WO2002046113A1

WO2002046113A1 - Fabrication of substrates for planar waveguide devices and planarwaveguide devices

Info

Publication number: WO2002046113A1
Application number: PCT/GB2001/005055
Authority: WO
Inventors: Duncan Harwood
Original assignee: University Of Southampton
Priority date: 2000-12-08
Filing date: 2001-11-15
Publication date: 2002-06-13
Also published as: GB2369820A; GB2369820B; AU2002215113A1; GB0030014D0

Abstract

Using MCVD or some other deposition process one or more glass layers are deposited on the inside of a glass tube (20). Rather than performing a conventional preform collapse step, the glass tube is instead expanded by blow moulding it into a facetted mould (40). The mould is a two-piece graphite mould of square internal cross-section. Blow moulding is performed by sealing the tube (20) at one end and pressuring it internally by supplying gas into its other end. The formed structure, referred to as a blank (60) herein, is then removed from the mould. The flat panels (62) of the blank are then cut out to form planar waveguide substrates with integral waveguiding layers. As well as planar waveguides, the method may also find application for preparing substrates for display.

Description

TITLE OF THE INVENTION

FABRICATION OF SUBSTRATES FOR PLANAR WAVEGUIDE DEVICES AND PLANAR WAVEGUIDE DEVICES

BACKGROUND OF THE INVENTION

The invention relates to methods of fabrication of substrates for planar waveguide devices and to planar waveguide devices. The market for optical telecommunications is rapidly growing particularly with the advent of dense wavelength division multiplexing (DWDM). The next generation of devices will be based on planar circuits in order to produce the required integrated functions such as add/drop multiplexing or switching.

Silica based devices are particularly advantageous because of their compatibility to the installed fibre networks and their excellent properties of high transmission and environmental stability. Integrated silica circuits are currently fabricated by costly methods such as flame hydrolysis deposition (FHD) or plasma enhanced chemical vapour deposition (PECVD). Both FHD and PECVD are quite limited in the types of materials that can be deposited and the size of the devices that can be produced.

Moreover, FHD and PECVD methods are usually both used with a silicon substrate wafer (so-called silica-on-silicon technology) for which the maximum substrate size is limited to those available. Normal current equipment is limited to handling wafers of up to 6 inches in diameter (15 cm). This is not ideal because there is a need to make some planar waveguide devices physically large, for example multistage Mach-Zehnder data modulators may be 20 to 30 cm long.

More generally, in practice, it is often quite difficult to mass-produce planar waveguide devices even after successful fabrication of small numbers of devices. The yield problems are often a result of complexities associated with the specific FHD or PECVD method adopted for the devices concerned. There is thus a desire to be able to mass-produce large-area planar waveguide optical circuits, and to mass-produce planar waveguide optical circuits incorporating dopants that are difficult to incorporate with FHD or PECVD.

SUMMARY OF THE INVENTION

The invention takes the novel approach of dispensing with conventional FHD or PECVD methods and instead fabricating planar waveguide circuits starting from technology conventionally associated with optical fibre preform fabrication.

In conventional optical fibre preform fabrication, a soot is deposited on the inside of a glass tube by vapour deposition, typically by modified chemical vapour deposition (MCVD). The soot is then consolidated into a glass by heating. The glass tube with deposited glass layer on its inner surface is then collapsed under its own surface tension by heating it to above its glass temperature. The central hole of the tube thus reduces down and finally closes to form a solid preform. What was the deposited glass layer thus ends up being confined along the central axis of the preform, ready to form the core of an optical fibre. The preform is subsequently inserted into a fibre drawing tower and pulled into an optical fibre. The basic idea behind the invention is to make a planar waveguide structure starting from optical fibre preform fabrication technology. More specifically, in an embodiment of the invention, the above-described conventional optical fibre preform fabrication process is initially followed by taking a glass tube and using MCVD or some other deposition process to deposit one or more glass layers on the inside of the glass tube. Rather than contracting the tube with the conventional preform collapse step, the glass tube is instead expanded by blow moulding it into a facetted mould which is advantageously of polygonal cross-section, e.g. square or hexagonal. A substrate with integral glass films suitable for planar waveguide fabrication can then be cut out of the flat facets of the blow-moulded object. In this way, it is possible to fabricate in planar waveguide technology any glass composition that can be fabricated in an optical fibre. For example, it is straightforward to fabricate planar waveguides that incorporate tin, germanium, Er/Yb, thulium or phosphorous which all present difficulties with conventional PECVD and FHD based planar waveguide fabrication. This embodiment is described by way of a brief introduction to the invention.

However, the inventive concept of blow moulding to provide substrate material can be generally applied to a wide variety of glass or polymer materials and is not limited to MCVD deposition. Indeed, the blow moulding idea can be applied to fabricating substrates whether or not an additional layer is deposited on the inside of the glass or polymer tube. Moreover, the invention may have applications beyond the field of planar waveguides. For example, the invention may be of use for fabricating large- area substrates for display devices, e.g. polymer displays or flexible glass displays.

According to one aspect of the invention there is provided a method of fabricating a blank for a substrate, the method comprising: (a) providing a glass or polymer tube having an inner surface; and (b) blow moulding the glass tube into a mould to form the blank. The method may further comprise depositing a layer of glass or polymer on the inner surface before blow moulding the glass tube. In this way, large area substrates with integral thin film layers can be fabricated.

The blow-moulding process has proved to be successful from the very first trials. It has been possible to fabricate channel waveguides into blow moulded films using direct ultra-violet (UV) writing. The waveguides exhibit propagation losses and UV-induced refractive index changes as good or better than many conventional techniques and far better than the first trials of other methods such as FHD or sputtering.

According to another aspect of the invention there is provided a method of fabricating a substrate, the method comprising: (a) providing a blank for a substrate fabricated as described above and (b) cutting an area from the blank to form the substrate.

In either of the above aspects of the invention, the mould may have at least one flat surface and the area cut from the blank is taken from a flat part of the blank formed by contact with the flat surface of the mould. In preferred embodiments of the invention, the mould has the cross-section of a regular polygon, e.g. a square or hexagon. However, any desired shape may be used. For example, curved surfaces may be preferred for some applications, e.g. for some display substrates.

The mould may have a length dimension of at least 10-100 cm. The mould may be arranged in a furnace for performing the blow moulding. The furnace may be an RF furnace. Furnaces are commercially available with diameters of up to 50 cm. Alternatively the furnace may be a graphite or other resistance furnace. Deposition of layers on the inner surface of the tube may be performed by a variety of techniques.

The depositing may be carried out by external heating of the glass tube and supply of a gas stream containing molecules carrying a material to be deposited, the external heating serving to disassociate the material from the molecules. The external heating may be directional onto the glass tube in which case the glass tube is preferably rotated during the depositing. Alternatively, the external heating may be omnidirectional to the tube in which case rotation of the glass tube will not be so beneficial. As well as MCVD, other vapour deposition techniques may be used. The depositing may include deposition of a layer comprising a volatile constituent such as tin, phosphor or lead. The method is particularly suited to incorporation of volatile constituents, because the blow moulding can be performed at lower temperatures than are required for preform collapse, and because the blow moulded structure is naturally enclosed. Typically the blow moulding temperature can be 100 - 200 degrees Celsius lower than the preform collapse temperature. This is because preform collapse relies on surface tension, whereas blowing can be performed when the glass is more viscous.

Alternative deposition techniques amenable to blow moulding include: spin coating a layer onto the inner surface of the tube; and spraying a layer onto the inner surface of the tube.

The method may advantageously comprise an intermediate step of pre-blowing the glass tube to an increased cross-sectional diameter before the blow moulding into the mould.

The method may also comprise an intermediate step of axially pre-stretching the glass tube before blow moulding. The tube surface is expanded axially during the pre-stretching and then radially during the pre-blowing (if performed) and the blow moulding.

According to a further aspect of the invention, there is provided a blow- moulded glass or polymer substrate fabricated as described above. The substrate may advantageously have one or more integral films or layers, for example a waveguiding layer. The waveguiding layer may be buried beneath an overcladding layer. Additional buffer or cladding layers may also be provided underneath the waveguiding layer.

The blow-moulded substrate may have a variety of length and width dimensions. For example, the substrate may have a length dimension of at least 10, 20, 30, 40, 50, 60, 70, 80, 90 or 100 cm or a width dimension of at least 0.5, 1, 2, 3, 4, 5, 10, 20, 30 or 50 cm.

It is not believed to be possible to make such large substrates with integral waveguiding layers with prior art techniques.

The substrate can be made of any glass material. In a preferred embodiment the substrate is silica. The substrate may alternatively be made of a polymer that is amenable to blow moulding. For example, the substrate may be made of PMMA.

Glass or polymer substrates with or without integral thin films and fabricated as described above may be used as the basis for making an optical waveguide, optical device, display or optical circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same may be carried into effect reference is now made by way of example to the accompanying drawings in which:

Figure 1 is a schematic drawing of a modified chemical vapour deposition (MCVD) reactor for depositing glass layers inside a glass tube;

Figure 2A is a cross-section of a glass tube after MCVD deposition of a sequence of glass layers;

Figure 2B is a cross-section of the glass tube of Figure 2A after blow moulding into a mould of square cross-section;

Figures 3 A to 3D show four stages of blow moulding to form a substrate blank for fabricating planar waveguide circuits thereon;

Figures 4A and 4B show first and second stages for blow moulding using a manual process with a ring burner;

Figure 5 is a microscope image of a waveguide structure, blow moulded using the manual burner method of Figures 4 A and 4B;

Figures 6A to 6E show five stages of an alternative blow moulding process using a vertical tube furnace;

Figures 7A and 7B show in cross-section a pre-blowing step that may be performed prior to blow moulding;

Figures 8A to 8C show in perspective view three stages of a blow-moulding process using the pre-blowing step of Figures 7 A and 7B; Figures 9A to 9D show four stages of a stretching step that may be preformed prior to blow moulding or pre-blowing;

Figure 10 is a schematic perspective view showing UV-writing of a waveguide into a blow-moulded substrate with integral waveguiding layer; and

Figure 11 shows an example integrated optical circuit.

DETAILED DESCRIPTION

Figure 1 is a schematic drawing of a modified chemical vapour deposition (MCVD) reactor used to implement a first stage in fabricating a glass substrate suitable for making planar waveguide devices. MCVD is a mature technology developed over about 30 years for the manufacture of preforms for optical fibre fabrication. Description of this technique as applied to optical fibre preform manufacture can be found in text books.

The MCVD reactor illustrated comprises a quartz glass deposition tube 20 of outside diameter 20 mm and inside diameter 16 mm. The glass deposition tube 20 is held by a glass chuck 14. At an inlet end of the glass tube 20, there is arranged a rotatable gas seal 12 connected to receive a gas stream carrying the vapour to be deposited through an inlet conduit 10. At an outlet end thereof, the glass deposition tube 20 opens out into a larger diameter quartz glass soot tube 15 of outside diameter 40 mm which is used as an exhaust during the growth process. The deposition tube 20 and soot tube 15 are formed from a single piece of glass. A further glass chuck 19 holds the soot tube 15. The MCVD reactor also has an axially movable gas burner 16 arranged to apply heat externally to one side of the deposition tube, thereby inducing deposition of the soot from the incoming vapour. In use, the deposition and soot tube are rotated as indicated by the arrow at the right hand end of the figure to ensure radially even heating of the vapour in the deposition tube. In an alternative embodiment, a ring gas burner could be provided, in which case the tube rotation could be omitted. The section of the deposition tube to which heat is applied by the burner 16 is conventionally referred to as the hot zone. In use, the hot zone is moved axially along the deposition tube 20 to provide for uniform deposition along the tube. A multi-layer structure may be deposited by changing the vapour species or vapour species mix supplied by the inlet conduit. Deposition of layers of tens of microns in thickness is possible. Graded refractive index profiles may be realised if desired. High concentrations of photosensitive agents, such as tin, boron or germanium, and rare earth dopants, such as Yb, Er, Th etc., can be incorporated using solution doping or organometallic vapour deposition. Moving the hot zone can also be used to vary the composition of the material axially along the deposition tube. For example, a zone of rare earth doped material may be deposited bounded by adjacent zones doped with a photosensitive element such as tin. The utility of such an approach will be understood further below in the context of an integrated optical circuit example.

In use, a vapour mixture of chemicals such as SiCl or GeCl₄ is passed through the inside of the glass tube 20 in an oxygen atmosphere. Using the gas burner 16 as the external heat source, a soot of unconsolidated glass particles is decomposed onto the inside of the heated glass tube 20. The soot is then consolidated. If desired, the soot may be doped with another material such as a rare earth ion before the consolidation into a glass layer. Many layers can be deposited on the inside of the tube to create the required structure.

Figure 2A shows in cross section the glass tube 20 after deposition of a layer sequence. The silica glass tube 20 has radially inwardly deposited thereon a cladding layer 22 followed by a waveguiding layer 24 and an overcladding layer 26 to protect the waveguiding layer 24. Each of these layers is deposited using multiple passes of the burner to build up the desired layer thickness. In one example, the deposition tube 20 is pure silica and the waveguiding layer 24 is germanosilicate. The waveguiding layer 24 is deposited with a flow rate of 160 seem GeCl₄ and 100 seem of SiCl₃ using six individual depositions to build up the desired thickness.

At this point in conventional optical fibre preform fabrication, the glass tube would then be converted into an optical fibre perform. This would be performed by heating the glass tube containing the deposited glass layers so that it collapses under its own surface tension. In this way, a solid central region would be formed by the last deposited layer and a series of glass rings formed by the preceding layers. This is the so-called preform collapse step of conventional optical fibre fabrication. The preform would then be loaded into a drawing tower for the next step of fibre pulling. However, instead of collapsing the tube into a preform as for optical fibre manufacture, the tube is instead expanded by blow moulding. Blow moulding is a well known process used for several centuries. Blow moulding is based on "working" a molten glass to deform it into a desired form by using an internal pressure to press the glass against a surface of desired shape, usually a surface that is enclosed in cross-section. Figure 2B is a schematic illustration in cross-section of the tube of Figure 2A after blow moulding into a square cross-section mould to form a blank. In an example, the mould is 50 mm square and, after the blow moulding, the waveguiding layer is 20 microns thick. The blow moulding step is now described in more detail. Figures 3A to 3D show in perspective view the steps of blow moulding the tube 20 into the square section blank. The tube of Figure 3A is inserted into a two- piece graphite mould 40 of square internal cross-section, as illustrated in Figure 3B. The tube 20 is then sealed at one end and pressurised by supplying gas into its other end, thereby expanding the tube 20 into the mould 40, as illustrated in Figure 3C. The formed structure, referred to as a blank 60 in the following, is then removed from the mould, as illustrated in Figure 3D. It is this blank that has the cross-section illustrated in Figure 2B. The flat panels, facets or faces 62 of the blank are then cut out to form four or more planar waveguide substrates with integral waveguiding layers 24.

Figures 4A and 4B show schematically a blow moulding process in more detail. The glass tube 20 is first pre-inflated to a larger tube 30 of circular cross- section. After pre-inflation, the pre-inflated tube 30 is lowered into the carbon mould 40 past a ring burner 32 which softens the glass (see Figure 4A). A lower portion of the softened glass tube 30 is then blown into the mould 40 (see Figure 4B) by gas injection into its upper aperture. During this process, the carbon mould 40 is rotated (arrows at bottom of figures) at the same speed as the glass tube 20 (arrows at top of figure). The tube 30 is then further lowered into the mould 40 and once more injected with gas 34 in order to blow the next section of the tube 30 into the mould. In this way, the full length of the pre-inflated tube 30 is blow moulded in a small number of steps. Several steps are necessary, since at each step it is only possible to blow mould a section of the pre-inflated tube that corresponds to the axial length of the ring burner 32.

Experiments have shown that the step of pre-inflating or pre-blowing the deposition tube prior to blow moulding improves layer uniformity in the blow moulded structure. Preferably, the deposition tube is pre-inflated to a diameter close to the internal dimension of the faceted blow mould. The pre-inflation step reduces the wall thickness of the glass tube and allows lower temperatures and pressures to be used for the subsequent blow-moulding step. An advantage of including a pre-blowing step is that this can be performed on a conventional glass-working lathe, which gives good control over the pre-blown tube diameter.

Figure 5 is a photograph of a blow moulded substrate with integral waveguiding layer fabricated according to this procedure. The structure comprises a silica substrate 20, germanosilicate waveguiding layer 24 of thickness 20 microns and a silica overcladding layer 26. As is evident, the film uniformity and substrate flatness is very good. For the 20 micron thick germanosilicate layer shown in Figure 5, 4 cm long multimode channel waveguides were fabricated with a total insertion loss (fibre- to-lens) of less than 2dB @ 633nm. Germanoborosilicate layers were also produced. These had an 8 micron thick guiding layer that produced channel waveguides with a loss of 4 dB in a 2 cm long waveguide which was dominated by fibre-to-waveguide coupling loss. A refractive index change of 0.0005 was achieved without deuterium loading. With deuterium loading, a refractive index change of 0.003 was achieved. A Y-junction splitter was also successfully written into the germanoborosilicate sample with a negligible excess loss over an equivalent single channel guide of the same UV-writing fluence.

A summary of the results for the germanoborosilicate sample is shown in the table below.

Waveguide writing parameters and performance

Total Insertion Loss (fibre to lens: 5dB) Spot diameter (1/e² intensity diameter) y: 8μm Spot diameter (1/e² intensity diameter) x: 9.5μm Waveguides written at 200mW (lOOmm/min) Device length: 2.1cm (Measurements @633nm)

Figures 6A to 6E show an alternative to the manual blow moulding process described with reference to Figures 4 A and 4B. This process is more suited for mass- production. The blow moulding is performed in a furnace 50. This may be a tube furnace energised by RF induction or resistive heating. The furnace provides a uniform hot zone along the length of the tube that is to be blown. If the mould is carbon, it can itself be used as the resistive heating element. Referring to Figure 6A, the deposition tube 20 is arranged in the vertically aligned furnace 50 and held by a high temperature resistant iris 52 at the upper end of the furnace 50. A mould 40 is arranged in the furnace. The mould is of hexagonal cross-section in this example. After loading the tube 20 into the furnace the iris 52 is closed. Pincers 54 at the lower end of the furnace are then closed around a lower end of the tube to pinch and seal it, as illustrated in Figure 6B. Gas 56 is then introduced through the upper end of the tube 20 (Figure 6B) to inflate the tube into the hexagonal mould (Figure 6C). The pincers 54 are then opened and the blow moulded blank is removed from the furnace (Figure 6D). The hexagonally faceted blank 60 is illustrated in perspective view in Figure 6E. The blank has six large rectangular facets 62 from which flat substrates can be cut.

It will be understood that this process may be modified by inserting a pre- inflated blank rather than the deposited tube 20.

Figure 7A show in cross-section a deposited tube 20 arranged in a hexagonal cross-section blow mould 40.

Figure 7B is a corresponding figure showing a pre-inflated tube 30 expanded to close to the internal size of the mould 40.

Figures 8A to 8C are perspective views showing a deposited tube 20, a pre- inflated tube 30 and a blow moulded blank 60 respectively, showing the three main stages in the blank formation from the deposited tube.

Figures 9A to 9D illustrate a further modification of the fabrication process. In this modification, the tube 20 is stretched prior to pre-blowing or blow-moulding. The stretching is performed in a drawing process (also called pulling or caning) of the kind familiar from optical fibre fabrication to draw fibre from glass preforms. Drawing the tube prior to blow moulding reduces the thickness of the glass tube wall and hence also the thickness of the deposited glass layer or layers. Drawing the tube thus provides a further parameter for adjusting the layer thicknesses in the finished article. This additional parameter has the advantage of being controllable independently of the mould size. Without using the additional drawing step, the thickness of the layers will depend solely on the tube size relative to the mould size and the thickness of the layers deposited during the vapour deposition process. The deposited tube 20 (Figure 9A) is caned down to form a thinner diameter tube 25 extending between upper and lower residual portions 27 and 29 of the tube 20

(Figure 9B). The caned tube 25 is then inserted into the furnace 50 (Figure 9C) and blow moulded (Figure 9D) as previously described with reference to Figures 6A to 6D. This additional step can serve to further improve the uniformity of layer thickness.

Figure 10 shows a planar waveguide being written into a substrate with integral waveguiding layer as fabricated by one of the above-described blow moulding processes. The substrate 20 has arranged thereon a waveguiding layer 24 made of photosensitive material (e.g. tin-doped silica) and a silica overcladding layer 26. An ultra-violet (UV) laser beam 76 is focused by a lens 74 onto the substrate and writes a line of elevated refractive index into the photosensitive waveguiding layer 72 to form a waveguide in the waveguiding layer. The writing may be performed by motion of the substrate, the laser beam or both. Waveguides of any desired shape may be written to fabricate a variety of components, such as splitters or Mach-Zehnder modulators. After fabrication, the UV- written planar waveguide may be connectorised or pigtailed to an optical fibre 70 as schematically illustrated.

Figure 11 shows an example optical integrated circuit fabricated using the above-described processes. The substrate 20 has deposited thereon a waveguiding layer with five different zones labelled I-V in the figure, each zone corresponding to a different axial length section of the original glass tube. The different zones are produceable with MCVD by moving the external heat source, as described above. Zones I and V are standard waveguiding layers of silica material such as phosphoursilicate or germanosilicate. Zones II and IV have waveguiding layers that are highly photosensitive, for example borosilicate or germanosilicate glass or silica glass doped with tin or some other photosensitive dopant. Zone III has a waveguiding layer of silica glass doped with one or more rare earth elements to provide gain.

By standard etching techniques such as reactive ion etching, ion beam milling or plasma etching, a ridge waveguide structure is formed in Zone I comprising a 1 :32 splitter 82. Each of the outputs from the splitter is provided with a Mach-Zehnder variable optical attenuator (VOA). The splitter serves to divide up a pump beam input from the substrate edge by a pigtailed input fibre 80. Each of the 32 outputs from the splitter continues into Zone II which is photosensitive. The photosensitivity is used to write distributed feed back (DFB) gratings 86 in each of the 32 ridge waveguides etched into the waveguiding layer. The 32 ridge waveguides then continue into Zone III, which is rare earth doped with Er and Yb, and into Zone IV which is photosensitive and has written into it a further set of DFB gratings 90. The DFB gratings 86 and 90 provide mirrors and the rare earth doped sections gain media 88, thus forming a set of resonant cavities for lasing. The waveguides then continue into Zone V in which is formed, again using standard etching techniques, an arrayed waveguide grating (AWG) 92 and a set of straight output waveguides 94 leading to the right-hand substrate edge. The AWG acts as a multiplexer. A pigtailed output fibre 96 for carrying the multiplexed output signal is also shown.

The above-described blow moulding techniques have the following advantages:

1. Waveguides with ultra low loss can be fabricated and fabrication costs are low. The MCVD process allows waveguides to be fabricated with bulk losses of a few dB's per km.

2. MCVD is a mature technology. With the invention, any materials which can be formed into an optical fibre preform can also be fabricated into optical waveguides, such as Er/Yb silica, thulium doped silica, phosphosilicates. For the first time, the invention enables the whole body of knowledge and expertise developed over decades for fibre preform fabrication technology to be directly transferred to the planar waveguide arena. The need to develop new fabrication technologies specifically for planar waveguide fabrication is thus removed. 3. The blow moulding process is well understood and based on established technology. Excellent uniformity over large areas can be achieved.

4. No physical contact with the planar waveguide layers takes place during fabrication, other than contact with high purity gases.

5. Very long waveguide structures (e.g. up to several metres) and very wide waveguide structures (e.g. up to 50 cm with commercially available tube furnaces) can be produced which allow integrated rare earth doped devices to be fabricated for which long absorption lengths prevent fabrication in short devices, such as those prepared by FHD or PECVD.

6. The all-silica nature of the substrates and epitaxial layers makes for easy fusion splicing to optical fibres. 7. High volatility materials such as tin can be incorporated into the epitaxial layers. This is because the blow moulding can be performed at lower temperature than preform collapse.

8. Different compositions or doped regions can be fabricated spatially along the waveguides by, for example, consolidating only certain regions of a solution doped MCVD tube, thereby allowing multi-functional devices or integrated pump sources.

9. Excellent control over waveguide properties such as dopant profile, thickness and numerical aperture can be achieved.

10. Blow moulding can be achieved at a range of pressures, temperatures and glass viscosities to achieve the required uniformity.

11. If a carbon mould is used it can be incorporated as the element or liner of a furnace to achieve excellent temperature uniformity.

12. The high strength of silica means that it can be formed and removed from the mould without any need for slow annealing. 13. The blow moulding process has enormous potential for mass production of optical waveguides, whereas PECVD or flame hydrolysis processes are much more difficult to scale up to mass production.

The invention may find a variety of device applications. Some further examples are now given:

1. Substrates for planar devices such as AWG's, splitters, Mach-Zehnders

2. High photosensitivity doped planar waveguides (e.g. tin doped). 3. Long rare earth doped integrated devices such as multi-channel amplifier laser sources, lossless splitters and grating based devices.

4. Thin film filters for UV using cerium doped core layers.

Claims

1. A method of fabricating a blank for a substrate, the method comprising: providing a glass or polymer tube having an inner surface; and blow moulding the glass tube into a mould to form the blank.

2. A method according to claim 1, the method further comprising: depositing a layer of glass or polymer on the inner surface before blow moulding the glass tube.

3. A method of fabricating a substrate, the method comprising: providing a blank for a substrate fabricated according to claim 1 or claim 2; and cutting an area from the blank to form the substrate.

4. A method according to claim 1 or 2, wherein the depositing is carried out by external heating of the glass tube and supply of a gas stream containing molecules carrying a material to be deposited, the external heating serving to disassociate the material from the molecules.

5. A method according to claim 1 or 2, wherein the depositing includes deposition of a layer comprising a volatile constituent.

6. A method according to claim 1 or 2, wherein the depositing comprises spin coating a layer onto the inner surface of the tube.

7. A method according to claim 1 or 2, wherein the depositing comprises spraying a layer onto the inner surface of the tube.

8. A method according to claim 2 or 3, or any one of claims 4 to 7 when appended to claim 2, wherein the mould has a flat surface and the area cut from the blank is from a flat part of the blank formed by contact with the flat surface of the mould.

9. A method according to any one of the preceding claims, further comprising an intermediate step of pre-blowing the glass tube to an increased cross-sectional size before the blow moulding.

10. A method according to any one of the preceding claims, further comprising an intermediate step of axially pre-stretching the glass tube before the blow moulding.

11. A blow-moulded glass or polymer substrate fabricated according to claim 3.

12. A substrate according to claim 11, wherein the substrate has at least one integral glass or polymer film.

13. A method or substrate according to any one of the preceding claims, wherein the substrate has a length dimension of at least 10, 20, 30, 40, 50, 60, 70, 80, 90 or 100 cm.

14. A method or substrate according to any one of the preceding claims, wherein the substrate has a width dimension of at least 0.5, 1, 2, 3, 4, 5, 10, 20, 30 or 50 cm.

15. A method or substrate according to any one of the preceding claims, wherein the substrate is made of silica.

16. A method or substrate according to any one of the preceding claims, wherein the substrate is made of PMMA.

17. An optical waveguide, optical device, display or optical circuit fabricated from a blow-moulded glass or polymer substrate according to claim 11.