, however, a rough estimate of the appropriate process parameters can be obtained from experience or from theoretical considerations. Nevertheless, it is envisaged that an examination of a particular material is necessary prior to the actual production. It is proposed here to make a "master curve" for the material at issue. The master curve describes the resulting refractive index changes in a CCG as a function of annealing temperature and annealing time. From the master curve, it is then straightforward to select the appropriate temperature and time. For example, one may be restricted to a certain annealing time (e.g. due to production requirements), and the appropriate annealing temperature for that annealing time is found from the master curve.

Some preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Fig. 1 is a flow chart showing the main steps in a method of forming a chemical composition grating according to the invention; Fig. 2 is a diagram showing a first type of master curve for a waveguide material; and

Fig. 3 is a diagram showing a second type of master curve for a waveguide material.

Basically, the inventive method comprises four basic process steps (hydrogen loading, hydroxyl formation, annealing and developing) and one optional process step (pre-annealing), as schematically illustrated in figure 1. The steps are as follows.

Hydrogen loading (First step - Step 1)

The purpose of this step is to sensitize the fiber so that a chemical composition grating can be written into the core of the fiber. In this step, molecular hydrogen is loaded into the fiber in order to allow OH formation therein when it is subjected to energy (e.g. by heating or by UN exposure, as described below). Hydrogen is loaded into the fiber at an elevated pressure, typically at 100 Bar.

However, higher pressures are conceivable, and is expected to result in an increase of the maximum final refractive index modulation. The hydrogen loading is typically performed at temperatures from about room temperature to about 50°C. However, higher temperatures are also possible, provided that the temperature is kept below a hydroxyl formation temperature at which OH groups are formed in the fiber. Hydroxyl groups are normally formed at temperatures above 200-250°C.

To obtain good hydrogen loading of the fiber, a temperature of 50°C, a pressure of 100 Bar and a loading time of about 5 days are appropriate. Of course, the atmosphere surrounding the fiber during the hydrogen loading step should be comprised of hydrogen.

In order to prevent hydrogen that has been loaded into the fiber from escaping, a hydrogen loaded fiber should preferably be stored at low temperatures. Preferably, the hydrogen loaded fiber should be stored in liquid nitrogen within a few hours after removal from the hydrogen loading chamber.

Hydroxyl formation (Second step - Step 2)

In this step, hydroxyl groups are formed in the fiber by causing the molecular hydrogen that has been loaded into the material to react with species of the fiber material, such that OH groups are formed. This is accomplished by exposing the hydrogen loaded fiber to energy at the portions where hydroxyl groups should be localized. The energizing of the hydrogen loaded fiber can be performed by heating, exposure to electromagnetic radiation or other suitable means. For high pitch gratings (gratings with a short period), exposure by means of electromagnetic radiation is preferred. One particularly preferred method of exposing the fiber is by interfering laser beams, producing a periodic intensity profile on the fiber.

In this way, periodic formation of OH groups can be achieved with very short periods. For gratings with longer periods, energizing (heating) by means of a localized flame or similar may be preferred, by virtue of its simplicity compared to elaborate interference apparatus. Other means of energizing the fiber are also available, such as exposure to actinic radiation or to radiation from a CO₂-laser.

In order to arrive at a good result with a chemical composition grating of high visibility, careful control of the OH formation is believed to be tantamount. If the OH grating produced in this step has a low visibility, the resulting chemical composition grating will also exhibit poor visibility.

Before energizing the fiber in the OH formation step, the fiber should be carefully cleaned, to avoid surface heating and a decreased grating visibility that could arise from such surface heating. Also, removal of the coating from the fiber may be necessary.

Pre-annealing (Optional step - Step 2a)

Pre-annealing, or hydrogen annealing, is an optional step that may be performed after the OH formation step. In this step, residual or excess molecular hydrogen that has not formed hydroxyl groups is removed from the fiber material.

Residual hydrogen is removed from the fiber by subjecting the fiber to an elevated temperature. The pre-annealing temperature should, of course, be below the hydroxyl formation temperature, which is typically about 200-250°C. Preferably, the pre-annealing temperature should be below 150°C, as a safety measure. The lower the pre-annealing temperature is, the longer time is required in order to remove the majority of molecular hydrogen from the fiber. Below, some examples of pre-annealing time and temperature, and residual hydrogen as a result thereof, are given.

For the best result of the pre-annealing, the fibers should be stored in an inert atmosphere, such as N₂, during the pre-annealing time. This maintains, or minimizes the degradation of, the mechanical stability of the fiber.

Annealing (Third step - Step 3)

In the annealing step, the fiber is subjected to a relatively high temperature between 400°C and 1000°C, at which the hydroxyl groups in the fiber are caused to react with a refractive index modifying species (e.g. F) to form a mobile compound (e.g. HF) that is subsequently removed from the material. Prior to insertion into the annealing chamber, the fibers should be thoroughly cleaned, hi order to minimize any decrease of mechanical strength, the annealing should preferably be performed in an inert atmosphere such as N₂.

The annealing time and temperature is preferably selected from a previously recorded "master curve", as shown in fig. 2 and 3. The selected time and temperature is often determined by the acceptable manufacturing time. In some cases, a longer annealing time can be accepted, while in another case, the annealing time must be minimized. Also, a high annealing temperature often cause a decrease of the grating visibility. Therefore, higher quality gratings can be obtained at the expense of longer manufacturing times.

During annealing, two competing processes occur in the material, namely:

(i) formation and diffusion of HF, and

(ii) diffusion of the periodic OH pattern.

As mentioned above, the formation and diffusion (removal) of HF is the mechanism behind the actual formation of the chemical composition grating. Remember that F is a refractive index lowering species, and removal thereof hence results in an increase of the refractive index. Periodic removal of F from the fiber material is obtained by the preceding step of periodic formation of hydroxyl groups, which are required for the formation of HF.

Diffusion of the OH pattern lowers the visibility of the grating and leads to formation of HF at undesired positions within the fiber. The final refractive index profile in the fiber is the product of the OH visibility and the rate of HF formation. Since both these processes are promoted by a higher temperature, the following behavior will usually occur during the annealing step:

- At low annealing temperatures, the decrease in OH visibility is negligible and the rate of HF formation and removal is low. - At higher annealing temperatures, the rate of HF formation and removal is increased, but the OH visibility decreases faster.

- Low annealing temperatures can be used for obtaining high quality gratings, but at the expense of long annealing times.

- High annealing temperatures can be used in order to have a short annealing time, but at the expense of a decrease in the grating quality.

Hence, for each acceptable annealing time, there should be a corresponding optimum annealing temperature.

Typically, the annealing temperature is selected within the range from about 500°C to about 800°C. In figure 2, a first example of a "master curve" is shown. In this case, a series of gratings were produced by first hydrogen loading a Ge/F doped fiber, and annealing at temperatures of 600°C, 700°C and 800°C for 3, 6, 12 and 24 minutes. The resulting refractive index modulation is shown in figure 2. For this master curve, the fibers were not pre-annealed prior to annealing. After annealing, the gratings were developed at a temperature of 1000-1050°C. As seen from the figure, the best results were obtained at an annealing temperature of 700°C. For the annealing temperature of 600°C, the lower modulation is explained by a low rate of HF formation and removal, whilst for the annealing temperature of 800°C, the lower modulation is explained by high OH visibility diffusion. From the master curve shown in figure 2, it can be seen that the best results are obtained for an annealing time of 700°C, except possibly for very short annealing times.

In figure 3, another example of a master curve is shown. In this case, the fibers were pre-annealed for about 20 hours at 100°C prior to annealing. The gratings were annealed at 500°C, 600°C, 700°C and 800°C for an annealing time of 24 minutes. This may be regarded as an example where there is a maximum annealing time that is acceptable in the manufacturing process. As can be seen from the figure, the best results were obtained at annealing temperatures between 600°C and 700°C. Again, the lower value at 500°C is explained by low rate of HF formation and removal, and the low value at 800°C is explained by a rapid decrease in OH visibility. Included in figure 3 are also results attained when using pre-annealing temperatures of 150°C and 230°C for annealing times ranging from 30 minutes to 72 hours. Under these conditions, none or only very weak gratings were formed. An additional data point at 1000°C is also included, as this annealing temperature corresponds to the developing temperature at which no (or only very weak) gratings are formed. From this figure, it is clear that there exists an optimal range of annealing temperatures for the selected annealing time, namely between 600°C and 700°C. Similar master curves could be taken for other types of fibers and for other annealing times in order to find the appropriate annealing temperature in each case.

Developing (Fourth step - Step 4)

Finally, the grating is developed at a temperature above the annealing temperature. During developing, the actual chemical composition grating (CCG) is formed. The developing temperature should be higher than a relaxation temperature at which the glass network structurally relaxes to a stable state. It is to be noted that the glass matrix is perturbed, or distorted, by the chemical reactions taking place during the previous steps, and that these distortions are relaxed during the developing step.

In fact, before developing, there is a comparatively strong grating present in the fiber. However, this grating is not a true chemical composition grating, and it is very sensitive to temperature. This grating is more like a traditional grating, in which defects and the like produce a varying refractive index. By developing the grating, these defects are removed and the glass matrix is relaxed to a homogeneous form. After developing, the remaining grating is a true chemical composition grating that is formed by a varying chemical composition. Therefore, the temperature stability of this chemical composition grating is very high, being limited only by the weak diffusion of the material itself. Hence, during developing of the grating, an intermediate grating is first erased and subsequently replaced by a true chemical composition grating having the same or similar period.

Example A practical example of how to carry out the method according to the present invention is outlined below.

- Start with an single mode optical fiber of standard dimensions (core diameter = 8-9 μm, cladding diameter = 125 μm), the fiber being doped with Ge and containing a small amount (about 1-3 mol%) F in its core. - Load the fiber with molecular hydrogen by putting it in a container having a hydrogen atmosphere, a temperature of 50°C and a pressure of 100 Bar, and leave the fiber there for five days.

- After the hydrogen loading, the fiber should be stored in liquid nitrogen within a few hours to prevent the hydrogen from leaking out. - Expose the fiber to a periodic radiation pattern obtained by splitting and then interfering a laser beam from a frequency doubled Ar-ion laser, such that hydroxyl groups are formed periodically in the fiber according to the interference pattern.

- Remove excess molecular hydrogen from the fiber by pre-annealing the grating at 100°C for 15-20 hours. Preferably, use an inert atmosphere such as nitrogen during the pre-annealing.

- Anneal the grating for 30 minutes at 700°C in order to form HF and remove it from the fiber. These values for the annealing time and temperature can be used as a rule of thumb. However, the time and temperature should preferably be selected from a previously recorded master curve.

- Develop the grating at 1000°C for approximately 15-20 minutes.

Conclusion

In conclusion, a method of forming a chemical composition grating in an optical fiber is provided. The method comprises, in order, the steps of (i) loading the fiber with molecular hydrogen in order to sensitize the fiber to refractive index changes, (ii) energizing the hydrogen loaded fiber in order for hydroxyl groups to form at selected locations, (iii) annealing the fiber at a predetermined temperature for a predetermined time, to cause the hydroxyl groups to react with an index modifying species in the fiber material and thereby form a mobile compound, and (iv) developing the chemical composition grating by subjecting the fiber to a temperature above the relaxation temperature, as mentioned above, for the fiber material, such that the waveguide material is structurally relaxed and a true chemical composition grating is formed. Preferably, the annealing step is preceded by a pre-annealing step, in which residual molecular hydrogen (that has not formed hydroxyl groups) is removed from the fiber material. In particular, suitable parameters for the annealing step (viz. annealing time and annealing temperature) are selected from a previously recorded master curve for the material at issue, showing the resulting refractive index changes at different times and temperatures.

In particular, the previously used, inadequate temperature ramping has been replaced by an annealing step and a subsequent developing step.

The invention may also be described by the following numbered clauses:

1. A method of forming a chemical composition grating in an optical waveguide, said waveguide including a core of silica containing a first species X and a second species Y, wherein the first species X has the property of lowering the activation energy for formation of hydroxyl groups and the second species Y is a refractive index modifying species that has an ability to react with a hydroxyl group to form compounds containing H and N, the method comprising, in order, the steps of:

(i) loading the core of the waveguide with molecular hydrogen;

(ii) energizing said core such that said molecular hydrogen forms hydroxyl groups in the waveguide material; (iii) annealing the waveguide at a predetermined annealing temperature and for a predetermined annealing time, to cause said hydroxyl groups to react with the second species Y to form mobile compounds containing H and Y, which are diffused out from the waveguide such that the content of species Y is reduced; and

(iv) developing the grating by subjecting the waveguide to a developing temperature, which is higher than the annealing temperature and higher than a relaxation temperature for the waveguide material, such that the waveguide material is structurally relaxed and a chemical composition grating is formed. 2. A method as described in clause 1 comprising, after the step of energizing (ii) and before the step of annealing (iii), the further step of removing remaining molecular hydrogen from the core via out-diffusion by pre- annealing the waveguide at a temperature that is lower than the annealing temperature and sufficiently low for undesired formation of hydroxyl groups to be prevented.

3. A method as described in clause 1 or 2, wherein the annealing temperature and the annealing time are taken from a master curve for the waveguide material, said master curve describing the resulting refractive index modulation of the chemical composition grating as a function of annealing times and annealing temperatures.

4. A method as described in clause 3, wherein the master curve is recorded by determining a resulting refractive index modulation for a plurality of annealing times and a plurality of annealing temperatures for a sample of the waveguide material.

5. A method as described in clause 3 or 4, wherein a desired annealing time or a desired annealing temperature is selected and a corresponding annealing temperature or annealing time is taken from the master curve, thereby obtaining suitable parameters for the annealing step (iii).

6. A method as described in any one of the preceding clauses, wherein the annealing temperature is in the range from about 450°C to about 850°C.

7. A method as described in any one of the preceding clauses, wherein the annealing time is in the range from about one minute to about 45 minutes.

8. A method as described any one of the preceding clauses, wherein the step of energizing the core (ii) is performed by exposing the core to an interference pattern of electromagnetic radiation, in order to achieve a spatially varying hydroxyl content in said core.

9. A method as described in any one of the preceding clauses, wherein the step of loading the core with molecular hydrogen (i) is performed by subjecting the waveguide to a hydrogen atmosphere of a pressure at or above about 10 bar and at a temperature in the range from about room temperature to about 200°C.

10. A method as described in any one of the preceding clauses, wherein the step of developing the grating is performed at a developing temperature higher than about 900°C.

11. A method as described in any one of the preceding clauses, wherein the species X is germanium.

12. A method as described in any one of the preceding clauses, wherein the species Y is fluorine.

Although the invention has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.

Claims

CLAIMS:

1. A method of varying the chemical composition of at least a portion of an optical device, said device comprising silica containing a first species X and a second species Y, wherein the first species X has the property of lowering the activation energy for formation of hydroxyl groups and the second species Y is a refractive index modifying species that has an ability to react with a hydroxyl group to form compounds containing H and Y, the method comprising, in order, the steps of:

(i) loading the device with molecular hydrogen; (ii) energizing said portion of the device such that said molecular hydrogen forms hydroxyl groups in the material of the device;

(iii) annealing the device at a predetermined annealing temperature and for a predetermined annealing time, to cause said hydroxyl groups to react with the second species Y to form mobile compounds containing H and Y, which are diffused out from the device such that the content of species Y is reduced; and

(iv) developing the variations in the chemical composition by subjecting the device to a developing temperature, which is higher than the annealing temperature and higher than a relaxation temperature for the material of the device, such that the material of the device is structurally relaxed and a chemical composition variation is formed.

2. A method as claimed in claim 1 comprising, after the step of energizing (ii) and before the step of annealing (iii), the further step of removing remaining molecular hydrogen from the device via out-diffusion by pre-annealing the device at a temperature that is lower than the annealing temperature and sufficiently low so as to substantially prevent the undesired formation of hydroxyl groups.

3. A method as claimed in claim 1 or 2, wherein the annealing temperature and the annealing time are taken from a master curve for the material of the device, said master curve describing the resulting refractive index modulation of the chemical composition variation as a function of annealing times and annealing temperatures.

4. A method as claimed in claim 3, wherein the master curve is recorded by determining a resulting refractive index modulation for a plurality of annealing times and a plurality of annealing temperatures for a sample of the material of the device.

5. A method as claimed in claim 3 or 4, wherein a desired annealing time or a desired annealing temperature is selected and a corresponding annealing temperature or annealing time is taken from the master curve, thereby obtaining suitable parameters for the annealing step (iii).

6. A method as claimed in any of claims 1 to 5, wherein the annealing temperature is in the range from about 450°C to about 850°C.

7. A method as claimed in any of claims 1 to 6, wherein the annealing time is in the range from about one minute to about 45 minutes.

8. A method as claimed in any of claims 1 to 7, wherein the step (ii) of energizing said portion of the device is performed by exposing said portion of the device to an interference pattern of electromagnetic radiation, in order to achieve a spatially varying hydroxyl content in said portion of the device.

9. A method as claimed in any of claims 1 to 8, wherein the step (i) of loading the device with molecular hydrogen is performed by subjecting the device to a hydrogen atmosphere of a pressure at or above about 10 bar and at a temperature in the range from about room temperature to about 200°C.

10. A method as claimed in any of claims 1 to 9, wherein the step of developing the variations in the chemical composition is performed at a developing temperature higher than about 900°C.

11. A method as claimed in any of claims 1 to 10, wherein the species X is germanium.

12. A method as claimed in any of claims 1 to 11, wherein the species Y is fluorine.

13. A method as claimed in any of claims 1 to 12, wherein the device comprises a core of a waveguide.

14. A method as claimed in any of claims 1 to 13, wherein varying the chemical composition of said portion of the device comprises forming a chemical composition grating.