GB2355078A

GB2355078A - Waveguide with composite cladding layer

Info

Publication number: GB2355078A
Application number: GB9923596A
Authority: GB
Inventors: James Ronald Bonar; Richard Ian Laming
Original assignee: Kymata Ltd
Current assignee: Kymata Ltd
Priority date: 1999-10-07
Filing date: 1999-10-07
Publication date: 2001-04-11
Also published as: AU7802600A; WO2001025829A1; GB9923596D0

Abstract

An optical waveguide (1) with a composite cladding layer (5) comprises a substrate (2), a waveguide core (4b) formed on the substrate (2) and upper cladding layer (5) embedding said waveguide core (4b). The composite cladding layer (5) consists of a cladding portion (5a) formed in proximity to the waveguide channel core (4b) with a first composition and at least one cladding outer portion (5b) which substantially embeds said first portion (5a) and which has a second composition. The composition of each of the cladding interface layer (5a) and the cladding outer layer (5b) is selected such that their refractive indices are substantially equal whilst their other characteristics, for example, the temperature range over which they consolidate and soften differ.

Description

2355078 1 1 OPTICAL WAVEGUIDE WITH A COMPOSITE CLADDING LAYER AND 2 METHOD

OF FABRICATION THEREOF 3 4 This invention relates to an optical waveguide with a S composite cladding and to a method of fabricating such 6 a waveguide. In particular, the invention relates to 7 an optical waveguide in which a cladding layer with a 8 structured composition embeds a waveguide core, the 9 cladding composition being varied with the depth of the 10 cladding layer. 11 12 Planar optical waveguides are usually fabricated by 13 forming several layers on top of a substrate, usually a 14 silica waf er. The layers can be deposited by a variety 15 of techniques, f or example, plasma enhanced chemical 16 vapour deposition (PECVD), low pressure chemical vapour 17 deposition (LPCVD), and flame hydrolysis deposition 18 (FHD). In the FHD fabrication process, the layers 19 which make up the waveguide are first deposited as a 20 layer of fine glass particles or "soot". The soot is 21 subsequently heated in situ so that the particles fuse 22 to form a consolidated glass layer. 23 24 The composition of each layer of the waveguide is 25 usually selected so that certain desirable 2 1 characteristics are obtained. For example, so that the 2 refractive index of the layer is uniform within the 3 layer and/or matches the refractive index of other 4 layers of the waveguide. Another desirable 5 characteristic is for the coefficient of expansion of 6 each layer to match that of the substrate and/or 7 underlying layer. This minimises the amount of warpage 8 that occurs as the waveguide is heated during its 9 fabrication and post-fabrication processing. 10 11 once deposited, a glass layer is heated so that it 12 consolidates into a denser glass layer. Individual 13 layers may be consolidated immediately after they are 14 deposited or several layers may be deposited and 15 consolidated together. If a layer is heated to a 16 sufficiently high temperature in excess of its 17 consolidation temperature, the viscosity of the 18 consolidated layer is reduced until eventually the 19 glass is able to flow. The smoother the surface of any 20 layer of the waveguide, the less light is scattered at 21 that surface. Thus, heating a layer to its softening 22 temperature for a period of time is desirable if a 23 high- quality waveguide is to be fabricated. 24 25 During the consolidation of a layer a temperature cycle 26 is used in which at one stage the layer is heated to 27 the "softening" temperature, which is significantly 28 higher than the actual consolidation temperature. This 29 enhanced temperature stage ensures that the glass 30 forming the layer is sufficiently softened to flow and 31 form a relatively smooth and level layer. 32 33 To ensure that the underlying layers are not deformed 34 during the consolidation and/or softening of subsequent 35 layers, the consolidation and softening temperatures of 36 each subsequent layer are usually less than the 3 1 softening temperature of the underlying layer. 2 3 It is desirable, moreover, for the full consolidation 4 of any overlying layer not to occur bef ore the 5 underlying layer has fully consolidated as this could 6 potentially result in gas expelled from the lower layer 7 being trapped under the overlying consolidated layer. 8 Such "outgassing" occurs as the deposited soot layer 9 begins to consolidate and the open network of pores 10 formed by the deposited soot begins to collapse. The 11 density of the glass layer is increased during its 12 consolidation phase as any gas pockets are expelled. 13 14 If a layer becomes fully consolidated and further 15 outgassing occurs in the underlying layer, the gas is 16 trapped beneath the consolidated layer. Moreover, in 17 waveguide devices such as Y-branch splitters and 18 arrayed waveguide gratings (AWGs), narrow junctions 19 with gaps of the order of 1 micron are formed where, 20 for example, two waveguides meet and gas can become 21 trapped in such gaps if the pore network of any 22 cladding layer collapses prematurely. 23 24 To ensure that gas is not trapped in such regions as 25 the glass consolidates, the cladding is usually 26 deposited in multiple stages using a slowly rising 27 temperature gradient. However, this greatly increases 28 the complexity of the cladding stage of the waveguide 29 fabrication. It is therefore desirable if a waveguide 30 can be fabricated by depositing several layers and 31 subsequently heating these layers together in a single 32 consolidation stage. 33 34 To achieve high quality waveguides which can be 35 consolidated in such a manner it is desirable for the 36 composition of each layer to be carefully selected so 4 1 that its consolidation and softening temperatures are 2 controlled. Also, multimode devices which have large 3 waveguide geometries (>10 Am) require thick cladding 4 layers which are also susceptible to gas trapping.

Large aspect ratio devices can also be encountered for 6 narrow slot devices; e.g. couplers with 8 ym deep 7 waveguides and 1 Am edge to edge spacing. Surface 8 relief gratings also require the filling, of narrow 9 corrugations.

11 The present invention seeks to obviate or mitigate the 12 aforementioned disadvantages by providing a waveguide 13 with a graded, or composite cladding layer.

14 A first aspect of the invention seeks to provide an 16 optical waveguide with a composite cladding layer. A 17 second aspect of the invention seeks to provide a 18 method of fabricating an optical waveguide with a 19 composite cladding layer.

21 According to the first aspect of the invention, an 22 optical waveguide is provided having 23 a substrate; 24 a waveguide core formed on the substrate and embedded by a cladding layer, wherein the cladding 26 layer composition is varied so that the composition of 27 a cladding interface portion located in the proximity 28 of an interface between the waveguide core and the 29 cladding is different from the composition of at least one cladding outer portion.

31 32 Preferably, the consolidation temperature of the 33 cladding interface portion is lower than the 34 consolidation temperature of the said at least one cladding outer portion.

36 1 More preferably, the softening temperature of the 2 cladding interface portion is lower than the 3 consolidation temperature of the said as least one 4 cladding outer portion.

s 6 Preferably, said at least one cladding outer portion 7 embeds said cladding interface portion.

8 9 The cladding layer composition may be varied by changing the concentration of at least one dopant ion 11 species in the cladding layer.

12 13 Preferably, the dopant concentration of the cladding 14 layer varies as a function of distance from the is substrate.

16 17 More preferably, the dopant concentration of the 18 cladding layer varies approximately as a function of 19 distance from the interface between the cladding layer and the waveguide core.

21 22 The substrate may be a silicon wafer. The substrate 23 may further comprise at least one intermediate layer 24 formed thereon. At least one intermediate layer may be a cladding layer. Preferably, at least one 26 intermediate layer is a buffer layer which comprises a 27 thermally oxidised layer of the substrate.

28 29 The cladding layer may be doped at least one ion species taken from the group consisting of: a 31 transition element, a rare earth ion species and/or a 32 heavy metal ion species.

33 34 Preferably, the cladding layer is doped with at least one ion species taken from the group consisting of:

36 phosphorus, boron, titanium, tantalum, aluminium, 6 1 lanthanum, niobium, and/or zirconium.

2 3 The volume of the cladding interface portion may be 4 substantially less than the volume of said at least one cladding outer portion.

6 7 Preferably, the depth of the cladding interface portion 8 upon the substrate is substantially less than the 9 maximum depth of the said at least one cladding outer portion.

11 12 The cladding layer may be doped with Boron and 13 Phosphorus. Preferably, the relative dopant 14 concentrations of Boron and Phosphorus in the cladding interface portion and the cladding outer portion 16 provide a homogeneous refractive index throughout the 17 cladding layer.

18 19 Preferably, the coefficient of thermal expansion of the cladding layer is substantially the same as the 21 coefficient of thermal expansion of the substrate.

22 23 Preferably, the cladding layer composition is smoothly 24 varied between said cladding interface portion and said at least one cladding outer portion.

26 27 According to a second aspect of the invention, a method 28 for fabricating an optical waveguide is provided, the 29 method having the steps of:

forming a substrate; 31 forming a waveguide core on the substrate; and 32 forming a cladding layer to embed said waveguide 33 core wherein the cladding layer composition is varied 34 so that the composition of a cladding interface portion located in the proximity of an interface between the 36 waveguide core and the cladding layer is different from 7 1 the composition of at least one cladding outer portion.

2 3 The step of forming said substrate may include the step 4 of forming an intermediate layer on said substrate.

The intermediate layer so formed is preferably a buffer 6 layer.

7 8 The cladding layer may be formed by depositing a 9 particulate cladding soot and subsequently consolidating the cladding soot.

11 12 Preferably, the cladding layer forming the cladding 13 interface portion is not consolidated before the said 14 at least one cladding outer portion is deposited.

16 Preferably, the cladding layer is consolidated in a 17 single process step.

is 19 The cladding interface portion may be at or above its softening temperature when the said at least one 21 cladding outer portion reaches its consolidation 22 temperature.

23 24 The consolidation temperature of the cladding interface portion is lower than the consolidation temperature of 26 the said at least one cladding outer portion.

27 28 The waveguide core and/or cladding are deposited using 29 a flame hydrolysis deposition process and/or a plasma enhanced chemical vapour deposition process and/or a 31 low pressure chemical vapour deposition process.

32 33 At least one portion of said cladding layer may be 34 doped with at least one dopant ion species taken from the group consisting of:

36 a transition element, a rare earth element and/or 8 1 a heavy metal element.

2 3 Preferably, at least one portion of said cladding layer 4 is doped with at least one dopant ion species taken from the group consisting of:

6 phosphorus, boron, titanium, tantalum, aluminium, 7 lanthanum, niobium, zirconium.

8 9 Preferably, the concentrations of the selected dopant ion species provide a refractive index for the buffer 11 layer and cladding interface layer which differs from 12 the refractive index of the waveguide core by between 13 0.2-201.

14 is Preferably, during the consolidation of the cladding 16 layer, the consolidation conditions include a stage 17 where the temperature remains above the softening 18 temperature of the cladding interface portion.

19 The present invention will be further illustrated by 21 way of example, with reference to the accompanying 22 drawings in which:

23 24 Fig 1 is a flow chart illustrating the fabrication steps of an optical waveguide according to a preferred 26 embodiment of the invention; 27 28 Figs 2A to 2D are schematic diagrams showing the 29 formation of an optical waveguide according to a preferred embodiment of the invention; 31 32 Fig 3 illustrates the variation of the refractive index 33 of the dopants T'02, A1203, Ge02, P20., B203, and F as a 34 function of the dopant concentration; 36 Fig 4 illustrates how the coefficient of expansion of 9 1 an Sio, layer varies as the dopant concentration of 2 Ge02, P205, B203 and T02 varies; 3 4 Fig S illustrates the variation of the softening 5 temperature of the dopant concentration of Ge021 P2051 6 B203 7 8 Fig 6 illustrates how the concentration of dopants 9 varies within the cladding layer in one embodiment of 10 the invention; 11 12 Fig 7 illustrates how the consolidation and softening 13 temperatures of the cladding layer and core layer vary 14 in one embodiment of the invention; and 15 16 Fig 8 illustrates how the temperature cycle varies 17 during the fabrication of the cladding layer according 18 to one embodiment of the invention. 19 20 As illustrated in Figs 1 and 2, in one embodiment of 21 the invention, an optical waveguide 1 has a composite 22 cladding layer 5 embedding a waveguide core 4b. The 23 waveguide 1 is fabricated in a series of steps as is 24 shown in Fig 1. 25 2G Referring now to Fig 2A, an intermediate layer 3, for 27 example a buffer or under-cladding layer, is formed on 28 top of a substrate 2. In this example, a S'02 buffer 29 layer 3 is formed by thermally oxidising a silicon 30 substrate 2. Alternatively, more than one intermediate 31 layer 3 may be formed by any suitable fabrication 32 process. 33 34 Fig 2B sketches how a core layer 4 is f ormed on top of 35 the buffer layer 3. Suitable fabrication processes for 36 the core layer 4 and/or the buffer layer 3 include, for 1 example, a flame hydrolysis deposition process (FHD) 2 In the FHD process, a soot layer of fine, particulate 3 glass material(s) is deposited. Other suitable 4 deposition processes may be used including, for example, plasma enhanced chemical vapour deposition 6 (PECVD) and low pressure chemical vapour deposition 7 (LPCVD) or a combination of deposition processes. The 8 deposited layers are then consolidated either before 9 the next layer is deposited or subsequently. Suitable consolidation processes include heating the optical 11 waveguide 1 in a furnace or repassing an FHD burner 12 flame over the deposited soot so that the soot layer 13 consolidates.

14 is The layers of the optical waveguide 1 typically include 16 glass materials such as, for example, germanium and/or 17 silicon oxides, in particular Ge02 and/or S'02 18 19 In one embodiment of the invention, the glass materials are doped during the deposition stage. Typical 21 dopants, chosen for their effect on the thermal 22 characteristics, refractive index and coefficient of 23 expansion of the layer are selected quantities of, for 24 example, boron, phosphorus, and/or titanium compounds (B2031 P201, T'02) - Certain characteristics of the glass 26 are enhanced by introducing other transition elements 27 and/or heavier dopant species, such as rare earths 28 and/or heavy metals, which may be introduced using 29 specialised techniques, for example an aerosol doping technique such as disclosed in United Kingdom Patent 31 Application No.9902476.2. other suitable dopants which 32 produce desirable properties include, for example, 33 tantalum, aluminium, lanthanum, niobium, and/or 34 zirconium.

36 Fig 2C illustrates how a waveguide core 4b is formed by 11 1 removing unwanted portions 4a of the core layer 4 using 2 a suitable etching technique, for example 3 photolithographic process(es) and dry etching. The 4 remaining core layer 4 forms the waveguide core 4b. 5 6 Fig 2D sketches how the waveguide core 4b is then 7 embedded in a cladding layer 5. To achieve certain 8 desirable characteristics, the composition of the 9 cladding layer 5 is varied so that it has a composite 10 structure. It is desirable for the composition to be 11 varied smoothly in the invention, but alternatively, 12 the composition may be varied more abruptly. The 13 cladding layer 5 is formed generally by depositing and 14 consolidating a glass material. 15 16 Any suitable deposition process, for example FHD, 17 PECVD, LPCVD, is used to deposit a cladding layer 5 of 18 glass material about the waveguide core 4b. The 19 cladding layer 5 may be deposited in one stage or more 20 than one stage, and the deposition may be varied 21 smoothly or abruptly between stages or within any one 22 stage. A cladding interface portion 5a has a 23 substantially consistent composition which differs from 24 a the composition of the cladding outer portion 5b. 25 Additional cladding portions may be provided, for 26 example, by a transition region between the two 27 cladding portions. 28 29 In one embodiment of the invention, glass material 30 forming the cladding interface portion 5a is deposited 31 about the waveguide core 4b and over a part of the 32 surrounding underlying surface presented by the 33 substrate 2 or the buffer layer 3 to form a cladding 34 interface portion. For example, a soot layer of 35 suitable glass cladding material can be deposited 3G around the core waveguide 4b using FHD to form the 12 1 cladding interface portion 5a.

2 3 The composition of the cladding is varied during the 4 deposition process, for example, by varying the concentration of dopants within the glass material, so 6 that at least one cladding outer portion 5b is formed 7 with a composition differing from that of the cladding 8 interface portion 5a. Using a FHD process, the 9 cladding composition is varied during the deposition stage. The dopant concentration is varied in relation 11 to the depth of the cladding layer 5 and/or in relation 12 to the proximity of the waveguide core 4b.

13 14 By varying the composition of the cladding layer 5 by introducing dopants, the cladding layer 5 can be 16 selected to possess certain desirable characteristics.

17 18 In this embodiment of the invention, the glass 19 materials are boron and phosphorous doped S'02 However, other suitable glass materials may be used 21 such as, for example, other silicon and/or germanium 22 oxides, which may be doped to achieve certain desired 23 properties. Dopants typically include transition 24 elements and may further include rare earths and/or heavy metal elements. Dopants such as phosphorus, 26 boron, titanium, tantalum, aluminium, lanthanum, 27 niobium, and/or zirconium may be used. These dopants 28 are usually chosen for their effect on the thermal 29 characteristics, refractive index and coefficient of expansion.

31 32 In this embodiment of the invention, the glass 33 materials are doped during the FHD deposition stage, 34 however the doping may be achieved using other conventional methods.

36 13 1 The cladding layer 5 has the same refractive index as 2 the refractive index of the buffer layer 3 in this 3 embodiment of the invention and has a consolidation 4 temperature Tc in the range lower than that of the 5 softening temperature Ts of the waveguide core 4b. 6 7 Figs 3 to 5 illustrate the effect the dopant 8 concentration has on the refractive index, coefficient 9 of thermal expansion and softening temperatures of a 10 silica cladding material. Fig 5 indicates that the 11 higher the concentration of phosphorus, boron and 12 germanium oxide in a layer, the lower the softening 13 temperature. Fig. 3 sketches how the presence of such 14 dopants also affects the refractive index of the is cladding material: increasing the quantity of 16 phosphorus and germanium oxide increases the refractive 17 index, whereas the presence of boron oxide tends to 18 reduce the refractive index. 19 20 By maintaining the relative concentrations of the 21 selected dopant species constant, a substantially 22 constant refractive index across the cladding layer 5 23 can be obtained. For example, by doping the cladding 24 layer 5 with phosphorus and boron it is possible to 25 reduce the sintering temperature and still maintain the 26 refractive index close to or matching that of the 27 buffer layer 3. Thus by increasing the phosphorus and 28 boron levels in the cladding interface portion 5a the 29 same refractive index as the buffer layer 3 is obtained 30 but the cladding interface portion 5a has a lower 31 sintering temperature than the sintering temperature of 32 the buffer layer 3. This provides a smoother interface 33 but also provides the advantage that the composite 34 layer is less susceptible to gas trapping. 35 36 The cladding composition is thus selected so that each 14 1 of the cladding interface portion 5a and the cladding 2 outer portion 5b have substantially the same refractive 3 index and so that this refractive index matches the 4 refractive index of the substrate 2 (or thermal oxide 5 buffer layer 3). For example, the cladding layer 5 can 6 be matched to the substrate /buffer layer so that 7 the thermal expansion coefficients are substantially 8 equal to 25 x 10-7. 9 10 Referring now to Fig. 6, the concentration of dopants 11 in the cladding layer 5 is varied so that the cladding 12 material at the cladding interface portion 5a has the 13 lowest consolidation temperature TSAC whereas the 14 consolidation temperature T5Bs of the cladding outer 15 portion 5b is higher. Away from the immediate vicinity 16 of the core 4b, the gradation of the cladding 17 composition may be increased to vary the consolidation 18 temperature as the cladding layer depth increases. 19 20 The thermal characteristics and conditions of the 21 optical waveguide and its method of fabrication will 22 now be discussed in more detail. 23 24 The temperatures to which the optical waveguide 1 is 25 subjected to during the consolidation phase of the 26 cladding layer 5 are varied at a rate determined by the 27 composition of the cladding layer 5 and by the 28 variation of the dopant concentrations as a function of 29 depth within the optical waveguide 1. 30 31 During consolidation of the cladding layer 5, the 32 temperature increases at such a rate as to ensure that 33 the cladding outer portion Sb consolidates fully only 34 once all gas trapped within the cladding interface 35 portion 5a has been fully expelled. This prevents gas 36 remaining in a partially consolidated layer from being is trapped by an overlying fully consolidated layer.

2 3 In one embodiment of the invention, the cladding 4 interface portion 5a has a softening temperature of 11000C whereas the remaining cladding portion 5b has a 6 consolidation temperature of approx 11500C. The 7 cladding interface portion 5a is thus fully 8 consolidated whilst the surrounding cladding outer 9 portion 5b is still only partially consolidated.

11 Fig. 7 indicates how the softening temperatures and 12 consolidation temperatures of each of the cladding 13 portions 5a and 5b vary in relation to each other.

14 The cladding layer 5, core layer 4 and substrate 2 16 compositions are selected to ensure that the 17 consolidation of any one of these does not cause any 18 thermal deformation of the rest of the optical 19 waveguide 1. Each of the cladding layer 5, core layer 4 and substrate 2 has a consolidation temperature which 21 is lower than the softening temperature of the 22 underlying portion. Alternatively, an additional 23 cladding and/or buffer layers can be formed in between 24 two layers of the waveguide.

26 The fabrication conditions for the cladding interface 27 portion 5a formed around the waveguide core 4b, are 28 provided below. These can be compared to the 29 conditions for forming the cladding outer portion 5b.

The cladding outer portion 5b has a composition 31 substantially different from that of the first cladding 32 portion 5a. The FHD conditions for forming the 33 cladding portions 5a, 5b are as follows:

34 16 1 Core/Clad Interface Remaining Cladding 2 Portion (5a) Portion (5b) 3 4 Bubbler Flow Rate Bubbler Flow Rate Gas (sccm) Gas (sccm) 6 7 SiC14 150 S'C14 150 8 PCL3 90 PCL3 73 9 BC13 32 BC13 26 11 Transport Flow Rate Transport Flow Rate 12 Gases Gases 13 14 H2:02 2 Lmin-1:4 Lmin-1 H2:02 2 Lmin-1:4 Lmin- 16 The above flow rates are controlled so that the 17 resulting composition of the cladding interface portion 18 5a produces a refractive index for the cladding 19 interface portion 5a which is substantially the same as 20 the refractive index of the cladding outer portion 5b. 21 This refractive index is selected to substantially 22 match the refractive index of the buffer layer 3. 23 24 The compositions of both the cladding interface portion 25 5a and the cladding outer portion 5b are controlled so 26 that index matching can be achieved whilst minimising 27 the potential for thermal deformation of the cladding 28 layer 5 during the consolidation stage of fabrication. 29 30 Fig 8 illustrates a suitable temperature cycle 31 according to the invention. In this example, during 32 the consolidation process the temperature conditions 33 are initially 6500C rising at 150C min-' to 8500C, and 34 then further increasing to 10500C at 50C min-'. The 35 optical waveguide 1 remains substantially at 10500C for 36 approximately 60 minutes in an helium oxygen atmosphere 17 1 (0.6 L min-' He and 0.2 L min-- 02) The temperature 2 further rises to 11500C min-3- and remains at this upper 3 temperature for approximately 60 minutes before being 4 cooled to 6500C at -50C min-'. To summarise, the temperature cycle is thus as follows:

6 7 i) 6500C to 8500C at 150C min-1 8 ii) 8500C to 10500C at 50C min-' 9 iii) 10500C for 60 minutes iv) 10500C to 11500C at 50C min-' 11 V) 11500C for 60 minutes 12 vi) 11500C to 6500C at - 50C min-' 13 14 The softening temperature is the temperature at which is the viscosity of a consolidated layer is reduced 16 sufficiently for the consolidated layer to begin to 17 flow'. During fabrication of the optical waveguide 1, 18 the softening temperatures of the cladding interface 19 portion 5a and at least one cladding outer portion 5b are each controlled by the selection-of suitable 21 dopants and dopant concentrations.

22 23 The cladding interface portion 5a has a softening 24 temperature T5. = 11000C. The cladding outer portion 5b has a consolidation temperature Tsac 11500C which has 26 been selected to exceed the softening temperature T5AS 27 of the cladding interface portion 5a by a preferred 28 amount, 500C.

29 If a temperature cycle such as Fig. 8 illustrates is 31 used to consolidate the cladding layer 5, then by 32 increasing the temperature from 6000C to 11000C at 50C 33 min-', the cladding interface portion 5a consolidates 34 first. This enables gas to be expelled through the overlying partially consolidated cladding outer portion 36 5b.

18 1 To prevent premature consolidation of the cladding 2 outer portion 5b, the temperature range over which the 3 cladding layer 5 is heated includes a suitable 4 consolidation ramp rate of 50C min-'. This removes the 5 possibility of any portion of the cladding interface 6 portion 5a prematurely consolidating. other means to 7 promote pore collapse may also be used, for example, 8 He gas may be included during the consolidation phase 9 to promote core collapse. 10 11 The high temperatures required to consolidate the 12 waveguide layers may be achieved by known techniques, 13 for example, passing a burner flame from a flame 14 hydrolysis burner over the deposited soot layer or by 15 placing the waveguide wafer 1 in a suitable furnace. 16 17 While several embodiments of the present invention have 18 been described and illustrated, it will be apparent to 19 those skilled in the art once given this disclosure 20 that various modifications, changes, improvements and 21 variations may be made without departing from the 22 spirit or scope of this invention. 23 24 For example, more than two cladding layers may be 25 formed in the composite multi-layer cladding, and the 26 composition of each cladding layer selected so that 27 joint or separate consolidation can occur. 28 29 Any range given herein may be extended or altered 30 without losing the effects sought, as will be apparent 31 to the skilled person for an understanding of the 32 teachings herein.

19

Claims

1 CLAIMS:

2 3 1 An optical waveguide (1) having:

4 a substrate (2); a waveguide core (4b) formed on the substrate (2) 6 and embedded by a cladding layer (5), wherein the 7 composition of the cladding layer (5) is varied so that 8 the composition of a cladding interface portion (5a) 9 located in the proximity of an interface between the waveguide core (4b) and the cladding layer (5) is 11 different from the composition of at least one cladding 12 outer portion (5b).

13 14 2. An optical waveguide (1) as claimed in claim 1, wherein the consolidation temperature of the cladding 16 interface portion (5a) is lower than the consolidation 17 temperature of the said at least one cladding outer 18 portion (5b).

19 3. An optical waveguide (1) as claimed in claim 1 or 21 claim 2, wherein the softening temperature of the 22 cladding interface portion (5a) is lower than the 23 consolidation temperature of the said as least one 24 cladding outer portion (5b).

26 4. An optical waveguide (1) as claimed in any 27 preceding claim, wherein said at least one cladding 28 outer portion (5b) embeds said cladding interface 29 portion (5a).

31 S. An optical waveguide (1) as claimed in any 32 preceding claim, wherein said cladding composition is 33 varied by changing the concentration of at least one 34 dopant ion species in the cladding layer (5).

36 6. An optical waveguide (1) as claimed in claim 5, 1 wherein the dopant concentration of the cladding layer 2 (5) varies as a function of distance from the substrate 3 (2) 4 7. An optical waveguide (1) as claimed in claim 6, 6 wherein the dopant concentration of the cladding layer 7 (5) varies approximately as a function of distance from 8 the interface between the cladding layer (5) and the 9 waveguide core (4b) 11 8. An optical waveguide (1) as claimed in any 12 preceding claim, wherein said substrate (2) is a 13 silicon wafer.

14 9. An optical waveguide (1) as claimed any preceding 16 claim, wherein said substrate (2) further comprises at 17 least one buffer layer (3) formed thereon.

18 19 10. An optical waveguide (1) as claimed in claim 9, wherein at least one of said at least one buffer layer 21 (3) is a thermally oxidised layer of the substrate (2) 22 23 11. An optical waveguide (1) as claimed in any one of 24 claims 5 to 10, wherein the cladding layer (5) is doped with at least one ion species taken from the group 26 consisting of:

27 a transition element, a rare earth ion 28 species and/or a heavy metal ion species.

29 12. An optical waveguide (1) as claimed in claim 11, 31 wherein the cladding layer (5) is doped with at least 32 one ion species taken from the group consisting of:

33 phosphorus, boron, titanium, tantalum, aluminium, 34 lanthanum, niobium, and/or zirconium.

36 13. An optical waveguide (1) as claimed in any 21 1 preceding claim wherein the volume of the cladding 2 interface portion (5a) is substantially less than the 3 volume of said at least one cladding outer portion 4 (5b).

6 14. An optical waveguide (1) as claimed in any 7 preceding claim, wherein the depth of the cladding 8 interface portion (5a) upon the substrate (2) 9 substantially less than the maximum depth of the said at least one cladding outer portion (5b) 11 12 15. An optical waveguide (1) as claimed in any one of 13 Claims 11 to 14, wherein the relative dopant 14 concentrations of Boron and Phosphorus in the cladding interface portion (5a) and the cladding outer portion 16 (5b) provide a homogeneous refractive index throughout 17 the cladding layer.

18 19 16. An optical waveguide (1) as claimed in any preceding claim, wherein the coefficient of thermal 21 expansion of the cladding layer (5) is substantially 22 the same as the coefficient of thermal expansion of the 23 substrate (2).

24 17. An optical waveguide (1) as claimed in any 26 preceding claim, wherein said cladding composition is 27 smoothly varied between said core/interface cladding 28 portion (5a) and said at least one cladding outer 29 portion (5b).

31 18. A method for fabricating an optical waveguide 32 having the steps of:

33 forming a substrate (2) 34 forming a waveguide core (4b) on the substrate (2); and 36 forming a cladding layer (5) to embed said 22 1 waveguide core (4b) wherein the cladding composition is 2 varied so that the composition of a cladding interface 3 portion (5a) located in the proximity of an interface 4 between the waveguide core (4b) and the cladding layer (5) is different from the composition of at least one 6 cladding outer portion (5b).

7 8 19. A method as claimed in Claim 18, wherein the step 9 of forming said substrate (2) includes the step of forming an buffer layer (3) on said substrate.

11 12 20. A method as claimed in Claim 18 or Claim 19, 13 wherein the cladding layer (5) is formed by depositing 14 a particulate cladding soot and subsequently consolidating the cladding soot.

16 17 21. A method as claimed in Claim 20, wherein the 18 cladding layer (5) forming the cladding interface 19 portion (5a) is not consolidated before the said at least one cladding outer portion (5b) is deposited.

21 22 22. A method as claimed in Claim 21, wherein the 23 cladding layer (5) is consolidated in a single process 24 step.

26 23. A method as claimed in Claim 22, wherein the 27 cladding interface portion (5a) is at or above its 28 softening temperature when the said at least one 29 cladding outer portion (5b) reaches its consolidation temperature.

31 32 24. A method as claimed in claim 23, wherein the 33 consolidation temperature of the cladding interface 34 portion (5a) is lower than the consolidation temperature of the said at least one cladding outer 36 portion (5b).

23 1 25. A method as claimed in claim 23 to 24, wherein the 2 waveguide core (4b) and/or cladding layer (5)are 3 deposited using a flame hydrolysis deposition process 4 and/or a plasma enhanced chemical vapour deposition 5 process and/or a low pressure chemical vapour 6 deposition process. 7 8 26. A method as claimed in any one of Claims 18 to 25, 9 wherein in at least one portion of said cladding layer (5) is doped with at least one dopant ion species taken 11 from the group consisting of:

12 a transition element, a rare earth element and/or 13 a heavy metal element.

14 27. A method as claimed in claim 25, wherein at least 16 one portion of said cladding layer (5) is doped with at 17 least one dopant ion species taken from the group 18 consisting of:

19 phosphorus, boron, titanium, tantalum, aluminium, lanthanum, niobium, zirconium.

21 22 28. A method as claimed in any one of claims 18 to 27, 23 wherein the concentrations of the selected dopant ion 24 provide a refractive index for the buffer layer (3) and cladding interface layer (5a) which differs from the 26 refractive index of the waveguide core (4b) by between 27 0.2-2%.

28 29 29. A method as claimed in any one of claims 20 to 28, wherein during the consolidation stage of at least one 31 portion of said cladding layer (5), the consolidation 32 temperature conditions include a stage where the 33 temperature remains above the softening temperature of 34 the cladding interface portion.

36 30.An optical waveguide (1) with a cladding layer (5) 24 1 including at least two distinct portions (5a, 5b) with 2 different compositions as described substantially 3 herein and with reference to the accompanying drawings.