Optical couplers with trenched waveguides

Abstract

Described herein are systems and techniques for providing photonic devices having efficient optical coupling between waveguides. The photonic devices comprise a glass substrate having glass waveguides and a photonic integrated circuit (PIC) having PIC waveguides evanescently coupled with the glass waveguides. The waveguides may have recesses disposed between adjacent waveguides to allow for excess adhesive to be squeezed out. The PIC may have a trench exposing the waveguide layer of the PIC, thus reducing the gap between the PIC waveguides and the glass waveguides when coupled.

Description

Atorney Docket No. L0858.70111WO00- 1 -OPTICAL COUPLERS WITH TRENCHED WAVEGUIDESCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application Serial No. 63 / 736,205, filed on December 19, 2024, under Attorney Docket No. L0858.70111US01 and entitled "OPTICAL COUPLERS WITH TRENCHED WAVEGUIDES," which is hereby incorporated herein by reference in its entirety.BACKGROUND

[0002] Optical interconnects are a type of communication technology employed by optical communication systems that use light signals to transmit data between different components or devices within the system. Optical communication systems employ various optical components to transmit an optical signal throughout the network. Optical communication systems thus benefit from efficient, low-loss coupling mechanisms at the optical interconnects.SUMMARY

[0003] The inventors have developed the devices and techniques described herein to provide low-loss coupling mechanisms for the optical interconnects of an optical communication system. For example, some optical interconnects employ evanescent coupling to optically couple waveguides on separate substrates. The insertion loss due to evanescent coupling is proportional to the distance between the waveguides and thus, some embodiments provide for systems and techniques that reduce that distance between evanescently coupled waveguides, thereby minimizing the insertion loss.

[0004] In some aspects, the techniques described herein relate to a photonic device including: a photonic integrated circuit (PIC) including a plurality of PIC waveguides and a trench etched in the PIC; and a glass substrate, disposed in the trench, the glass substrate including a plurality of glass waveguides evanescently coupled with respective PIC waveguides of the plurality of PIC waveguides, wherein: a first glass waveguide and a second glass waveguide of the plurality of glass waveguides are separated from each other by a recess in the glass substrate.

[0005] In some aspects, the techniques described herein relate to a photonic device, wherein a first PIC waveguide and a second PIC waveguide of the plurality of PIC waveguides are separated from each other by a recess in the PIC, the recess in the PIC being aligned with the#14739475vlrecess in the glass substrate to form a cavity between the first and second glass waveguides and first and second PIC waveguides.

[0006] In some aspects, the techniques described herein relate to a photonic device, further including an index-matching adhesive layer disposed between the glass waveguides and PIC waveguides.

[0007] In some aspects, the techniques described herein relate to a photonic device, wherein the adhesive layer has a thickness of 2 pm or less.

[0008] In some aspects, the techniques described herein relate to a photonic device, wherein a portion of the adhesive layer extends into the cavity when the glass substrate is attached to the PIC.

[0009] In some aspects, the techniques described herein relate to a photonic device, wherein the recess in the PIC is formed by dry etching, wet etching, or anisotropic etching.

[0010] In some aspects, the techniques described herein relate to a photonic device, further including one or more electronic stacks disposed on the PIC, and wherein: the glass substrate includes trenches configured to receive respective electronic stacks of the electronic stacks disposed on the PIC when the glass substrate is attached to the PIC.

[0011] In some aspects, the techniques described herein relate to a photonic device, wherein the electronic stacks include redistribution layers electronically coupled to application-specific integrated circuits (ASICs) disposed on the PIC, the redistribution layers having conductive pads configured to abut the glass substrate when the glass substrate is attached to the PIC.

[0012] In some aspects, the techniques described herein relate to a photonic device, wherein the trenches are disposed in the glass substrate between groups of glass waveguides of the plurality of waveguides.

[0013] In some aspects, the techniques described herein relate to a photonic device including: a photonic integrated circuit (PIC) including a plurality of PIC waveguides and a trench etched in the PIC, wherein: a first PIC waveguide and a second PIC waveguide of the plurality of PIC waveguides are separated from each other by a recess in the PIC; and a glass substrate, disposed in the trench, the glass substrate including a plurality of glass waveguides evanescently coupled with respective PIC waveguides of the plurality of PIC waveguides.

[0014] In some aspects, the techniques described herein relate to a photonic device, further including an index-matching adhesive layer disposed between the PIC waveguides and the glass waveguides.#14739475v1

[0015] In some aspects, the techniques described herein relate to a photonic device, wherein at least some of the adhesive layer extends into the recesses when the glass substrate is attached to the PIC.

[0016] In some aspects, the techniques described herein relate to a photonic device, wherein a thickness of the adhesive layer between the PIC waveguides and glass waveguides is 2 pm or less.

[0017] In some aspects, the techniques described herein relate to a photonic device, wherein the trench is configured to extend through the PIC so that a tail of an optical mode of the PIC waveguide is located within the trench.

[0018] In some aspects, the techniques described herein relate to a photonic device, wherein the recess in the PIC is formed by dry etching, wet etching, or anisotropic etching.

[0019] In some aspects, the techniques described herein relate to a method of manufacturing a photonic device, the method including: obtaining a photonic integrated circuit (PIC) patterned with a plurality of PIC waveguides and a trench etched in a coupling region of the PIC; obtaining a glass substrate patterned with a plurality of glass waveguides, wherein a first glass waveguide and a second glass waveguide of the plurality of glass waveguides disposed in a coupling region of the glass substrate are separated from each other by a recess in the glass substrate; and optically coupling at least some of the plurality of PIC waveguides with respective glass waveguides of the plurality of glass waveguides by: placing the glass substrate in the trench; and compressing the coupling regions of the glass substrate and the PIC.

[0020] In some aspects, the techniques described herein relate to a method, the method further including: prior to optically coupling at least some of the plurality of PIC waveguides with the respective glass waveguides, disposing adhesive between the PIC waveguides and the glass waveguides; and wherein compressing the coupling regions of the glass substrate and the PIC includes compressing the adhesive between the PIC waveguides and the glass waveguides to form an index-matching adhesive layer.

[0021] In some aspects, the techniques described herein relate to a method, wherein compressing the adhesive to form the index-matching adhesive layer includes pressing the glass substrate into the PIC until the index-matching adhesive layer has a thickness of 2 pm or less.

[0022] In some aspects, the techniques described herein relate to a method, wherein at least a portion of the adhesive is pressed into the trenches of the glass substrate when compressing the index-matching adhesive layer.

[0023] In some aspects, the techniques described herein relate to a method, further including aligning the glass waveguides with the PIC waveguides within the trench.#14739475v1BRIEF DESCRIPTION OF DRAWINGS

[0024] Various aspects and embodiments of the application will be described with reference to the following figures. It should be appreciated that the figures are not necessarily drawn to scale. Items appearing in multiple figures are indicated by the same or similar reference number in the figures in which they appear. In the figures:

[0025] FIG. 1A illustrates a side view of an example photonic device, according to some embodiments;

[0026] FIG. IB illustrates a top view of the photonic device of FIG. 1A, according to some embodiments;

[0027] FIG. 2A illustrates an enlarged view of example coupling regions of the photonic integrated circuit (PIC) and the glass substrate of a photonic device, according to some embodiments;

[0028] FIG. 2B illustrates an enlarged view of other example coupling regions of the photonic integrated circuit (PIC) and the glass substrate of a photonic device, according to some embodiments;

[0029] FIG. 3 A illustrates an example coupling mechanism between the PIC and the glass substrate of a photonic device, according to some embodiments;

[0030] FIG. 3B illustrates another example coupling mechanism between the PIC and the glass substrate of a photonic device, according to some embodiments;

[0031] FIG. 3C illustrates an enlarged view of the gap between PIC waveguides and glass waveguides of a photonic device, according to some embodiments;

[0032] FIG. 3D illustrates the gap of FIG. 3C with the coupling regions of the PIC and glass substrate having been compressed, according to some embodiments; and

[0033] FIG. 4 is a flowchart showing an example method of manufacturing a photonic device, according to some embodiments.DETAILED DESCRIPTION

[0034] Optical communication systems that utilize light signals to transmit data between different components or devices in the system utilize various optical components to transmit the signals. For example, light signals may be transmitted through waveguides on various substrates, including photonic integrated circuits and glass interposers. Accordingly, the inventors have recognized and appreciated that optical communication systems benefit from low-loss coupling mechanisms to maintain the quality of the light signal during transmission.#14739475v1

[0035] The inventors have further recognized and appreciated that evanescent coupling may provide a low-loss optical coupling between waveguides on two substrates. Evanescent coupling typically exhibits low-insertion losses approximately 0.5 dB or less. However, the insertion loss in evanescent coupling is proportional to the gap between the two waveguides. Accordingly, the techniques and systems described herein relate to photonic devices that provide low-loss optical coupling by reducing the gap between evanescently coupled waveguides.

[0036] Conventional techniques such as fusion bonding (e.g., oxide-to-oxide bonding), typically require that waveguides be in close proximity to both of two planarized surfaces of the substrates. In many applications, this condition renders the conventional direct bonding techniques infeasible for many applications as conventional photonic integrated circuits (PICs) have routing and bumping requirements that make the condition difficult to meet.

[0037] To overcome this constraint, the photonic devices described herein include one or more recesses disposed in a coupling region of a PIC of the photonic device. The cavities may be etched to expose a waveguide layer of the PIC patterned with a plurality of PIC waveguides. In that way, the PIC waveguides can be brought in closer proximity with the waveguides on the other optical component. In some embodiments, the other optical component is a glass substrate patterned with a plurality of glass waveguides configured to be evanescently coupled with the PIC waveguides.

[0038] To bond the glass substrate to the PIC, some embodiments may employ an indexmatching adhesive to facilitate physical coupling while minimizing the interference with the evanescent coupling between the PIC and glass waveguides. However, the inventors have recognized and appreciated that conventional device architectures make it difficult to form a thin layer of adhesive between the glass substrate and PIC due to the high viscous forces that arise when attempting to flatten the adhesive to a thin film over a large area. Accordingly, in some embodiments, the photonic devices described herein may utilize recesses etched between the PIC waveguides and / or between the glass waveguides. As the glass substrate is pressed into the PIC during manufacture, the adhesive disposed between the two is pressed into a thin film with the recesses allowing for excess adhesive to be squeezed out from between the PIC and glass substrate. In that way, the high viscous forces can be avoided, enabling thinner adhesive layers to be formed between the waveguides. In some embodiments, the techniques described herein provide for a gap between the PIC waveguides and glass waveguides to be approximately 2 pm or less.

[0039] The inventors have further recognized and appreciated that the glass substrate can be further configured to accommodate other components of the photonic device to enable electrical#14739475v1connectivity alongside efficient optical coupling. In some embodiments, the glass substrate may include one or more trenches configured to receive electronic stacks of the photonic device. In some embodiments, optical adhesive and / or capillary underfill may be used in these trench(es), enabling the electrical connectivity and efficient optical coupling. For example, in some embodiments, an electronic stack may comprise a redistribution layer electronically coupled to application-specific integrated circuits (ASICs) disposed on the PIC. The redistribution layer may further include one or more conductive pads that abut the glass substrate when the PIC is coupled with the glass substrate

[0040] FIG. 1A illustrates a side view of an example photonic device 100, according to some embodiments. In the illustrated embodiment, photonic device 100 comprises an optoelectronic assembly 110 and two glass substrates 120 coupled to the assembly. FIG. IB illustrates a top view of the photonic device 100 of FIG. 1A, according to some embodiments.

[0041] Optoelectronic assembly 110 includes a PIC 111 and one or more application- specific integrated circuits (ASICs) 113 disposed on PIC 111, and a substrate 117 upon which PIC 111 is disposed. This arrangement allows the PIC waveguides 112 to be aligned and optically coupled with glass waveguides 122, as described in detail further below. ASICs 113 may comprise any suitable electronic integrated circuit (EIC) for performing electronic functions (e.g., memory storage, processing, electronic control of PIC 111, electronic switching components). For example, each ASIC 113 may comprise a memory chip (e.g., high bandwidth memory), a compute chip (e.g., a central processing units (CPU), a graphics processing (GPU), a tensor processing unit (TPU), an accelerator or any other suitable xPU), a switching chip, an input / output (VO) chip, a serializer / deserializer (SerDes) and / or any other suitable electronic chip component. Substrate 117 may comprise an organic substrate attached to, and electrically coupled to PIC 111.

[0042] PIC 111 may include one or more photonic and electronic components. For example, PIC 111 includes one or more PIC waveguides 112. In some embodiments, PIC 111 may include active components including modulators, photodetectors, optical switches, optical transmitters, optical receivers, optical transceivers, or any other active component. PIC waveguides 112 may be disposed on a waveguide layer of PIC 111. As will be described further below with respect to FIGS. 3 A and 3B, in some embodiments, PIC 111 may include one or more trenches etched in a coupling region A of PIC 111 to expose the waveguide layer. In some embodiments, the trench may be etched through enough material of PIC 111 to place the tail of the waveguide mode within the trench to enable efficient evanescent coupling. In some embodiments, the trench may expose a cladding material of the waveguide layer, the cladding being disposed between the PIC#14739475v1waveguides 112 and a surface of the PIC 111. In some embodiments, the trench may expose the PIC waveguides 112 themselves. In that way, the gap between glass waveguides 122 and PIC waveguides 112 may be reduced when glass substrate 120 is attached to PIC 111.

[0043] Glass substrate 120 may be made of any suitable type of glass, including for example SiO2, fused silica, or borosilicate glass. The glass substrate 120 may be passive in nature in that it may include passive optical devices (e.g., waveguides, passive couplers, waveguide crossings, wavelength multiplexers / demultiplexers, etc.) but may omit active optical devices (e.g., modulators, detectors, switches, etc.). In the illustrated embodiment, glass substrate 120 includes a plurality of glass waveguides 122. In the illustrated embodiment, two glass substrates 120 are utilized in photonic device 100 — one on each side of the optoelectronic assembly 110. However, it can be appreciated that a single glass substrate may be used (e.g., on one side or an integral piece surrounding optoelectronic assembly 110), or more than two glass substrates may be used (e.g., one for each ASIC 113 disposed on PIC 111). Optical fiber 103 (e.g., a single fiber or fiber array) may be coupled to glass substrate 120 in any suitable manner. For example, the optical fiber 103 may be coupled using an edge coupler, grating coupler, pluggable coupler, lens and mirror array, fiber array unit 102 or any other suitable coupler.

[0044] Glass waveguides 122 may be used to route light to and from the optoelectronic assembly 110 to transmit signals to and from the ASICs 113 of the assembly. In some embodiments, glass waveguides 122 may be disposed as groups 121, with each group 121 corresponding to a different ASIC 113 of the assembly.

[0045] The glass waveguides 122 may be made of any suitable material that is compatible with the technology used to fabricate the glass substrate 120. For example, the glass waveguides 122 can be made in-situ within the glass substrate 120 itself with lithography or laser writing. In another embodiment, the glass waveguides 122 and the passive optical components within the glass substrate 120 can be manufactured using ion-exchange processes. Different glass compositions may inform which manufacturing process may be used. Further, the glass waveguides 122 may be made of a material having a refractive index greater than the refractive index of the surrounding material, thus ensuring that the optical signal mode is sufficiently contained and guided within the waveguide. For example, the glass substrate 120 may be made of SiO2 and the waveguides may be made of SiO2 doped to have a larger refractive index and / or be made of silicon nitride (e.g., grown, deposited, or bonded). In some embodiments, the glass waveguides 122 may be formed near the surface of the glass substrate 120. In that way, the waveguides may be easily accessed for coupling with the optoelectronic assemblies 110 and fiber array couplers.#14739475v1

[0046] FIG. 2A illustrates an enlarged view of example coupling regions (e.g., coupling region A of FIG. 1A) of the photonic integrated circuit (PIC) and glass substrate of a photonic device, according to some embodiments. In the coupling region, the glass waveguides 122 are evanescently coupled with PIC waveguides 112. In the illustrated embodiment, to secure the glass substrate 120 to PIC 111 while minimally interfering with the optical coupling, an adhesive layer 130 may be used. In some embodiments, the adhesive layer has a thickness of approximately 2 pm or less. As noted above and described further herein with respect to FIGS. 3A-3D, to facilitate forming the adhesive layer, recesses may be formed in the glass substrate 120 and / or the PIC 111 between adjacent waveguides to avoid viscous forces that arise when forming a thin layer over a large surface area. For example, in the illustrated embodiment, glass substrate 120 includes recesses 124 disposed between adjacent glass waveguides 122. In some embodiments, the adhesive may be made of an index-matching material. In some embodiments, the index-matching material may have a refractive index between the refractive indices of the PIC waveguides 112 and glass waveguides 122. In some embodiments, the index-matching material may have a refractive index between 1.50 and 1.57.

[0047] In some embodiments, optoelectronic assemblies 110 include electronic stacks 116 disposed partially or fully in portions of the coupling region of PIC 111. For example, electronic stacks 116 may be disposed between groups of PIC waveguides 112 (e.g., where each group corresponds to a separate ASIC 113). Accordingly, glass substrate 120 may include one or more trenches 126 for receiving electronic stacks 116 when glass substrate 120 is attached to PIC 111. In some embodiments, electronic stacks 116 may comprise redistribution layers electrically coupled to different ASICs 113 of the assembly to enable inter-ASIC communication.

[0048] In some embodiments, trench 126 may be configured to accommodate bumping on the surface of electronic stacks 116. FIG. 2B illustrates an enlarged view of other example coupling regions of the photonic integrated circuit (PIC) and glass substrate of a photonic device, according to some embodiments. Although the coupling region illustrated is similar to that of FIG. 2A, electronic stacks 116 include one or more conductive pads 115. Conductive pads 115 may abut glass substrate 120 when the substrate is coupled with PIC 111. By providing a trench 126 for electronic stacks 116 (with or without conductive pads 115), the architecture enables a mix of optical adhesive and capillary underfill to be used, allowing for electrical connectivity alongside efficient optical coupling.

[0049] As noted above, evanescent coupling provides efficient optical coupling between the glass waveguides and PIC waveguides. However, the insertion loss attributed to the evanescent coupling is proportional to the gap between the PIC and glass waveguides. Accordingly, the#14739475v1coupling regions of the PIC and glass substrate may have one or more features to reduce the gap between the waveguides. In some embodiments, the gap between the waveguides may be reduced to approximately 2 pm or less. FIG. 3A illustrates an example coupling region between a group of PIC and glass waveguides of a photonic device, according to some embodiments. In the illustrated embodiment, the group of waveguides includes three coupled pairs of PIC waveguides 312 and glass waveguides 322, however a group may consist of any number of coupled pairs.

[0050] Glass substrate 320 is coupled to PIC 311 within trench 318 of PIC 311. Trench 318 may be etched (e.g., by dry etching) from a surface through PIC 311. Enough material may be removed from PIC 311 to place the tail of the waveguide mode within trench 318, thus enabling efficient optical coupling. In some embodiments, trench 318 may be etched to expose waveguide layer 313 of PIC 311. In that way, the distance between the glass waveguides 322 and PIC waveguides 312 is reduced. Exposing the waveguide layer 313 may comprise exposing the PIC waveguides 312 themselves. In some embodiments, waveguide layer 313 may include the PIC waveguides 312 and a cladding disposed at least between the PIC waveguides 312 and the surface of PIC 311 to provide protection and reduce signal leak from the waveguide. Thus, in some embodiments, exposing waveguide layer 313 may comprise exposing the cladding.

[0051] PIC 311 further includes recesses 314A extending through waveguide layer 313 between adjacent PIC waveguides 312. Alternatively or additionally, glass substrate 320 further includes recesses 324A between adjacent glass waveguides 322. Although the illustrated embodiment includes recesses 314A forming cavities with corresponding recesses 324A, it can be appreciated that only one of recesses 314A or recesses 324A may be present. Recesses 314A and / or recesses 324A may facilitate formation of an adhesive layer between PIC waveguides 312 and glass waveguides 322. As will be discussed further below, forming the photonic devices may comprise compressing the coupling regions of the glass substrate 320 and PIC 311. As will be discussed further below with respect to FIGS. 3C and 3D, adhesive may be disposed between the components which expands during compression, for example, in gap 330. Where conventional architectures utilize flat coupling surfaces, the adhesive gets compressed over a large surface area, introducing high viscous forces and preventing the adhesive layer from achieving lower thicknesses. Accordingly, in the illustrated embodiment, during compression, at least a portion of the adhesive (e.g., excess adhesive) may be squeezed into recesses 314A and / or 324A, avoiding the high viscous forces of the adhesive being compressed over a large surface area. Accordingly, the recesses 314A and / or 324A enable thinner adhesive layers to be achieved, thereby reducing the gap, and thus, the insertion loss due to the evanescent coupling of#14739475v1the waveguides. This allows for evanescent coupling over larger areas while maintaining high waveguide density.

[0052] FIG. 3B illustrates another example coupling region between a group of PIC and glass waveguides of a photonic device, according to some embodiments. The coupling region of FIG. 3B is similar to the one shown in FIG. 3 A but differs in the shape of the recesses and cavities formed by recesses 314B and 324B. Where recess 314A has a rectangular shape, recess 314B has a rectangular portion through waveguide layer 313 and a triangular portion extending beyond waveguide layer 313 into PIC 311. In some embodiments, the shape of the recess may be determined in part based on the manufacturing process used to form the recesses. For example, the recesses may be etched using dry etching, anisotropic etching, or any other suitable etching process. Further, although illustrated as having flat surfaces, in some embodiments, the recesses may be shaped and sized to secure glass substrate 320 within trench 318.

[0053] As described throughout, manufacturing the photonic devices described herein may include disposing adhesive between the glass substrate and PIC of the photonic device and then compressing the coupling regions of the glass substrate and PIC. Compressing the components forms a thin adhesive layer between the waveguides to facilitate physical coupling while minimally interfering with the optical coupling of the waveguides. FIG. 3C illustrates an enlarged view of the gap between PIC waveguides and glass waveguides of a photonic device, according to some embodiments. In the illustrated embodiment, an adhesive layer 340 is disposed between glass substrate 320 and PIC 311 in gap 330 when the substrate and PIC are not compressed.

[0054] FIG. 3D illustrates the gap of FIG. 3C with the coupling regions of the PIC and glass substrate having been compressed, according to some embodiments. Once the substrate and PIC are compressed against each other, the adhesive layer 340 gets compressed (in the illustrated vertical direction) and expands outward (in the illustrated horizontal direction) to form a thin layer between the waveguides. As the adhesive expands outward, at least some of the adhesive spills over into the recess (e.g., recesses 314A and / or 324A). Having a recess in which excess adhesive spills over avoids the high viscous forces caused by compressing the adhesive of a large surface area. Thus, thinner adhesive layers 340 can be achieved. In some embodiments, the adhesive layer has a thickness of approximately 2 pm or less.

[0055] FIG. 4 is a flowchart showing an example method 400 of manufacturing a photonic device, according to some embodiments. The method begins at act 402 with obtaining a PIC patterned with waveguides thereon. The waveguides may be disposed on a waveguide layer of the PIC. In some embodiments, the PIC may further include one or more trenches in a coupling#14739475v1region of the PIC to expose the waveguide layer of the PIC. The coupling region may additionally or alternatively include recesses disposed between adjacent PIC waveguides. The PIC may further include one or more electronic coupling components for coupling electronic and optoelectronic components of an optoelectronic assembly. For example, the PIC may include conductive traces, through- silicon vias (TSVs), or other electronic coupling components.

[0056] In some embodiments, the PICs may be obtained from a foundry with the trenches and / or recesses having already been formed in the PIC. Alternatively, in some embodiments, obtaining the PIC patterned with waveguides comprises forming the trenches and / or recesses in the PIC in house using etching techniques. The trenches and / or recesses may be formed in any suitable manner, including, but not limited to, dry etching, wet etching, or anisotropic etching, as described herein.

[0057] The method proceeds to act 404 to obtain a glass substrate patterned with glass waveguides. The glass waveguides may be formed near one side of the glass substrate to facilitate evanescent coupling with the PIC waveguides. In some embodiments, recesses may be disposed in the glass substrate between adjacent glass waveguides. The glass waveguides can be made in-situ within the glass substrate itself with lithography or laser writing. In another embodiment, the glass waveguides and the passive optical components within the glass substrate can be manufactured using ion-exchange processes. Different glass compositions may inform which manufacturing process may be used.

[0058] The method proceeds to act 406, wherein adhesive is disposed between the PIC and glass substrate. Adhesive may be disposed in the coupling region of the PIC. For example, the adhesive may be disposed in the trench(es) exposing the waveguide layer of the PIC. The adhesive may be index-matching adhesive. For example, the adhesive may be index-matching epoxy. The index-matching adhesive may have an index of refraction between those of the PIC and glass waveguides. In some embodiments, the index-matching adhesive has an index of refraction between 1.50 and 1.57.

[0059] At 408, the PIC waveguides are optically coupled with the glass waveguides. At act 408A, the glass substrate is placed within the trench etched within the PIC. In that way, the tail of the optical mode of the glass waveguides may overlap with the tail of the optical mode of the PIC waveguides, optically coupling the PIC waveguides and glass waveguides. In some embodiments, placing the glass substrate within the trench may comprise aligning the glass waveguides with the PIC waveguides. The alignment may be performed either passively or actively. For example, visual markings may be used to indicate alignment. The visual markings may be disposed on the glass substrate and PIC. In some embodiments, the PIC and / or glass#14739475v1substrate may include alignment features. For example, the sidewall of the trench(es) exposing the waveguide layer of the PIC may position the glass substrate so that the waveguides are selfaligned.

[0060] At act 408B, the coupling regions of the glass substrate and the PIC are compressed against each other. As noted above, compressing the components may cause the adhesive to form a thin layer between the PIC and glass waveguides. Excess adhesive may expand out from between the waveguides and into the recesses of the PIC and / or glass substrate. As such, thinner adhesive layers can be achieved. In some embodiments, compressing the components may comprise compressing the components until the adhesive layer has a thickness of approximately 2 pm or less. In some embodiments, compressing the components comprises pressing the glass substrate into the PIC. Additionally or alternatively, compressing the components comprises pressing the PIC into the glass substrate.

[0061] Having thus described several aspects and embodiments of the technology of this application, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those of ordinary skill in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the technology described in the application. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, inventive embodiments may be practiced otherwise than as specifically described. In addition, any combination of two or more features, systems, articles, materials, and / or methods described herein, if such features, systems, articles, materials, and / or methods are not mutually inconsistent, is included within the scope of the present disclosure.

[0062] Also, as described, some aspects may be embodied as one or more methods. The acts performed as part of the method may be ordered in any suitable way. Accordingly, embodiments may be constructed in which acts are performed in an order different than described, which may include performing some acts simultaneously, even though shown as sequential acts in illustrative embodiments.

[0063] All definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and / or ordinary meanings of the defined terms.

[0064] The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.”#14739475v1

[0065] The phrase “and / or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases.

[0066] As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified.

[0067] The terms “approximately” and “about” may be used to mean within ±20% of a target value in some embodiments, within ±10% of a target value in some embodiments, within ±5% of a target value in some embodiments, and yet within ±2% of a target value in some embodiments. The terms “approximately” and “about” may include the target value.#14739475v1

Claims

CLAIMSWhat is claimed is:

1. A photonic device comprising: a photonic integrated circuit (PIC) comprising a plurality of PIC waveguides and a trench etched in the PIC; and a glass substrate, disposed in the trench, the glass substrate comprising a plurality of glass waveguides evanescently coupled with respective PIC waveguides of the plurality of PIC waveguides, wherein: a first glass waveguide and a second glass waveguide of the plurality of glass waveguides are separated from each other by a recess in the glass substrate.

2. The photonic device of claim 1, wherein a first PIC waveguide and a second PIC waveguide of the plurality of PIC waveguides are separated from each other by a recess in the PIC, the recess in the PIC being aligned with the recess in the glass substrate to form a cavity between the first and second glass waveguides and first and second PIC waveguides.

3. The photonic device of claim 2, further comprising an index-matching adhesive layer disposed between the glass waveguides and PIC waveguides.

4. The photonic device of claim 3, wherein the adhesive layer has a thickness of 2 pm or less.

5. The photonic device of claim 3, wherein a portion of the adhesive layer extends into the cavity when the glass substrate is attached to the PIC.

6. The photonic device of claim 2, wherein the recess in the PIC is formed by dry etching, wet etching, or anisotropic etching.

7. The photonic device of claim 1, further comprising one or more electronic stacks disposed on the PIC, and wherein: the glass substrate comprises trenches configured to receive respective electronic stacks of the electronic stacks disposed on the PIC when the glass substrate is attached to the PIC.#14739475v18. The photonic device of claim 7, wherein the electronic stacks comprise redistribution layers electronically coupled to application- specific integrated circuits (ASICs) disposed on the PIC, the redistribution layers having conductive pads configured to abut the glass substrate when the glass substrate is attached to the PIC.

9. The photonic device of claim 7, wherein the trenches are disposed in the glass substrate between groups of glass waveguides of the plurality of waveguides.

10. A photonic device comprising: a photonic integrated circuit (PIC) comprising a plurality of PIC waveguides and a trench etched in the PIC, wherein: a first PIC waveguide and a second PIC waveguide of the plurality of PIC waveguides are separated from each other by a recess in the PIC; and a glass substrate, disposed in the trench, the glass substrate comprising a plurality of glass waveguides evanescently coupled with respective PIC waveguides of the plurality of PIC waveguides.

11. The photonic device of claim 10, further comprising an index-matching adhesive layer disposed between the PIC waveguides and the glass waveguides.

12. The photonic device of claim 11, wherein at least some of the adhesive layer extends into the recesses when the glass substrate is attached to the PIC.

13. The photonic device of claim 11, wherein a thickness of the adhesive layer between the PIC waveguides and glass waveguides is 2 pm or less.

14. The photonic device of claim 10, wherein the trench is configured to extend through the PIC so that a tail of an optical mode of the PIC waveguide is located within the trench.

15. The photonic device of claim 10, wherein the recess in the PIC is formed by dry etching, wet etching, or anisotropic etching.

16. A method of manufacturing a photonic device, the method comprising:#14739475v1obtaining a photonic integrated circuit (PIC) patterned with a plurality of PIC waveguides and a trench etched in a coupling region of the PIC; obtaining a glass substrate patterned with a plurality of glass waveguides, wherein a first glass waveguide and a second glass waveguide of the plurality of glass waveguides disposed in a coupling region of the glass substrate are separated from each other by a recess in the glass substrate; and optically coupling at least some of the plurality of PIC waveguides with respective glass waveguides of the plurality of glass waveguides by: placing the glass substrate in the trench; and compressing the coupling regions of the glass substrate and the PIC.

17. The method of claim 16, the method further comprising: prior to optically coupling at least some of the plurality of PIC waveguides with the respective glass waveguides, disposing adhesive between the PIC waveguides and the glass waveguides; and wherein compressing the coupling regions of the glass substrate and the PIC comprises compressing the adhesive between the PIC waveguides and the glass waveguides to form an index-matching adhesive layer.

18. The method of claim 17, wherein compressing the adhesive to form the index-matching adhesive layer comprises pressing the glass substrate into the PIC until the index-matching adhesive layer has a thickness of 2 pm or less.

19. The method of claim 17, wherein at least a portion of the adhesive is pressed into the trenches of the glass substrate when compressing the index-matching adhesive layer.

20. The method of claim 17, further comprising aligning the glass waveguides with the PIC waveguides within the trench.#14739475v1

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