Fibre-to-waveguide optical mode coupler

Abstract

An optical mode coupler is provided. The optical mode coupler comprises a waveguide structure, the waveguide structure comprising: a waveguide, the waveguide having a proximal end, the proximal end situated at a distance from an edge facet of the waveguide structure; a cladding material encasing the proximal end of the waveguide; and a mode-matching region formed of the cladding material and situated between the proximal end of the waveguide and the edge facet of the waveguide structure, the mode-matching region configured to match a mode of light supported by the waveguide to a fibre mode supported by an optical fibre at the edge facet of the waveguide structure.

Description

Fibre-to-waveguide optical mode couplerTechnical Field

[0001] The present disclosure relates to optical interconnects. More particularly, the present disclosure relates to optical mode couplers for coupling light from an optical fibre to a waveguide (or vice versa) and methods of fabrication of such optical mode couplers.Background

[0002] Integrated photonics provides many advantages over free-space or fibre optics. By integrating multiple photonic functions onto a single chip, the size, weight, and power consumption of optical systems may be significantly reduced, enabling more compact and energy -efficient devices. The scalability of photonic integrated circuits (PICs) allows for mass production using established semiconductor manufacturing processes, leading to lower costs and greater accessibility for advanced optical technologies.

[0003] A key challenge in photonics is to provide repeatable, high yield optical interconnects to couple light between an optical fibre and a waveguide of a PIC with very low loss. For many applications, notably quantum communication and quantum computation, loss at the interconnect is a critical challenge. The difficulty in efficiently coupling light from an optical fibre to a waveguide on a PIC stems at least in part from the mismatch between the size, shape and position of an optical mode in fibre and the size, shape and position of an optical mode in a (often rectangular) waveguide of a PIC. The coupling efficiency may be sensitive to both the alignment of the fibre with the waveguide and the precision of fabrication of the PIC.

[0004] Current approaches to provide low -loss interconnects include (i) inverse tapering of the PIC waveguide during manufacture to expand the optical mode so that it better matches the size of the mode in optical fibre, (ii) transitioning from a single waveguide to a multi-layer / pronged coupling region in the photonic chip so that the optical mode’s shape and size better matches the mode in optical fibre, and (iii) manipulation of individual fibre cores in a fibre array to correct for misalignment with waveguides. These solutions help but still run into limitations in terms of fabrication precision, cost, optical alignment or scalability.Summary

[0005] The present disclosure provides for efficient, low -loss optical interconnects between fibre optic cables and PICs, which are essential components in modern optical communication and signal processing systems. By introducing a novel coupling mechanism, this disclosure significantly enhances the alignment precision, and mode transfer efficiency between the optical fibres and the photonic waveguides on the PIC, thereby improving overall performance and reducing insertion losses. The present disclosure is particularly advantageous when coupling a fibre array to a plurality of PIC waveguides.

[0006] According to an aspect of the present disclosure, a method of manufacturing an optical mode coupler is provided. The method comprises providing a waveguide structure, wherein providing the waveguide structure comprises: providing a waveguide layer, the waveguide layer comprising a waveguide having a proximal end; and providing a cladding material around the waveguide layer and between the proximal end of the waveguide and an edge facet of the waveguide structure. The method further comprises laser writing the cladding material between the proximal end of the waveguide and the edge facet of the waveguide structure to form a mode-matching region, the mode-matching region configured to match a mode of light supported by the waveguide to a mode of light (fibre mode) supported by an optical fibre at the edge facet of the waveguide structure.

[0007] According to an aspect of the present disclosure, an optical mode coupler is provided. The optical mode coupler comprises a waveguide structure. The waveguide structure comprises a waveguide, the waveguide having a proximal end, the proximal end situated at a distance from an edge facet of the waveguide structure. The waveguide structure further comprises a cladding material encasing the proximal end of the waveguide. The waveguide structure further comprises a mode -matching region formed of the cladding material and situated between the proximal end of the waveguide and the edge facet of the waveguide structure, the mode-matching region configured to match a mode of light supported by the waveguide to a mode of light (fibre mode) supported by an optical fibre at the edge facet of the waveguide structure.

[0008] At least a portion of the cladding material inside the mode-matching region may have a different refractive index profile to a refractive index profile of the cladding material outside of the mode -matching region. The mode-matching region may be laser-written.

[0009] The waveguide structure may further comprise further a second waveguide, the second waveguide having a proximal end, the proximal end terminating a distance from the edge facet of the waveguide structure The waveguide structure may further comprise a second mode-matching region formed of the cladding material and situated between the proximal end of the second waveguide and the edge facet of the waveguide structure, the second mode-matching region configured to match a mode of light supported by the second waveguide to a mode of light supported by a second optical fibre at the edge facet of the waveguide structure.

[0010] In some examples, at least a part of the waveguide may be tapered.

[0011] The cladding material may comprise any material having a lower refractive index than the waveguide material and capable of being laser-written. For example, the cladding may comprise silica.

[0012] The waveguide may comprise any material having a higher refractive index than the cladding material (outside of the mode-matching region). For example, the waveguide may comprise silicon nitride.

[0013] The optical mode coupler may comprise a substrate layer, wherein the waveguide is provided (directly or indirectly) above the substrate layer. In some examples, the substrate layer does not coincide with the edge facet of the waveguide structure. The substrate layer may be truncated such that the mode-matching region overhangs the edge of the substrate layer.

[0014] By employing laser-writing techniques to create a mode-matching region directly in the cladding material of a PIC, the refractive index profile of the cladding material may be designed to better match an optical mode of an optical fibre to an optical mode of the waveguide. This approach is particularly useful to provide an interconnect between a fibre array (in which a plurality of optical fibre cores may have inter-fibre spacings that are not consistent) and a PIC comprising multiple waveguides, as each mode-matching region can be individually written to best match the fibre mode of the optical fibre to which a waveguide is to be coupled.

[0015] Many modifications and other embodiments set out herein will come to mind to a person skilled in the art in light of the teachings presented herein. Therefore, it will be understood that the disclosure herein is not to be limited to the specific embodiments disclosed herein. Moreover, although the description provided herein provides example embodiments in the context of certain example combinations of elements, steps and / or functions, it will be appreciated that different combinations of elements, steps and / or functions may be provided by alternative embodiments without departing from the spirit or scope of the disclosure.Brief description of the Drawings

[0016] Illustrative embodiments of the present disclosure will now be described by way of example only, with reference to the accompanying figures.

[0017] Fig. 1 shows an illustration of a cross-sectional side view of a fibre-to-waveguide optical mode coupler according to an example.

[0018] Fig. 2 shows an illustration of a cross-sectional top view of the fibre-to-waveguide optical mode coupler according to the example of Fig. 1.

[0019] Fig. 3 shows an illustration of a cross-sectional angled view of the fibre-to-waveguide optical mode coupler according to the example of Fig. 1.

[0020] Figs 4A-4J illustrate a waveguide structure at intermediate stages during manufacture (according to an example) of a photonic integrated circuit having an optical mode coupler.

[0021] Fig. 5 shows an illustration of an optical fibre array according to an example.

[0022] Fig. 6 shows an illustration of a cross-sectional top view of an optical mode coupler for coupling a fibre array to a PIC comprising a plurality of waveguides according to an example.

[0023] Fig. 7 shows an illustration of a cross-sectional angled view of an optical mode coupler for coupling a fibre array to a PIC comprising a plurality of waveguides according to an example.

[0024] The features shown in the figures are not drawn to scale. Throughout the description and the drawings, like reference numerals refer to like parts.Detailed Description

[0025] Embodiments of the disclosure are described with reference to the accompanying drawings. However, it should be appreciated that the disclosure is not limited to the embodiments, and all changes and / or equivalents or replacements thereto also belong to the scope of the disclosure. The same or similar reference denotations may be used to refer to the same or similar elements throughout the specification and the drawings.

[0026] As used herein, the terms “have”, “may have”, “include”, or “may include” a feature (e.g. a number, function, operation, or a component such as a part) indicate the existence of the feature and do not exclude the existence of other features. Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0027] As used herein, the terms “A or B”, “at least one of A and / or B”, or “one or more of A and / or B” may include all possible combinations of A and B. For example, “A or B”, “at least one of A or B”, “at least one of A and B” may indicate all of (1) including at least one A, (2) including at least one B, or (3) including at least one A and at least one B.

[0028] As used herein, the terms “first” and “second” may modify various components regardless of importance and do not limit the components. These terms are only used to distinguish one component from another. For example, reference to a first component and a second component may indicate different components from eachother regardless of the order or importance of the components. Similarly, terms such as “proximal” and “distal” are intended only to distinguish one end of e.g. a waveguide from the other end.

[0029] It will be understood that when an element (e.g. a first element) is referred to as being (physically, operatively or communicatively) “coupled with / to”, or “connected with / to” another element (e.g. a second element), it can be coupled with / to the other element directly or via a third element. In contrast, it will be understood that when an element (e.g. a first element) is referred to as being “directly coupled with / to” or “directly connected with / to” another element (e.g. a second element), no element (e.g. a third element) intervenes between the element and the other element.

[0030] The terms as used herein are provided merely to describe some embodiments thereof, but not to limit the scope of other embodiments of the disclosure. It is to be understood that the singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. All terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the embodiments of the disclosure belong. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealised or overly formal sense unless expressly so defined herein.

[0031] Fig. 1 shows an illustration of a cross-sectional side view of a fibre-to-waveguide optical mode coupler 100 according to an example. The optical mode coupler 100 is coupled to an optical fibre 120 comprising an optical fibre core 130. The optical mode coupler 100 is formed in a photonic integrated circuit (PIC) comprising a waveguide 140. More particularly, the optical mode coupler 100 comprises a waveguide structure 110 of the photonic integrated circuit. The waveguide structure 110 comprises a substrate layer 190, a waveguide 140, and a cladding material 150 encasing (e.g. surrounding or enclosing or encapsulating) a proximal end 160 of the waveguide 140. The proximal end 160 of the waveguide 140 is accordingly situated at a distance from an edge facet 180 of the waveguide structure 110 to which the optical fibre 120 is coupled. The proximal end 160 of the waveguide 140 is separated from the edge facet by an amount of cladding material 150. The waveguide 140 extends from the proximal end 160 to a distal end (not shown).

[0032] The waveguide structure 110 also includes a mode-matching region 170 in and formed of the cladding material between the waveguide 140 and the edge facet 180 of the waveguide structure 110. In particular, in the mode-matching region 170 the cladding material has been adapted or modified (for example by damaging or deforming the cladding material) to provide a different refractive index profile to the refractive index profile of the cladding material 150 outside of the mode-matching region. The mode-matching region 170 is configured to match a mode of light supported by the waveguide 140 to a fibre mode of light supported by the optical fibre core 130 at the edge facet 180 of the waveguide structure 110.

[0033] The waveguide structure 110 of the optical mode coupler 100 is accordingly configured to couple an optical fibre 120 to the photonic integrated circuit such that light may be sent from the optical fibre 120 to the photonic integrated circuit (or vice versa) with low loss.

[0034] The waveguide 140 in this example is composed of silicon nitride (in this example Si3N4), which is an optically transmissive material. However, in other examples, the waveguide 140 may be composed of any of various optically transmissive materials having a high refractive index relative to the refractive index of thecladding material. For example, the waveguide may in some examples comprise silicon, silicon carbide or lithium niobate.

[0035] The cladding material 150 in this example comprises silica (SiO2). However, in other examples, the cladding material may be composed of any of various materials having a low refractive index relative to the refractive index of the waveguide material and suitable for laser writing. For example, the cladding material may in some examples comprise hydrogen silsesquioxane (HSQ).

[0036] The thickness of a cladding layer comprising the cladding material above the waveguide may be thicker than in traditional PICS in order to provide enough space to enable a light mode to expand within the cladding material of the mode-matching region (when propagating from the waveguide 140 towards the edge facet)

[0037] The substrate layer 190 in this example comprises a bulk silicon layer. However, the skilled person would appreciate that other substrate materials may be used. In some examples, the substrate layer 190 may be truncated such that an edge of the substrate layer does not coincide with the edge facet of the waveguide structure.

[0038] Fig. 2 shows an illustration of a cross-sectional top view of the fibre-to-waveguide optical mode coupler 100 according to the example described in relation to Fig. 1. Fig. 3 shows an angled view of the optical mode coupler 100.

[0039] As can be seen in Fig. 2 and Fig. 3, the proximal end 160 of the waveguide 140 is tapered to a termination point. The waveguide 140 widens along a portion of the waveguide from the proximal end in a direction towards the distal end (this portion of the waveguide is referred to herein as the waveguide taper 200). The waveguide taper 200 may assist in transitioning a fibre mode of the optical fibre 120 to the optical mode of the waveguide 140 beyond the waveguide taper 200. In other words, the mode-matching region 170 and the waveguide taper 200 may be configured to together match a mode of light supported by the waveguide 140 (beyond the taper 200) to a mode of light supported by the optical fibre core 130 at the edge facet 180 of the waveguide structure 110.

[0040] The skilled person will appreciate that the optical mode coupler 100 may be varied in any of a number of ways. For example, the waveguide 140 may be one of a plurality of waveguides provided in a waveguide layer of the waveguide structure. Several waveguides of the waveguide layer may be configured to support light from corresponding optical fibres, for example the waveguide structure may comprise a second waveguide and a second mode-matching region configured to match a fibre mode of a second optical fibre to an optical mode of the second waveguide. In some examples, the waveguide 140 may be a primary waveguide and the waveguide layer may comprise one or more secondary waveguides tapered in a direction opposite to the taper 200 of the primary waveguide 140 (e.g. narrowing in a portion from their respective proximal ends towards their respective distal ends) in order to support the transition of the optical mode from the fibre 120 through the mode -matching region 170 and the waveguide taper 200 into the primary waveguide 140 (and vice versa). In some examples, the waveguide 140 may not be tapered.

[0041] In some examples, the waveguide structure 110 may comprise further layers, for example further waveguide layers and further layers of cladding material. Different layers may comprise different materials, for example a first waveguide layer may be formed from a different material to a second waveguide layer.

[0042] Figs. 4A to 4J depict cross-sections corresponding to intermediate steps of manufacturing an example simplified photonic integrated circuit that includes an optical mode coupler as described herein.

[0043] In particular, Fig. 4A depicts a cross-sectional side view of a first intermediate stage of manufacturing the photonic integrated circuit having the optical mode coupler, and Fig. 4B depicts a cross-sectional end view ofthe first intermediate stage. The cross-section depicts a bulk silicon layer 190 that may be of any suitable thickness. Formed onto and / or into the bulk silicon layer 190 is a first cladding layer comprising a cladding material 150. The cladding material 150 in this example comprises silicon dioxide (SiCh) although other materials may be used. The cladding layer can be formed by any suitable process or technique, whether active or passive. The cladding layer may be a thermal oxide and / or a deposited oxide. In other cases, certain regions of the cladding layer may be thermal oxide whereas other regions may be deposited. In some cases, a multi-layer process may be used to form the cladding layer. For example, a thermal oxide may be formed into the bulk silicon layer 190 after which one or more deposited oxide layers may be disposed, grown, or otherwise formed. One example process or technique by which the cladding layer may be formed includes placing the bulk silicon layer 190 in a furnace to form a thermal oxide layer. The cladding layer may be formed in a layer of uniform thickness or, in some cases, the cladding layer may be formed to a particular desired profile.

[0044] As with other layers of material described herein, the cladding layer can be formed to any suitable thickness. One example thickness is on the order of 2-10 micrometres. In other cases, a thinner or thicker cladding layer may be formed. In many cases, a thickness of the cladding layer is determined based on an optical property of the cladding layer at the selected thickness. In other words, the thickness of the first cladding layer may be determined so that the first cladding layer can provide mechanical support and / or optical functionality to other layers of the photonic integrated circuit.

[0045] The first cladding layer or a portion thereof may be smoothed, polished and / or otherwise planarized with a suitable process (e.g. chemical-mechanical polishing). In some examples, the first cladding layer is mechanically polished, whereas in other examples, the first cladding layer is chemically polished. In some examples, the first cladding layer may be both mechanically and chemically polished. These foregoing examples are not exhaustive and a person of skill in the art may readily appreciate that many suitable techniques can be used to form a flat surface in the first cladding layer.

[0046] Fig. 4C depicts a cross-sectional side view of a second intermediate stage of manufacturing the photonic integrated circuit having the optical mode coupler, and Fig. 4D depicts a cross-sectional end view of the second intermediate stage. In Fig. 4C and 4D a waveguide layer 240 of silicon nitride is provided onto the first cladding layer, although the skilled person will appreciate that other materials may be used in the waveguide layer). The silicon nitride of the waveguide layer 240 can be deposited in any suitable manner. In one example, the silicon nitride is deposited by low pressure chemical vapour deposition.

[0047] As with other layers described herein, the waveguide layer 240 is shown as extending across an entire surface of the starting substrate and the first cladding layer, but it may be appreciated that this is not required of all embodiments. In some examples, the silicon nitride may be formed in a pattern, for example by leveraging a lithographic masking or other local deposition technique. As with other layers of material described herein, the waveguide layer 240 can be formed to any suitable thickness. One example thickness is on the order of 80-150 nanometres. In other cases, a thinner or thicker waveguide layer 240 may be determined based on an optical or photonic property of the waveguide layer 240 at the selected thickness.

[0048] Once the silicon nitride layer 240 is formed over the first cladding layer, a lithographic mask may be disposed that is configured to define one or more features resulting from a subsequent manufacturing step (e.g. etching, mechanical ablation, laser ablation, and so on). The lithographic mask may be formed from, and / or may include any of a number of suitable materials including resins, polystyrenes, lacquers, and so on including one ormore photoactive additives or compounds. The lithographic mask may be formed to any suitable thickness which may vary from process to process.

[0049] Fig. 4E depicts a cross-sectional side view of a third intermediate stage of manufacturing the photonic integrated circuit having the optical mode coupler, and Fig. 4F depicts a cross-sectional end view of the third intermediate stage. In particular, Figs. 4E and 4F depict the waveguide structure of Figs. 4C and 4D after etching through apertures defined by the lithographic mask. The etching operation, a result of which is shown in Fig. 4E, may have any suitable selectivity and any suitable isotropy / anisotropy. The etch may be a wet etch process or a plasma-phase process, and selection therebetween (or of another process or process type) may be based at least in part on design or manufacturing constraints. Any suitable etching process compatible with the lithographic mask can be used.

[0050] Regardless of the process (or process characteristics, such as etchant, duration, temperature, pressure, and so on) selected to leverage the pattern defined by the lithographic mask, once the etch is completed, a set of one or more waveguide features may be defined. In particular, as is depicted in the figures, a waveguide 140 of the photonic integrated circuit is defined. Furthermore, the silicon nitride has been etched away from the edge of the waveguide structure such that there is a distance between the proximal end 160 of the resulting waveguide 140 and the edge facet 180 of the waveguide structure that may be formed by a later stage in manufacture.

[0051] Once the etch is completed, as well as the waveguide 140 further waveguide layer features (not shown) may also be defined for use in alignment of the photonic integrated circuit with one or more optical fibres (not shown) in a later stage of manufacture. For example, such further waveguide layer features may provide a looped path from a first further edge facet of the waveguide structure to a second further edge facet of the waveguide structure, designed to align with optical modes emerging from or entering particular optical fibres, such that the alignment of another optical fibre with the mode-matching region 170 and waveguide 140 can be checked.

[0052] Fig. 4G depicts a cross-sectional side view of a fourth intermediate stage of manufacturing the photonic integrated circuit having the optical mode coupler, and Fig. 4H depicts a cross-sectional end view of the fourth intermediate stage. In particular, Figs. 4G and 4H depict the waveguide structure of Figs. 4E and 4F after deposition of a second cladding layer formed of the same cladding material as the first cladding layer. More particularly, once the waveguide 140 of the waveguide layer is defined another cladding layer can be added over the proximal end of the waveguide 140 and the exposed part of the first cladding layer beneath. In many cases, the extended thickness of the cladding material 150 can be selected based at least in part on an optical property of the cladding material. For example, the thickness may be selected, at least in part, to reduce transition losses and / or propagation losses of light traversing the silicon nitride waveguide 140, or to reduce transition losses in the modematching region 170 that is to be formed in the cladding material between the edge facet 180 of the waveguide structure and the waveguide 140. Once deposited, the extended region of the second cladding layer may be annealed or densified.

[0053] As depicted in Fig. 4G, after the second cladding layer has been provided, the waveguide structure comprises a waveguide 140, the proximal end 160 of which is set back from the edge facet 180 of the waveguide structure and separated from the edge facet 180 by an amount of cladding material.

[0054] In some examples, (not depicted in Figs. 4A to 4 J), a portion of the substrate layer 190 may be removed from under the part of the optical mode coupler in which a mode -matching region 170 is to be formed. This may reduce (e.g. eliminate) any degradation in the fabrication of the mode-matching region 170 caused by the substrate.

[0055] Next, a mode-matching region 170 is formed in the cladding material 150 between the proximal end 160 of the waveguide 140 and the edge facet 180. The mode-matching region 170 is formed by laser writing the modematching region 170 into the waveguide structure. For example, a fabrication laser system comprising a femtosecond laser and a spatial light modulator may be used to write a mode-matching region 140 into the cladding material 150 between the edge facet 180 and the waveguide 140. For example, the mode-matching region fabrication system may comprise a regenerative amplified Yb:KGW laser with MHz repetition rate, and optical pulse duration on the order of 200fs in conjunction with a spatial light modulator. The skilled person will appreciate that the type of laser used in the fabrication laser system may be varied, as may other parameters such as the power, repetition rate and pulse duration of the laser.

[0056] In some examples, the fabrication laser system may be operated in a non-heating regime with low laser repetition rate (e.g. <10 kHz). In other examples, the fabrication laser system may be operated with high laser repetition rate (e.g. >500 kHz). The focus of the fabrication laser system may be scanned across the cladding material between the edge facet 180 and waveguide 140 multiple times to produce the desired refractive index profile of the mode-matching region 170. Other parameters of the fabrication laser system, such as pulse energy and scanning speed, may be selected (or varied) in order to achieve the desired refractive index profde.

[0057] Fig. 41 depicts a cross-sectional side view of a fifth intermediate stage of manufacturing the photonic integrated circuit having the optical mode coupler, and Fig. 4J depicts a cross-sectional end view of the fifth intermediate stage. In Figs. 41 and 4J, the waveguide structure 110 is shown with a mode-matching region 170 formed of the cladding material between the edge facet 180 of the waveguide structure 110 and the proximal end 160 of the waveguide 140. The mode-matching region is configured to match a mode of light supported by the waveguide 140 to a fibre mode supported by an optical fibre at the edge facet of the waveguide structure 110.

[0058] Following laser-writing of the mode-matching region, further operations may be performed, such as the provision of further cladding layers and / or or waveguide layers, or finishing operations such as cleaning, polishing and / or coupling to other optical or electrical circuits. In particular, an optical fibre may be positioned into alignment with the edge facet 180 of the waveguide structure 110. In some examples, the alignment of the optical fibre may be checked using e.g. a reference beam and additional waveguide features provided as part of the waveguide lay er 240. The optical fibre may be brought into direct contact with the edge facet 180 of the waveguide structure 110. The optical fibre may optionally be bonded to the edge facet 180 of the fibre-to-waveguide optical mode coupler using e.g. index matching epoxy. Fusion splicing techniques may be utilised to secure the optical fibre to the chip facet, minimising interface losses and improving the robustness of the optical packaging.

[0059] The foregoing examples described in relation to Fig. 4A-4J and the various alternatives thereof and variations thereto are presented, generally, for purposes of explanation, and to facilitate an understanding of various configurations and constructions of method of manufacturing an optical structure, such as described herein. However, it will be apparent to one skilled in the art that some of the specific details presented herein may not be required in order to practice a particular described embodiment, or an equivalent thereof.

[0060] Thus, it is understood that the foregoing and following descriptions of specific embodiments are presented for the limited purposes of illustration and description. These descriptions are not targeted to be exhaustive or to limit the disclosure of the precise forms recited herein. To the contrary it will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings.

[0061] As such, generally and broadly, the method(s) of manufacturing described in reference to Figs. 4A to 4J may be summarised as (i) providing a waveguide structure, wherein providing the waveguide structure comprises: providing a substrate layer; providing a waveguide layer above the substrate layer, the waveguide layer comprising a waveguide having a proximal end; providing a cladding material around the waveguide layer and between the proximal end of the waveguide and an edge facet of the waveguide structure; and (ii) laser writing the cladding material between the proximal end of the waveguide and the edge facet of the waveguide structure to form a mode-matching region, the mode-matching region configured to match a mode of light supported by the waveguide to a fibre mode supported by an optical fibre at the edge facet of the waveguide structure.

[0062] The techniques described herein may be particularly useful for coupling arrays of optical fibres to waveguides on a photonic integrated circuit. Fig. 5 depicts a fibre array 210. As shown in Fig. 5, the optical fibres 220 in a fibre array 210 can suffer from misalignment, for example the optical fibre cores 130 may be misaligned from their ideal position (depicted by a cross with reference numeral 230) in their respective optical fibres 220. Accordingly, the fibre cores 130 may be unevenly spaced apart from one another and may be centred at different heights. However, according to some examples herein, the fibre array 210 may be imaged and characterised to determine the relative alignments of the optical fibre cores 130. The relative alignments of the optical fibre cores 130 may then be taken into account when laser-writing the respective mode-matching regions of the optical mode coupler. The mode-matching regions 170 may accordingly provide further adaptability to mitigate the fixed boundary conditions set by the waveguides of the photonic integrated circuit and the fibre core positions in the fibre array.

[0063] Fig. 6 and Fig. 7 respectively depict a cross-sectional top view and a cross-sectional angled view of a portion of an optical mode coupler of a photonic integrated circuit coupled to several optical fibres of a fibre array. The optical mode coupler 110 comprises a waveguide 140 for each of the fibre cores 130. A mode-matching region 170 is laser written into the cladding material between each waveguide 140 and the corresponding edge facet 180. The mode-matching regions can correct for alignment problems with the optical fibre cores 130.

[0064] Each feature disclosed in this specification (including any accompanying claims, abstract or drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. The disclosure is not restricted to the details of any foregoing embodiments. The disclosure extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. The claims should not be construed to cover merely the foregoing embodiments, but also any embodiments which fall within the scope of the claims.

Claims

CLAIMS1. An optical mode coupler comprising: a waveguide structure comprising: a waveguide, the waveguide having a proximal end, the proximal end situated at a distance from an edge facet of the waveguide structure; a cladding material encasing the proximal end of the waveguide; and a mode-matching region formed of the cladding material and situated between the proximal end of the waveguide and the edge facet of the waveguide structure, the mode-matching region configured to match a mode of light supported by the waveguide to a fibre mode supported by an optical fibre at the edge facet of the waveguide structure.

2. An optical mode coupler according to claim 1, wherein at least a portion of the cladding material inside the mode-matching region has a different refractive index profile to a refractive index profde of the cladding material outside of the mode-matching region.

3. An optical mode coupler according to claim 2, wherein the mode-matching region has been laser-written.

4. An optical mode coupler according to any preceding claim, wherein the waveguide structure further comprises: a second waveguide, the second waveguide having a proximal end, the proximal end terminating a distance from the edge facet of the waveguide structure; and a second mode-matching region formed of the cladding material and situated between the proximal end of the second waveguide and the edge facet of the waveguide structure, the second mode -matching region configured to match a mode of light supported by the second waveguide to a fibre mode supported by a second optical fibre at the edge facet of the waveguide structure.

5. An optical mode coupler according to any preceding claim, wherein at least a part of the waveguide is tapered.

6. An optical mode coupler according to any preceding claim, wherein the cladding material comprises silica.

7. An optical mode coupler according to any preceding claim, wherein the waveguide comprises silicon nitride.

8. An optical mode coupler according to any preceding claim, further comprising a substrate layer, wherein the waveguide is provided above the substrate layer, and wherein an edge of the substrate layer does not coincide with the edge facet of the waveguide structure.

9. A method of manufacturing an optical mode coupler, the method comprising: providing a waveguide structure, wherein providing the waveguide structure comprises: providing a waveguide layer, the waveguide layer comprising a waveguide having a proximal end;providing a cladding material around the waveguide layer and between the proximal end of the waveguide and an edge facet of the waveguide structure; and laser writing the cladding material between the proximal end of the waveguide and the edge facet of the waveguide structure to form a mode-matching region, the mode-matching region configured to match a mode of light supported by the waveguide to a mode of light supported by an optical fibre at the edge facet of the waveguide structure.

10. The method of claim 9, wherein providing the waveguide structure comprises providing a substrate layer, and wherein the waveguide layer is provided above the substrate layer.

11. The method of claim 10, wherein providing the waveguide structure further comprises removing a portion of the substrate layer.

12. The method of claim 9 or claim 10 or claim 11, further comprising polishing the cladding material at the edge facet.

13. The method of any of claims 9 to 12, further comprising affixing an optical fibre to the edge facet of the waveguide structure in alignment with the mode -matching region.

14. The method of any of claims 9 to 13, wherein the waveguide layer further comprises a second waveguide having a proximal end, the method further comprising: providing the cladding material between the proximal end of the second waveguide and the edge facet of the waveguide structure; and laser writing the cladding material between the proximal end of the second waveguide and the edge facet of the waveguide structure to form a second mode-matching region, the mode-matching region configured to match a mode of light supported by the second waveguide to a fibre mode supported by a second optical fibre at the edge facet.

15. The method of any of claims 9 to 14, wherein the cladding material comprises silica.

16. The method of any of claims 9 to 15, wherein the waveguide layer comprises silicon nitride.

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