WO1993018420A1

WO1993018420A1 - Silica germania glass compositions

Info

Publication number: WO1993018420A1
Application number: PCT/GB1993/000462
Authority: WO
Inventors: Benjamin James Ainslie; Douglas Lawrence Williams; Jonathan Richard Armitage; Raman Kashyap; Graeme Douglas Maxwell
Original assignee: British Telecommunications Public Limited Company
Priority date: 1992-03-09
Filing date: 1993-03-05
Publication date: 1993-09-16

Abstract

It has been demonstrated that silica/germania glasses in reduced form are more sensitive to radiation in the band 225-275 nm and, therefore, reduced silica/germania glasses are particularly adapted to receive refractive index modulation, e.g. to make reflection gratings. The reduced compositions of silica/germania are identified by an absorption of more than 200 dB/cm/wt% of Ge. The reduced compositions are conveniently obtained by subjecting the glass to a non-oxydising atmosphere, e.g. helium preferably while it is in a porous state or in the form of a thin film.

Description

SILICA GERMANIA GLASS COMPOSITIONS

This invention relates to reflection waveguides and, more particularly, to silica/germania glass compositions which are adapted to receive refractive index modulation, e. g. for making reflection waveguides.

Reflection waveguides have a path region with a modulated refracted index. The waveguiding structure is often in the form of a fibre or a planar waveguide. The modulation takes the form of alternare regions of higher and lower refractive index. When radiation traverses the modulation, it is selectively reflected. The period of the refractive index modulation is usually equal to the wavelength to be reflected or to a multiple or sub-multiple of said wavelength. Thus periods in the range 0.25 to 0.6 μm preferentially reflect selected wavelengths within the range 800 - 1650 μm.

Reflection gratings have many applications in optical signalling. For example, a reflection grating can be associated with a fibre laser in order to narrow.the lasing band. When the refractive index bands are inclined to the waveguide axis, the grating can be used for the selective removal of unwanted wavelengths. In addition to reflection gratings, refractive index modulation has other applications, e. g. to achieve phase matching in waveguides and to control spot size and/or shape of a guided mode.

Refractive index modulation is conveniently produced by an optical process in which a photosensitive glass is exposed to radiation which causes an adequate change in its refractive index. The radiation has higher and lower intensities corresponding to the intended modulation of the refractive index of the glass. In many commonly used embodiments, the mutual interference of two beams of radiation produces the variation of intensity. Silica/germania glasses are widely used in optical telecommunications and it has been noticed that these glasses have an optical absorption band extending approximately over the wavelength range 225 - 275 nm and exposure to radiation within this band increases the refractive index of the silica/germania composition. The peak of the band occurs at a wavelength which is close to 240 nm. It has, therefore, been proposed to produce refractive index modulation, e. g. to make reflection gratings, by exposing silica/germania glass compositions to radiation within the wavelength band 225 - 275 nm. Radiation close to 240 nm is particularly suitable. High powers of radiation, e. g. 2 - 10 mW continuous, are needed to produce adequate changes in the refractive index and writing times of 5 - 15 mins are appropriate.

The sensitivity of the glass is important, and this invention relates to a form of silica/germania glass which is particularly sensitive to said radiation.

The glass according to the invention can be recognised by the height of the peak which displays its maximum absorption at a wavelength close to 240 nm. The peak height is related to the Ge content of the glass. However, the composition according to the invention is in a reduced state and the Ge may be present in various forms, e. g. GeO as well as Ge0₂. Therefore the peak height should be related to the total Ge content of the glass .composition, and the glass is characterised by an absorption of at least 200 dB/cm/wt% of Ge, preferably at least 300 dB/cm/wt% of Ge and most preferably at least 500 dB/cm/wt% of Ge, said absorption being measured at 240 nm.

In this specification it is appropriate to quote quantitative values for the concentration of various germanium compounds (oxides) present in the glass. There is a minor problem in that, being a reduced glass, the composition will be oxygen deficient and various oxides are likely to be- present. To take this circumstance into account the concentrations are quoted as the total Ge content calculated as the element. However this mode of expression does not imply that any of the Ge is present in the elementary state.

The glass compositions according to the invention are obtained by subjecting the glass to non-oxidising conditions, eg by heating the glass in the presence of a non-oxidising atmosphere. The non-oxidising atmosphere might be provided by an inert gas such as nitrogen or helium or the other noble gases. Alternatively a reducing atmosphere, eg hydrogen, may be used.

After the glass composition has been subjected to non- oxidising conditions it is recommended that oxidising conditions should be avoided in case the effect is wholly or partially reversed.

Wide temperature ranges are available for the non-oxidising treatment. Temperatures as low as 400°C may be used, but in this case very long treatments e.g. many hours may'be needed. ^•At 600-800°C, times of about 1 hour are usually sufficient. At temperatures of 1000°C - 1200°C times as short as 5 seconds to 5 minutes may be sufficient. The upper temperature is often set by the melting point of the glass. Where the treatment is applied to an article, the maximum temperature is usually set by the melting point of the most easily fused part of the article. It is to be expected that hydrogen, being a more potent reducing agent, will require shorter times than inert gases such as helium.

It is believed that the treatment of glass in the solid phase depends upon the diffusion of the treatment gas from the surface to the interior of the glass. It is thus desirable to conduct the heating under conditions such that there are only short, e. g. below 250 μm, path lengths from the centre of the glass composition to the atmosphere. • This can be achieved by heating the glass composition in the form of a fine powder, or while it is in a porous state. As an alternative, the glass composition can be heated while it is in the form of a thin film, e. g. up to 250 μm thick.

Glass compositions in accordance with the invention are primarily intended for use in waveguiding structures having path regions of small dimension, e. g. below 50 μm, so that the dimensions are favourable for reduction as described above. In addition, many processes used to prepare waveguides initially deposit the glass in porous layers. Thus the physical states encountered during the preparation of a waveguiding structure favour chemical reaction with a gaseous atmosphere. Thus, waveguides and reflection gratings in accordance with the invention are produced following conventional methods with non-oxidising treatment incorporated at the appropriate stage. The appropriate stage for waveguiding structures in the form of a fibre usually occurs during the preparation of a path region precursor and before the structure is drawn into fibre. For structures in the form of planar waveguides the non-oxidising treatment is usually most appropriate as the final stage of processing, that is after the guides have been fully formed.

It is now desired to indicate certain significant differences between the preparation of fibre and the preparation of planar waveguides.

In most methods of making fibre waveguides the glass is first formed as small particles which coalesce to give a porous structure. In this form, the glass displays what is in effect a large surface area whereby its reactivity with gaseous reactants is enhanced. It is convenient, in accordance with this invention, to reduce the glass while it is in this form but photosensitivity is associated with mild reduction, e. g. with oxygen deficient glasses. In order to subj ct the porous structure to mild reduction it is convenient to use non-oxidising atmospheres as described above. It is also appropriate to maintain the non-oxidising conditions while the porous structure is consolidated. After consolidation, the photosensitive region will be located in the centre of a rod or fibre and the outer layers of this structure provide substantial protection for the reduced composition and this protection makes it unlikely that the reduced composition will become reoxidised.

Planar waveguides are deposited in layers and various shaping processes may be included. It is particular important to make path regions photosensitive and it is unlikely that a path region will be the last deposited layer of" a structure.

For example, a confining region is usually deposited after a path region. Even if no final confining region is required it is usually appropriate to deposit a capping layer for mechanical protection. Thus there is a difference from fibre structures in that all path regions of a planar waveguide are likely to become re-oxidised when a subsequent layer is deposited since said deposition utilises highly oxidising conditions.

It is, therefore, usually necessary to reduce the path region of a planar waveguiding structure after an intervening layer has been deposited and it is necessary for the reducing conditions to penetrate this intervening layer. In order to penetrate the intervening layer it is appropriate to use active reducing atmospheres and especially hydrogen. Hydrogen is particularly important because it has a high rate of diffusion and it is particularly adapted to penetrate the intervening layer and reduce the path region. After this reduction it may sometimes be desired to subject the structure to further oxidising conditions, e. g. in order to deposit further path regions. Oxygen defuses far more slowly than hydrogen and, therefore, the intervening region which permitted reduction will permit oxidation to a far less extent. In view of the structural differences just mentioned it is preferred to use non-oxidising atmospheres in the preparation of fibre waveguides in accordance with the invention whereas reducing atmospheres, and especially hydrogen, are more suitable for planar waveguides.

The invention will now be further described by way of example with reference to a fibre waveguide (example 1) and a planar waveguide (example 2). In both examples the waveguide structure includes a silica/germania glass composition in accordance with the invention and a reflection grating is formed in said glass composition. The examples will refer to the accompanying drawings in which: -

Figure 1 illustrates the exposure of a waveguide to radiation in order to produce a reflection grating,

Figure 2 is a schematic showing a cross-section through a planar waveguide structure.

Figure 3 shows the absorption spectrum of a 4mole% Ge0₂/Si0₂ layer before hydrogenation (curve (a)) and after hydrogenation (curve (b)),

Figure 4 shows the transmission spectrum of a 3cm long single mode planar waveguide before hydrogenation (curve (a) ) and after hydrogenation (curve (b)),

Figure 5 shows the transmission spectrum of the planar waveguide of Figure 3 after a reflection grating has been written in the path region of the waveguide.

Example 1

In a first embodiment the invention comprises a waveguiding structure in the form of a fibre having a path region formed of a silica/germania glass composition in accordance with the invention.

It is convenient to divide Example 1 into two stages. In the first stage, a photosensitive fibre is prepared in accordance with the method of the invention.

In the second stage the photosensitive fibre is treated with radiation in order to prepare a reflection grating in accordance with the invention.

The fibre according to the invention was prepared by a modification of the well-known inside deposition process for making optical fibre. In this process, the appropriate number of layers are deposited on the inner surface of a tube which serves as a substrate. Thus the outermost layers are deposited first and the innermost layers are deposited last. After all the layers have been deposited, the tube is collapsed into a solid rod, and the solid rod is drawn into fibre.

Individual layers are produced by passing a mixture of oxygen with SiCl₄ and/or GeCl₄ through the tube and heating a small section thereof to temperatures in the range 1200^*C - 2000"C. Under these conditions the chlorides are converted into the corresponding oxides which initially deposit in the form of a fine "soot" on the wall of the tube. After deposition, a higher temperature is used so that the "soot" fuses to give consolidated glass. Melting point depressants such as phosphous and fluorine may be incorporated in the mixture to facilitate processing by causing fusing at lower temperatures.

The heating is carried out by causing a flame to traverse along the length of the tube. The flame heats a short section of the tube so that a portion, about 20 mm long, is heated to the working temperature. This technique of heating is used for all stages of the process, i. e. for the deposition, for consolidating porous layers to solid layers and for the collapse of the tube. Multiple passes are used at all stages of the process.

The speed (in mm/min) at which the flame traverses is given in the following examples. In addition, the time taken to traverse 20 mm is given.

To make the fibre according to the invention, the following sequence was used. The starting tube was made of pure silica. It had an external diameter of 18 mm and an internal diameter of 15 mm.

Cladding Deposition

The deposited cladding took the form of Si0₂ wi'th phosphous and fluorine to reduce its melting point. Six layers of cladding were deposited, and the conditions used for the deposition of each layer were as follows: -

For deposition, the flame traversed at 110 mm/min (i.e. the time to traverse 20 mm was about 10 seconds). For deposition the working temperature was approximately 1525"C. It is emphasised, that after each traverse, each cladding layer was in the form of a clear glass layer before the next layer was deposited.

For the purposes of this specification, the cladding layers are considered to be part of the substrate tube upon which the core layers were deposited. The deposition of cladding layers as described above could be omitted. The main purpose of the cladding layers is to reduce the risk of contamination from the original tube affecting core layers.

Core Deposition

Core was deposited in two layers and each layer was left in the porous state. The conditions for the deposition of each of the two layers were as follows:

The flame traversed at 110 mm/min (i.e. the same as for the cladding), but the working temperature was only 1220'C. This lower temperature ensured that the core layers remained in a porous condition for reduction.

The procedure described above constitutes a conventional preparation of a fibre preform. Once the core cursor has been deposited on the tubular substrate it is reduced by exposure to a non-oxidising atmosphere in accordance with the invention. The reduction procedure will now be described.

The procedure was carried out in five stages each of which comprised heating the porous core precursor in the presence of an atmosphere of helium which was flowed through the tube at a rate of 2 litres/min. During each of the five stages, the flame traversed at 110 mm/min whereby, for each portion of glass, a total heating time of about 50 seconds was achieved. The working temperatures used in each of the five stages were as follows: -

During the process described above, the core precursor was converted from a porous layer about 250 μm thick in an oxidised state into a thin consolidated layer about 5 μm thick in a less oxidised state. The glass was reduced because the atmosphere was helium.

After the reduction and consolidation as described above, the tube was collapsed into a solid rod. The collapse was also carried out in the presence of helium so that the reducing conditions were maintained. The mechanics of the collapse are substantially conventional (but, in normal practice, the atmosphere inside the tube contains oxygen).

Five traverses of the flame were used to collapse the tube into a rod. During each traverse, the diameter narrowed in the hot zone and, in the last traverse the bore became completely closed. The conditions during each traverse are given in the following table in which the flow rate of helium is given in litres/min, the working temperature in given in "C, the speed at which the flame traversed is given in mm/min and the time taken to traverse the 20 mm zone is given in seconds. FLOW TEMP SPEED TIME

The flow rate in stage 5 is zero because the tube is converted to a rod and flow becomes impossible. Nevertheless, the atmosphere in the remaining bore is still helium.

It is believed that further reduction occurs during the collapse.

The preform prepared as described above was drawn into fibre of 120 μm diameter at a temperature of 2,000"C. The fibre was produced at a rate of 18 metres/sec.

Using very low intensity radiation, in accordance with the standard techniques of absorption spectroscopy the absorption at 240 nm was measured by passing light longitudinally through samples of the fibre. The samples had a path length of 20 μm. The absorption was 900 dB/σm/wt%. (The radiation intensities used to measure the absorbtion are too low to affect the refractive index. )

Short lengths of the fibre described above were converted into reflection waveguides using the technique illustrated in the drawing.

A short portion 14 of the fibre 15 was illuminated by a source 10. This radiation was, in the first instance, produced by a Ar⁺ laser, frequency doubled to give output at a wavelength of 244 nm. The beam from the source 10 was directed onto a splitter 11 so that two beams were directed onto mirrors 12 and 13. The mirrors 12 and 13 caused the beams to converge onto the target section 14. Thus an interference pattern is produced with alternating regions of higher and lower intensity. Because the fibre 15 is photosensitive, the region 14 (whereon the beams are focused) is affected by the beams and the refractive . index is inσreased in the areas of high intensity. Thus a reflection grating is produced in the region 14.

It will be appreciated that the spacing of the interference pattern is affected by the angle at which the two beams intersect one another, and hence the spacing of the grating can be adjusted by adjusting the relative position of the splitter 11 and the mirrors 12 and 13.

In the example described above, the fibre was subj cted to the interference pattern for 10 mins (i.e. 10 mins writing time) under a continuous power of 5 mW in the beams which impinge on the core of the fibre.

Important measurements on the reflection grating and its fibre waveguide are given in the following table.

For comparison, two reflection gratings were prepared from two conventional fibres which were prepared under the usual oxidising conditions throughout. Both conventional fibres had the same absorption, namely 125 dB/cm/wt% (i. e. seven times less than the fibre according to the invention).

Key parameters of these two gratings are given in the following table.

The "RIC" is the relative index change and it is calculated as [(index modulation)/Δn) ] x 100 to convert to percentage.

Germania is incorporated into the core in order to increase its refractive index. Δn is the direct measure of the change of refractive index and it is approximately proportional to the amount of germania in the core. ince all three fibres were exposed to the same radiation, the index modulation could be taken as a measure of the photosensitivity of the fibre. However, it is easier to achieve a larger modulation if the fibre contains more germania. Hence the RIC involves both the concentration of the germania measured by Δn and the effect of the radiation.

Both of the prior art fibres have the same RIC even though fibre 1 contains more germania. Prior art fibres tend to have RIC values below 0.5% whereas the fibre according to the invention has an RIC value above 1%. These values clearly illustrate that the glass according to the invention is more photosensitive than prior art glass.

(In optical technology, refractive index matching of components is important to avoid unwanted reflections from component interfaces. Thus reflection gratings need to be refractive index-matched to adjacent components and, therefore, the amount of germanium in the grating cannot be adjusted to suit the purposes of the grating. Thus, RIC is an important parameter for assessing sensitivity of glasses intended to be converted into reflection gratings by exposure to radiation. ) Prior art fibres 1 and 2 illustrate that grating reflectivity is dependent upon the amount of germania, but even the fibre with high germania only had a reflectivity of 25%. The fibre in accordance with the invention had a reflectivity of 40%. This also illustrates the superiority of the reduced silica/germania glasses for making reflection waveguides.

Example 2

A planar waveguide according to the invention was prepared b a modification of the well known Flame Hydrolysis Process. Once a planar waveguiding structure has been formed in a conventional way by this technique it was reduced by exposure to a non-oxidising atmosphere in accordance with the invention.

Flame hydrolysis is a process which enables layers of a specific glass composition to be deposited, generally onto a planar silicon or fused silica substrate. Waveguides can be formed form these layers by etching path regions defined, for example by photolithography, from the layers.

Flame hydrolysis is carried out at high temperature in a oxygen/hydrogen flame, which provides the water for the hydrolysis part of the reaction. Reagents are introduced into the flame in order to produce the desired layers. Thus

SiCl₄ is introduced to the flame, to produce Si0₂. In order to increase the refractive index of the Si0₂ it is necessary to introduced ancillary reagents into the flame, and three important ancillary reagents are GeCl₄ which produces Ge0₂,

TiCl₄ which produces Ti0₂ and PC1₃ or P0C1₃ which are converted to £₂°₅- ^A1-¹- ° these ancillary reagents increase the refractive index of the silica to produce layers which can form path regions.

In addition to the reagents used to vary the refractive index, other reagents can be used to reduce the melting point of the deposited layer. For example, introducing BC1₃ and/or PCI₃ into the flame dopes the Si0₂ (and any additives used to adjust the refractive index) with B₂0₃ and/or P₂0₅, careful selection of the correct ratio will avoid unwanted modifications of the refractive index. Reagents to introduce a lasing additive may also be used.

The reactions which occur in the flame hydrolysis process produce, in the first instance, a product which is in the form of very small particles, and for this reason the product is often known as "soot". Flame hydrolysis produces a very thick layer of material having a low bulk density and this material must be consolidated by melting it.

After the deposition of a layer able to form a path region, e. g. a layer containing Ge0₂, the consolidated layer is etched to give a desired path configuration. This etching is achieved by conventional photolithography followed by etching, e. g. reactive ion etching.

The Flame Hydrolysis Process was used to fabricate the planar waveguide structure shown schematically in Figure 2.

With reference to Figure 2 the planar waveguide comprises a silicon substrate 1 approximately 1mm thick, a Si0₂ thermal oxide layer 2 approximately lOμm thick, a buffer layer 3 approximately 15μm thick, a path region 4 approximately 6μm square in cross-section, and a cladding layer 5 approximately 15μm thick.

The glass compositions of respectively, the buffer, path and cladding regions are choosen so that their melting points decrease in that order. This ensures that a previously deposited region is dimensionally stable during the deposition of a subsequent region. The buffer layer 3 comprises approximately 2 to 4 wt% B₂0₃ and 1 to 3 wt% P₂0₅ doped Si0₂. The path region 4 comprises approximately 4 to 10 wt% Ge0₂, 2 to 5 wt% B₂0₃ and 1 to 3 wt% P₂0₅ doped Si0₂. The cladding region 5 comprises approximately 10 to 20 wt% B₂0₃ and 5 to 10 wt% P-0₅ doped Si0₂.

The proportions of B₂0₃ and ₂0₅ in the .cladding and buffer layer are σhoosen so that these two layers have approximately the same refractive index. The path region 4 is thus surrounded by material of uniform refractive index, which index is lower than the refractive index of the path region and which acts to confine optical radiation to the path region.

The completed waveguide structure was then placed in an oven at 750°C and surrounded by hydrogen at atmospheric pressure for one hour. This hydrogenation schedule was selected to allow hydrogen to diffuse a distance of greater than 1mm, and so the whole of the waveguide structure should be affected by the treatment.

The hydrogenation treatment increases the absorbtion at 240nm of the glass forming the waveguide and, in accordance with the invention, increases the photosensitivity of the glass. Figure 3 shows the absorption spectrum of a layer of path region glass, containing approximately 4mole% Ge0₂, before and after hydrogenation. The 240nm absorption band measured on the untreated sample (labelled (a) in Figure 3) was extremely small (approximately 2dB/cm) and planar waveguides based on these prior art low UV absorbing layers show negligible photosensitivity. The absorption of the treated path region layer (labelled (b) in Figure 3) at 240nm is substantially higher, having a peak absorption of approximately 12, OOOdB/cm (corresponding to approximately 2, 500dB/cm/wt% of Ge). It is believed that high absorption levels at 240nm in planar Ge0₂/Si0₂ layers give a strong indication of their potential photosensitivity because the extent to which this UV band can be bleached and the UV absorption spectrum changed, directly relates to any index change via the Kramers-Kronig relationship.

Figure 4 shows the transmission spectrum of the 3cm planar waveguide before and after hydrogenation. The untreated guide (labelled (a) in Figure 4) shows a relatively flat wavelength response over the range 1.0-1.6μm and the loss of the guide in this region is approximately 0. IdB/cm. However, after hydrogenation the transmission spectrum (labelled (b) in Figure 4) shows strong attenuation towards shorter wavelengths, although in the 1.5μm region the guide is still low loss. Only a relatively weak shoulder at 1.38μm is apparent indicating that OH absorption should not significantly affect the loss in planar devices. The increase in attenuation could be due to a low energy tail from defect related absorptions or to scatter. It is anticipated that by optimising the fabrication conditions this loss can be reduced so that grating devices can be formed for use in the 1.3μm window in addition to the 1.5μm window.

A reflection grating was written into the planar waveguide using the arrangement shown in Figure 1, a continuous power level of 25mW, and an exposure time of 40 minutes.

Figure 5 shows the transmission spectrum of the waveguide after grating writing.

Measurements on the planar waveguide reflection grating are given in the following table.

It should be noted that prior art planar waveguides have such low levels of photosensitivity that it is not possible to write a grating of any significant reflectivity in such waveguides for comparison with planar waveguide reflection gratings according to the present invention.

Gratings formed according to the invention have been found to be stable, showing no change in reflectivity after several weeks in the laboratory at room temperature. In addition after heating samples in air for 15 hours at 100°C followed by a further 15 hours at 150°C there was no change in the grating reflectivity.

Claims

CLAIMSCASE A24520

1. A silica/germania glass composition sensitive to radiation of wavelength 0.24μm, said sensitivity being such that the refractive index of said glass is increased by exposure to said wavelength, wherein said glass is characterised by an optical absorption of at least 200dB/cm/wt% of Ge, said absorption being measured by radiation at 240 nm.

2. A glass article a portion of which is adapted to receive refractive index modulation by exposure to a modulated intensity of radiation within the band 225 - 275 nm, characterised in that said portion is formed of a glass according to claim 1.

3. A glass article comprising refractive index modulation applied to a silica/germania glass composition, wherein the regions of lowest refractive index have optical absorptions of at least 200 dB/cm/wt% of Ge, said absorptions being measured at 240 nm.

4. A reflection grating formed of a glass article according to claim 3, wherein the modulation takes the form of alternate regions of higher and lower refractive index whereby said modulation is adapted for use as a reflection grating.

5. A reflection grating according to claim 4, wherein the period of said refractive index modulation is within the range 0.25 to 0.6 μm whereby said modulation selectively reflects radiation within the wavelength band 800 - 1650 nm.

6. A reflection grating according to either claim 4 or claim 5, wherein said modulation is located in the path region of a waveguide.

7. A reflection grating according to claim 6, wherein the waveguide is in the form of a fibre.

8. A reflection grating according to claim 6, wherein the waveguide is a planar waveguide.

9. A method of preparing a glass composition according to claim 1, which method comprises subjecting a silica/germania glass composition to non-oxidising conditions.

10. A method of making a glass article adapted to receive refractive index modulation upon a portion thereof, said portion being formed of a photosensitive silica/germania glass composition, which method comprises subjecting said portion to non-oxidising conditions.

11. A method according to either claim 9 or claim 10, which comprises heating the silica/germania glass composition while it is porous or in the form of a thin film, said heating being carried out in the presence of a non-oxidising atmosphere and thereafter consolidating said porous glass composition.

12. A method of making a fibre waveguide adapted to receive refractive index modulation upon its path region, which method comprises: -

(i) preparing a fibre preform having a path region precursor formed of a silica/germania glass composition in porous form; (ii) heating said path region precursor in the presence of a non-oxidising atmosphere while it is still porous;

(iii) consolidating said path region precursor. (iv) pulling the fibre preform into a fibre

13. A method according to claim 12, which method uses the inside deposition process wherein said process comprises: -

(a) depositing in porous form, a core precursor being a silica/germania glass composition on the inner surface of a tubular substrate which constitutes the cladding precursor;

(b) heating said core precursor while it is still porous in the presence of a non-oxidising atmosphere;

(c) consolidating said core precursor;

(d) collapsing the tubular preform produced in (c) into a solid fibre preform; and

(e) drawing said fibre preform into a fibre.

14. A method according to any one of claims 9 - 13, wherein the consolidation is performed in the presence of a non-oxidising atmosphere of the same composition as was used in the previous heating stage.

15. A method according to any one of claims 9 - 14, wherein the non-oxidising atmosphere is composed of gases selected from hydrogen, nitrogen and the noble gases.

16. A method according to claim 15, wherein the non- oxidising atmosphere is helium.

17. A method of producing refractive index modulation into a glass article which includes a glass prepared by a method as specified in any one of claims 9 - 16, which method comprises exposing a portion of said reduced glass to a modulated intensity of radiation within the band 225 - 275 nm whereby the refractive index of said portion is modulated to reproduce the modulation pattern of said radiation.

18. A method of making a planar waveguide adapted to receive refractive index modulation upon its path region, which method comprises

i) forming a planar waveguide having a path region formed of a silica/germania glass composition, and

ii) heating said planar waveguide in the presence of a non-oxidising atmosphere.

19. A method of making a planar waveguide adapted to receive refractive index modulation upon its path region as claimed in claim 18 in which in step (ii) the atmosphere is composed of hydrogen.

20. A method of making a planar waveguide adapted to receive refractive index modulation upon its path as claimed in claim 19, in which the planar waveguide is heated to at least 400^QC.

21. A method of making a planar waveguide adapted to receive refractive index modulation upon its path as claimed in claim 19, in which the planar waveguide is heated for at least 10 minutes.

22. A method of making a planar waveguide as claimed in any one of claims 18 to 21 wherein the Flame Hydrolysis Process is used to form the path region.